HVAC · Psychrometrics · Problem 9PDFSolution in PDF ↓
HVAC · Psychrometrics · Problem 9
Problem & Solution
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Problem: Psychometrics Problem 9.
Given: 000 CFM air handling unit? Supplying 60 degree dry bulb 58 degree wet bulb air; 000 CFM air stream in GPN; 000 CFM wh...
Approach: How much condensate is produced by 10,000 CFM air handling unit?
Calc: The return air conditions are 76 degrees and 60% relative humidity.
Calc: This problem wants us to quantify how much condensate what is the flow rate of condensate that's being removed from that 10,000...
Result: So picking up with this formula mass flow rate of water is equal to 10,000 CFM which is cubic feet per minute divided by 13.8 feet cubed per pound.
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Student questions asked in live office hours about this problem
OH 36: HVAC: Psych-9
Q: Why do we use the specific volume of state 1 instead of state 2 when calculating mass flow rate across a coil?
A: Use the entering coil condition (state 1) specific volume to calculate the mass flow rate of air entering the coil, since mass flow rate equals volume flow rate divided by specific volume. While averaging the two states or using state 2 might occasionally vary the answer slightly in extreme cases like very cold outside air, using state 1 specific volume is correct 90-95% of the time and won't drastically change your answer in practice.
OH 74: HVAC: Psychometrics Module #9
Q: Should I use air density at room temperature (entering coil) or supply condition for this dehumidifying coil problem, and why did I get 0.161 as an answer when using the standard specific volume of 13.34?
A: Use the entering condition (V1) for density—it only creates about 3% difference in the answer either way, so don't lose sleep over it. Your main issue is that you didn't take the delta of humidity ratios (W1 - W2); you used just one humidity ratio value instead of the difference, which is why you got a trap answer.
OH 104: HVAC: Psychrometrics Module #9
Q: Why does this problem require the high temperature psychrometric chart, and is it acceptable to use average density between entering and leaving conditions when calculating GPM?
A: You don't need the high temperature chart unless the problem involves high temperature conditions—use the regular chart. For density/specific volume, use the entering condition; 95% of the time it doesn't matter which you choose, but only worry about averaging if you have a very dramatic process (like going from cold/dry to hot/humid), which rarely happens in typical HVAC applications.