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Psychrometric of mixing hot and cold air

Psychrometric of mixing hot and cold air

Psychrometric of mixing hot and cold air

(OP)
I needed to calculate a mixed air temperature so I used the formula and found the number. To double check my numbers, I used the McQuay Psychrometric Analyser, but the number was different. I was a bit confused so I got the psychrometric chart and did a manual calculation by drawing a straight line across the two points. I realized that the drawn line cut across the saturation line. If the line cut across the saturation line, does the straight line method still stand, or does it require a different method to calculate the mixed air temperature?

Below are some sample numbers:
O/A temp: -40F
O/A flow: 4615 cfm
R/A temp: 90F
R/A flow: 10385 cfm
Calculated MAT: 50F

McQuay calculated MAT: 42.3F (assuming everything is 50%RH)


How would you find the mixed air temperature in this case?

RE: Psychrometric of mixing hot and cold air

Mass flow, not volumetric, however pay attention to where your fan is.

Ignoring relative humidity for now which actually makes air LESS dense as it increases. I also assumed sea level pressure.

-40 degree F and 4,615 CFM = 437 lbs/min
90 degree F and 10,385 CFM = 751 lbs/min

total mass flow is now 1,188 lbs/min

Mixed air conditions is.....

((-40)x 437 + 90x751)/1188 = 42.18 degrees F.

McQuay maybe giving you an answer but you need to look at your system. A fan, at any given speed is a CONSTANT VOLUME device, meaning a fan at sea level set to deliver 1,000 cfm will try to deliver 1,000 CFM at 5,500 feet or at 100 degrees F or -40 degree F. What changes is the motor horsepower due to air density (as well as friction loss which is proportional to air density). A fan is shovel, the blade is the same size if moving sand or gravel, work required changes.

Apparently there is an equation writer in the forum that I need to figure out...

I also developed a little spread sheet that figures this stuff out pretty quick and includes altitude, temperature, relative humidity and inlet static pressure. Remember a fan 'sees' what it's inlet conditions are.....







RE: Psychrometric of mixing hot and cold air

mixing line could hypothetically cross saturation line only if both air streams that mix have near 100% relative humidity.


as you are mentioning 50% RH, i believe that you found wrong air state points. 50% RH line is far away from saturation line.

RE: Psychrometric of mixing hot and cold air

(OP)
Thanks 11241. I've thought it was volumetric.

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