Economizing with outside air

[pwn01]

If my outdoor air enthalpy is less than my air handler return air enthalpy is it always best to bring in outside air if the indoor air humidity can be maintained within acceptable limits? I have heard that outside air dampers should be shut above 70 or 75 degrees F. Sometimes my return air is at a higher enthalpy than the outside air, but both are above 75 degrees.

[key]

the purpose of an economizer is to offer free cooling and reduce the cooling load. I would do a proper enthalpy check and adjust your controls accordingly. Is this application designed for make-up air aswell?

[pwn01]

To my knowledge, the single OSA damper in the system is the only inlet for make-up air.

An example may help. . .

If my return air is 74.90 deg F at 44.8% RH, the enthalpy is about 27.5 BTU/Lb. If the outside air is 90 deg F at 10% RH, the enthalpy is about 23.1 BTU/Lb. It is more economical to use outside air even though the temperature is very high because there is less water vapor and water vapor takes more energy to cool? Or is the savings offset by the greater temperature difference between the outside air and the desired AHU supply air temperature of 58 deg F (90-58=32 as opposed to 74.9-58=16.9)?

[sean88]

I've gotten away from using enthalpy sensors. They seem to be more problem than they are worth. Being a firm believer in the KISS principle (keep is simple stupid) I use dry bulb change over now. The sensors are better (just keep them out of the sun) and are less likely to get out of calibration.

If you are monitoring the system on a daily basis you can actually adjust the set point to suit the buildings needs. For example on a 60 F day, I'll run our unit on full economizer to keep the strip heat coming on in the terminal boxes.

[hvac3901]

enthalpy comparative economizer control is best. when using enthalpy comparative control you are looking at the total heat content of the air and you cannot solely reply upon dry bulb readings.

[john dalton]

Dear Pwn01,

Although you have neglected to give us any equipment information, I’m going to “assume” you have either a matched split or packaged system. Based upon that assumption, and the fact that most HVAC systems have a delta T of approximately 20 degrees F, then opening the economizer under the above conditions would be extremely detrimental to your HVAC system.

I would have to agree with Hvac3901 in his option that a typical economizer works much better and more efficiently with entropy rather than dry bulb temperature controls.

I also agree with Sean88 in his option that the KISS method is always the best method to design and work toward, but it is not a determining factor in the converting or elimination of humidity as a controlling factor for your econimiser.

Respectfully Submitted,
John J. Dalton

[djken]

Where are you ... I don't believe I've erer seen it 90* outside and 10% rh??

[pwn01]

Thanks for the replies.

I live in central NC. It is currently 88F with 19% RH. The 90F/10% was what it looked like it may arrive at today the way things were moving this morning.

The chiller is a 250 or 300 ton Carrier. It has a primary-secondary loop chill water setup. The heat is an LP fired boiler circulating through VAV box reheat coils (also a primary-secondary loop setup).

The chiller was donated and installed before my time and it is oversized for the building. It usually only runs one or two out of six (or eight?) compressors. (I'm sitting at home and can't remember exact specs on the chiller).

[john dalton]

Dear Djken,

Pwn01 didn’t state that the 90 degrees F and the 10% RH was his outside conditions, but I can tell you that at times here in Los Angeles the outside conditions can reach 112 degrees F and 8% RH. It’s happened twice in the last three years here.

Respectfully Submitted,
John J. Dalton

[fixerup]

Stick with enthalpy it is your TOTAL heat load.

[ac/dc]

I cant imagine trying to cool a space with with outdoor
air above 65F regaurdless of its enthalpy. I agree enthalpy
is far better than dry bulb in this part of the country.
But I will say dry bulb is simple and more reliable if you
can get away with not sensing the humidity

[tab-tech]

... is the cats meow. I've seen conditions in early mornings where OSA temps are 55 - 60 degrees with RH levels at 90% or higher. (dense fog from dew lifting off the ground and surface water sources). Enthalpy comparison is the standard design here . In most cases I find that you do not want the economizer damper opening until the minimum osa damper begins to open from its minimum (ventilation) position. If the unit also incorporates a motorized relief damper, lag it a little behind the economizer damper to keep your building "slightly positive pressure". If your building goes negative, you'll suck humid air into the building from doors and loading docks (not good).
When OSA and RAT conditions are available, energy management control systems can save big bucks.

[NormChris]

Perhaps this short article will help.


http://hvac-talk.com/vbb/showthread.php?threadid=34033


Norm

[Carnak]

Originally posted by sean88
I've gotten away from using enthalpy sensors. They seem to be more problem than they are worth. Being a firm believer in the KISS principle (keep is simple stupid) I use dry bulb change over now. The sensors are better (just keep them out of the sun) and are less likely to get out of calibration.

If you are monitoring the system on a daily basis you can actually adjust the set point to suit the buildings needs. For example on a 60 F day, I'll run our unit on full economizer to keep the strip heat coming on in the terminal boxes.

Running the risk of pumping humidity into the building at 60F. Maybe consider 57 or 55.

[rubobornot]

I was in Canyon City Colorado one summer when the dry bulb temperature was 115 degrees and the RH was 5%. That was the year of all the fires in the hill country NW of Colorado Springs.

[tab-tech]

Originally posted by rubobornot
I was in Canyon City Colorado one summer when the dry bulb temperature was 115 degrees and the RH was 5%. That was the year of all the fires in the hill country NW of Colorado Springs.

...dryer than a Popcorn Fart.

[pwn01]

If the return air temperature is 77 deg F @ 50% RH, the enthalpy is about 29.5 BTU/LB. If the outside air is 90 deg F @ 27% RH, the enthalpy is about 30.5 BTU/LB. If I go with the lower enthalpy air, I would use my return air. But with a supply air temperature setpoint of 56 deg F, this will cause condensation on the coils which dumps all the energy of condensation into my chill water causing my chiller to work more. If I bring in the higher enthalpy outside air (with less humidity), and dump the lower enthalpy return air out the end of the building, its going to avoid condensation on the coils which will keep the latent heat in the air and out of my chill water (in addition to bringing the indoor humidity off of the top edge of the comfort zone.) Won't this be more economical than simply using the lower enthalpy air?

[Edited by pwn01 on 04-25-2005 at 02:10 PM]

[john dalton]

Dear Pwn01,

Simply put……no.

The following calculations are based upon your previous post:

Return air being used:

(77 degrees F dry bulb) – (56 degrees F dry bulb) = 21 degree F dry bulb delta T

Outside air being used:

(90 degrees F dry bulb) – (56 degrees F dry bulb) = 34 degrees F dry bulb delta T

My question to you is what is the capacity of your existing chilled water coil, both sensible and latent in terms of air temperature drop? Does your existing chilled water coil have the capacity to reduce the return or outside air by 34 degrees F, most I would contend do not, at least the several hundred I’ve seen here in the Los Angeles area.

A large part of our refrigeration business at the company I work for is in the Hospital and defense industry. These areas do sometimes requires 100% outside air all the time, and in these cases the chilled water or even DX coil assemblies have the capacity to reduce the air by 30, 40, 50, and even 60 degrees F dry bulb. The question to you is: Does your existing coil have the capability?

That would be the true starting point to a discussion of this type.

We here at this thread await your reply………………………

Respectfully Submitted,
John J. Dalton

[pwn01]

Thanks for your help on this.

Let me tell you where I am in this. I am a maintenance person and not an HVAC person. I must make decisions regarding the HVAC system as to whether to call for contracted service people to address problems. Because of this responsibility I am familiarizing myself with the theory of operation of all of these systems. Hopefully this will allow me to make intelligent decisions about the need for contracted service.

