Outdoor Wet-Bulb Temperature – the Defining Measurement for Cooling Tower Operations

/ APPLICATION NOTE

Outdoor Wet-bulb Temperature – The Defining Measurement for Cooling Tower Operations

Cooling towers are used as part of industrial HVAC systems to remove heat from process water by evaporative cooling. When evaporative cooling can occur is based on local environmental variables that can be provided by one measurement: outdoor wet-bulb temperature.

Cooling towers can be operated for water is circulated out of the chiller specified upper and lower limits of peak efficiency under the following to a cooling tower, where it is temperature and humidity. Cooling conditions: outdoor wet-bulb distributed onto complex porous tower controls are based on a single temperature is used as the basis surfaces and exposed to increased lower-limit set point, outdoor wet- for the control system set point, air flow provided by fans to maximize bulb temperature. measurement instruments are evaporation and remove heat from the accurate and reliable, and operators water. The cooled condenser water is Evaporative cooling benefits cease routinely check instruments for returned to the chiller to accept heat when a combination of environmental sensor drift. Shortcomings in any again from the refrigerant. variables – relative humidity, water of these areas can result in cooling vapor saturation pressure, and tower overuse, which leads to Water, Energy, and temperature – reach a point where increased water and energy usage Equipment Costs liquid water cannot vaporize into the and reduces the functional life of air. Outdoor wet-bulb temperature fans and pumps. Water and energy use are two is the lowest temperature that can operational cost drivers of a cooling be achieved purely by evaporative Industrial-scale tower. Condenser water lost to cooling and is the lowest possible Evaporative Cooling evaporation needs to be replaced and temperature of the process water energy is consumed to run the tower’s leaving the tower. Wet-bulb The function of the cooling tower fans and pumps. A cooling tower temperature is always less than or is to remove heat from the process with an automated control system is equal to the dry-bulb, or ambient fluids, in this case from the designed to operate at peak efficiency, air, temperature. condenser water used to cool the minimizing these operational costs refrigerant in the chiller unit of the and reducing maintenance and repair “Cooling towers are designed to HVAC system. In the chiller, there costs to fan and pump equipment. operate to a range of wet-bulb is an exchange of heat from the temperatures in a certain location. refrigerant to the condenser water. Set Point Based on Tower operators need to be able to This exchange cools the refrigerant Wet-bulb Temperature answer the question ‘What is the and heats the condenser water. wet-bulb temperature?’ If they can’t Like the rest of the HVAC system, answer that question, they are not The waste heat must be removed the cooling tower is operated by a using the best measurement for from the condenser water before control system that uses set points to their operations,” says Tim Wilcox, it can accept more heat from the govern operations. The most common an international energy efficiency refrigerant. The heated condenser set points for HVAC systems are consultant with U.S.-based WPI. Dew Point Temperature Due to its familiarity, relative Best Practices for is an Option humidity is often used in cooling Best Results tower operations, but it does not Dew point is the temperature provide the temperature at which Wilcox recommends tower owners to where condensation begins. This evaporative cooling effects stop. adopt four best practices for operating is the temperature at which air However, relative humidity and the cooling tower as it was designed. becomes fully saturated with temperature from sensing devices water vapor resulting in the should both be used as inputs to First, the mechanical engineer formation of liquid water. For use calculate wet-bulb temperature. designing the system should specify in cooling tower control systems, that the control system will be pre- dew point temperature is the next Measuring Wet-bulb programmed to automatically calculate best measurement to wet-bulb Temperature wet-bulb temperature based on direct temperature since it measures measurements of relative humidity a temperature where water is Traditionally, wet-bulb temperature and temperature. The formulas can condensing (not vaporizing) and it was measured by a thermometer be found online and in white papers, correlates with the amount of water covered with wet cloth (or wet sock) including from Vaisala. vapor in the air at that temperature. and exposed to air flow. A sling Like wet-bulb temperature, dew point psychrometer is a device with two Second, the system designer should temperature is always less than or thermometers, one thermometer specify high-quality sensors that are equal to the dry-bulb temperature. measures ambient temperature accurate, not prone to sensor drift, (dry-bulb temperature) and the other and stay in calibration. “Operators do Relative Humidity measures wet-bulb temperature. The not always understand the significant Falls Short wet-bulb thermometer is fitted with financial consequence on relying on a wet wick and then whirled around low-cost, low-quality sensors,” Wilcox Relative humidity is the most to generate air flow and evaporative says. “These inexpensive, error- commonly used humidity cooling. These low-precision prone sensors can cost hundreds measurement. It is the ratio, measurement instruments require of thousands of dollars in wasted expressed as a percentage, between manual operation and do not provide energy use and equipment damage the amount of water vapor present the accuracy and reliability required from overuse if the sensor produces and the maximum amount that for industrial-scale cooling tower inaccurate readings that indicate is physically possible at that control systems. evaporative cooling can occur when temperature. The major drawback it can’t.” of using relative humidity is that it is Today, wet-bulb temperature is heavily dependent on temperature. calculated based on measurements Third, during commissioning, the The capacity of air to hold water of relative humidity and temperature operator owner should confirm that vapor is dependent on temperature. by accurate and reliable instruments. the sensors and the control system For example, if the temperature is These calculations can be program are in compliance with 18°C and the relative humidity is 96%, programmed into the control system specification. Finally, tower operators an increase in the air temperature to eliminate manual calculation need to perform routine maintenance of only 2°C reduces the relative errors. on the sensor as recommended by humidity to 85%. the manufacturer for accuracy and reliability over the long run.

Humidity Formulas: Conversions Made Easy Vaisala publishes free tools to calculate or convert relative humidity, dew point / frost point, absolute humidity, water content, mixing ratio, vapor pressure, parts per million, wet-bulb temperature. A white paper containing the essential humidity conversion formulas used by Vaisala digital instrument is available at www.vaisala.com (keyword: humidity conversion formulas). Online and downloadable calculators are available for several humidity parameters from one known value to make unit conversions quickly and see the effects of changing ambient conditions, like temperature and pressure. Available at www.vaisala.com/humiditycalculator.

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