dehumidification question

[stoshka]

i need help about dehumidification
how can I calculate the relative humidity of air after cooling it on cooling coil?
I have these data: air velocity, air volume, air temperature before cooling coli, cooling coil capacity. Can I calculate the relative humidity of air after cooling it?
thank you

[BACnet]

I suppose you could find some way to measure the liquid coming off of the coil but that would be fairly complex and perhaps expensive.

If you want to know the humidity, why not use a cheap relative humidity sensor and be done with it?

[The_USC_2001]

I agree. Get a humidity sensor.

[TSASteve]

You can take a digital pocket thermometer and cut a shoelace to create a "sock" wet the sock and put it over the probe. Insert this into the supply plenum/duct and leave for 5 minutes, this will give you the wetbulb temperature of the supply air. Next take the sock off and put the thermometer in the same place and leave for 5 minutes. This will give you dry bulb. Make sure and take the readings while the thermometer is still in place. With these two readings you can find the RH of the supply air.

[BACnet]

Individual components aren't for everyone, but I can get my hands on RH sensors for close to $17, and that's buying them one at a time with no price break.

The sock idea is valid and it does work. But you can't exactly install that in your control scheme. A digital RH sensor on the other hand...

DigiKey (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=445-6140-5-ND)

[TSASteve]

Sorry, misunderstood what we were trying to do here.

[BACnet]

Or perhaps I did...

:)

[ispip]

If you are truely dehumidifing your RH will always be 100% at whatever temperature is coming off the coil. If you want to calculate the RH after the coil, then you need to know what the temp and humid before it is, or the wet and dry bulb before. but if you are able to get that info before the coil. then you should be able to get after. But when all else fails the pyschometric chart.

[stoshka]

thanks for for fast answering
but, I am interesting in calculating the humidity, is it possible, do I have to collect more data...
I am not dehumidifier expert :(
Is it possible?
Than You

[teddy bear]

i need help about dehumidification
how can I calculate the relative humidity of air after cooling it on cooling coil?
I have these data: air velocity, air volume, air temperature before cooling coli, cooling coil capacity. Can I calculate the relative humidity of air after cooling it?
thank you

No.
You know the dew point of the air will be at best several degrees above the coil temperature. Best to deal with dew points or grains of moisture per lb. of air. Search net for more info.
Regards TB

[ACCMan]

Carrier has a free software program for psychrometrics (all known properties of air).
With this program you can enter the wetbulb and drybulb temperatures and determine almost anything about the air. The problem will be in knowing what you are looking for...
or trying to accomplish with the information.
There are many reasons for dehumidifying and, depending on the requirement, many reasons a system may not be meeting your needs.
(high evap. coil temp., too much air, dirty coil, poor fin contact, coil too small for the purpose, not enough capacity for the area you are conditioning, etc....

[TSASteve]

Carrier has a free software program for psychrometrics (all known properties of air).
With this program you can enter the wetbulb and drybulb temperatures and determine almost anything about the air. The problem will be in knowing what you are looking for...
or trying to accomplish with the information.
There are many reasons for dehumidifying and, depending on the requirement, many reasons a system may not be meeting your needs.
(high evap. coil temp., too much air, dirty coil, poor fin contact, coil too small for the purpose, not enough capacity for the area you are conditioning, etc....
Where can one get said software? Hvac-partners?

[BACnet]

Where can one get said software? Hvac-partners?

There are a number of them online, some free, some for sale, some as apps and some as cloud/web based (http://www.coolit.co.za/airstate/airmoistobject.htm) programs.

[ACCMan]

There are a number of them online, some free, some for sale, some as apps and some as cloud/web based (http://www.coolit.co.za/airstate/airmoistobject.htm) programs.

http://www.filebuzz.com/findsoftware/Carrier_Hvac_Psychrometric/1.html

[chad_nc]

DB and WB temp is all you need to calculate RH%, as someone already stated the RH% in the supply duct is going to be 100% or right there at it, if you are truley dehumidifying. Psycometrics is not that complicated, but a psych chart is a must. When dehumidifying air picture a cubic foot of air say 72 degrees at 60% rh coming down the return duct getting ready to go through the evap coil. The air hits the coil and cools (shrinks) down to 52 degrees the rh goes up over 100% (which we all know is impossible) so the excess moisture runs out down in your drain pan and out of the unit. When the air goes out of your supply duct back in to the space your cooling it warms back up to room temp and now has less moisture in it. Now if that same air 72 degrees at 60% rh is only cooled down to say 60 degrees when it hits the evap coil you will not be taking any moisture out of the air and the coil will not sweat. The air would have to be down to at least 57 degrees for moisture to be taken out in this example. Hope this helps.