In a link posted in this thread to a post on economizers in another thread, proper adjustment of an economizer is emphasized as essential. It is emphasized that the cost of energy wasted because of an out of adjustment economizer can quickly exceed the cost of calling a service person to adjust it properly. Because of this, I am attempting to fully understand the theory and operation of economizers so that I may ensure that ours is operating efficiently. Or if it is out of reach for me, I can call someone in.

This being the case, the questions that I have posted are posted in the hope of clarifying some points that I have yet to understand. I realize that some things just take sitting in a classroom or being a technician for a while to understand. I have had two years of aerospace engineering studies but very little practical experience (before I took this job). I understand that there is a vast multitude of knowledge that could never be related in a forum. But I also know that the men that discovered the laws upon which these machines operate used only their eyes, hands and brains and did not have teachers and classrooms to tell them these things.

These things are said respectfully and not sarcastically. I just want to let you know were I am in the food chain.

. . .

I can tell you that the coil spec sheet states a capacity of 538029 BTU's and 20350 CFM. Since it has been here, I don't believe that it has ever been run to capacity (8 staged compressors) and it always keeps the building cool. The return air temperature tends to follow the OSA temperature during the summer (maybe because of the steel roof on the building which serves as the top of the return plenum). It gets 90 and 95 on a regular basis here in the summer. I can't say for certain if it can handle 34 degrees, but I think to keep this building cool it would have to be able to.

If this is not enough information and it is not possible to answer my questions on this forum, I apologize.

I have a question that arises based upon your response, Mr. Dalton. Is the upper limit cut-off of the economizer to be determined by the ability of the coils to drop the dry bulb temperature? or is this an oversimplified conclusion?

Respectfully,

pwn01

[Edited by pwn01 on 04-26-2005 at 05:01 PM]

[Carnak]

PWN

Your economizer most likely has a minimum position setting, that allows enough outside air to meet your ventilation requirements.

In cooler ambients, the economizer can be used to bring in more cool air than required for ventilation to reduce mechanical cooling.

An enthalpy controller or a high ambient set back will set your outdoor air level back to the minimum position required for ventilation. There would always be mixed air entering your coil.

Your examples make you sound like you are in an arid region.

In more humid areas setting back by temperature alone can cause humidity to be pumped into the space if the set back temperature is high, especially when the outdoor air has a higher dewpoint than the indoor air.

Example

Indoor space is 75F @ 50% RH, has a dewpoint of about 55F.

Early morning outdoor air is 64F at 90%RH and has a dewpoint of about 61F. Economizer could be wide open, no mechanical cooling being run, and building humidity could rise up to about 62%.

Actually this example could fool an enthalpy controller as well.

A previous poster mentioned using 60 as a set back, and applying the same logic, would mean a slight chance of trouble on a rainy 60 degree day.

A dewpoint of 60F is being called neutral air these days however there are rumblings to drop that dewpoint down to 57F.

[top mechanic]

I think he was talking about comparing return air against outdoor air and using whichever one had less heat, then temper it with the mechanical system, I don't think he was trying to use it to cool the building as it is.Is this the correct assumption ?

[crab master]

I agree with Top Mechanic, that's how I understand this post as well. I am not a humidity expert and live in a relatively arid climate, but I was taught that latent heat has far more heat load than sensible heat. I can't remember the numbers but I do know that it is a substantial difference. Like 940 btu's for 1 lb of water at 212 deg F to become 1 lb of steam at 212 deg F, then once it's all steam it only takes one btu to make it 213 degree steam! Or that same water at 80 deg would take one btu to make it 81 deg. Latent heat is a major factor! I wish I could remember the condensation factor - I know its less like 400-500. Anyway the point is that you want to use whatever air is cooler to further cool by your mech. cooling if needed. If outside air has less total heat content than your return air you would want to use outside air and then be exhausting your return air. With an oversized chiller I would think that even at high temps, maybe not as high as 90 plus, that you could cool your building well enough. If this is a DDC system have your programmer set up the program to run the economizer if the total heat load of the outside air is less than that of the return air. Then incorporated in with that have an outside air lockout setpoint (dry bulb only) for your economizer that is user adjustable - and set it for 80 deg. You'll be there on site so if you see that setting is too high you can lower it or if you find your system is still working fine at that setting begin increasing it till you find the balance point.
With that being said have the mechanical cooling compressors sequenced on based on supply air temp - if this is a DX coil.

PS "Maybe I should have paid more attention to the psychometric chart!"

[pwn01]

The post by crab master answers my original question. Thanks.

P.S. I get to play DDC programmer, also.

. . .

Now, tell me if this thinking is correct. . . The chill water coils don't know the difference between sensible heat and latent heat. Its all BTU's. Therefore, it would always be more economical to use the air with an enthalpy that is closer to the enthalpy of my supply air, unless there is the condition that the lower enthalpy air has a dew point higher than my supply temperature while the higher enthalpy air does not in which case my chill water would have to absorb lots of heat of vaporization during the condensation process which would kill my savings. . . (is that a run on sentence?)


[Edited by pwn01 on 04-26-2005 at 10:46 PM]

[Carnak]

The chiller does not know the difference between sensible and latent however the coil will.

[pwn01]

Isn't this true only if condensation must take place under which condition the coil/chill water would have to suddenly absorb a large quantity of BTU's.

[Edited by pwn01 on 04-26-2005 at 11:01 PM]

[Carnak]

You change your question half way through my response. However.

In your case your airflow is fixed, the coil has a fixed amount of rows and fins per inch.

So you can basically alter the entering condition of the air, the chilled water flow rate, and the chilled water temperature.

Work out a few scenarios and talk to the coil manufacturer and see what will happen.

[pwn01]

Could you expound a bit? I don't follow you.

With respect. . .

[Edited by pwn01 on 04-26-2005 at 11:30 PM]

[Carnak]

The way a cooling coil works is that some air makes contact with a coil surface and some air makes it right through the coil without contacting the coil surface.

Providing the surface temperature of this coil is at or lower than the dewpoint of the entering air, then the portion of the air that makes contact with the coil leaves saturated at the 'average' coil surface temperature.

The other air sails through, unchanged. The mixture of the air that makes contact, with the air that does not make contact, is your supply air condition off of that coil.

Your coil was selected to be so many rows deep and have so many fins per inch, to have a certain [ editted in "chilled water"] flow rate through it, and a certain temperature rise of the chilled water to take a design entering condition down to a design leaving condition.

Off peak loads, perhaps the flow rate is reduced through the coil, maybe the chilled water supply temp is set up.

The heat transfer is all happening when air contacts the coil. Very humid air will release a lot more energy than hot dry air when it contacts a coil.

In your case the airflow is fixed, you are not altering it.

You are altering the entering air condition from the design level. You can contact the manufacturer and ask them to tell you the rated capcity as you change the entering condition. As well if you increase the chilled water flow through the coil or change the chilled water supply temperature to the coil, you will also alter the coils capacity.






[Edited by Carnak on 04-27-2005 at 12:42 AM]

[pwn01]

Maybe I do follow you. The airflow is constant, so even if the more moist air has a lower enthalpy, more moist air has more water content, water vapor temperature is harder to move than dry air temperature. If a given quantity of water vapor and a give quantity of dry air remain in contact with the coils for the same amount of time, the air temperature will be changed more than the water vapor temperature? So even lower enthalpy air that does not condense water on the coils may take more energy to change because more of the heat is latent and is less affected by the BTU's being applied by the coil as a comparable amount of dry air. (thermally speaking, water vapor has more inertia? it is more at rest than dry air and so is harder to get moving?)

[pwn01]

My last post was typed before I saw your last post. Your last post helps. I'll have to take some time to process it.

Thanks for your time, sir.

