Calculating removal of air humidy

[fastline12]

A bit outside my area of work but I am mostly trying to understand some concepts. It seems most dehumidifiers rate performance in volume/time, not BTUs, watts, etc. Air conditioners are rated with BTUs cooling with no regard to air moisture.

I realize and have no problem calculating the dew point based on air temp, RH, pressure, etc. I also realize that simply passing air across an evap that has a surface temp below the dew point will condense the moisture. However, I have never been able to quantify the effects of an air conditioner in regards to separation of air cooling and water condensing.

What am I missing? Or am I simply over thinking this? I might need to dust off the thermo book...

 

[fastline12]

Doh, I think I have my problem solved and just needed to think for a moment. The latent heat of condensation of water. By knowing the volume of water in the air, air volume, and evap temp, I can calculate how much energy it is taking to condense the water at eh evap this the evap carries that away rather than the heat in the air. Probably safe to assume there is no exacting science in how much is condensed vs how much air cooling because some air may exchange enough on one pass to reduce the water content to dew point and still cool the air a bit.

 

[zdas04]

Glad we could help.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[Compositepro]

The difference between a dehumidifier and an air conditioner is that the dehumidifier will take the air that has been cooled below the dew point and pass that through the condenser where it is reheated to above room temperature. The latent heat that was extracted from the water is dumped into the room, which adds to the reduction in relative humidity. An air conditioner removes mainly sensible heat (lowers air temp) and some latent heat, which is dumped to outside air. Most modern window A/C units will use any condensed water to help cool the condenser by evaporation. So these units don't drip water like the old ones.

 

[fastline12]

I thought I had a firm grasp on this but seems that I need to build some calcs in excel for psychrometric data. I found a couple calcs but I am trying to arrive at the density, water/air ratio, and thus the specifheat of the gas. From a calc I have looked at, they have enthalpy listed but do not explain nor are the equations available so I have no idea how the value is derived. The enthalpy value as btu/lb will vary with temp and was my understanding that specific heat of any water/air mixture would be completely dependent on the ratio of water and have nothing to do with temperature thus density?

In short, would anyone have the calcs available to arrive at specific heat for air/water?

 

[EnergyProfessional]

Calculating in excelt is rather difficult since the psycrometric chart is complex. You can use a software like EES that has proeprty functions built in.

The dehumdifier values are based on standard conditions (i.e. 80°F and 70%RH and in that type of room it removes X gallons per hour etc.). this removal rate changes with space conditions rapidly.

 

[chicopee]

Perry's CE and Kent ME handbooks have a lots of formulae from which psychometric charts were developed or vice versa.