HVAC · Psychrometrics · Problem 13PDFSolution in PDF ↓
HVAC · Psychrometrics · Problem 13
Problem & Solution
Video Synthesis
Problem: Psychometrics Problem 13.
Given: 0.0153 pounds of water per pound of dry air and then the other point that we needed Was 80 degrees that's interesting...
Approach: We've got some entering air At state one that's the 90 degrees dry bulb 75 degrees wet bulb and then we've got some leaving air...
Calc: Well it's Got an enthalpy that's higher right the enthalpy's now almost 45 where's down here.
Calc: It was less than 40 So it is picking up energy, but it's being cooled in the process.
Result: So let's record some of these values We had 0.053 pounds of water per pound of dry air for the entering condition 0.0223 For the leaving condition ...
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Student questions asked in live office hours about this problem
OH 64: HVAC: Fluids Module #22
Q: If a cooling tower works similarly to an air washer along a constant wet bulb line, why does the air leave saturated at 80°F when the entering wet bulb was 75°F—how can the leaving wet bulb be higher?
A: An air washer and a cooling tower are similar but not identical—in an air washer, the water recirculates and reaches equilibrium at the adiabatic saturation temperature, so the wet bulb stays constant. In a cooling tower, the water being cooled is warmer than the air's adiabatic saturation temperature, so the process isn't adiabatic and the leaving air can exit at a higher wet bulb temperature than it entered. The key difference is the temperature of the water source driving the process.
OH 72: HVAC: Psychrometrics #13
Q: How do you know to multiply delta W by (1/specific volume) to get pounds of water per cubic foot, rather than multiplying W by (1/delta specific volume)?
A: Delta W (change in humidity ratio) is in pounds of water per pound of dry air; dividing by specific volume (ft³/lb of dry air) cancels the dry air unit and gives pounds of water per cubic foot of moist air—which is what we want for a volumetric calculation. Multiplying by (1/delta specific volume) doesn't produce a meaningful unit combination. It comes down to careful unit tracking: set up the expression so that the units you want remain after cancellation.
OH 104: HVAC: Fluids Module #25
Q: Why do you use the specific volume at state 1 rather than state 2 or the average of both states?
A: Using either state 1 or state 2 gives a close enough answer for PE exam purposes—the difference is rarely significant. I default to the entering condition (state 1) because it's a consistent habit and saves the time of computing an average. Unless the problem specifically asks you to use an average or the two states differ dramatically, just pick one and move on.