[crab master]

PWN01,
Maybe it is a run on sentence, and maybe I am stating the obvious, but as you begin to condense water on your coil you reduce surface area and therefore will affect airflow and begin absorbing latent heat; however, I guess if you really wanted to get technical and try to provide for the most energy savings you probably could figure out a way to reset your chilled water setpoint based off of supply air total heat content and if you have a VFD on your fan(s) you could adjust the speed accordingly as well. I don't know if that would work, but if I am thinking along the same lines you are just trying to keep you coil temp above the dewpoint. All Carnak is saying is that you are not altering your cooling coil size, but you can alter other things, airflow, mixed air temp, chilled water supply/return temp, etc. The more I think about this the more I think you are getting too technical and what you are trying to do won't work. Add in a reset for the Chilled water temp based on overall building load/return air thc/outdoor air temp to maximize savings. No need to run a 50 deg Chilled water loop when you only have a small building load. You'll have to figure out what reset settings work best for your building. Alot of systems I see have a resets based on OAT. There are better ways to do resets in some applications. Anyway good luck with it and if anyone has another/better way of controlling this I would really like to hear about it.

*I posted this in response to the post at 8:10 PM PST* After I posted I read the ones written after that.

[Edited by crab master on 04-27-2005 at 12:05 AM]

[Carnak]

Download this booklet on some air handlers.

http://www.magicaire.com/BMW.pdf

Go to page 8 for example and see how capacity changes as the entering air changes

Look at the change in total and sensible cooling.

[pwn01]

The question that started all of this was simply, should I always allow my outside air damper to open above minimum if the outside air is of lower enthalpy than return air? Or is there a high temperature lockout at which I should return the OSA damper to its minimum position and use only return air? But, a greater knowledge of the theory is always helpful.

Carnak, are you saying that very humid air will release more energy into the coil than hot dry air even if condensation is not taking place?

[Carnak]

The humid air releases more energy as condensation is taking place.

Hot dry air will release less energy.

Set economizer to minimum when outdoor enthalpy reaches a certain level.

Maybe I am wrong, I did not think they were comparing return air to outdoor air, I thought they were just set for the outdoor enthalpy.

[pwn01]

This system compares the Zone 1 (OSA) enthalpy to the Zone 2 (ISA) enthalpy. If Zone 1 + 2.00 BTU/LB is less than Zone 2, the enthalpy "switch" turns "ON". The OSA damper is sequenced after the enthalpy "switch". When the enthalpy "switch" is "ON" the OSA damper is allowed to open. If not, the OSA damper remains in its minimum position.

[crab master]

pwn01 - does your building have relief/exhaust dampers/fans incorporated with the return ducting? If it does then the answer is in my first post on this topic.

[pwn01]

Crab Master, the building does have relief dampers. Your original post did answer the question. My posts after that have been in response to Carnack's indication that my thinking was wrong when I stated that the coils couldn't tell the difference between sensible heat and latent heat.

[pwn01]

Carnak, I'm not sure what "MBH" stands for on the document that you linked me to, but I gather that "TTL MBH" is the total heat and "Sens MBH" is the sensible heat and so if I take the difference I will get latent heat?

[Carnak]

40 MBH is 40,000 Btu/hr

You are correct, the difference is latent heat.

[pwn01]

If I am reading it correctly, it seems evident that that the sensible cooling capacity becomes a greater percentage of the total cooling capacity as the water content decreases.

But why is it that the total cooling capacity drops as the dry bulb temperature of the inlet air drops? I'm assuming that the "85 degF DB/71 degF WB" indicates the state of the inlet air.

[Edited by pwn01 on 04-27-2005 at 02:15 AM]

[Carnak]

Originally posted by pwn01
If I am reading it correctly, it seems evident that that the sensible cooling capacity becomes a greater percentage of the total cooling capacity as the water content decreases.

But why is it that the total cooling capacity drops as the dry bulb temperature of the inlet air drops? I'm assuming that the "85 degF DB/71 degF WB" indicates the state of the inlet air.

[Edited by pwn01 on 04-27-2005 at 02:15 AM]

You are reading it correctly.

85/71 is an entering condition, the inlet air

Dry bulb and wet bulb are both dropping in those tables as you move to the right.

The chilled water supply temperature is constant. With less moisture in the entering air, there is less latent heat removal. As well, there is less of a temperature difference between the dry bulb temperature and the chilled water supply temperature, therefore less sensible heat transfer as well.

[top mechanic]

But getting back to the original Q, Doesn't comparing the total heat before choosing which air to use eliminate all of the other factors, if your coil capacity goes down does it matter ? if you don't need it anyway. Losing capacity doesn't necessarily mean losing efficeincy,You have already chosen the load with the lower total heat content therefore you have reduced the capacity requirement of the chiller as it is,I can't do any math that shows me that it would be more economical to cool air with a higher heat content than one with a lower one.Lets keep in my mind that we are talking about airconditioning conditions, and we are talking about full season efficeincy.Overall he will save that customer money by implementing a comparative enthalpy schedule into his programming logic, I don't think that there is any doubt about it. JMO

[coolairman]

Gentlemen:
You may find this website useful-www.climatequest.com.

They list degree day information, economizer hours info and lots of other good data pertinant to economizer usage.

Regards

Coolairman

[pwn01]

But if I draw in 90 degree air with less enthalpy instead of air of a lower dry bulb temp with a higher enthalpy, will the coil be able to change the 90 degree air to 56 degree air (delta T of 34 degrees) in the time the given mass of air is in contact with the coils. Is it purely a matter of the amount of BTU's that the coil can supply or is their a time factor due to the physical thickness of the coil. (If the coil can supply enough BTU's to change the dry-bulb, will it have enough time to with such a large delta T and having to work with a constant airflow?) Would this be figured into the spec'd capacity of the coil? (Maybe this is the point John Dalton was trying to make?)

Tell me what your thoughts are on this. If this were true, there would be an exception to the lowest enthalpy rule.

I'll take a look at the link posted by coolairman.

[hvac3901]

PWN01,

you have gathered alot here i take it, from reading your replys. It is'nt so much a matter of time, its a matter of capacity. if as you say you only use normally no more than three out of eight compressors, than you sir have ample capacity, unless there is a funtionality problem with the remaining compressors on your system.

when talking comparitive elthalpy economizer try not to think so much in terms of delta t, or differential temperature. if operating properly the economizer is selecting and cooling the air which is easier to cool regardless of the differential, hence more economical.

find and attend a class on psychrometrics, in doing so your questions regarding economizing with air should be answered. coupled with what you know and what you have learned here you will find the answers you seek. it is also my recommendation because unless you learn how to obtain WB DB readings and translate them into usefull chart readings then you will only be guessing as to wether or not yur economizer is functioning properly.

[pwn01]

Originally posted by hvac3901
It is'nt so much a matter of time, its a matter of capacity. if as you say you only use normally no more than three out of eight compressors, than you sir have ample capacity, unless there is a funtionality problem with the remaining compressors on your system.

Then you are saying that it is purely an issue of BTU's. This is why enthalpy is a better control variable than dry-bulb temperature when it comes to economizing. In reality then, if its a question of enthalpy versus capacity, if the lower enthalpy air exceeds the capacity of the coils, then certainly the higher enthalpy air will also.

I would translate this to mean that there should be no high temperature cut-off for the outside air damper as long as the OSA damper is sequenced to an enthalpy comparitor that compares the return air enthalpy with the OSA enthalpy.


Originally posted by hvac3901
it is also my recommendation because unless you learn how to obtain WB DB readings and translate them into usefull chart readings then you will only be guessing as to wether or not yur economizer is functioning properly

My control software has three readings that pertain to enthalpy. It has dry-bulb, RH and enthalpy itself. If I'm not mistaken the sensors are dry-bulb and RH with the enthalpy being taken from a lookup table in the software. I don't believe that there is an "enthalpy sensor" on the system. As you know, the wet-bulb temperature can be read from the psychrometric chart with dry-bulb and RH information. What information does knowledge of the wet-bulb temp yield that knowing enthalpy does not yield?

