HVAC Fundamentals and Psychrometrics
This domain tests whether you can describe moist air quantitatively: reading the psychrometric chart, converting between dry-bulb, wet-bulb, dew point, relative humidity and humidity ratio, and turning those properties into sensible, latent and total load numbers.
Core relationships
- Q_sensible = 1.08 x CFM x delta-T (dry bulb), Btu/h.
- Q_latent = 4.5 x CFM x delta-W (lb/lb), or 0.68 x CFM x delta-grains.
- Q_total = 4.5 x CFM x delta-h (enthalpy).
- The 1.08 and 4.5 constants assume standard air: 0.075 lb/ft3 at sea level. At altitude, correct by the density ratio.
- SHR = sensible / total. Coil performance is set by apparatus dew point and bypass factor, not by room RH alone.
Worked example
2,000 CFM enters a coil at 75F dry bulb and leaves at 57F dry bulb. Sensible load = 1.08 x 2,000 x 18 = 38,880 Btu/h. Humidity ratio drops from 0.0095 to 0.0080 lb/lb, so latent = 4.5 x 2,000 x 0.0015 = 13,500 Btu/h. Total is about 52,380 Btu/h and SHR = 38,880 / 52,380 = 0.74. Cross-check total from enthalpy: 52,380 / (4.5 x 2,000) = 5.8 Btu/lb of air.
Traps that cost marks
- Treating relative humidity as an absolute moisture measure. RH changes with temperature; humidity ratio does not.
- Using wet bulb and dew point interchangeably. They coincide only at saturation.
- Applying 1.08 at 5,000 ft without correcting for reduced air density.
- Choosing equipment on total capacity when the space load is mostly sensible, so the unit cannot hold temperature.