psychrometric chart question
psychrometric chart question
(OP)
If you look at the enthalpy scale on a sea level psych chart, it appears to go to zero at about 32 deg F and 0% RH...following the line, it also appears to be 0 at 30 deg F and 10% RH. I thought the complete absence of energy only occurs at 0 deg K.
I'm trying to calculate the heating requirements to achieve 88 deg F and 50% RH...this has an enthalpy of about 37 Btu/lb. During winter design conditions, I'd like to plan for 0 deg F and 0% RH.
How much energy is required to heat 0 deg F 0% RH air to 30 deg F 10% RH?
I'm trying to calculate the heating requirements to achieve 88 deg F and 50% RH...this has an enthalpy of about 37 Btu/lb. During winter design conditions, I'd like to plan for 0 deg F and 0% RH.
How much energy is required to heat 0 deg F 0% RH air to 30 deg F 10% RH?





RE: psychrometric chart question
At 30°F & 10% RH, enthalphy is 7.6 Btu/Lb of dry air.
Average specific volume between these conditions is 12 CF/Lb dry air.
Therefore Btu/hr = CFM x (7.6-0) x 60/12 = CFM x 38
Note however that this is total heat (sensible + latent).
Sensible heat Btu/hr = CFM x 1.1 x (30-0)= CFM x 33
RE: psychrometric chart question
RE: psychrometric chart question
The enthalpy that you read on an ASHRAE chart or table is not absolute; it is relative to an assigned value of 0 BTU/LBda at 0 degrees F and standard atmospheric pressure.
So you can find an answer to your question by looking up the enthalpy at 30F, which I read as 7.2 (from the table). As far as the humidification aspect of your question, that is a separate calculation, and depends on how you are adding the water vapor.
---KenRad
RE: psychrometric chart question
If you're below freezing, BTU definition goes out the window and we use latent heat of fusion to define energy in a phase change to ice. Cooling the ice block toward abs. zero I don't really know - we start thinking more in terms of entropy perhaps.
RE: psychrometric chart question
Q = m cp dT
m = mass flow rate of air
cp = specific heat at constant pressure of air at the average of the two temperatures
dT = 30°F
RE: psychrometric chart question
Since we are ultimately concerned with delta-h in our calculations, it make no matter where the zero point is assigned.