HVAC · Systems-and-Components · Problem 1PDFSolution in PDF ↓
HVAC · Systems-and-Components · Problem 1
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Student questions asked in live office hours about this problem
OH 40: HVAC: Systems and Components-1
Q: For Systems and Components 1, how did you interpolate the specific volume at 20°F and 50% RH from the table — the values are very close; can I just use the midpoint?
A: Yes, essentially — use the 'Thermodynamic Properties of Moist Air' table directly below the psychrometric chart (search that phrase) for low-temperature lookups. The values at 20°F are close enough that simple midpoint interpolation is acceptable.
OH 41: HVAC: Systems and Components-1
Q: For Systems and Components 1, how do I find the humidity ratio at low temperatures when the handbook only has standard- and high-temperature psychrometric charts?
A: Search for 'thermodynamic properties of moist air' to find the low-temperature table directly below the second psychrometric chart. That table gives saturation humidity ratios; for partial saturation, multiply the saturation value by your relative humidity fraction.
OH 44: HVAC: Systems and Components-1
Q: For Systems and Components 1 (humidification), how do you decide whether to use the cold entering air density/specific volume versus the warmer downstream air — some outside problems use warm air density?
A: Use the entering conditions — the specific volume at the cold outdoor state converts the volume flow rate to mass flow rate at the right location. Using warm air density would overestimate the mass flow rate and lead to the wrong answer.
OH 59: HVAC: Systems and Components-1
Q: Can you explain how effectiveness is calculated differently for ERVs (energy recovery ventilators) versus HRVs (heat recovery ventilators)?
A: HRV effectiveness uses dry-bulb temperature change (sensible heat only), while ERV effectiveness uses enthalpy change (both sensible and latent). The key distinction is that ERVs transfer moisture as well as heat — use enthalpy in the ERV formula, temperature in the HRV formula.
OH 66: HVAC: Systems and Components-1
Q: For Systems and Components 1, I tried solving using GPM of humidification water times latent heat of vaporization rather than finding mass flow rate — can you find the error in my approach?
A: The two approaches are related but can diverge depending on how you account for the enthalpy of the water being added. The psychrometric process involves adding moisture at a specific enthalpy state, and getting that enthalpy of addition correct is where these approaches can split.