HVAC · Thermodynamics · Problem 22PDFSolution in PDF ↓
HVAC · Thermodynamics · Problem 22
Problem & Solution
Video Synthesis
Problem: Thermo-22, our Friduration cycle using R-22 operates between zero degrees and 80 degrees with a refrigerant flow rate of 300 pounds per hour.
Given: 300 pounds per hour; 300 pounds per hour times the difference between H1 and H3 which we worked out separately 71; 3 ...
Approach: So let's start with a little sketch of the cycle.
Calc: So QL equals M dot which we now have times H1 minus H4.
Calc: Can we go and find the enthalpy at state one and state four by working our way around the cycle for R22 with those temperature ...
Result: So back over the chart higher temperature now 80 degrees but we're interested in the saturated liquid 33.3 and now the difference between H1 and H3...
Office Hours
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Student questions asked in live office hours about this problem
OH 18: OH18-03-THERMO-22
Q: When a temperature range (0-80°C) is given for a refrigeration cycle, should the 80°C correspond to state 3 (saturated liquid) or state 2 (higher temperature)? Is the range assumed to be condenser outlet to evaporator inlet rather than the entire cycle?
A: There's some ambiguity here, but the intent is that 80°C on the high side falls inside the vapor dome at state 3, not in the superheated region at state 2. On the low side, T and P are linked, but on the high side, this only holds inside the dome—superheated vapor breaks that relationship. The latent heat during condensation is what matters most, so assuming the high-side conditions stay within the dome is the practical approach.
OH 70: HVAC: Thermo Module #22
Q: If the cycle operates between 0 and 80°F and state 2 is superheated at constant entropy (isentropic), wouldn't state 2 be at 80°F and state 3 be lower, not state 3 at 80°F as shown in the solution?
A: You're absolutely right—that's a valid observation. The problem statement should have been clearer that the 80°F specifically corresponds to state 3 (saturated vapor at the condenser exit). I added a clarification note below the solution video because without specifying this, the problem is under-defined on the T-S diagram and you can't determine where the high-pressure side should be.