Moisture loss calculation

[gregorm]

Hi Guys,
Can one of you clever guys tell me how to work out how much moisture is lost when air travels over a coil. If for example air on is 32°C and 85% RH, and the coil temperature is +5°C, what would the RH be of the air leaving the coil if it was, say 12°C?
Can someone explain how to read this from a psychrometric chart please?

Thanks

[NoNickName]

It is impossible to determine from your data.
I can tell you which RH is the air leaving the coil, from the air temperature leaving the coil. But not just on the coil temperature, because the efficiency of heat transfer depends on the type and geometry of the coil.

[gregorm]

Thanks for your response NoNickName. So what would the RH be on air leaving at 12°C then, and what other data would you need to know?

[NoNickName]

32°C 85% have an absolute water content of 25.8 g/Kg and dewpoint of 29.2°C
Any temperature lower than the dewpoint is saturated, that is 12°C is 100% humidity.
The water content at 12°C 100% is 8.7 g/Kg.
The difference is 17.1 g/Kg of dehumidification.
1 Kg of air is 1m3 approx.
That is your moisture removal would 17grams of water for each cubic meter crossing the coil.
For 1000m3/h of air flow, that would sum up in 17Kg of condensate per hour.

[gregorm]

Thanks again NoNickName, that helps. So would I be correct in saying then, if I wanted 12°C off at 50% RH, then I would need to have 2°C off, and re-heat it to 12°C, or am I way off the mark?
Sorry if I am sounding real thick, but I am just trying to learn a bit how it works.

[NoNickName]

Yes, you are right, but I don't see any commercially feasible way to have 12°C 50% starting from 32°C 85%. Unless it is all recirculated air, and in this case the off and on coil condition will continuously change until setpoint.

[gregorm]

Hi NoNickName,
What I have here is fresh air on at 32°C 85% RH, the coil has a glycol mixture entering at 5°C and leaving at 8°C. The air off is 12°C 70% RH. It then travels through ducting to supply some cabins. When it reaches the cabins the temperature has increased to around 22 - 23 °C 38% RH.
I would like to have about 23°C 50% RH in the cabins. With the coil as it is, would the only way of increasing the humidity be to have a water mist injection in the air after the coil?

[NoNickName]

Hi NoNickName,
What I have here is fresh air on at 32°C 85% RH, the coil has a glycol mixture entering at 5°C and leaving at 8°C. The air off is 12°C 70% RH.

This is impossible. Can't be 70%, it must be 99%.
Or it is not 32°C 85%

[gregorm]

Apologies, it is drawn through the coil by a belt driven centrifugal fan, and therefore picks up heat off the motor. The point I was able to measure it was at the fan outlet, here it was 12°C 70% RH. Does this make more sense?

[NoNickName]

Apologies, it is drawn through the coil by a belt driven centrifugal fan, and therefore picks up heat off the motor. The point I was able to measure it was at the fan outlet, here it was 12°C 70% RH. Does this make more sense?

Well, it depends how much is the fan gain to the air flow.
Why are you using total fresh air? Is it a requirement?

For your question, there are many possibilities of increasing humidity, from ultrasonic humidifiers, to electrodes steam humidifiers, to microsprayers...

[gregorm]

I don't know why it was designed like that, the other ones have a return and mix the fresh air with the recirculated air.
Thanks for bearing with me, I think I have a better understanding now of the temperature & humidity relationships.

[NoNickName]

Glad I helped somehow.

[rbartlett]

This is impossible. Can't be 70%, it must be 99%.
Or it is not 32°C 85%

I'm not too sure but is this related to this confusion?

http://www.hvacrforums.com/showthread.php?p=91#post91


cheers

richard

[gregorm]

That looks an interesting one, do you know the answer Richard?

Greg

[US Iceman]

I'm not too sure but is this related to this confusion?

Quite an interesting method to hide advertising.

[autt]

Except the words I don't understand, it looks like frosting