[hvac3901]

pwn01,
you got it pretty good. DB and WB is what are used to derive RH when using a sling psychrometer and transfering those readings to a chart which will tell you RH. Your computer tells you the readings but you wanted info to tell you when the economizer is working properly, right? Then you need to be able to rule out sensor problems with your economizer system. In order to do that you need some psychrometric training, and a sling psychrometer, imo. Based on the questions you are asking it is my opinion that you attend that type of class. Or keep asking questions here and trust your sensors only (not really a good idea imo).

and yes when using enthalpy comparison there is not a high temp lock out for the same reason you stated. if the lower enthalpy will not attain set point then neigther will the higher of the two readings.

[top mechanic]

Originally posted by pwn01
But if I draw in 90 degree air with less enthalpy instead of air of a lower dry bulb temp with a higher enthalpy, will the coil be able to change the 90 degree air to 56 degree air (delta T of 34 degrees) in the time the given mass of air is in contact with the coils. Is it purely a matter of the amount of BTU's that the coil can supply or is their a time factor due to the physical thickness of the coil. (If the coil can supply enough BTU's to change the dry-bulb, will it have enough time to with such a large delta T and having to work with a constant airflow?) Would this be figured into the spec'd capacity of the coil? (Maybe this is the point John Dalton was trying to make?)

Tell me what your thoughts are on this. If this were true, there would be an exception to the lowest enthalpy rule.

I'll take a look at the link posted by coolairman.

I think you will rarely find that a 90 degree day even with low humidity is going to be the choice of the enthalpy control, keep in mind that if the humidity is low outside , it probably is also low inside the building, so even though the scenario of not geting enough sensible drop out of the coil because of contact time could theoretically come in to play, it really doesn't. If it is 90 and 10% outside , you could almost bet that without a humidification system that the building is going have a pretty dry return air also. SO if we are dealing in the real world , comparative enthalpy is the way to go to save the client energy costs,besides if you only drop it tp 60 degree discharge it would cool the building anyway because there is no appreciable latent load.

This is a common control approach, you should also be resetting the CWV to the lower of the two dewpoints,

[pwn01]

Everything pertaining to my original questions is pretty clear now. Thanks to all who contributed.

HVAC3901, your right, I do need to get something to verify my sensor calibration. I have read about the various kinds of psychrometers and they seem to be pretty easy to use.

Top mechanic, I have observed that the inside air humidity tends to follow the outside air humidity since there is no humidification in this building. The only exception of course is this time of year when there are large temperature/humidity swings in the OSA from night to day. This is one of the things that prompted my original questions. I am certain that in the summer, the OSA damper will stay at its minimum position since the humidity in this area tends to be high at that time of year.

. . .

You mentioned that the "CWV should be resetting on the lower of the two dewpoints." Could you explain this? And I assume that CWV is "chill water valve"?

[john dalton]

Dear Pwn01,

Regarding your posted question:

“I have a question that arises based upon your response, Mr. Dalton. Is the upper limit cut-off of the economizer to be determined by the ability of the coils to drop the dry bulb temperature? or is this an oversimplified conclusion?”

Although it is an oversimplied conclusion, in its pure essence it’s a correct conclusion, as far as the dry bulb temperature is concerned.

Based upon the information that you’ve already posted on this thread, let me recap your HVAC system as I “assume” it is, please correct me if any of my assumptions are not correct so I may alter my conclusions accordingly.

Your existing HVAC system as “assumed by me” according to your posted information:

The single 250/300 ton packaged Carrier water cooled chiller working at approximately 15 to 18% of rated capacity supplies chilled water to your single chilled water cooling coil that has a total cooling capacity of 538,029 BTU/Hr or 44.8 tons of sensible and latent cooling, based upon the cooling coils total airflow of 20,350 CFM, that would calculate out to approximately 454 CFM/ton of airflow through your coil and assuming the chilled water coil assembly is a standard, off the shelve, factory unit, then the latent/sensible heat ratio would be approximately 28/72, therefore the latent heat removing capacity of this coil would be approximately 150,648 BTU/Hr or 12.5 tons, and the sensible heat removing capacity of this coil would be approximately 387,381 BTU/Hr or 32.3 tons.

Now, lets apply your original posted question of wheatear a return building condition of 74.90 degrees F dry bulb and 44.8% RH or wheatear an outside condition of 90 degrees F dry bulb and 10% RH is better to be used as the return air for the above HVAC system. We will be using the industrial standard formula of:

BTU/Hr = 1.08 x CFM x delta T

With this formula, let’s calculate the maximum dry bulb temperature drop through the above HVAC system assuming only sensible cooling would be taking place, this would be the ultimate maximum that this system could ever do under perfect ideal conditions:

538,028 BTU/Hr = 1.08 x 20,350 CFM x delta T

24.48 degrees F = delta T

Therefore using the outside air, the supply air dry bulb temperature would be:

90.00 – 24.48 = 65.52 degrees F

Therefore using the return air, the supply air dry bulb temperature would be:

74.90 – 24.48 = 50.42 degrees F

If the set point for the supply air is 58 degrees F dry bulb temperature, then the maxium ideal dry bulb temperature for this HVAC system would be:

58.00 + 24.48 = 82.48 degrees F dry bulb

I think now you can clearly see that the limitations of this chilled water coil assembly would require that the building’s return air of 74.90 degrees F dry bulb and 44.8% RH would be far superior to the 90 degree F dry bulb and 10% RH of the outside air.

Lastly, because the of dynamic nature of any chilled water or DX evaporative coil assembly in relationship to its entering and leaving dry and wet bulb temperatures, it is essential that the manufactures operating conditions be taken into account when trying to dial in the economizer’s entropy controls. Also, as pointed out throughout the above example and calculations, dry bulb temperature control of the system’s economizer is vastly inadequate because of the very nature of the dynamic conditions that exist inside and outside the building.

Hopefully this answered your question to me, if not let me know and I can elaborate in any areas outlined above.

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear Hvac3901,

Regarding your previous post:

“…and yes when using enthalpy comparison there is not a high temp lock out for the same reason you stated. if the lower enthalpy will not attain set point then neigther will the higher of the two readings.”

Not a correct statement based upon my above calculations my friend.

Respectfully Submitted,
John J. Dalton

[pwn01]

The only correction to be made is that the chiller is a 200 ton chiller which I noted in a later post.

So you are saying that the high temperature cut off point for the OSA damper should be 82.48 deg F?

[john dalton]

Dear Pwn01,

Regarding your most previous post:

“The only correction to be made is that the chiller is a 200 ton chiller which I noted in a later post.”

Having checked all eighteen (18) of your posts in this thread regarding the above statement, I think you are inaccurate regarding your stated “…200 ton in a later post…” statement friend. I do see the only reference to tonnage, your third post on page one of this thread, where you stated the tonnage as 250 to 300. But, in order to be more accurate in my assumption of your system, the only difference in my previous post would be that the Carrier chiller is only utilizing approximately 22.4% instead of the 15 to 18% as previously stated, of its rated capacity in the servicing of your existing chilled water coil assembly, all other data and calculations would remain the same.


”So you are saying that the high temperature cut off point for the OSA damper should be 82.48 deg F?”

No. What I am saying is that anytime the outside dry bulb temperature is above 82.48 degrees F, then your desired set point of 58 degrees F will be higher, even under the most ideal and perfect outside conditions (ie no humidity). The actual dry bulb temperature drop will be a function of the dry and wet bulb temperatures entering the chilled water coil assembly.

I’m also answering your original posted question that started this thread, and stating that under your posted air conditions that the return air from the building would be the air to use instead of the outside air.

