HVAC · Thermodynamics · Problem 4PDFSolution in PDF ↓
HVAC · Thermodynamics · Problem 4
Problem & Solution
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Problem: Thermo 4, what is the density of propane at 77 degrees and atmospheric pressure?
Given: 14.7 psi; 14.7 psi instead of writing psi, I'll actually write pound force per inch squared so that we can see all th...
Approach: So if we assume it's an ideal gas, then that should be enough to find out pretty much anything else we would want to know in th...
Calc: So hopping over to the reference manual, we can search properties of ideal gases and we'll get to this table, which has the spe...
Calc: So let's note that down r for propane is 35.04 and that has units of foot pound force per pound mass rankin.
Result: So plugging that in we get 0.11 pound mass per cubic foot, which is closest to answer choice c.
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Student questions asked in live office hours about this problem
OH 115: HVAC: Thermo #4
Q: Can I use the R value for propane at room temperature for all cases, and can I use a similar setup to find mass even when volume isn't given?
A: Yes, R is practically constant across a wide temperature range for gases like propane unless you're dealing with extreme temperatures (thousands of degrees), in which the problem would give you R anyway. For finding mass, you need volume in addition to pressure and temperature; without it, you can only find density or specific volume using ρ = P/RT, but mass requires knowing the actual volume occupied.
OH 96: HVAC: Thermo Module #4
Q: I got 0.112 for density using specific volume from Table 10.1 (inverse method)—is this just a coincidence, and at what point does temperature impact specific volume?
A: Not a coincidence—you're good. Temperature continuously impacts specific volume, but it's a matter of degree (linear or nonlinear). You need to recognize the relationship and judge how far out you are. Your method is correct; learn both approaches and check the solution video to verify you can work both ways.