HVAC · Thermodynamics · Problem 11PDFSolution in PDF ↓
HVAC · Thermodynamics · Problem 11
Problem & Solution
Video Synthesis
Problem: Thermo 11 and outside air handling unit delivers 5,000 CFN of outside air.
Approach: But there's a couple of rules of thumb that don't really get introduced until a little bit later when we get into HVAC, but I'm...
Calc: What's going on here is that we're heating air that's coming from outside, a specific volume of air 5,000 CFN with hot water.
Calc: So for sensible heating or cooling of air, we use the rule of thumb 1.08 times CFN.
Result: So the volume flow it has to be in CFN.
Office Hours
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Student questions asked in live office hours about this problem
OH 79: HVAC: Thermo Module #11
Q: Why is it acceptable to assume outside air has no latent component when real-world air virtually always contains moisture? Is it just because the problem doesn't give wet bulb temperature information?
A: The outside air does have moisture, but that doesn't mean the process has a latent component—the key is distinguishing between the state of the air and the process applied to it. This problem involves only sensible heating (a horizontal line on the psychrometric chart), not humidification, so you use the sensible heat formula Q = 1.08 × CFM × ΔT. The real clue is that latent heat doesn't affect dry bulb temperature, and since the problem asks about increasing dry bulb temperature, it's purely sensible heating.
OH 89: HVAC: Thermo Module #11
Q: Can you solve the heating coil air handler problem using specific heat and mass flow rates (MCP·ΔT approach) instead of the rule of thumb equations, as an alternative method for exam confidence?
A: Absolutely, you can use M·Cp·ΔT for both water and air sides and set them equal—it yields the same result. The key is converting volume flow rates to mass flow rates using density or specific volume (13.5 cf/lb for air at standard conditions) and the specific heats (0.24 Btu/lb·°F for air, 1.0 for water), then solving for the water mass flow rate and converting back to GPM.