Estimating dehumidification rate with A/C unit

[preet]

Hey all,

I'm working on a project where I'm trying to use the inherent dehumidification ability of air conditioners to remove moisture from a closed environment. I would like to estimate the amount of condensation (and hence moisture removal) that occurs. I don't have access to the air conditioning unit -- we're trying to spec the unit based on our estimations. To approximate the condensation rate, I'm modeling the problem as heat transfer through a cooling coil.

[PLAIN]http://img201.imageshack.us/img201/1413/72151171.png

I'd like to know whether the method I'm using is appropriate, or I've overlooked something. Any other feedback/advice would also be awesome :)

 

[russ_watters]

If you have the capacity of the coil, you can't figure out how much sensible vs latent heat cooling you get by calculation alone. You need to use a psychrometric chart to calculate it based on the incoming air conditions and the assumption that the leaving air is saturated.

 

[preet]

Hey Russ,

Thanks for the reply. Why do I assume the air leaving the unit is saturated? Doesn't the A/C unit dehumidify the air? I understand that the w.vap would be saturated around the coil, but if you put your hand in front of the outlet of an A/C unit the air isn't wet (doesn't feel like RH of 100).

Just to confirm: If I used psychrometric charts, all I'd need to do is guess an output temperature and use an RH of 100%. That'd get me the specific humidity through the chart. Since I can calculate specific humidity at the inlet as well, the difference between the two states would give me the condensation rate right?

 

[preet]

I read up on this problem more and I think I understand it a bit better. My system needs to go from 60% relative humidity at 26 deg C to 30% relative humidity at 10 deg C. I plotted this process on a psychrometric chart... It's denoted by the '1' line.

[PLAIN]http://img405.imageshack.us/img405/6366/20125266.png

In my case I need a coil surface temp of -5 degrees C to attain 30% RH at 10 deg C.

The chart tells me the ADP of an ideal system where all my air becomes the ADP temp and is 100% saturated at the outlet of the AC unit. This gives me the coil surface temperature of the ideal system. I'm stuck on how to choose an existing AC unit based on this information because all I'm given is the cooling capacity (in BTU for example), but I wouldn't know the surface temperature of the coils. How do I proceed from here?

Preet

 

[russ_watters]

Sorry, it's been a long week at work and I kinda forgot about you, but you seem to have mostly figured it out on your own...

Hey Russ,

Thanks for the reply. Why do I assume the air leaving the unit is saturated? Doesn't the A/C unit dehumidify the air? I understand that the w.vap would be saturated around the coil, but if you put your hand in front of the outlet of an A/C unit the air isn't wet (doesn't feel like RH of 100).

In front of the outlet, the air quickly mixes with room air to yield warmer, lower relative humidity air. The air in the unit all passes over the coil and thus all gets cooled. To use another example, a glass of ice water sitting in a room will not cool the air across the room from it, but it does cool the air in a thin film surrounding the glass down to 0C.

Just to confirm: If I used psychrometric charts, all I'd need to do is guess an output temperature and use an RH of 100%. That'd get me the specific humidity through the chart. Since I can calculate specific humidity at the inlet as well, the difference between the two states would give me the condensation rate right?

Correct.

In my case I need a coil surface temp of -5 degrees C to attain 30% RH at 10 deg C.

That's a pretty tall order, particularly because you will get ice on your coil and not water. You may need to pursue an altogether different dehumidification method, such as dessicant dehumidification. If I may ask, what is the reason for this low humidity need?

I'm stuck on how to choose an existing AC unit based on this information because all I'm given is the cooling capacity (in BTU for example), but I wouldn't know the surface temperature of the coils. How do I proceed from here?

Coils are rated under certain conditions (inlet temp, humidity and airflow), so the manufacturer's rating should include the inlet temperature, humidity, airflow and the coil capacity, from which you calculate the leaving temperature, as you did above.