Problem: a 1000 CFM fan coil unit installed for dehumidification maintains an apparatus dew point of 40 degrees and has a coil bypass factor of 10%.
Given: 1000 CFM fan coil unit installed for dehumidification maintains an apparatus dew point of 40 degrees and has a coil b...
Approach: The coil has an apparatus dew point of 40 degrees but that doesn't mean that the error is actually leaving the coil of 40 degre...
Key formula: formula for the coil efficiency is 1 minus the coil efficiency equals the bypass factor so if you know one you know t...
Calc: We've got a 1000 CFM coming through this thing and we know the conditions leaving the coil.
Calc: The coil has an apparatus dew point of 40 degrees but that doesn't mean that the error is actually leaving the coil of 40 degre...
Result: So let's start with a little cartoon of the psychometric chart I'm not going to show this on the actual chart because I don't want to deprive you o...
Office Hours
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Student questions asked in live office hours about this problem
OH 19: HVAC 3
Q: In HVAC-3, what's the purpose of finding the humidity ratio, and is it coincidental that drawing a line between the entering air condition and the ADP lets me find the enthalpy at the intersection?
A: Not coincidental at all — you're using the psychrometric chart to define the process line from entering conditions to the ADP, and any state on that line is physically valid. The humidity ratio helps you locate your state on that process line precisely.
OH 20: HVAC 3
Q: I solved HVAC-3 a different way — I applied the 90% efficiency at the end rather than multiplying delta-H earlier. Is this okay?
A: Yes, this is fine — you're just multiplying by 0.9 regardless of when you do it in the process. Whether you apply the 90% earlier or later, you get the same answer.
OH 29: HVAC 3
Q: How do you derive the latent heat of vaporization of water (970 BTU/lb) from the reference handbook — is this the latent heat of fusion?
A: The 970 BTU/lb is the latent heat of vaporization (h_fg from the steam tables), not the latent heat of fusion — those are two completely different things. Latent heat of vaporization converts liquid water to steam; latent heat of fusion melts ice — don't mix them up.
OH 31: HVAC 3
Q: For Psychrometrics-3, do I need to interpolate for specific volume when the state is at 25% relative humidity?
A: No interpolation needed — with both temperature and relative humidity known, the state is fully defined. Read the specific volume directly from the psychrometric chart at that defined state.
OH 47: HVAC: HVAC-3
Q: Where in the reference handbook does the bypass factor formula (BF = 1 − n_coil) come from?
A: That relationship is conceptual rather than a specific handbook equation — and my goal is for you to internalize it, not memorize it. Think about what physically happens at a coil: some air bypasses completely at entering conditions, and the rest exits at the ADP; that picture gives you the formula.
OH 95: HVAC: HVAC-3
Q: For HVAC-3, is there a reason not to use the bypass fraction of CFM and solve using the ADP as the leaving state — I was within 2000 BTU/hr with this approach?
A: This was an air handling unit with cooling and dehumidification, so the psychrometric picture matters here. The bypass factor approach is valid in principle, but you need to verify the assumptions hold before relying on it for a different problem.
OH 105: HVAC: HVAC-3
Q: I drew the process line from state 1 to the ADP, placed the supply temperature of 44°F on that line, and found an enthalpy of 17 BTU/lb — is that a valid approach?
A: Yes, absolutely — that's a valid approach. Going forward, if you've reviewed the solution and feel confident, try to argue the case for your own method rather than seeking validation — that habit will serve you well on exam day.