HVAC · Thermodynamics · Problem 21PDFSolution in PDF ↓
HVAC · Thermodynamics · Problem 21
Problem & Solution
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Problem: Thermal 21 and R410 refrigeration cycle operates between 25 and 1.5 psi and 135 psi with 20 degrees of superheat and 20 degrees of sub cooling.
Given: 1.5 psi and 135 psi with 20 degrees of superheat and 20 degrees of sub cooling; 135 psi i
Approach: We're finding the COP for refrigeration cycle, but we have superheat and sub cooling, and we now have to deal with the compress...
Calc: We have our pressure enthalpy curve, and since we have sub cooling, we're starting with state 3 being outside the saturation cu...
Calc: Then as we go to the right, we go past the saturated vapor line out into the superheat region for state 1 before entering the c...
Result: Best answer choice is A.
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Student questions asked in live office hours about this problem
OH 15: Thermo 21
Q: I got 4.6 instead of the correct answer of 4.3 on thermo 21 by using state 1-9 instead of 1-8 when reading enthalpy values off a pH diagram for a refrigeration cycle COP calculation—how precise do we need to be on the PE exam?
A: You're noticing the right things about precision. The key insight is identifying where the problem is sensitive: the denominator (H2 - H1) is much smaller than the numerator, so small errors there have outsized effects—a 1 BTU/lb mistake becomes 10-20% error, whereas the numerator is forgiving. Use your line tool on the exam and focus your precision efforts on the sensitive terms rather than worrying uniformly about all values.
OH 23: Thermo 21
Q: I'm having trouble reading data accurately from pressure-enthalpy diagrams on screen without PDF tools—will there be a more accurate way to do this on the CBT exam, or do I need to improve my reading skills?
A: You'll have a line tool on exam day to draw straight lines (though not perfectly horizontal/vertical), so use annotation tools like Zoom to practice now. More importantly, recognize that some problems are highly sensitive to precision—like refrigeration cycle COPs where small errors in enthalpy readings get amplified in small denominators—so experience and understanding problem sensitivity matter more than perfect accuracy.
OH 62: HVAC: Thermo 21
Q: When interpolating thermodynamic values on a pH diagram, how can I determine values quickly and get close to the answer without spending excessive time on full interpolation?
A: For this refrigeration cycle problem, we're actually reading values graphically from the pH diagram rather than interpolating from tables, since the superheating and subcooling mean states don't fall directly on saturation curves. The key is to be as careful and detailed as possible locating each point and reading enthalpy values vertically, recognizing that some graphical error is inevitable and acceptable—you don't need to be perfect for the PE exam, just reasonably precise.
OH 78: HVAC: Thermo Module #21
Q: For Thermo 21 (R410 refrigeration cycle COP problem), I solved it using tables for states 1 and 3 but the P-H chart for state 2. Is mixing tables and the chart acceptable, and how accurate is this approach?
A: Yes, absolutely—tables and charts contain the same information for the saturated region, so use whichever is more convenient and accurate for you. However, be cautious: state 1 has 20° of superheat and state 3 has subcooling, so both are outside the saturated region where tables won't help. You may have gotten lucky on state 3, but in general, tables are actually more precise than the chart when applicable, so if there's a discrepancy between your answer and the solution, yours might be more accurate.
OH 94: HVAC: Thermo Module #21
Q: Is there a general rule for when to account for motor efficiency in HVAC calculations?
A: Motors are rated by their output (BHP), so input is always larger than output due to losses. The key HVAC rule is: if only the motor is in the conditioned space, cool for motor losses only; if both motor and driven equipment are in the space, conservatively assume all motor output becomes heat that needs removal.