Psychrometric (Wet Bulb and Dry Bulb) Temperature Calculations for Cooling Towers Using Spreadsheet

In any industrial utility network, a cooling tower’s efficiency is bounded by two vital meteorological baselines: Dry-Bulb Temperature (DBT) and Wet-Bulb Temperature (WBT). Understanding how these properties govern air-water interaction is what separates reactive troubleshooting from proactive process design. This article provides an engineering breakdown of psychrometrics in action. We will explore the physical mechanics behind these temperatures, how they set the theoretical limit for heat rejection, and how you can leverage this math to minimize water loss and maximize your tower's cooling effectiveness.

Measing Dry Bulb Temperature

Measuring Dry Bulb Temperature (DBT) is much simpler than measuring wet bulb temperature because DBT is just the true ambient air temperature we are all familiar with. However, to get an accurate, scientifically valid DBT reading in an industrial environment or weather station, you must follow specific rules to prevent surrounding elements from distorting the reading. Any temperature measuring instrument will do the job, even a normal thermometer is okay.

To ensure you are measuring the actual temperature of the air and not something else, the sensor must meet three criteria-

1. It Must Be Completely Dry - Unlike the wet bulb thermometer, the sensor bulb or probe for DBT must be completely dry and exposed directly to the air. Any moisture on the sensor will cause water to evaporate, cooling the bulb and accidentally pulling the reading down toward the wet bulb temperature.
2. It Must Be Shielded from Radiation (Sunlight & Equipment) - This is the most common mistake in the field. If a thermometer is placed in direct sunlight, the sensor absorbs radiant energy from the sun, heating it well above the actual air temperature. The same thing happens inside a plant if the sensor is near hot pipes, boilers, or reactors. The sensor must be placed in a shaded area or inside a specialized ventilated enclosure called a Stevenson screen or a solar radiation shield. These shields block direct radiation while allowing air to pass through freely.
3. It Must Have Adequate Ventilation - Air must be moving across the sensor. If the air is completely stagnant, a micro-climate can form around the thermometer, leading to an inaccurate reading. In professional setups, a small fan often draws air across the dry bulb sensor (aspirated thermometer) to ensure a continuous, representative sample of the ambient air.

Summary of checklist - If you are walking around your cooling tower with a handheld digital meter to check the ambient DBT -

1. Do not stand in direct sunlight.
2. Do not stand directly downwind of the cooling tower plume (which is full of hot, humid air).
3. Do not hold the sensor near hot process piping.
4. Allow the reading to stabilize for a minute or two until the numbers stop moving.

Read why approach, range and wet bulb temperature matter in cooling tower design and operation

Measuring Wet Bulb Temperature

Determining the Wet Bulb Temperature (WBT) is the most critical step in evaluating or designing a cooling tower. The WBT represents the theoretical absolute lowest temperature to which a cooling tower can cool the circulating water, as it factors in the cooling effect of water evaporation into the air.

There are three primary methods used in the industry to determine the wet bulb temperature - 

1. Direct physical measurement
2. Using a psychrometric chart
3. Utilizing mathematical formulas

Method 1: Direct Physical Measurement (Psychrometer)

Historically and practically in the field, WBT is measured using a tool called a psychrometer (or a sling psychrometer).

• Setup - The instrument consists of two identical thermometers mounted side-by-side.
• Dry Bulb Thermometer - Left exposed to the ambient air to measure standard ambient temperature.
• Wet Bulb Thermometer - The bulb is covered with a cloth wick (usually muslin) that is saturated with distilled water.
• The Process - Air is forced over both thermometers (either by spinning a sling psychrometer manually or using a motorized fan in an electronic version).
• Result - As air passes over the wet wick, water evaporates, absorbing latent heat and lowering the temperature of that thermometer. The temperature will drop until the rate of evaporative cooling matches the heat absorbed from the surrounding air. The lowest steady temperature reached is your Wet Bulb Temperature.

Method 2: Using a Psychrometric Chart

If you already know the Dry Bulb Temperature (ambient temperature) and the Relative Humidity (RH %) from a standard weather station or plant sensor, you can easily find the WBT using a psychrometric chart.

1. Locate your measured Dry Bulb Temperature on the horizontal axis at the bottom of the chart.
2. Move vertically up along that temperature line until you intersect with the curved line representing your current Relative Humidity (%).
3. From that intersection point, look for the diagonally sloping lines labeled Wet Bulb Temperature (these lines run diagonally up and to the left).
4. Follow that diagonal line up to the leftmost boundary curve (the 100% saturation curve). The temperature value at that intersection on the curve is your Wet Bulb Temperature.

Method 3: Using a Mathematical Approximation (Stull’s Formula)

For control systems, computers, or quick spreadsheets, you can calculate the wet bulb temperature mathematically. One of the most widely used empirical equations for standard sea-level atmospheric pressure is Stull’s Formula.

If you have the Dry Bulb Temperature (DBT in °C) and Relative Humidity (RH as a percentage, e.g., 55 for 55%), the Wet Bulb Temperature (WBT in °C) can be calculated as:

Twb = T · arctan[0.151977 · (RH + 8.313659)½] + arctan(T + RH) - arctan(RH - 1.676331) + 0.00391838 · (RH)¹·⁵ · arctan(0.023101 · RH) - 4.686035

(Note: The arctangent functions in this equation must be calculated in radians, not degrees.)

Google Sheet tool to calculate Wet Bulb Temperature using "Stull's Formula".

(Make a Copy of this template to Google Drive for personal use)

Find Psychrometric Chart here.

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