Help with cooling problem

[Mech_Engineer]

Folks, could someone help me understand when using "q=m-dot x Cp x Delta T" will not work with a gas or air? I get mixed up thinking I can use this formula for most gas/air situations, but I recently worked an HVAC problem that the solution used an energy balance across the condenser to solve the problem. I tried both methods and the answers are different when using the two methods. Please help me understand when and when not to use Cp x Delta T. I thought "Cp x Delta T" was the same as "Delta h"?

I feel like I'm ok using the formula for many heat transfer problems, but I'm starting to get confused on when and when not to use it. 

Problem attached. Forgive my handwriting. I appreciate the feedback.

View attachment 20181017145909389.pdf

 

[Mech_Engineer]

Is it because the air going into the coil is a mixture of air/water? To use mCpdT, it must be ideal?

 

[Slay the P.E.]

Is it because the air going into the coil is a mixture of air/water? To use mCpdT, it must be ideal?

Close.

m*Cp*DELTA-T is accurate for sensible cooling/heating. This would be somewhat accurate even for moist air as long as there is no condensation or humidification (i.e., the process is a horizontal line on a psych chart).

If there is condensation (like in this problem) or humidification, then you need to account for latent heat effects associated with phase change. Thus, it is best to use the psych chart to get DELTA-H, as m*Cp*DELTAT fails to capture this effect.

 

[Mech_Engineer]

Ah. Thanks for the explanation! That makes sense now. I was struggling with this problem today. 

 

[Abraham5G PE]

Did you go to UT Tyler by any chance?

 

[Audi Driver P.E.]

Note that, like in your problem, the given information gives you a clue as to how to solve. h1 is provided, and so that should tip you off to the fact h is important.

 

[Mech_Engineer]

Did you go to UT Tyler by any chance?

Negative. From Louisiana but now in Houston area.

 

[Mech_Engineer]

Note that, like in your problem, the given information gives you a clue as to how to solve. h1 is provided, and so that should tip you off to the fact h is important.

Good point. I think I'm wrapping my head around it now. For some reason, HVAC type problems give me fits and I'm trying to get better at them. It was my weak area on my first attempt in April. I'm also always nervous I don't read the graph correctly or cross up lines, etc when using the graphs.

 

[Slay the P.E.]

Note that, like in your problem, the given information gives you a clue as to how to solve. h1 is provided, and so that should tip you off to the fact h is important.

Interesting observation. The dry bulb T and the relative humidity at the inlet are also given, so giving h_1 makes the problem over-specified. Luckily 33.6 Btu/lbm is the right enthalpy for the given values of T and r.h. I think it is more likely that the h_1 in the handwritten notes was looked up in a psych chart and was not part of the given data.

 

[Mech_Engineer]

Interesting observation. The dry bulb T and the relative humidity at the inlet are also given, so giving h_1 makes the problem over-specified. Luckily 33.6 Btu/lbm is the right enthalpy for the given values of T and r.h. I think it is more likely that the h_1 in the handwritten notes was looked up in a psych chart and was not part of the given data.

I went back and checked the problem statement. You're right. I found h1 in the psych chart, and when I transcribed my work to upload here, I had it as given data. Good catch.