psychrometric chart question

[new2hvac]

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?

 

[lilliput1]

ASHRAE has a low temperature psychrometric chart. At 30°F & 0% RH, enthalphy is 7.2 Btu/Lb of dry air. At 0°F & 0% RH, enthalphy is 0 Btu/Lb of dry air.
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

 

[new2hvac]

That's helpful...thank you. Still I wonder how air at 0 deg F and 0% RH can contain no energy. It must take some amount of energy to heat air from say -10 deg F and 0% RH to 0 deg F and 0% RH. How can you calculate that?

 

[KenRad]

new2hvac,

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

 

[ChasBean1]

A BTU is the amount of thermal energy needed to raise 1 pound of water 1°F (starting from its highest density, ~39°F).

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.

 

[ChasBean1]

I didn't notice the last question before - I would disregard any possible latent change and use:

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

 

[ProfSporlan]

The assignment of zero enthalpy on a psych chart or refrigerant thermodynamic chart is arbitrary, and usually has nothing to do with true zero enthalpy, i.e., no usefully energy in the substance.

Since we are ultimately concerned with delta-h in our calculations, it make no matter where the zero point is assigned.