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.

📖Belt Conveyor System Design Drawing Set (Full Access)

View →

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.

Topics covered

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