HVAC · Thermodynamics · Problem 7PDFSolution in PDF ↓
HVAC · Thermodynamics · Problem 7
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Problem: Thermo 7 in a refrigeration cycle R134A leaves the compressor at 20 psi with 20 degrees of super heat.
Given: 7 in a refrigeration cycle R134A leaves the compressor at 20 psi with 20 degrees of super heat
Approach: So first of all you have to find the pressure enthalpy curve for R134A.
Calc: So in the reference manual you can just search 134A and eventually you'll find this diagram and I've brought that over onto the...
Calc: So the pressure in the evaporator is 20 let's draw a line across the straight as we reasonably can to here.
Result: Draw lines I can see it looks like it's closer to 0.24 than it is to 0.22 but not by much so maybe it's 0.235 let's say and that has units of B to ...
Office Hours
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Student questions asked in live office hours about this problem
OH 35: HVAC: Thermo-7
Q: Can you go over pressure versus enthalpy diagrams for refrigerants, and does the handbook have an interpolation equation?
A: I'd recommend watching the fundamentals in daily insights first for a comprehensive overview, but I can cover the basics: the P-h diagram has pressure (logarithmic scale) on the vertical axis and enthalpy on the horizontal axis, with a vapor dome separating subcooled liquid (left), saturated mixture (inside), and superheated vapor (right). Just note that the logarithmic pressure scale means 150 PSI isn't halfway between 100 and 200.
OH 77: HVAC: Thermo Module #7
Q: For this R134A superheated state problem, would it be more precise to use tabular interpolation instead of the graphical P-H diagram method to find entropy?
A: No, because R134A tables only provide saturated region data—there's no superheated table like steam has. To find entropy in the superheated region, you'd have to extrapolate from the saturated table, which is unreliable. That's why the graphical method is necessary here; interpolation between close data points is fairly reliable, but extrapolation introduces too much uncertainty.
OH 92: HVAC: Thermo Module #7
Q: Can you show how to find the entropy after the evaporator on an R-134 refrigeration cycle with superheat using the pH diagram?
A: Find the constant entropy line on the pH diagram that passes closest to your state point after the evaporator; if no exact line exists, interpolate between the nearest lines on either side. Remember that the evaporator process is always horizontal on a pH diagram since pressure is constant, and superheat extends horizontally to the right by following isothermal lines until you reach the desired temperature increase.