Cooling Coil Dehumidification Limit

[BronYrAur]

Just need to confirm my thinking. When using a cooling coil to dehumidify, you are limited by the temperature of the coil, correct? In other words, if I have a 55 deg F coil, the best I can do it achieve 55 deg dewpoint, which gives a HR around 65 Gr/lb. Reheating the air is a horizontal line to the right on the psyc chart, which does nothing for absolute humidity (only relative). So, if I have a 70 deg discharge air temp, I am sitting at around 60% RH. If I need 70 deg and 50% RH, I either need to lower my coil temp or introduce a desiccant, etc. Agree? It will not help me to run the unit longer or try to push more air through it, right? No matter how much air I move and no matter how much coil "contact time" I have, I can never do better than the dewpoint temp of the coil. Please share your thoughts.

 

[insult2injury]

The saturation temperature of the cooling coil determines the maximum dehumidification you can achieve through cooling methods; however, other forms of dehumidification can be used in conjunction with the coil (i.e. desiccant) to achieve further dehumidification below the cooling coil limit. If you try to increase the dehumidification by increasing the flow rate, you will actually adding more water content to the conditioned space. (same humidity level with higher flowrate)

I2I

 

[AbbyNormal]

The air leaving a coil will not be any colder than the apparatus dewpoint which I always think of as being the average surface temperature of the coil.

When you slow the air flow down on a DX coil, the suction pressure could drop on you and you would get a colder coil, but less capacity on your compressor.

When you slow the air down on chilled water, the leaving water temperature does not rise as much.

70F @50% is about a 50.5F dewpoint. Start looking for lower than say 48 dewpoint and I think you need either dessicants or defrost.

 

[quark]

Yes, conceptually you are right but don't ignore coil bypass factor. Coil bypass factor and mixed condition of air both affect the dew point.

You can go to as low as 45F DP by using DX systems but below that it is not economical. Desiccant dehimidifiers are preferable below that DP.

However, 70F and 50%RH is not very difficult to acheive if you have 6 C chilled water. It was a very tight operation but we managed even in humid coastal area (ofcourse, with reheat).

 

[BronYrAur]

Quark,

What is your leaving air temp out of the chilled coil before reheat?

 

[quark]

It was 10⁰C. The design ADP was 12⁰C. The SHR was about 0.7 (if I recall correctly) and fresh air was 10% maximum. The cooling coil valve was modulated by a temperature sensor in the return duct and hot water coil was modulated by an RH sensor in the return duct. I never bothered to continuously monitor the air temperature after reheat coil but it was about 16⁰C when we were commissioning. This was for our pharma facility for OSDs, Aerosols and Opthalmics (FFS).

We used the condeser return water for reheating and this was directly fed to the cooling tower basin.

If your SHR is lower, then plot it on a psychro chart and have a look into it.

 

[hmm97]

You got it exactly right. That is how to decide what your minimum LVG coil air temperature should be.

 

[lilliput1]

quark,
I think you have it reversed. The reheat coil should be controlled to maintain the space temperature. The cooling coil should be controlled to maintain the RH.
lilliput1

 

[quark]

Lilliput,

Unlike comfortable air conditioning, we had a huge band of 4⁰C for temperature but the RH band was just 5%. Further, reheating for temperature comfort was not required due to the equipment load inside. Also, there was overgowning that took care of any low temperatures. The RH was critical with respect to product stability.

 

[lilliput1]

Quark,
You can only control humidity by cooling , by use of dessicant or by humidifying. Heating is just a sensible process, a horizontal line on the psychrometric chart.
If your RH is that tight it means you have to control RH with humidification & dehumidification then keep the space temperature fixed with reheat. This will definetely give you tighter control envelope than allowing space temperature to float within the % RH band. You can mark up the areas on the psychrometric chart to see the envelope of operation for both case.
lilliput1