View Full Version : Psychrometric question
crusty demon
26-06-2009, 09:50 PM
Senerio ---- 100% outside air unit moving 2000 cfm with an outside air temperatureof 30 degrees F.
How many watts of electric heat will it take to raise the supply air temperature to 70 degrees F ?
Altitude = sea level
specific density = .075 lb/cubic foot
The answer i came up with just doesn't look right, need to see someone elses calculcations.
thanks
frank
26-06-2009, 10:46 PM
30F = 1.1111C
70F = 21.1111C
therefore dt = 20C
1CFM = 0.028317m3/s therefore 2000CFM = 56.634m3/s
56.634m3/s x 20C = 1381.86kW
It's a little difficult when you mix SI units with Imperial units in your question.
crusty demon
27-06-2009, 12:50 AM
86,400 btu
25,307.55 watts
25.3 kw
is this correct?
Abby Normal
27-06-2009, 04:39 AM
2000/12.4x60x(19.81-10.16)/3413= 27.4 kW :)
30 degree air is a little denser, probably some water vapour in it as well
end up with more than 2000 CFM of supply
nike123
27-06-2009, 06:32 PM
1CFM = 0.028317m3/s therefore 2000CFM = 56.634m3/s
1cfm = 0.0004719 m3/s
2000CFM = 0.9438948864 m3/s
Required heat is: 24,5 kW
http://farm4.static.flickr.com/3310/3665099907_f8b7445219_o_d.png
Abby Normal
28-06-2009, 12:22 AM
2000 / 60 x .3048^3 = 0.9438948...... cubic meters per second
30F is -1.11111C, its below freezing
24.49 x 22.1/20= 27.06 Kw :)
I allowed for about 80% RH to get 27.4
Abby Normal
28-06-2009, 12:25 AM
the answer depends on whether you measure the cold air or the heated air flow
nike123
28-06-2009, 07:11 AM
30F is -1.11111C, its below freezing
It seems that Frank has got hard Saturday.;)
30F = 1.1111C
1CFM = 0.028317m3/s therefore 2000CFM = 56.634m3/s
Abby Normal
29-06-2009, 04:44 AM
NASA crashed 9 figures worth of probes into Mars because they had problems with IP and metric
frank
29-06-2009, 10:40 PM
I know, I know. I saw it and then realised that you would all start pulling my leg before too long :confused:
Note to self. DON'T do calc's after BEER!:D
vijay_study
30-06-2009, 02:08 PM
Sensible Heat (Btu/hr) = 1.08 x cfm x (Delta T)
=1.08 x 2000 x (70 - 30)
=86400 Btu/hr
=25Kw.
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