The Figure 7.2 illustrates various cooling tower types. An induced draft fan draws the air across the wetted fill and expels it through the top of the structure. In cross flow induced draft towers, the water enters at the top and passes over the fill. The air, however, is introduced at the side either on one side (single-flow tower) or opposite sides (double-flow tower). Both forced and induced draft fans are used. In the counter flow induced draft design, hot water enters at the top, while the air is intro- duced at the bottom and exits at the top. Mechanical draft towers are available in the following airflow arrangements: ![]() Cooling rates of Mechanical draft towers depend upon their fan diameterand speed of operation. Since, the mechanical draft cooling towers are much more widely used, the focus is onthem in this chapter. Mechanical draft towers utilize large fans to force or suck air through circulated water. The water fallsdownward over fill surfaces, which help increase the contact time between the water and the air - this helps maximise heat transfer between the two. These types of towers are used only by utility power stations. Due to the large size of these towers, they are generally used for water flow rates above 45,000 m3/hr. Natural draft towers use very large concrete chimneys to introduce air through the media. Typical closed loop cooling tower system is shown in Figure 7.1.Ĭooling towers fall into two main categories: Natural draft and Mechanical draft. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling. Hot water from heat exchangers is sent to the cooling tower. The make-up water source is used to replenish water lost to evaporation. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water. The primary task of a cool- ing tower is to reject heat into the atmosphere. Take, for example, ratio control system (with remote input) of a metered-air combustion process in the figure below.Cooling Tower: Types and performance evaluation, Efficient system operation, Flow control strategies and energy saving opportunities, Assessment of cooling towersĬooling towers are a very important part of many chemical plants. Often times, these select elements are used as an override element in ratio control. However, some flow meters like orifice meters require additional computations to achieve a linear relationship between the flow rate and signal. ![]() Turbine flow meters provide signals that change linearly with flow rates. The output signals from the flow transmitters, Fw and Fc, must change linearly with a change in flow rate. A disadvantage is that if the mix ratio does need to be changed quickly, operation may be shut down while waiting for the appropriate person to change it.Īnother disadvantage is that linear flow signals are required. This higher level of security may be an advantage so that only permitted people can change the mix ratio and decrease the chance that an accidental error occurs. The mix ratio (Fc/Fw) is not easy to access, so it requires a high level of authorization to change. Image adapted from Houtz, Allen and Cooper, Doug "The Ratio Control Architecture" ![]() The flow controller then adjusts the controlled feed flow rate so that it matches the set point (3). The ratio relay outputs the calculated controlled feed flow rate which is compared to the actually flow rate of the controlled feed stream. The wild feed flow rate is received by the ratio relay and then multiplied by the desired mix ratio.
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