HVAC · Psychrometrics · Problem 17PDFSolution in PDF ↓
HVAC · Psychrometrics · Problem 17
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Problem: Psychometrics Problem 17 The combustion products from an industrial process are passed through a heat exchanger for energy recovery at 30 psi It th...
Given: 30 psi It the humidity ratio is 800 grains of water per pound of drier What is the dew point of the exhaust stream so...
Approach: We can convert that to pounds of water There's 7,000 grains of water per pound so that's fairly easy unit conversion To get the...
Calc: We'll say Omega times p minus p sub w equals 0.62 to times p sub w And then we'll distribute the humidity ratio.
Calc: I'll divide by this entire term here And we get omega p over Omega plus 0.62.
Result: So let's do that Properties of saturated water by pressure and our pressure is 4.66 will call it 4.7 So it's between 4 and 5 and these are the corr...
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Student questions asked in live office hours about this problem
OH 80: HVAC: Psychrometrics #17
Q: I realized the psychrometric chart is invalid at pressures above 14.7 PSIA—how do we know to use the steam tables to find dew point temperature in this problem?
A: Exactly right: the psychrometric chart assumes standard atmospheric pressure (14.7 PSIA), so at elevated pressures it can't be used directly. The steam tables give saturation temperature as a function of partial pressure of water vapor, which is exactly the definition of dew point—the temperature at which the water vapor in the air becomes saturated. Once you recognize that the humidity ratio gives you the partial pressure of water vapor, you can look up the corresponding saturation temperature in the steam tables.