HVAC · Heat-Transfer · Problem 4PDFSolution in PDF ↓
HVAC · Heat-Transfer · Problem 4
Problem & Solution
Video Synthesis
Problem: A skylight is comprised of two layers of half-inch thick laminated glass with a 1-inch air gap with a resistance of 0.5 hour foot squared degree F ...
Given: 1.5 Btu per hour degree Fahrenheit, foot squared and 2 to the same units respectively; 2 in the denominator; 1.5 BTU ...
Approach: So, from that we can use what we know about psychometrics to find out the dew point.
Key formula: formula for the total resistance as a function of the thermal conductivity K for the glass, so we can set the equal
Calc: Because if it is condensation will form and we're going to use the design conditions that they've given us, 10 degrees and 70 d...
Calc: Now, the inside conditions we know are 70 degrees.
Result: And so we end up with a K value of 0.075 BTU per hour foot F, which is closest to answer choice B.
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Student questions asked in live office hours about this problem
OH 104: HVAC: Heat Transfer #4
Q: Is it the water vapor in the surrounding air that condenses on a cooler glass surface, not water passing through the glass?
A: Yes—condensation on a cold glass surface is caused by water vapor in the surrounding room air being cooled below its dew point when it contacts the cold surface. It has nothing to do with moisture passing through the glass; the water is sourced entirely from the air in the room. Any time you cool a surface below the dew point of the adjacent air, that water vapor will change phase and condense on that surface.