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

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Outdoor Wet-Bulb Temperature the Defining Measurement for Cooling Tower Operations Measuring Outdoor Rative Cooling Yani Bettencourt / Market Manager, Americas / Vaisala / Boulder, CO, USA Outdoor Wet-bulb Temperature The Defining Measurement for Cooling Tower Operations Measuring outdoor rative cooling. When evaporative drift. Shortcomings in any of these cooling can occur is based on local areas can result in cooling tower wet-bulb temperature environmental variables that can overuse, which leads to increased be provided by one measurement: water and energy usage and reduces helps operate cooling outdoor wet-bulb temperature. the functional life of fans and pumps. In order to operate cooling towers for peak towers for peak efficiency, certain Industrial-scale conditions need to be met. Outdoor Evaporative Cooling wet-bulb temperature need to be efficiency. used as the basis for the control The function of the cooling tower system set point, measurement is to remove heat from the process Cooling towers are used as part of instruments need to be accurate and fluids, for example from the industrial HVAC systems to remove reliable, and operators need to rou- condenser water used to cool the heat from process water by evapo- tinely check instruments for sensor refrigerant in the chiller unit of the 188-189/2012 19 Vaisala HUMICAP® Humidity and Temperature Transmitter Series HMT333 or HMT120/HMT130 are recommended for outdoor wet-bulb temperature measurement. HVAC system. In the chiller, there is Consequently, water and energy control system that uses set points an exchange of heat from the refriger- use are the two main operational to govern operations. The most ant to the condenser water. This cost drivers of a cooling tower. common set points for HVAC systems exchange cools the refrigerant and Condenser water lost to evaporation are specified upper and lower limits heats the condenser water. needs to be replaced and energy is of temperature and humidity. Cooling The waste heat must be removed consumed to run the tower’s fans tower controls are based on a single from the condenser water before it and pumps. A cooling tower with lower-limit set point, outdoor wet- can accept more heat from the refrig- an automated control system is bulb temperature. erant. The heated condenser water designed to operate at peak effi- Evaporative cooling benefits is circulated out of the chiller to a ciency, minimizing the operational cease when a combination of envi- cooling tower, where is it distributed costs and reducing maintenance and ronmental variables – relative humid- onto complex porous surfaces and repair costs. ity, water vapor saturation pressure, exposed to increased air flow pro- and temperature – reach a point vided by fans to maximize evapora- Set Point Based on where liquid water cannot vapor- tion and remove heat from the water. Wet-bulb Temperature ize into the air. Outdoor wet-bulb The cooled condenser water is then temperature is the lowest tempera- returned to the chiller to accept heat Like the rest of the HVAC system, ture that can be achieved purely by again from the refrigerant. the cooling tower is operated by a evaporative cooling. It is also the 20 188-189/2012 lowest possible temperature of the air temperature of only 2°C reduces Best Practices for process water leaving the tower. Wet- the relative humidity to 85%. Best Results bulb temperature is always less than Due to its familiarity, relative or equal to the dry-bulb, or ambient humidity is often used in cooling Wilcox recommends tower owners air, temperature. tower operations, but it does not adopt four best practices for oper- “Cooling towers are designed to provide the temperature at which ating the cooling tower as it was operate to a range of wet-bulb tem- evaporative cooling effects stop. designed. peratures in a certain location. Tower However, relative humidity and tem- First, the mechanical engineer operators need to be able to answer perature from sensing devices should designing the system should specify the question ‘What is the wet-bulb both be used as inputs to calculate that the control system be pre-pro- temperature?’ If they can’t answer wet-bulb temperature. grammed to automatically calculate that, they are not using the best wet-bulb temperature based on measurement for their operations,” Measuring Wet- direct measurements of relative says Tim Wilcox, an international bulb Temperature humidity and temperature. energy efficiency consultant with the Second, the system designer U.S.-based company WPI. Traditionally, wet-bulb temperature should specify high-quality sensors was measured by a thermometer that are accurate, not prone to Dew Point Temperature covered with wet cloth (or wet sock) sensor drift, and stay in calibration. an Option and exposed to air flow. A sling “Operators do not always psychrometer is a device with two understand the significant financial Dew point is the temperature where thermometers, one thermometer consequence on relying on low-cost, condensation begins: the tem- measures ambient temperature (i.e. low-quality sensors,” Wilcox says. perature at which air becomes fully dry-bulb temperature) and the other “These inexpensive, error-prone saturated with water vapor resulting measures wet-bulb temperature. The sensors can cost hundreds of thou- in the formation of liquid water. wet-bulb thermometer is fitted with sands of dollars in wasted energy use For use in cooling tower control a wet wick and then whirled around and equipment damage from overuse systems, dew point temperature is to generate air flow and evapora- if the sensor produces inaccurate the next best measurement to wet- tive cooling. These low-precision readings that indicate evaporative bulb temperature since it measures a measurement instruments require cooling can occur when it can’t.” temperature where water is condens- manual operation and do not provide Third, during commissioning, ing (not vaporizing) and it correlates the accuracy and reliability required the operator should confirm that to the amount of water vapor in the for industrial-scale cooling tower the sensors and the control system air at that temperature. Like wet-bulb control systems. program are in compliance with temperature, dew point temperature Today, wet-bulb temperature is specification. Finally, tower operators is always less than or equal to the calculated based on measurements need to perform routine maintenance dry-bulb temperature. of relative humidity and temperature. on the sensor as recommended by These calculations can be pro- the manufacturer for accuracy and Relative Humidity grammed into the control system to reliability over the long run. Falls Short eliminate manual calculation errors. Further information: Relative humidity is the most com- www.vaisala.com/HVAC monly used humidity measurement. It is the ratio, expressed as a percent- age, between the amount of water Humidity Formulas: Conversions Made Easy vapor present and the maximum amount that is physically possible at Vaisala publishes free tools to calculate or convert relative humidity, dew that temperature. point/frostpoint, absolute humidity, water content, mixing ratio, vapor The major drawback of using pressure, parts per million, and wet-bulb temperature. relative humidity in cooling tower A white paper containing the essential humidity conversion formu- operations is that it is heavily depen- las used by Vaisala digital instrument is available at www.vaisala.com dent on temperature. The capacity of (keyword: humidity conversion formulas). air to hold water vapor is dependent Online and downloadable calculators are available for several humidity on temperature; for example, if the parameters from one known value to make unit conversions quickly and temperature is 18°C and the relative see the effects of changing ambient conditions, like temperature and pres- humidity is 96%, an increase in the sure. Available at www.vaisala.com/humiditycalculator. 188-189/2012 21.
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