Psychrometric of mixing hot and cold air 1

[CKwan]

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?

 

[MTES93]

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.....

 

[Drazen]

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.

 

[CKwan]

Thanks 11241. I've thought it was volumetric.