TFS · Practice Exam 1 · Problem 8PDFSolution in PDF ↓
TFS · Practice Exam 1 · Problem 8
Problem & Solution
Video Synthesis
- Problem: A thousand CFM of 350 degree 80 PSIA air is supplied by an air compressor.
- Approach: We could use the table in the reference handbook.
- Key values: 80 PSIA, 80 PSI, 14.7 PSI
- Reference: reference handbook
- Result: When in fact the temperature is higher which means at standard conditions it's going to be cooler and therefore more dense and that's going to make it...
- ✅ Answer: C
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Student questions asked in live office hours about this problem
OH 108
Q: I used the ideal gas law to find density at given conditions, then took the ratio to standard density and multiplied the CFM by that ratio—is that valid? And how do I know to multiply versus divide when I can't intuitively tell which direction the volume flow rate should go?
A: Your approach is absolutely correct—density depends on pressure and temperature via the ideal gas law, so taking the density ratio and applying it to CFM is the right way. The key to knowing whether to multiply or divide is that pressure's effect on density far outweighs temperature's effect unless temperatures are extreme; here the pressure dominates, making the air denser at elevation, so standard CFM must be larger than the measured low-pressure CFM.