Respectfully Submitted,
John J. Dalton

[pwn01]

I stand corrected in regard to the "200 ton" statement. Apparently, I edited it out before I submitted the post that it was in (the post in which I refered to the chiller having 8 compressors but not the one in which I was unsure about whether the chiller has 6 or 8 compressors). Nevertheless, it is a 200 ton chiller.


I’m also answering your original posted question that started this thread, and stating that under your posted air conditions that the return air from the building would be the air to use instead of the outside air.

As I stated in my second post, the enthlapy of the return air at the said conditions would be about 27.5 BTU/LB. The enthalpy of the outside air at the said conditions would be about 23.1 BTU/LB. With a OSA damper that is sequenced to an enthalpy comparitor set to use the lowest enthalpy air, the outside air damper would be open in this case, which according to your calculations would be the bad decision.

How then should the enthalpy comparitor be used to determine if the OSA should be used instead of the return air?

[john dalton]

Dear Pwn01,

Regarding your posted question:

“How then should the enthalpy comparitor be used to determine if the OSA should be used instead of the return air?”

Good question, unfortunately I see no accurate way of doing so under all inside, and outside dry and wet bulb conditions. Maybe there’s your answer my friend.

Respectfully Submitted,
John J. Dalton

[pwn01]

That's a reasonable answer.

. . .

While you're here, could you answer this for me:

Currently the chill water valve is reset on the supply air temperature. As it currently sits (which is the setting from before my time here) the setpoint for the supply air scales from 58 to 53 deg F as the OSA temperature scales from 34 to 80 degrees. This of course would mean as the temperature outside gets hotter, the system is trying to keep the supply air colder. Is this set correctly? or should it be allowing the supply air to creep up in temperature (to a limit) as it gets hotter outside.

[top mechanic]

[QUOTE]Originally posted by pwn01
Everything pertaining to my original questions is pretty clear now. Thanks to all who contributed.

Top mechanic, I have observed that the inside air humidity tends to follow the outside air humidity since there is no humidification in this building. The only exception of course is this time of year when there are large temperature/humidity swings in the OSA from night to day. This is one of the things that prompted my original questions. I am certain that in the summer, the OSA damper will stay at its minimum position since the humidity in this area tends to be high at that time of year.

. . .

You mentioned that the "CWV should be resetting on the lower of the two dewpoints." Could you explain this? And I assume that CWV is "chill water valve"? [/QUOT

I do mean chilled water valve, the comment on resetting the chilled water to a dew-point ( Dewpoint is a lot more stable than an RH reading), is just stating that if you were to use outdoor air or indoor air , you will still need to reset the chilled water temp as the load goes up or down, we happen to do this by using the dew-point of the air entering to the coil. You probably don't need to get this complicated and can just do it on dry-bulb for comfort cooling apps.I do think it makes sense to lock out the comparative sequence when you reach the temps that John stated for the coil that you have.So what you'll end up with is a about a twentyfive degree range of outside air that the comparative sequence will be choosing which airstream to use.I have done many buildings to this schedule and it does save on energy costs, and it does require maintenance service to keep it in order, but that is good for everybody.

[john dalton]

Dear Pwn01,

In checking the psychometric chart in my office I can see that the three(3) outside air conditions outlined below have the exact same entropy of 30 BTU / pound of air:

a. 65 degrees F dry bulb / 65 degrees F wet bulb / 100% RH

b. 103 degrees F dry bulb / 65 degrees F wet bulb / 5% RH

c. 125 degrees F dry bulb / 65 degrees F wet bulb / 0% RH

Given the fact that all three of these air conditions have the exact same entropy, which one would you think would require the least amount of energy to reduce the dry bulb temperature to 58 degrees F? Answer: a

Respectfully Submitted,
John J. Dalton

[Carnak]

The wet bulb temperature is pretty well indicative of total heat. The enthalpy lines are pretty much parallel to the wet bulb lines.

[john dalton]

Dear Pwn01,

Regarding your previous posted comments:

“Currently the chill water valve is reset on the supply air temperature. As it currently sits (which is the setting from before my time here) the setpoint for the supply air scales from 58 to 53 deg F as the OSA temperature scales from 34 to 80 degrees. This of course would mean as the temperature outside gets hotter, the system is trying to keep the supply air colder. Is this set correctly? or should it be allowing the supply air to creep up in temperature (to a limit) as it gets hotter outside.”

I think what you mean is that the set point of the supply air discharged dry bulb temperature from the chilled water coil assembly is reset according to the outside dry bulb temperature. As the outside dry bulb temperature increases, the supply air dry bulb temperature set point decreases, in other words they are mutually exclusive of one another in their operation. The decrease of supply air dry bulb temperature set point is needed to compensate for the additional structure and makeup air heat load that will increase the overall heat load of the HVAC zone or building because of the increased dry bulb temperature outside.

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear Top mechanic,

Regarding your previous posted comments:

“So what you'll end up with is a about a twentyfive degree range of outside air that the comparative sequence will be choosing which airstream to use.I have done many buildings to this schedule and it does save on energy costs, and it does require maintenance service to keep it in order, but that is good for everybody.”

I’m more than a bit curious as to what twenty-five degree range a comparative sequence controller would work for this or any economizer? Please elaborate on this range. As far as I can see according to my psychometric chart, this range simply doesn’t exist for the purposes of controlling an economizer.

Respectfully Submitted,
John J. Dalton

[pwn01]

Here are my calculation for John's recent post. Check my math and tell me if my initial equations are approximately correct.

Total enthalpy of dry air = 0.25T
Total enthalpy of water vapor = g(970+0.5*T) where g is water content.

Working with one pound of air for the argument.

A. Initial state 65 deg F DB/65 deg F WB / 100%RH
Final state 58 deg F DB/58 deg F WB / 100%RH

Initial state (dry) = 0.25(65)=16.25 BTU/LB
Final state (dry) = 0.25(58)=14.5 BTU/LB
Change in BTUs = 8.25-6.5=1.75 BTU

Initial state(vapor) = 0.0135(970+0.5(65))=13.53 BTU/LB
Final state (vapor) = 0.010(970+0.5(58))=9.99 BTU/LB
Change in BTUs = 13.53-9.99=3.54 BTU

Total BTUs required to change moist air from initial to final state = 1.75 + 3.54 = 5.29 BTU

B. Initial state 103 deg F DB/65 deg F WB / (10%RH)
Final state 58 deg F DB/47.5 deg F WB / 45%RH

Initial state (dry) = 0.25(103)=25.75 BTU/LB
Final state (dry) = 0.25(58)=14.5 BTU/LB
Change in BTUs = 25.75-14.5=11.25 BTU

Initial state(vapor) = 0.0045(970+0.5(103))=4.6 BTU/LB
Final state (vapor) = 0.0045(970+0.5(58))=4.5 BTU/LB
Change in BTUs = 4.6-4.5=0.1 BTU

Total BTUs required to change 1 LB of moist air from initial to final state = 11.25 + 0.1 = 11.26 BTU

C. Initial state 125 deg F DB/65 deg F WB / 0%RH
Final state 58 deg F/37 deg F WB / 0%RH

Initial state (dry) 0.25(125) = 31.25 BTU/LB
Final state (dry) 0.25(58) = 14.5 BTU/LB
Change in BTUs = 31.25-14.5 = 16.75 BTU

No water vapor

Total BTUs required to change 1 LB of air from initial state to final state = 16.75 BTU

To summarize:

A. 5.29 BTUs to move to cool 1 LB of this air to desired dry-bulb.
B. 11.26 BTUs to move to cool 1 LB of this air to desired dry-bulb.
C. 16.75 BTUs to move to cool 1 LB of this air to desired dry-bulb.

