mixed condition

[gilla]

Dear Braintrust,

I am trying to understand the best way to find a mixed condition (ie temperature, enthalpy, etc.). When for example...return air is mixed with outside air. A text I am studying says you can find this by the following equation:

mixed temp. condition=(OATemp*CFMOA+RETTemp*CFMRET)/CFMT

CFMT=CFMOA+CFMRET

I get mixed results when I solve by this approach...ie I don't get the right answer according to other textbooks I am studying.

Please help.

 

[imok2]

gilla
your formula is correct, I used 200 CFM OA at 95*F and 1600 CFM RA at 75*F = (19000)+ (120,000)/1800 CFM = 139,000/1800 = 77.22*F

 

[quark]

That's quite right. But you should not use it to find wet bulb temperature. Apply the formula either to enthalpy or moisture content and correlate it with the DBT value(as per the formula) and get the final WBT.

Regards,

 

[KenRad]

gilla,

The easiest way is to go to the Trane website:

http://www.trane.com/commercial/software/index2.asp?flash=yes

There's a "Web Psychrometrics" section, which has a mixed air calculator.

---KenRad

 

[gilla]

Thanks for your help...people like Quark and others deserve a big thanks because they are always helping keep this website on the "cutting-edge"!!

 

[ChasBean1]

I've always liked this one - you can download it here:

http://pmilligan.net/pmtherm.htm

Install it (v 1.23). Go to "Psychro" on the menu bar, then "PsychroCalc." You can enter conditions in boxes 1 & 2 and calculate the result in 3 with "Processes."

 

[quark]

Yeah! That's really a nice program and the programmer is a friend of ProfSporlan (I remember he saying it). There is another software to calculate refrigerant properties (another nice program and this is Professor's work) available from the same site.

PS: PMTherm requires the file vbrun300.dll to work and there is also a link available.

Regards,

 

[wilg]

Your equation is correct, but you will only be able to
calculate drybulb temperature with it, either use a psychrometric chart or use one of the many software
utilities already mentioned in previous replies.
When using the psychrometric chart draw a stright line between the outside air and the return air conditions.
Then calculate the percentage of outside air, measure
the line that you have drawn and take the percentage of distance from the return air point using the outside air
percentage distance.