Thread: Questions to help my understanding of economizers

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Could someone explain the difference between the enthalpy and the specific heat of a volume of air? Also, if there is enough space, could someone give a quick "economizers for dummies" course and explain the thought behind using enthalpy rather than dry bulb temperature to control an economizer (I mean how does the combined temperature/humidity element factor in and how is this combined figure used in adjusting the OA inlet vanes)?

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I tried google all day yesterday and there is not much technical, only general info. It finds primarily product info for enthalpy sensors, although I did find a little dealing with theory that was helpful. I was hoping for a simple technical explanation. I think that I am within about ten minutes of understanding it if someone with a comprehensive knowledge would say the right things. I had thermodynamics, physics, etc, etc. (going for Aerospace Engineering) in college around '92, but haven't had formal training for HVAC, only OJT. I was just hoping to bring the whole picture together as it relates to HVAC.

Well. . . maybe this isn't the place for it.

4. Not simple

I think your question is more advanced than the majority of those on this board, and there might be very few people capable of answering it. I certainly cannot, although it someone explains things I will study that with interest.

Might it be possible to telephone a vendor, or get some of their sales stuff, which might shed light on things?

Best of luck -- P.Student

5. Enthalpy/Specific Heat

PWN,

I can take a stab at the first part of your question.

When considering an air volume, Specific Heat is the amount of heat necessary to raise the temperature of that air volume 1 degree C with NO moisture phase change. For illustration, plotting this out on a Psych chart, measuring specific heat would be a straight horizontal line between 20---21 C.

Enthalpy measures the total heat potential of a pound of air made up of Sensible Energy (energy measured by a thermometer) and the Latent energy of moisture in the air (the phase change Energy).

Now lets take a stab at the economizer question.

Economizers are set up to use outside air to condition the indoor air when the outside air condition permits. For example, a building is at cooling set point of 72 F, but it is 65 F outdoors, instead of turning on the compressor for cooling, a damper can be open and the outside air will do the cooling more economically.

If human comfort or building conditions were only dependendent on temperature, then setting up the economizer on temperature alone would be simple. But many buildings require a tighter indoor humidity range as well. In the above instance with 65 F air outdoors, if it was raining and the outside RH was close to 100%, then that air would not be desirable for conditioning and the compressor would be the better cooling logic.

So controlling the dampers to enthalpy prevents wide swings in space RH. Depending on the situation, controlling the dampers for just dry bulb temperature is fine.

Honeywell has put together an excellent self study course on the Psychrometric Chart which pulls it all together nicely (in about a half hour). I think I've seen that course on their web site.

If you have a specific situation in mind, I can tell you how I would set up the economizer logic to meet the space conditioning needs of that situation.

I hope this helped.

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Thanks for the response. The question about enthalpy and specific heat arose out of an attempt to calculate enthalpy. The economizer on the system that I maintain seems to use temp and humidity to calculate enthalpy. Tell me if I am wrong, but I think that the procedure would be to take the enthalpy of the dry air (1 BTU/LB-deg F) for the temp of the air and add it to the enthalpy of the water contained in the air (about 0.24 BTU/LB-deg F above freezing) and would I add the heat of vaporization for the amount of water vapor (in 1 lb of air) also? (Concerning the water, is the only significant energy, the heat of vaporization?) I am thinking that from the units all of this should come down to BTUs/lb which I think is the units for enthalpy. The water vapor content of the air (absolute humidity?) could be calculated from the relative humidity at the given temperature (correct?).
. . . Is there a concise equation for all of this?

About the economizer question. . . The things that you said confirm what I was thinking. Here is a further matter. . . I was looking at the system today and the OA humidity was above 60% and the OA temp was around 40 deg. The ISA Humidity was about 27% and the ISA Temp was about 60 deg and wanted to be about 57-58 deg. The economizer damper (OA Damper) is sequenced with the enthalpy sensing system. The enthalpy system was allowing the OA damper to open to draw in outside air. It seems that with the lower temperature this is okay, but with the high humidity, maybe it is not. But then, it is taking into account total energy and not just one factor or the other. But could 34 degree air at 100% humidity be drawn in or is it's total energy to high?

I'll see if I can find that self-study course.

