TFS · Thermodynamics · Problem 12PDFSolution in PDF ↓
TFS · Thermodynamics · Problem 12
Problem & Solution
Video Synthesis
- Problem: A steam condensate drain is designed to accommodate 50,000 pounds per hour of 5 PSIG water containing 5% vapor by mass.
- Approach: So we need to size the pipe to accommodate both the liquid water and the vapor.
- Key step: A steam condensate drain is designed to accommodate 50,000 pounds per hour of 5 PSIG water containing 5% vapor by mass.
- Watch out: So as long as the velocities are not more than these 15 and 100 respectively the area will be sufficient because we're assuming the worst case by assu...
- Result: So area equals pi over 4D squared.
- ✅ Answer: D
Office Hours
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Student questions asked in live office hours about this problem
OH 119 · April 28, 2026
Q: When sizing a pipe to carry simultaneous liquid water and steam vapor, why is it wrong to add the individual phase diameters instead of their areas?
A: Pipe cross-sectional area scales as diameter squared (A = πD²/4), so adding diameters algebraically overcounts the combined area — you must add the areas first (A_total = A_liquid + A_vapor) and then compute the final diameter from the total. A small extra area corresponds to only a small increase in diameter on a large pipe, which is exactly why the student's answer of 6.68 in was too large compared to the correct 5.8 in.