Problem: a 1000 square foot walk in cooler using refrigerant R134A, has a capacity of 4 tons.
Given: 4 tons
Approach: The total heat rejection from the condenser is 57MBH.
Calc: We have the compressor going from 1 to 2, and then the condenser going from 2 to 3, the expansion going from 3 to 4, and then e...
Calc: And just for consistency, let's get everything in VTU's power so I'm going to multiply that by 12,000 VTU's power per ton, that...
Result: So the COP is always the desired output over the required input.
Office Hours
4
Student questions asked in live office hours about this problem
OH 29: HVAC 2
Q: Can you walk through this DOAS (dedicated outdoor air system) problem — no specific question was submitted, just a request to work through it?
A: I'll outline the solution approach and draw a picture of the system, since understanding the geometry is the hardest part of DOAS problems. I won't go through all the math, but I'll give you the conceptual framework you need to handle problems like this.
OH 30: HVAC 2
Q: Can you check my work on this cooling tower problem — circulating water at 1000 GPM enters at 90°F and cools to 70°F — asking for the make-up water rate?
A: This is a classic make-up water problem — as the condenser water cools, some of it evaporates into the air stream, and you're finding that evaporation rate. Set up the moisture balance using the inlet and outlet air humidity ratios; the make-up water replaces exactly what evaporates.
OH 31: HVAC 2
Q: In HVAC-2, how do you determine the temperature of the air entering the coil — is it the mixed air temperature?
A: Yes — the air entering the coil is the mixed air, a blend of outside air and return air, and that's the source of the ambiguity. The mixed air temperature requires knowing both the outside air condition and the return air condition, then blending them proportionally by volume.
OH 32: HVAC 2
Q: Can you explain how to get the supply air temperature of 62°F using the SHR and the psychrometric chart?
A: I don't think you can solve this problem without the psychrometric chart — you need to draw the process line from the room condition using the SHR slope from the protractor. The supply air temperature of 62°F is where that process line intersects the supply condition, and that's something you have to read graphically.