dx cooling in 100% make up air unit

[rmac2]

our company wants to add a dx cooling coil inside an existing reznor make up air unit that provides air to five floors. each floof has a supply grille at end of hallway.We are just looking to improve the humidity levels in hall not cool space to 72 degrees. any thoughts out there if this is worth doing.Existing units are 10 years old.

[gasnowman]

you may want to consult reznor for your specific application. We have some MAPS 2 units that work well with a custom dewpoint controller. They provide spot cooling and require hot gas bypass due to enthalpy conditions.....

[freddy-b]

our company wants to add a dx cooling coil inside an existing reznor make up air unit that provides air to five floors. each floof has a supply grille at end of hallway.We are just looking to improve the humidity levels in hall not cool space to 72 degrees. any thoughts out there if this is worth doing.Existing units are 10 years old.

Do you have reheats on each trunk? Might get pretty nippy if not. Where were you thinking of putting your space humidity sensor? Also is there a BMS installed?

[rmac2]

There is no bms on unit. there is a space after blower outlet inside unit to install dx coil. these make up air units are gas fired for heating. Customer does not have money to install new makeup air units with cooling in them. Also, there is no room in discharge duct work to install new coil. We would probably have an ambient lockout for cooling and also fan clcling control on condenser for cooler weather. If anyone has any info on how they operate dx cooling on a 100% make up air unit your help would be appreciated. Thanks!!

[hvacker]

The problem with 100 % osa is you'll effectively run out of cooling when the outside temp is about 20 deg above your set point. You know how when you look for a 20 deg drop across the coil in most applications? Well the same thing happens with 100% systems. So at 95 outside you will be delivering about 75 deg air. There is little cooling with 75deg supply air. Everything would be ok up to maybe 85 deg depending on the load.
You can for fun run the numbers on say a 3 ton system with 1200 cfm and see where the cooling quits.
I've seen chillers do much better in these applications as you can dial in lower water temps.
You could stage 2 units one after the other. Like a cascade system.

Next problem is at 95 outside your coil will be very warm and will not be effective with humidity removal.


Best would be to duct a return and limit osa as much as possible.

[jackt]

Chilled water is no doubt the best way to go if you can. If you have to stay with DX cooling, I recommend several stages of cooling in stead of just one. When dealing with outside air and humidity, you need to design your system so it will work when the temperature is cold and humid as well as when its hot and humid. I always specify multiple circuits and compressors or compressors with unloaders. I also use intertwined cooling coils and add hot gas reheat coils. Hot gas reheat uses the hot gas from the compressor to reheat the discharge air when it's needed and its free energy. If you need to purchase a new unit for outside air, I would look at the Munter units. I just had four installed and they work great and are easy to service compared to some other ones that I use.

[Carnak]

where are you

how much air do you have to supply

[rmac2]

carnak, it is a small reznor make up air unit. about 200,000btu and 2000cfm. We were thinking 5ton.We dont have any room. Only a space inside unit between outlet of blower and supply duct work. We would have to move unit to fit coil in supply duct before it goes through roof.

[Carnak]

well if you want to do it to control humidity, you have to get the dewpoint of the supply air down under 60F.

WIth oustide air it is less than 400 CFM per ton. SO how much cooling you have to use depends on how hot and humid the air is.

Big difference between 2000 CFM in Duluth Minnesota vs 2000 CFM in Miami, so it sort of matters where you are

[mtntom]

Ive seen this type thing done before. its not perfect but will be an improvement over 100% O.A. with no cooling. Even if you don't get the supply below 60 you will still remove moisture, the coil will be cold and condense, just look at the drain. The application I saw was a RTU-100% O.A. space stat in hallway(same scenario you described-cooling hallways) controls unit, O.A. stat locks out cooling below 50-55*. Run the numbers, may need more than 5 ton as mentioned. multi stage a plus. Of course the best thing to do is install a unit designed for this application.

[madcad]

what mod. is the heater what size is the duct. Do you know which coil you have in mind. what pressure drop would the heater have to overcome.
Remember more rows (coil depth) the more moisture you can ring out. You would be better off checking with reznor who can give you the right info on the blower and air flow. What motor will over come different sp. You may find you will need to change motor or pullys but they will have all that info. Good luck let us know what you do

[cxagent]

Listen to Carnak - he seems to be pretty good at predicting the future

Ball park kind of numbers -

1) You want the supply air DEW POINT leaving the unit to be the same as the dew point you want to maintain in the space or even a bit lower. Or said another way, the supply air dew point is the lowest dew point you CAN reach in the space. Look on a psych chart to see what you need. 74 FDB and 50% comes out to about 55F dew point.

2) That supply air temp will probably overcool the space without reheat. Using hot gas for reheat can do a lot of goold things for you at low energy cost. One of the things it does is make the condenser coil more effective which can improve the energy efficiency.

3) Look VERY CAREFULLY at intermingled versus split face coils. In order for the space to be dehumidified the average dew point of the supply air has to be what you figured in number 1. Some intermingled coils can't do that unless all stages are running.

4) Unless your weather conditions are VERY constant (like Carnak's), you will need multiple stages. How many stages is determined by how acurately you want to control the space humidity and how much your weather changes.

5) Use the psych chart to figure out how many tons of cooling you need in a perfect system. Then add a little but because no system is perfect. In my area it comes out around 250 cfm per ton due to our hot & humid weather.

6) IMHO, 100% outside air units are not 'build it yourself' systems. If you have lots of time to kill, willing to spend LOTS of money, and a VERY UNDERSTANDING customer, you might make it work. But if any one of those are missing, I wouldn't even try. You are doomed to failure before you start. I would prefer to learn from someone else's mistakes while they built a lab rat system and slowly grew it into a reliable system.

[falkore24]

The RTU has a lifetime of about 20 years, so I'd say that it is worth the upgrade if dehumidification is what is needed. Rather than a cooling coil in the AHU, I'd consider this company as an add-on device to the incomming air. They have models for chilled water as well as DX.

www.msptechnology.com