TFS · Heat Transfer · Problem 15PDFSolution in PDF ↓
TFS · Heat Transfer · Problem 15
Problem & Solution
Video Synthesis
Topic: Heat exchanger / heat transfer — 200 degree feed water at a thousand PSIA enters a boiler and is heated to produce a hundred thousand pounds per hour of 90 percent quality steam.
Approach: So let's start with state one.
Key values: 100,000 pounds per hour times 1127, 1000 PSIA
Reference: table look up properties of saturated water and steam by pressure
✅ Answer: B
Office Hours
2
Student questions asked in live office hours about this problem
OH 53
Q: For liquid water at 200°F and 1000 PSIA (compressed liquid region), is it acceptable to use the saturated liquid enthalpy value (hf) from the steam table, or should I instead calculate enthalpy using the ΔH = cp·ΔT method with saturation temperature?
A: At elevated pressures like 1000 PSIA, using hf directly is actually more accurate than the cp·ΔT method I show in the solution video. The actual compressed liquid enthalpy is 170.4 Btu/lbm, which matches the hf table value of 168 much better than the cp·ΔT result of 190—though you won't have compressed water tables during the exam, this confirms that taking hf is a reliable approximation for high-pressure compressed liquids.
OH 90
Q: Should properties (thermal conductivity and viscosity) be interpolated at the film temperature rather than the given temperatures, and should solutions stick to SI units since the reference handbook presents natural convection correlations in SI?
A: Yes, technically you must use film temperature (average of surface and fluid temperatures), though small temperature differences won't significantly change results since heat transfer answers are typically only accurate to within 20-25%. Regarding units, both SI and US customary are fair game on the exam, but I emphasize US customary in practice problems because SI is easier—you need to be sharp on the harder unit system so exam day won't surprise you.