Humidity Question for make up air

[joeywpittman]

What should the discharge air be on a make up air handler unit in the summer (cooling) mode of operation? The make up air unit has 2 stages of DX cooling and a hot water reheat coil with a three way modulating valve. I want the coldest discharge temp so not to load the existing mech cooling units. This is a general question and a made up problem.

Outside conditions are: 90* F D.B. and 100% humidity
Inside conditions are: 70* F D.B. and 45% humidity

 

[tombmech]

Frankly, I am a little amazed to see hot water reheat combined with two-stage DX cooling. That is an odd combination of capability. Hot water reheat is a "Cadillac" temperature refinement while two-stage DX cooling is a hammer approach.

Given your scenario, relative humidity will fluctuate wildly. To maintain the Inside condition of 45% humidity at 70F, you must cool to saturation at approximately 47.5F. That will give you the amount of water vapor in the air to result in 45% RH at 70F. This is true for any condition of higher moisture content in the outside air.

The problem is trying to get 2-stage DX cooling coils to always cool to saturation, regardless of outside conditions. You can size for the peak condition - 90F and 100% RH, or the condition that will completely saturate with only the 1st stage. Every other condition will result in over-humidifying or de-humidifying. This is because the DX coils will only have 3 settings of cooling - OFF, 1 stage, or 2 stages - period. It matters little at that point if the hot water reheat is able to maintain precise temperature for an infinite number of conditions.

A throttling chilled water coil can reduce or increase flow - at will - to perfectly saturate and dehumidify the air under any condition, allowing the hot water reheat to return the dehumidified air to the precise temperature desired.

 

[cme]

I would highly suggest an enthalpy wheel for reduce the cooling/heating load if possible

Then use a preccoling coil to say mid point

Then a final cooling coil to say 44 deg F DB LAT

Use a DDC based LAT controller for each stage

Intertwined DX coils

Whats the air volume?

 

[haneyrm]

Your ambient conditions sound alot like the summer conditions in Tampa. When we do oa pre-conditioning with a DX unit, we will try to first of all select a unit that is designed for this duty. A Trane Morganizer is a good example. This type of unit will typically have multiple compressors and will continue to dehumidify on part load days with high humidity. Those types of days are typically the nail in the coffin for DX pre-conditioning systems. With that said, I have designed several systems which simply use a multiple compressor unit and some form of reheat (typically hot gas). The trick is to always force one compressor to continue to operate and dehumidify, especially on part load days. Through means such as hot gas bypass and selective staging (staging controllers), you can typically ensure that one compressor runs almost continuously. You can also custom select equipment with mis-matched compressor sizes which will increase the likelihood that the unit will continue to operate at part load conditions. The full load conditions you mentioned are easy. Part load is the killer. For your conditions and assuming that your room units are doing sensible only, Tombmech is correct that around 47 degrees saturated is the number. This is always a challenge with off-the-shelf DX equipment.