What is the difference between wet-bulb temperature and dew-point temperature?

Both are measures of moisture content, but they represent different physical concepts. Wet-bulb temperature (WBT) is the temperature reached when water evaporates into the air until the air is saturated — it represents the lowest temperature achievable by evaporative cooling and is a function of both DBT and humidity. Dew-point temperature (DPT) is the temperature at which air must be cooled at constant pressure (constant W) for condensation to begin — it depends only on the actual water vapor content (humidity ratio), not on the dry-bulb temperature. At 100% relative humidity, WBT = DBT = DPT. In practice, DPT is used for pipe condensation analysis and dehumidification coil design; WBT is used for cooling tower performance, evaporative cooling equipment, and HVAC equipment rating conditions.

How do I use the psychrometric chart to size a cooling coil?

The cooling coil sizing process on the chart: (1) plot the entering air state (mixed outdoor + return air at design conditions); (2) plot the desired leaving air state (supply air temperature and humidity ratio needed to maintain room conditions given the SHR); (3) draw the process line from entering to leaving state; (4) extend this line to intersect the saturation curve — this intersection is the apparatus dew point (ADP), representing the effective coil surface temperature; (5) the coil bypass factor = (leaving DBT - ADP) / (entering DBT - ADP); (6) total cooling load (tons) = mass flow rate (lb/min) × enthalpy difference (h_entering - h_leaving) / 200. Select a coil with sufficient rows and FPI to achieve the calculated ADP at the design water temperature.

What are typical design indoor conditions for a commercial office?

ASHRAE Standard 55-2023 (Thermal Environmental Conditions for Human Occupancy) defines the comfort zone. Typical commercial office design conditions are 74–76°F DBT and 40–55% RH for cooling season, and 70–72°F DBT and 30–40% RH for heating season. The ASHRAE 55 adaptive model allows slightly wider temperature ranges in naturally ventilated spaces. For energy modeling, ASHRAE 90.1 Appendix G uses 75°F / 50% RH as the cooling setpoint and 70°F as the heating setpoint for the baseline building. Always confirm with the owner's program requirements, especially for spaces with sensitive occupants, equipment, or processes.

How does the psychrometric chart help with preventing condensation on chilled water pipes?

Condensation occurs on any surface whose temperature is at or below the dew-point temperature of the surrounding air. To prevent pipe condensation: (1) determine the dew-point temperature of the warmest, most humid air that could contact the pipe (typically summer outdoor air conditions for outdoor piping, or the most humid interior zone for indoor piping); (2) ensure the pipe insulation surface temperature is above this dew point; (3) calculate required insulation thickness using ASHRAE or manufacturer methods accounting for insulation conductivity (k-value), pipe surface temperature, ambient temperature, and relative humidity; (4) seal all insulation joints and vapor retarder to prevent humid air from reaching the cold pipe surface through insulation gaps. Chilled water supply pipes (44°F) in a 75°F / 60% RH space have a dew point of approximately 59°F — all insulation must maintain a surface above 59°F.

Engineering·9 min read·June 15, 2025

🔧 Psychrometrics: Understanding and Using the Psychrometric Chart

A practical engineering guide to psychrometrics — reading the psychrometric chart, understanding air properties, calculating HVAC processes (cooling, heating, humidification, mixing), and applying psychrometrics to equipment selection.

Introduction to Psychrometrics

Psychrometrics is the science of the thermodynamic properties of moist air — a mixture of dry air and water vapor. For HVAC engineers, mastering the psychrometric chart is as fundamental as understanding electrical circuits for an EE. Every HVAC process — cooling, heating, dehumidifying, humidifying, mixing return and outside air — can be plotted and analyzed on the psychrometric chart, providing instant visual insight into the energy content of air streams and the performance of HVAC equipment.

The psychrometric chart used by ASHRAE (and defined in ASHRAE Fundamentals Handbook Chapter 1) is valid at sea-level atmospheric pressure (29.921 in. Hg, or 14.696 psia). For projects at elevations above 2,500 feet, altitude-corrected charts or software (such as ASHRAE CoolTools or carrier Hourly Analysis Program with altitude input) must be used — air density decreases with altitude, directly affecting coil capacity and fan performance.