[Edited by pwn01 on 04-28-2005 at 07:43 AM]

[pwn01]

Well, if these calculations are correct, the conclusion that I would come to from this example is that the air with a dry bulb temperature closer to the desired supply air dry-bulb temperature is the one to use as long as the dew point of the air being used is not above the desired supply air temperature since condensation causes the water content to become a large factor in energy use.

Applying this to the economizer settings. This example would lead me to believe that dry-bulb temperature is a better determining factor in use of outside air for economizing than anything else.

1. If the options are to use only OSA or only return air.
A. Use the air that has a dry-bulb temperature closer to the desired supply air dry-bulb temperature.
B. Return the OSA damper to minimum position if the dew point of the outside air rises above the desired supply air dry-bulb temperature.

2. If the only option is to mix the OSA with the return air, use the OSA only while it has a dry-bulb temperature below that of the return air and while the return air dry-bulb is above the desire supply air dry-bulb.

In both of these, ISA RH must be maintained at an acceptable level.

Do you think that these are valid conclusions to draw?

[Carnak]

As a side note

After your little trip along the constant enthalpy line of 65 wet bulb you are almost ready to take on evaporative cooling. Just something for you to read up on.

I never checked your math BUT 970 is a little on the low side. For calculating enthalpy with respect to dry air at 0F try enthalpy = 0.240 x T + W x (1061.2 + 0.444 x T)

where T is dry bulb in deg F and W is pounds moisture per pound of dry air. I believe you have been given this formula before.

If outside air dewpoint is greater than inside air dewpoint OR outside air dry bulb is greater than inside air dry bulb THEN GOTO minimum position.

Make sure you avoid that 65F db and 65F wet bulb air.

[top mechanic]

It basically works from 55OA to 85OA just compare the enthalpy of each and choose th elower one, at 85 outdoor air lock it to min pos, this is a standard sequence of operation. I'll post the sequence when I get a chance.All the other stuff your talking about doesn't really matter that much.

[pwn01]

Your right Carnack, I remember the other equation now. The 970 came from what I understood to be the heat of vaporization of water.
...
I suppose that if I brought air in from outside that had a dewpoint above the dewpoint of the inside air some fog may form? and condensation would form on inside surfaces?
...
Top Mechanic, thanks for those numbers. Any other info along that lines that you have would be helpful.

With a low dry-bulb cut-off of 55, I am reading that under normal conditions if you have outside air that is less than 55 for an extended period, probably your inside temperature is going to be such that reheating of the air is going to be needed anyway and you may end up spending more energy to reheat the colder outside air than to reheat the return air?

[Carnak]

When you start bringing in air, at a rate higher than required for ventilation, and this air has more moisture in it than the inside air (meaning outside air has a higher dewpoint than the indoor air), then you are pumping extra moisture into the system.

[top mechanic]

Originally posted by Carnak
When you start bringing in air, at a rate higher than required for ventilation, and this air has more moisture in it than the inside air (meaning outside air has a higher dewpoint than the indoor air), then you are pumping extra moisture into the system.






This sums it up on both ends, thats why we should choose the one with the lowest dewpoint, when we are in the 55-85 OA range, when we get above 85OA we may get out of the range of the coils ability to change the sensible temp enough to be useful.Under 55 ( or changeover ) the dampers should modulate to maintain a discharge air setpoint with no mechanical cooling.This setpoint should be reset also so as not to waste a lot of reheat energy,JMO

[pwn01]

All right. Thanks to everyone who posted. This has been a real help.

Here is what I am going to do. Set the OSA temp lockout to put the OSA damper in minimum position below 34 deg F and above 85 deg F. The OSA damper does modulate to control the AHU supply air dry-bulb temperature so it will open enough to mix the air down so that no mechanic cooling is needed. I will also leave the OSA damper sequenced after the enthalpy comparitor so that it will use OSA when the OSA enthalpy is less than the return air enthalpy.

What do you think of the lower limit on the OSA dry-bulb for locking out the OSA Damper? And does everything else look all right?

[crab master]

Make it user adjustable so you can find the point at which your heat loss exceeds your need for cooling and then easily make changes as needed.


[Edited by crab master on 04-28-2005 at 04:35 PM]

[top mechanic]

Only you can tell if the lower limit on the OSA will be ok at 34OA, at 34 outside I wouldn't assume that the volume of air at a minimum position will be enough to provide cooling, for the building. I don't see a need to lock it out, but I would incorparate a temperature that would allow the control to shut the damper completely if it needs to.

[pwn01]

So there is no need to lock out the damper at the low end at all?

[top mechanic]

If you lock the damper out at 34OA you have no way to cool the building if you need it,I've never seen a damper locked to minimum on low outdoor air temp, I have seen the damper disabled ( closed ) at a low outdoor air temp for freeze protection though.

[crab master]

Good point Top Mechanic. I was relating it to a job we just did. A built up multi-zone unit w/ econ. - what an energy waster. Had to put a low lockout on mixed air temp econ. for that particular application but it is mainly serving a big open space 2 zones with a couple of basement zones that won't need cooling when OAT is below 40. Of course that is an application out of the norm.

[pwn01]

Top mechanic, I was using "lockout" as freeze protection. Evidently I don't really understand the definition of "lockout". In my understanding, the effect of "lockout" was a fully closed OSA damper.

. . .

The AHU also has a reheat coil after the mixed air box and before the chill water coil. Is this normally used for freeze protection? If so, is it ever necessary to close the OSA damper even with low OSA dry-bulb?

It seems that the reheat coil and the OSA damper could just "fight" each other if the OSA damper were opened with below freezing OSA dry-bulb. This would only waste a bunch of energy.

[top mechanic]

its not neceesary to lockout the dampe r, only in a freeze condition, usually determined by a freezestat, I thought you were trying to incorporate some extra freeze protection into the logic by shutting down the damper at a very low OA temp, but it isn't typical.

I'm not quite clear on the congfig of the coils, you say you have a reheat before the cooling coil, do you have a preheat before the coil and a reheat after the coil, or are there reheats at each terminal ?

[john dalton]

Dear Pwn01,

In regards to your post:

“Top mechanic, I was using "lockout" as freeze protection. Evidently I don't really understand the definition of "lockout". In my understanding, the effect of "lockout" was a fully closed OSA damper.”

You never want a totally closed OSA damper for a multitude of reasons, such as without OSA you won’t be able to maintain the minimum 15 to 20 CFM of outside air per person in the building that’s required by almost every federal, state, municipality, or OSHA regulation, you won’t be able to maintain a positive pressure between the outside and the building’s envelope, you won’t be able to dilute, and remove the stale and contaminated air from the building, and so on, and so on. In other words, NEVER close the OSA damper.


”The AHU also has a reheat coil after the mixed air box and before the chill water coil. Is this normally used for freeze protection?

This is used to reheat the outside air only if the air temperature in the building is below the heating set point.


“If so, is it ever necessary to close the OSA damper even with low OSA dry-bulb?”

As outlined above, you never close the OSA damper 100%, you will modulate the OSA damper to its minimum setting as you approach the supply air discharge set point of your air handler.


“It seems that the reheat coil and the OSA damper could just "fight" each other if the OSA damper were opened with below freezing OSA dry-bulb. This would only waste a bunch of energy.

If the economizer control system is setup and working correctly, then under some extreme outside conditions, you would have the OSA damper assembly closed to its minimum position, drawing in an extremely low dry bulb air temperature, and having reduced the discharged air temperature below the air handler set point, would require the reheat coil to heat this air a certain amount. Although this may sound extremely wasteful to you, the component is in fact called a “reheat coil” hence the “reheating” of the cold air will be required to maintain the OSA damper to its minimum postion..

Hopefully this helps.

