Air Off Temperature

[dallan]

Can anyone give me any help in calculating an air off temeprature, using a coldroom as an example. If you know the evaporating temperature and the air on temperature how do you calculate the air off temperature.

[Dan]

Dallan, I was surprised to only see a degree or two between incoming and outgoing air temperatures in walk-ins. I think some of us, including Marc, discussed it, but not to my satisfaction, nor Marc's, nor the others. Your question is a process question.

We are so used to seeing dramatic temperature changes in air conditioning ingoing and outgoing temperatures. And there is little to play with:

Fin spacing (secondary surface)
Rows of evaporator piping (primary surface)
Temperature difference (design TD... the difference between saturated suction temperature and desired room temperature)
Air velocity
Latent versus sensible.
Wet surface compared to dry surface (not as redundant it seems)

Carrier's ADP (Apparatus dewpoint) is also a wonderful consideration to insert.

Your question is a good one and I think we can answer it. Does anybody notice I used the word "we"?

Because I sure as heck do not have the answer. I just think I do.

[RogGoetsch]

Can anyone give me any help in calculating an air off temeprature, using a coldroom as an example. If you know the evaporating temperature and the air on temperature how do you calculate the air off temperature.

I assume that "air off" refers to the air discharged from the coil. I wonder why you don't measure it instead of calculate. Can you tell us more about the purpose of your calculation? Concerned about freezing produce in the discharge air stream, for example?

Marc is dead-on accurate, IMHO. Does his information answer your question?

You would need to have the coil specifications: BTUH capacity (in our archaic units) and air flow in CFM.

If this question is academic, is latent heat removal (moisture condensation) from the air given as a factor? (i.e.: is relative humidity or dew point provided?)

For simplicity's sake, it may be reasonable to assume it is a cold room with no humidity load, which would allow the calculation to safely ignore latent heat. Depends on how accurate you need to be.

As Marc points out, this would not apply to the same calculations during pull-down from higher temps or higher relative humidity.

Also, as per Dan, discharge air rapidly entrains ambient air so its temperature can quickly rise a short distance from the coil.

More info?

Rog

[Dan]

discharge air rapidly entrains ambient air so its temperature can quickly rise a short distance from the coil

I think that is a key observation, Rog. A unit cooler more often than not is an unplenumed fan-evaporator assembly.

There is not a distinct trail of return air versus the discharge air. We can measure distinct and significant differences between the air in and the air out of an air conditioner or even a supermarket display case because the heating process is contained between the stream of the air in and the air out, as is the cooling process. With a unit cooler, you most often have a slightly warmer entering air than you do with the desired room temperature.

I am still intrigued, though. Has anybody measured more than 5 deg change between the inlet and outlet of walk-in evaporator?