HVAC · Practice-Exam-2 · Problem 7PDFSolution in PDF ↓
HVAC · Practice-Exam-2 · Problem 7
Problem & Solution
Video Synthesis
Problem: A fan delivers 10,000 CFM of 70 degree air at sea level.
Key step: For 4,000 feet, there's a density factor and I'll use the subscript A density factor based on the altitude of 4,000 feet is 0.864.
Watch out: Make sure you're actually finding that in the table and not just taking my word for it.
Result: And that gives us a new volume flow rate at 4,000 feet of 11,000 and 97 CFM, which is closest to answer choice C.
✅ Answer: C
Office Hours
1
Student questions asked in live office hours about this problem
OH 102: HVAC: Full Practice Exam #7
Q: Why can't the fan affinity laws be used in this question?
A: I would have thought that you could calculate the pressure adjustment from altitude as well as air density to get a revised CFM. However, the result in answer would give you a lower CFM instead. So this question was about an elevation correction, situation and you're being asked to find the CFM with the circumstances of being at elevation.