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patskE
18-10-2009, 04:32 AM
just wondering
when u are sizing up a evaportator, say for a cool room
has 2300 watts load at 2 degrees, but needs to be at 75% RH, how do u work it out? so it does run at the correct RH?
tx

lana
18-10-2009, 04:46 AM
Hi there,

There is no way to get the exact relative humidity you need, but you can get close. This depends to many factors first and the most important one is the evaporator TD (troom - tevaporation).

Higher TD then lower RH.
Lower TD then higher RH.

Room wall and floor construction materials (outside layer), kind of product ,... directly affect RH in the room.

Cheers

patskE
18-10-2009, 04:52 AM
how low/high of a td is say 50%? so i got a ball park figure :S

mad fridgie
18-10-2009, 05:06 AM
Imagine your coldest point Te (evap) the air "could" be 100% RH (it has a certain fixed amount of actual water in it "weight") If this air is warmed to room temp with this fixed amount of water then the RH will drop. So colder the evap the lower the actual amount of water will exit.
So answer to your question work out your actual water content at 2C and 75RH, then find out at what temperature this actual content reaches 100%, this then would be your Te temp.
No allowance for entering moisture

mad fridgie
18-10-2009, 05:18 AM
You need to use a LT chart, but can not spell "pyscrometric"

Gary
18-10-2009, 06:27 AM
Here's a psych chart:

http://www.nrc-cnrc.gc.ca/obj/irc/images/bsi/83-psy_E.gif

Gary
18-10-2009, 06:33 AM
Find 2'C at the bottom... follow this up to where it intersects with the 75%RH curve... then left to the 100%RH curve... then down. It ends up at -2'C. This is the coil surface temp/leaving air temp you need.

In order to get -2'C surface temp, the evaporating temp of the refrigerant will need to be somewhat lower than -2'C.

A 5'C TD coil should get you in the ballpark.