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.
Understanding HVAC Psychrometrics Reference
A psychrometric chart is a map of possible air conditions at one fixed pressure, usually standard atmospheric pressure. The horizontal position represents dry-bulb temperature. Moving upward means the air contains more water vapor per unit mass of dry air.
Curved lines near the upper edge show saturation. At saturation, liquid water can begin to form if the air cools further. Sloping lines help track enthalpy and wet-bulb temperature.
Learning the direction of each line matters more than memorizing every label. Start with two measured properties, locate their intersection, then read the remaining properties from the chart. Two independent properties are enough because the other properties must agree with that same physical air state.
Relative humidity often causes confusion because it changes when temperature changes, even if no water enters or leaves the air. Warm air has a much larger saturation capacity than cool air. Air that feels dry after being heated may contain exactly the same humidity ratio as it did before heating.
Its relative humidity fell because the saturation amount increased. Cooling produces the opposite effect. When air reaches its dew point, its relative humidity reaches one hundred percent.
Further cooling removes water as condensation. This is why cold drink cans collect droplets and why an air-conditioning coil produces condensate. The coil must be below the entering air dew point for real dehumidification to occur.
HVAC equipment changes air along recognizable paths. A heating coil raises dry-bulb temperature with little change in humidity ratio unless moisture is deliberately added. A cooling coil first lowers temperature.
Once the air reaches its dew point, continued cooling lowers both temperature and humidity ratio as water drains from the coil. An evaporative cooler adds water vapor while taking energy from the air, so dry-bulb temperature falls while humidity ratio rises. A humidifier can add moisture with little temperature change depending on its design.
These paths reveal whether a system is handling sensible load, latent load, or both. A classroom full of students, cooking, showers, wet clothing, and outdoor ventilation air all create latent loads that a temperature-only calculation misses.
Air mixing is important at the point where return air joins outdoor air before reaching a coil. The mixed state lies between the two incoming states on a chart. A larger airflow has more influence on the final condition than a smaller airflow.
Engineers use dry-air mass flow because water vapor is part of the moving air mass but the reference basis stays consistent. Mixing can create unexpected condensation if one stream is cold enough and the combined state reaches saturation.
This matters in ducts, energy recovery devices, and poorly insulated outdoor-air sections. It also explains why ventilation can make a building harder to cool on hot humid days.
Good measurements are essential before any chart work. A dry-bulb sensor should be shielded from direct sunlight, warm equipment, and wet surfaces. Relative-humidity sensors need time to stabilize and can drift when dirty.
Air pressure matters at high elevations because saturation behavior and air density change. Use the same unit system throughout a calculation, then check whether the answer makes physical sense. Heating should not remove moisture by itself.
A cooling coil should not claim large moisture removal when its surface is warmer than dew point. These simple checks catch many errors before they become equipment sizing mistakes.