Respectfully Submitted,
John J. Dalton

[crab master]

John Dalton,

You do want to close the outside air damper, during unoccupied periods for sure - why waste the enery - or if your mixed air temp ever gets below say 40 - at least a high enough setpoint to keep your coil from freezing, especially if there is not glycol in the system. Along with that your freeze stat should close your O.A damper (break power on a spring return damper) and shut down the air handler when it trips - in case your damper doesn't close on the spring return.

[pwn01]

That was a mistake when I stated "a fully closed OSA damper." I should have stated "an OSA damper in minimum position."

The configuration that I have is: the return air duct coming in the top of the mixing box with the OSA damper bringing air in the back of it. The filters are next in the airstream. The reheat (or what TM has called preheat) coil is next and I believe that it has a freezestat on it. After that is the chill water coil, and finally the fan.

John, that does help. It sounds like what you said is the configuration that I have. Your last paragraph on the use of the reheat coil when the OSA damper is in minimum position to reheat air that draws the supply air below setpoint was something I hadn't thought about yet. I'll need to check to ensure that this has been configured correctly.

Thanks again to everyone who posted. This has been a real help. I am very grateful.

[top mechanic]

Originally posted by john dalton
Dear Pwn01,

In regards to your post:

“Top mechanic, I was using "lockout" as freeze protection. Evidently I don't really understand the definition of "lockout". In my understanding, the effect of "lockout" was a fully closed OSA damper.”

You never want a totally closed OSA damper for a multitude of reasons, such as without OSA you won’t be able to maintain the minimum 15 to 20 CFM of outside air per person in the building that’s required by almost every federal, state, municipality, or OSHA regulation, you won’t be able to maintain a positive pressure between the outside and the building’s envelope, you won’t be able to dilute, and remove the stale and contaminated air from the building, and so on, and so on. In other words, NEVER close the OSA damper.


”The AHU also has a reheat coil after the mixed air box and before the chill water coil. Is this normally used for freeze protection?

This is used to reheat the outside air only if the air temperature in the building is below the heating set point.


“If so, is it ever necessary to close the OSA damper even with low OSA dry-bulb?”

As outlined above, you never close the OSA damper 100%, you will modulate the OSA damper to its minimum setting as you approach the supply air discharge set point of your air handler.


“It seems that the reheat coil and the OSA damper could just "fight" each other if the OSA damper were opened with below freezing OSA dry-bulb. This would only waste a bunch of energy.

If the economizer control system is setup and working correctly, then under some extreme outside conditions, you would have the OSA damper assembly closed to its minimum position, drawing in an extremely low dry bulb air temperature, and having reduced the discharged air temperature below the air handler set point, would require the reheat coil to heat this air a certain amount. Although this may sound extremely wasteful to you, the component is in fact called a “reheat coil” hence the “reheating” of the cold air will be required to maintain the OSA damper to its minimum postion..

Hopefully this helps.

Respectfully Submitted,
John J. Dalton


Well so much for morning warm-up, unocc cooling and dehum, and. Since when is a how water coil in front of the cooling coil called a reheat coil, its only a reheat if it is after a cooling coil, what he has is a preheat coil, it tempers the outdoor air, and locking a damper out for freeze protection is not uncommon, but it is rare that you need to, and it won't be there very long, the other things you mention will suffer but it will be better than the expense of freezing a coil.

[john dalton]

Dear Crab master,

Regarding your posted comments:

“You do want to close the outside air damper, during unoccupied periods for sure - why waste the enery”

Although I agree with you completely that the OSA damper should close 100% when no one is in the building, and when the building’s HVAC system is shut down, that is not what I was referring to in my previous post. “When the building is occupied, when there are people in the building, and when the building’s HVAC system is in operation, “then” you should never shut your OSA damper 100% closed for the reasons I’ve outlined in my previous post.


“… - or if your mixed air temp ever gets below say 40 - at least a high enough setpoint to keep your coil from freezing, especially if there is not glycol in the system.”


OPERATION OF THE ECONOMIZER AT LOW OUTSIDE TEMPERATURES:

If the economizer is controlled and operating correctly, and the mixed air temperature drops below the supply discharge air handler’s set point, then the OSA damper should be modulated to its minimum closed position. If the outside air being supplied from the minimum closed OSA damper is starting to drop the discharge supply air from the air handler below its set point, then the “preheat” or “reheat” coil, as it’s been described to us, should start to modulate open, and warm the OSA to maintain the discharge supply air set point accordingly. If the preheat, or reheat coil as it was called, fails to maintain the set point of the supply discharge air, and that air drops below a predetermined amount, such as 35/37/40 degrees F(personal pretence as to how much safety to build in your system) then the absolute last control of the system, the freeze stat, will cycle the OSA damper 100% closed, and shut down the air handler at the same time.


Along with that your freeze stat should close your O.A damper (break power on a spring return damper) and shut down the air handler when it trips - in case your damper doesn't close on the spring return.”

As described above………………..

Respectfully Submitted,
John J. Dalton

[crab master]

Ok that's fine. I just try to not rely on the freeze stat for my only low end protection. I also build another safety in the programming as well based on mixed air temp.

[john dalton]

Dear Top mechanic,

Regarding your posted comments:

“Well so much for morning warm-up, unocc cooling and dehum, and. Since when is a how water coil in front of the cooling coil called a reheat coil, its only a reheat if it is after a cooling coil, what he has is a preheat coil, it tempers the outdoor air, and locking a damper out for freeze protection is not uncommon, but it is rare that you need to, and it won't be there very long, the other things you mention will suffer but it will be better than the expense of freezing a coil.”

The sequence of operation that I was describing in my previous post was only a small section of the entire economizer program, I though that was obvious based upon the limited information that I described. And if you check my post, you will find that I use the term of “reheat as it was described to us” referring to the preheat coil. Preheat preheats the outside air because it’s too cold for the system to use, and yes reheat heats the supply discharged air from the air handler to dehumidify, and in some instances to temper the discharge air in some specialty application like environmental test cambers and rooms.

Lastly, the “section of” sequence of operation of the economizer that I outlined above would protect the system from freeze ups, unless you see a flaw in its operation that I might have missed, no ones perfect.

Any problem with my posted sequence, after the morning warm-up, in the occupied operation mode, and while not dehumidifying my friend?

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear Crab master,

More than one level of safety……that’s never a subject to debate against my friend, I can’t agree more. And thank you for bearing with me through my clarification of the sequence of operation; I obviously need to improve on my writing skills on this thread it seems.

Respectfully Submitted,
John J. Dalton

[top mechanic]

So we all agree to run econ in a normal mode, then changeover to mechanical and comparative control between outdoor air temps of 55 to 85, then lock the OAD to a minimum above 85.Then have freze protection be mechanical at the inlet to the hot water coil, ?

PS, I do close dampers at 0 through software where I am at, because the system was designed for 5 degrees OA, so I feel that I can take it upon myself to shut it when I am out of design specs,

[pwn01]

So if I set the DDC to "lockout" the damper below a certain temperature does that mean the damper will close 100% below that temperature?

[top mechanic]

Normally "lockout" means to close it and not let it operate at all,but I don't know hpw much control over it, who is setting the minimum, is it your controls or somethng in the unit ?

[pwn01]

I believe there is a variable resistor on the control circuitry at the damper itself which is used to manually adjust the minimum. But in the software there is the option of "OSA temperature lockout" on the damper configuration screen which allows the selection of "lockout" above, below, inside, or outside of certain temperatures. So since these are mutually exclusive controls I assume that one will be limited by the other.

. . .

Another question, in the final analysis is the hot water coil between the mixing box and the chill water coil in the AHU called a "reheat" or a "preheat" coil?

