# HVAC 6MS breadth #2



## navyasw02 (Sep 28, 2010)

I tried working the 6MS #2 HVAC and cant figure out why they chose 32 Btu/lbm over 28. I worked it exactly the same way as the US solution of ex 38.4 and it should give me the same result as the method they used. I used 13.65 for vb from psych chart giving me 5000/13.65 = mb = 366 lbm/min. I used 14.2 for va giving me 2500/14.2 = ma = 176 lbm/min. Using the mixing equation (eq 38.21) I got a Tmix = 80 F or so. Going back to the psych chart and got pretty much 28 for the final answer. Any thoughts? Maybe I just have to squint harder and this tiny psych chart I'm using in the MERM.


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## Bman (Sep 29, 2010)

navyasw02 said:


> I tried working the 6MS #2 HVAC and cant figure out why they chose 32 Btu/lbm over 28. I worked it exactly the same way as the US solution of ex 38.4 and it should give me the same result as the method they used. I used 13.65 for vb from psych chart giving me 5000/13.65 = mb = 366 lbm/min. I used 14.2 for va giving me 2500/14.2 = ma = 176 lbm/min. Using the mixing equation (eq 38.21) I got a Tmix = 80 F or so. Going back to the psych chart and got pretty much 28 for the final answer. Any thoughts? Maybe I just have to squint harder and this tiny psych chart I'm using in the MERM.


Navy,

I don't have the SMS book anymore, so if you write out the entire problem, I think I could help you out.


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## navyasw02 (Sep 29, 2010)

Bman said:


> navyasw02 said:
> 
> 
> > I tried working the 6MS #2 HVAC and cant figure out why they chose 32 Btu/lbm over 28. I worked it exactly the same way as the US solution of ex 38.4 and it should give me the same result as the method they used. I used 13.65 for vb from psych chart giving me 5000/13.65 = mb = 366 lbm/min. I used 14.2 for va giving me 2500/14.2 = ma = 176 lbm/min. Using the mixing equation (eq 38.21) I got a Tmix = 80 F or so. Going back to the psych chart and got pretty much 28 for the final answer. Any thoughts? Maybe I just have to squint harder and this tiny psych chart I'm using in the MERM.
> ...


The problem is:

A room is conditioned to 75F and 50% rh. A fan system combines 5000ft3/min of recirc air from the room with 2500 ft3/min of outdoor air at 90F db and 75wb temp. The mixed air enters through a chilled water cooling coil as shown. (Diagram doesn't really matter for the problem) The enthalpy of the air entering the cooling coil is 28, 32, 35, or 39 Btu/lbm.

Maybe I just need a bigger psych chart. Thanks for the help.


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## nashbmattPE (Oct 1, 2010)

navyasw02 said:


> Bman said:
> 
> 
> > navyasw02 said:
> ...


Navy

when going to the psych chart are plotting 50% rh or 50F WB. when i worked this on the psych chart it appears you may be using 50F WB which gives around 28.8 BTU/lbm. when plooted out using 50% rh i derived around 33 BTU/lbm. I calculated the MAT at 82.5.

hope this helps out


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