How much capacity needed to cool airflow

[LDS]

Forgive me if I do not ask this correctly, thermal topics are not by best.

I want to put a heat exchanger in an air duct. If I know the airflow and humidty of the airflow, how do I determine how much heat I need to pull from the air to cool it from 77 deg to 70 degrees?

Thank you.
Les Strickland
Fort Worth, TX

 

[Yeldud]

You can determine this from a psychrometric chart. Determine the entering and leaving enthalpy of the air stream at 77F and 70F. Also determine the mass flow of air by using the density from the psychrometric chart. Mass flow times enthalpy difference gives you heat removal rate required for your process. All heat removal may be sensible or some may be sensible and some latent depending on how high of an entering relative humidity you have. You can model the process on a psych chart to see.

 

[grim900]

Les
As Yeldud says, psycrometric chart is easiest for this. However, if you're only interested in reducing temperature and not humidity (ie sensible cooling) then cooling duty is:

Q = M x C x (t1 - t2)

Q = Duty (kW)
C = Specific Heat Capacity (approx 1.012 kJ/kg Kelvin)
t1 = On coil temperature (Kelvin)
t2= = Off coil temperature (K)
M = Mass flow rate of air = volume flow x density (density approx 1.17 kg / m3)


Sorry for my units but the equation will of course hold true using F and CFM, you will need to adjust the specific heat capacity and density figures accordingly.

Cheers,
Graham.

 

[lilliput1]

Sensible heat in btu/hr = CFM x 1.1 x (77-70)

Latent heat in btu/hr = CFM x 4840 x (W2-W1)

Where W1 = entering humidity in lb water vapor/lb dry air
W2 = leaving humidity in "ditto"

Total heat in btu/hr = Sens heat + Latent Heat

Gpm of cooling water needed = Total heat in btu per hr/(500 x (t1-t2))

Where t1 = °F entering water temperature
t2 = °F leraving water temperature