Psychrometrics help - dehumidifying with a cooling coil

[BronYrAur]

I'm scratching my head on this one and could use some help. I have 2 similar rooms that each has a similar air handling unit. The target condition in these rooms is 70 deg F at 50% RH. I'm having trouble holding humidity in one of the rooms and am trying to figure out why. I've been looking at the other room as a guide to see where things may differ. As I study the room that is working correctly, I find myself confused with the psychrometrics. It doesn't seem possible.

Here is the room is is functioning correctly:
Space conditions: 70 deg F @ 52%RH
Return air at the unit: 74 deg F. Humidity sensor bad
Outside air dampers completely shut with gasketed dampers
Discharge of the cooling coil: 57 deg F @65%
Chilled water temp in/out of coil: 44 deg / 58 deg
Reheat coil off

The supply air is much dryer than I would expect. With an average coil temp of let's say 51 deg, how can my supply air be so dry? It's well below the line from return conditions to average coil temp. This is again the room that is working correctly (or at least everyone thinks so).



My problem room conditions this morning are as follows:
Space conditions: 69 deg F @ 58%RH
Return air at the unit: 73 deg F @ 53%
Outside air dampers completely shut with gasketed dampers
Discharge of the cooling coil: 64 deg F @ 72%
Chilled water temp in/out of coil: 45 deg / 62 deg
Reheat coil off

The psychrometrics on my problem room actually make a little sense. My discharge air conditions fall on the line between return air and average water temp. It's just way too close to the center of the line. Maybe I'm just moving too much air???

Any thoughts?

 

[IRstuff]

How much cooling capacity is available? Are these rooms on separate A/C's?; seems like it, from the different coil outputs

Seems to me that the bad room's A/C is just not working at all. Typically, cooling coils condense moisture out of the return air, which is why A/C air tends to be dry. But, my rough calculation of absolute humidity between 73 deg F @ 53% vs. 64 deg F @ 72% says that the cooling coil is hardly condensing anything, which means that the cooling coils are nowhere close to being below the dewpoint of the return air, which means it's not working.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[317069]

"Maybe I'm just moving too much air???" you answered yourself.

 

[Drazen]

psychrometrics makes all sense, mere subtracting enthalpies before and after coil, comparing with cooling water flow findings can diagnose your coils.

you can also check dew point temperatures and compare with coil surface temperatures. simple heat exchanger logic will not work for wet coolers.

 

[MarineHVAC]

What is the face velocity of the coil?

 

[MatthewL]

BronYrAur,

Check the flow arrangement of the coils. Looking at a psychro chart, it looks like your exit air is the same absolute humidity as the inlet water temp on the room that is working "correctly". If the piping on one coil is opposite of the other, swap the "bad" coil inlet and outlet and see if that helps.

Regards,

Matt

Quality, quantity, cost. Pick two.

 

[ChasBean1]

Post the control sequences of operation for each space/AHU. You might be achieving good numbers by accident based on outdoor conditions.