Key Psychrometric Properties and Chart Axes

The psychrometric chart has the following key properties plotted on its axes and as curved lines:

Dry-bulb temperature (DBT): The horizontal axis. Temperature measured by an ordinary thermometer. The most familiar property, ranging from approximately -40°F to 120°F on a standard HVAC chart.
Humidity ratio (W): The vertical axis. Mass of water vapor per unit mass of dry air, in grains per pound (gr/lb) or pounds per pound (lb/lb). Also called specific humidity or moisture content. Standard room design conditions: approximately 54–65 gr/lb (0.0077–0.0093 lb/lb).
Relative humidity (RH): Curved lines sweeping from lower left to upper right. The ratio of actual water vapor pressure to saturation vapor pressure at the same temperature, expressed as a percentage. The 100% RH line is the saturation curve (left boundary of the chart).
Wet-bulb temperature (WBT): Diagonal lines sloping down-right from the saturation curve. Measured by a thermometer with a wet wick — evaporative cooling brings the temperature to the wet-bulb value. Used in cooling tower design, evaporative cooling, and equipment rating conditions (e.g., ARI rating: 80°F DB / 67°F WB entering air).
Dew-point temperature (DPT): Read by drawing a horizontal line left from any state point to the saturation curve. The temperature at which air must be cooled at constant pressure for condensation to begin. Critical for preventing condensation on building surfaces and chilled-water pipes.
Specific enthalpy (h): Lines parallel to wet-bulb lines, in Btu/lb dry air. Total energy content of the air-moisture mixture. The difference in enthalpy between two state points equals the energy added or removed per pound of dry air in that process. Standard room air at 75°F / 50% RH has enthalpy of approximately 28.1 Btu/lb dry air.
Specific volume (v): Nearly vertical lines in cubic feet per pound of dry air. At design room conditions (75°F, 50% RH), v ≈ 13.5 ft³/lb. Used to convert between airflow in CFM and mass flow rate in lb/min.

Plotting HVAC Processes on the Psychrometric Chart

Every fundamental HVAC process traces a specific path on the chart:

Sensible heating (no moisture change): Horizontal line moving right. DBT increases; humidity ratio W remains constant; RH decreases. Example: duct heating coil or heat pump heating coil.
Sensible cooling (above dew point): Horizontal line moving left, until the dew point is reached. DBT decreases; W constant; RH increases. Example: air cooled without condensation.
Cooling and dehumidification: A process line curving down and to the left toward the saturation curve. DBT and W both decrease. This is the typical cooling coil process. The apparatus dew point (ADP) is where the process line intersects the saturation curve — the effective coil surface temperature. The coil bypass factor (BF) = (leaving air DBT - ADP) / (entering air DBT - ADP).
Humidification at constant temperature (isothermal — steam injection): Vertical line moving upward. DBT constant; W and RH increase. Steam injection adds moisture with negligible temperature change.
Evaporative cooling (adiabatic humidification): Line following a constant wet-bulb line downward to the left. DBT decreases and W increases at constant enthalpy. Used in evaporative coolers in dry climates.
Mixing of two airstreams: A straight line between the state points of the two streams. The mixed state point divides the line in inverse proportion to the mass flow rates of the two streams. Example: mixing OA and return air in an AHU economizer.

Sensible Heat Ratio and Equipment Selection

The Sensible Heat Ratio (SHR) is the ratio of sensible cooling load to total (sensible + latent) cooling load. SHR is plotted on the psychrometric chart as the slope of the process line from room design conditions to the apparatus dew point. SHR drives coil selection:

High SHR (0.90–0.95): Dry climates, low latent load — less rows or higher leaving air temperature acceptable.
Medium SHR (0.75–0.85): Typical commercial applications in mixed climates.
Low SHR (0.60–0.70): Humid climates, spaces with high occupant density, or lab/clean room applications requiring strict humidity control. Requires deeper coils, lower leaving air temperatures, and often reheat.

The ASHRAE Handbook of Fundamentals provides psychrometric property tables and equations for detailed manual calculations. For daily practice, psychrometric software such as the ASHRAE Psychrometric Analysis CD, Carrier HAP, or free online calculators provide instant chart plotting and property lookup.

Altitude and Non-Standard Conditions

At elevations above sea level, atmospheric pressure decreases — approximately 1 in. Hg per 1,000 feet of elevation. Key effects on HVAC system design:

Air density decreases: the same CFM carries less mass (lb/min) of air, reducing coil heat transfer and requiring more airflow (higher CFM) to deliver the same Btu/h.
Fan performance derate: centrifugal fans produce less static pressure and less mass flow at altitude. The fan must be re-rated at altitude conditions — AMCA 99-0086 provides correction factors.
DX equipment derates: refrigerant condensing conditions change at altitude because the condenser must reject heat to lower-density air. Always check manufacturer altitude derating data for rooftop units in Denver, Albuquerque, or other high-altitude cities.
Evaporative cooling becomes more effective at altitude because low humidity and lower air density allow more evaporation per unit volume of airflow.

Topics covered

psychrometricspsychrometric chartdry bulb temperaturewet bulb temperaturedew pointrelative humidityenthalpyhumidity ratiosensible heat ratioSHRcooling processmixing airASHRAE psychrometrics

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