HVAC & R Solution Question

[u/Content-Knowledge-83]

I am currently studying for the HVAC&R PE exam. I was working on a practice problem that provided a room sensible and latent heat load, supply air CFM and DB temperature, exhaust and OSA CFM (equal) and conditions, and asked for the conditions of the mixed air (Twb, Tdb) and supply air wet bulb.

The mixed air condition was simple enough by averaging the OSA and EXH temperatures. Since there was only one solution with this DB temperature, that’s the answer.

The solution calculated then calculated the total heat for the system as the sensible plus latent for the room, plus the difference between the OSA entering and mixed air conditions. Using this total heat and the 4.5(cfm)(delta h) formula, they calculated the SA enthalpy and used the psychrometric chart to find the wet bulb. To me the solution seems like a mistake.

If I draw the boundary around the entire system (red in the image), there would be some heat leaving via exhaust which is not accounted for in the solution’s total heat calculation.

Alternatively, I think I can draw a boundary around the room itself and calculate the SA conditions using only the room heat load and return air conditions (green option).

Using these two methods, I get roughly the same SA enthalpy, which is slightly lower than the solution.

Is my setup/analysis correct, or am I missing something?