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

7. Take a course on pychrometrics, many manufacturers, such as Carrier, offer one.

The enthalpy of moist air equals is the sum of the enthalpy of dry air and the enthalpy of water vapour.

Usually on the pyschrometric charts the enthalpy is listed as a change above a certain temperature. It can be based on 0F for example. Most calcualtions only need a change in enthalpy as well.

Based on enthalpy being zero at 0F,

enthalpy= 0.240xT + W(1061.2 +0.444T) Btu/lb da

Where T would be a temp in degrees F, W is the specific humidity ratio (lb w)/(lb da)

1061.2 is the enthalpy of saturated water vapour at 0F.

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The fundamental concept of the economizer is pretty simple. Here it is in easy to understand terms.

Norm

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Originally posted by Carnak

enthalpy= 0.240xT + W(1061.2 +0.444T) Btu/lb da

Where T would be a temp in degrees F, W is the specific humidity ratio (lb w)/(lb da)
A few questions on this, why does the equation use 0.444T for the enthalpy of water? Doesn't it take 1 BTU/lb-deg F? Also, isn't the enthalpy of water zero at 32 deg F, but of air zero at 0 deg F (I read this somewhere). Finally, can I get the specific humidity ratio (W) easily from the relative humidity?

. . .

A question about humid air. Is it all right to pull in air that is cool but even 100% humid as long as it does not drive the ISA humidity above the 40-50% limit?

. . .

Thanks for all of the help. I think that I have the concept of the economizer down now. Hopefully, if I can understand some of the theory, I can keep the system here in top shape.

[Edited by pwn01 on 02-05-2005 at 11:16 AM]

10. Originally posted by pwn01
Originally posted by Carnak

enthalpy= 0.240xT + W(1061.2 +0.444T) Btu/lb da

Where T would be a temp in degrees F, W is the specific humidity ratio (lb w)/(lb da)
A few questions on this, why does the equation use 0.444T for the enthalpy of water? Doesn't it take 1 BTU/lb-deg F? Also, isn't the enthalpy of water zero at 32 deg F, but of air zero at 0 deg F (I read this somewhere). Finally, can I get the specific humidity ratio (W) easily from the relative humidity?
1 Btu/lb degF is for water as in a liquid. We are dealing with water vapour.

The equation I gave you gives enthalpy with the enthalpy of bone dry at 0F being set as zero. The water term then uses the vapour enthalpy at 0F (1061.2 Btu/lb w and adds 0.444 Btu/lb for every degree above 0F)

In metric quite often 0C (32F) is used as the reference point for zero enthalpy. The zeros make for easy math.

The easiest way to find W is the pyschrometric chart, but there are empirical equations out there, try posting at

The empirical equations can approximate W for you based on relative humidity. These empircal equations will be exponential.

The only equations I have on hand require you to program in saturated steam tables.

RH= (W x Pa)/(0.6219 Ps)

Where Pa is atmospheric pressure and Ps is the saturated vapour pressure for Water at that temperature. RH will be a decimal fraction here.

Originally posted by pwn01

A question about humid air. Is it all right to pull in air that is cool but even 100% humid as long as it does not drive the ISA humidity above the 40-50% limit?

[/B]
I think you need to look at what indoor dewpoint you want, and then you can bring in as much air as you want, as long as it is not higher than the dewpoint you are trying to maintain.

In the winter, I think you can use these formulae to find out how much moisture a humidifier will have to add to this outside air. Your economizer will be at minimum position, probably allowing the minimum outside air required. This will be drying out your building.

You can figure out the highest dewpoint allowable to avoid window condensation and then figure out how much moisture the humidifiers will add.

In cooling mode, you could add as much air so as not to raise your indoor dewpoint. I would cut off the economizers at a 57F dewpoint max.

You seem interested so I think it would be a good idea for you to take a course on pyschromentrics.

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I will take a look at the link that you posted, and will try to find out something about a course on psychrometrics. Maybe if anyone has info about a course being offered in central NC the person could post the info. I know the local community college may offer something, but it is already in session.

. . .

I'm going to take some time to process the info that you have given me and pursue some thoughts that have been provoked. If/when I have any more questions, I will post here again.

12. you're welcome

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If you can get an ASHRAE Handbook - Fundamentals, look in the section on psychrometrics.

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