HVAC psychrometrics describes the temperature, moisture, and energy condition of air in heating, ventilation, and air conditioning systems. Students need this reference to connect chart reading with engineering calculations for comfort, ventilation, cooling, heating, and dehumidification. This cheat sheet helps organize the main air properties and process formulas used in introductory HVAC design.
Key Facts
- Humidity ratio is the mass of water vapor per mass of dry air, usually written as w = mass of water vapor / mass of dry air.
- Relative humidity compares actual water vapor to saturation water vapor at the same temperature, so RH = actual vapor pressure / saturation vapor pressure x 100%.
- Moist air enthalpy can be estimated in IP units by h = 0.24Tdb + w(1061 + 0.444Tdb), where h is in Btu/lb dry air and Tdb is in degrees Fahrenheit.
- In SI units, moist air enthalpy can be estimated by h = 1.006Tdb + w(2501 + 1.86Tdb), where h is in kJ/kg dry air and Tdb is in degrees Celsius.
- Sensible heat transfer in air can be estimated by Qs = 1.08 x CFM x delta T in IP units.
- Latent heat transfer in air can be estimated by Ql = 4840 x CFM x delta w in IP units, where delta w is in lb water per lb dry air.
- Total heat transfer equals sensible heat plus latent heat, so Qt = Qs + Ql.
- For adiabatic mixing of two air streams, the mixed condition is found by mass-weighted averages such as hm = (m1h1 + m2h2) / (m1 + m2).
Vocabulary
- Dry-bulb temperature
- The ordinary air temperature measured by a standard thermometer shielded from radiation and moisture effects.
- Wet-bulb temperature
- The temperature indicated by a wetted thermometer bulb when evaporation cools the bulb.
- Humidity ratio
- The mass of water vapor contained in a unit mass of dry air.
- Relative humidity
- The percentage ratio of actual water vapor pressure to the saturation vapor pressure at the same temperature.
- Dew point
- The temperature at which air becomes saturated and water vapor begins to condense.
- Enthalpy
- The total heat content of moist air, including sensible heat in the air and latent heat in the water vapor.
Common Mistakes to Avoid
- Confusing humidity ratio with relative humidity is wrong because humidity ratio is a mass ratio, while relative humidity is a percentage of saturation.
- Using dry-bulb temperature alone to judge comfort is wrong because moisture content strongly affects evaporation from skin and perceived comfort.
- Adding air stream temperatures directly during mixing is wrong because mixed air conditions should be based on mass flow and energy balance.
- Ignoring latent load in cooling calculations is wrong because removing moisture often requires significant energy beyond lowering temperature.
- Reading a psychrometric chart with mixed unit systems is wrong because IP and SI charts use different scales and property units.
Practice Questions
- 1 An air stream has a dry-bulb temperature of 75 degrees F and a humidity ratio of 0.009 lb water/lb dry air. Estimate its enthalpy using h = 0.24Tdb + w(1061 + 0.444Tdb).
- 2 A cooling coil handles 1200 CFM of air and lowers the dry-bulb temperature by 18 degrees F. Estimate the sensible cooling rate using Qs = 1.08 x CFM x delta T.
- 3 Two air streams mix adiabatically: 300 lb/min at 28 Btu/lb dry air and 700 lb/min at 20 Btu/lb dry air. Find the mixed air enthalpy.
- 4 Explain why cooling air below its dew point can reduce both dry-bulb temperature and humidity ratio.