[top mechanic]

Some units will have a relay that can override the minimum, some will just kill power to the damper motor and let the spring return it to the closed position, you will probably need to determine this by punching in some values and seeing what happens, also when they say lockout, I don't know if they mean econ operation or OAD, if they mean econ operation then it will probably go to a minimum, if they say OAD then it will probably drive closed, I can't tell from here,

Typically a hotwater coil located before a cooling coil is called a preheat coil, or just the "heating coil", if it is after the cooling coil it is called a "reheat" coil or just a "heating" coil,

Some units will have both coils, and that is when you will see them named as such, What is the unit brand that you are working on ?

[pwn01]

It's a Pace AHU. The heating system for the building uses a VAV box with a reheat coil for each zone. I'm positive that there is no hot water coil after the chill water coil.

. . .

Is the name "economizer" also the name of a piece of hardware (or a hardware driven economizer)? This system does not have a piece of hardware that I would call an "economizer." It just has the ability to "economize" using outside air because it has an enthalpy comparitor function that may be sequenced in a way to control the outside air damper. It is software driven.

[Edited by pwn01 on 04-30-2005 at 02:31 PM]

[john dalton]

Dear Top mechanic

Regarding your previous post:

“Typically a hotwater coil located before a cooling coil is called a preheat coil, or just the "heating coil", if it is after the cooling coil it is called a "reheat" coil or just a "heating" coil,…”

You are correct in your explanation of the different locations and names of “typical” heating coils in the above answer, but…….and you know there’s always a but, technically you are incorrect.

In the truest technical name of the typical heating coils you outlined above, the following is the actual technically correct name and location:

Preheat coil: This heating coil is located between the OSA inlet and the inlet of the mixing box, it only preheats the OSA entering into the system.

Reheat coil: This heating coil is located downstream from the cooling coil and only reheats the air after the cooling coil.

Heating coil: This heating coil is located between the downstream side of the mixing box and the inlet to the cooling coil, and heats the air before the cooling coil.

Almost all service technicians, especially after they’ve been in the field for any length of time, either pickup or develop “pet or slang” names for different tools, equipment, or components in the HVAC/R industry. Is this a bad thing, absolutely not, after all, we’re all just human beings trying to get through the sometime difficult days we face in our choosen profession and this is a fun way of doing so. And like I always say, but, and you know there’s always a but, we can sometimes throw a fellow service technician, or parts counter person, off the right track because of our casual use of these pet or slang names. What is the solution then…….. well fortunately its an easy one, simply know when you are learning the many names of different parts, equipment, tools, and procedures, both the technically correct name, and then the slang or pet names almost all of us here have.

Thinking about that last paragraph of mine makes me wonder if someone started a thread by asking our members here their pet or slang names for the different parts, equipment, tools, or procedures, if that would be a popular thread………..maybe it would, and then again, maybe it would just die out like so many other great threads here on this forum.

Well, that’s my humbly option of the subject of pet or slang names.

By the way Top Mechanic, you seem to know your economizer control sequence quiet well I see.

Respectfully Submitted,
John J. Dalton

[NormChris]

Are any of you aware of a video tape on economizers by a guy named "Harlow"? I think he lives and works in Northern California.

I have been trying to locate a copy of his video but have not had any luck. I understand it is very well done and very complete.

Norm

[top mechanic]

John, I would have to disagree with your description of a preheat coil, a preheat coil is used and described as a coil that will heat air before it reaches another coil ( or any other tempering device) be it cooling or another heating coil,. This is often after a mixed air box, and is technically correct.

[john dalton]

Dear Pwn01,

Regarding your previous post:

“Is the name "economizer" also the name of a piece of hardware (or a hardware driven economizer)? This system does not have a piece of hardware that I would call an "economizer." It just has the ability to "economize" using outside air because it has an enthalpy comparitor function that may be sequenced in a way to control the outside air damper. It is software driven.”

The word economizer in the HVAC/R industry has grown in the few decades to mean several things. Among the several different meanings of the word, your described central HVAC system falls into one or more of them. The word economizer simply means to practice economy, or to make economical use of something.

In our industry the word has been used, and accurately so, to mean several things, some of which are the following:

A single or multiple module(s) that processes a portion, or all of a buildings return air, and replaces it with outside air to reduce the overall energy consumption of the HVAC system.

A part or component of the HVAC/R equipment or system, used exclusively for the variable process of the return/outside air for the system.

An overall HVAC/R program or process, be it mechanical, pneumatic, electrical, electronic, DDC, or anything else whose function is to reduce the overall energy consumption of the HVAC/R system by using a variable processing to control the return/outside air for the system.

In short, in our day and age of soaring energy use and costs, this word has been used to describe anything in our industry, from a single component, to an extremely complicated. large, and multifaceted process or program that helps the service technician, facility manager, or building owner save energy, and money.

Hope this helps.

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear top mechanic,

I stand corrected in your explanation of a “preheat” coil in terms of an industrial process, but in the terms of the HVAC system that Pwn01 has described to us in his central HVAC plant, I think my explanation and location description are still accurate.

Respectfully Submitted,
John J. Dalton

[pwn01]

John, that makes things clear. Thanks.

Top mechanic,


So we all agree to run econ in a normal mode, then changeover to mechanical and comparative control between outdoor air temps of 55 to 85, then lock the OAD to a minimum above 85.Then have freze protection be mechanical at the inlet to the hot water coil.

A question about your statement "to run econ in normal mode." From this I gather that "normal mode" for economization is maximum outside air. Should mechanical cooling be locked out when the OSA temp is below 55?

Does anyone else have a comment on this?

[top mechanic]

Originally posted by pwn01
John, that makes things clear. Thanks.

Top mechanic,


So we all agree to run econ in a normal mode, then changeover to mechanical and comparative control between outdoor air temps of 55 to 85, then lock the OAD to a minimum above 85.Then have freze protection be mechanical at the inlet to the hot water coil.

A question about your statement "to run econ in normal mode." From this I gather that "normal mode" for economization is maximum outside air. Should mechanical cooling be locked out when the OSA temp is below 55?

Does anyone else have a comment on this?

Yes the normal econ mode : A modulating outdoor air damper that works to maintain a discharge air temp by mixing outdoor air with return air. A simple economizer, that sounds like what you have.

You have to choose when to lockout the compressor, by whether or not you can cool your building while on outside air only, the unit is probably rated for an outdoor air temp that it should be ran down to, the type of pressure controls on the unit are what generally determine this, typically they will be setup around 50 if there are no extra pressure controls ordered with the unit.

If 50 degree outdoor air will coool the building then go ahead and lockout mech cooling at 50, locking it out at 55 is often risky. Because of sensor calibration and solar conditions, I wold typically start the econ at 55 and lockout mech cooling at 50, but you will need to monitor it and see if the building can be cooled with outdoor air only at 50 OA temp + or- the differential,Don't assume that because you have a low enough outdoor air temp, that you will actually get enough air through the dampers, it is not always true.So punch in the setpoints and monitor it,

PS , sometimes "monitoring it", means wait til you get complaints LOL.Or check on it the next time you are there.

[pwn01]

That helps. The way that I have it setup now is: the OSA damper attempts to maintain the AHU supply air (CW coil dicharge air) a degree or two lower than the CW valve does. So as long as the enthalpy comparitor does not shut the OSA damper, the damper will modulate to full open. If the AHU supply air begins to drift up and goes above the temp that the CW valve is trying to maintain, the CW valve opens to check the temperature rise.

With this setup, it looks to me like the system will always draw in as much outside air as possible (given that the enthalpy is okay) and then if it still can't maintain setpoint, the mechanical cooling will open up.

In this system the chiller operates independently and maintains the water in the primary loop at 45 degrees. There is no way to conveniently lock out the chiller, but if the heat load decreases, the chiller on-time does decrease. I can lockout the CW valve which I believe minimizes the chiller on-time. I think that this configuration tends to keep our electrical demand peak down.