Process Cooling Overview Process cooling can be an expensive In general we use the following guidelines when trying to reduce cooling costs 1 2 3 4 Eliminate once through cooling Use cooling towers rather than chillers when feasible Apply for sewer exemption on cooling tower make up water Use gas powered chillers rather than electric chillers when cost effective Cooling Towers Tower Performance A cooling tower is a counter flow or cross flow heat exchanger that removes heat from water and transfers it to air Cooling towers come in many configurations Induced draft cooling towers such as the one shown below generally use less fan power and have short circuit less air than forced draft cooling towers Figure 1 Induced draft cross flow cooling tower Source ASHRAE Handbook HVAC Systems and Equipment 2000 The temperature difference of water through a tower dT Tw1 Tw2 is determined by the load Ql and the mass flow rate of water mw Neither the size of the tower nor the state of the outside air influences the temperature difference however larger towers or lower outdoor air wet bulb temperatures will decrease the exit water temperature Tw2 1 Typically most towers are sized for a 10 F temperature difference and about 2 4 gpm ton of cooling Fan motor hp is about 0 1 hp ton and air flow rates are about 2 000 cfm hp The temperature of water from a cooling tower Tw2 can be calculated based on tower performance data such as that shown below water flow rate cooling load and the ambient wet bulb temperature This process can be automated in software to predict cooling tower performance with varying ambient conditions For example CoolSim Kissock 1997 calculates exit water temperatures and the fraction of time that a cooling tower can deliver water at a target temperature based on entering water temperature Tw1 and TMY2 weather data This information is useful in determining how often a cooling tower can replace a chiller in cooling applications Figure 2 Typical cooling tower performance curve Source ASHRAE Handbook HVAC Systems and Equipment 2000 Sensible and Latent Cooling Depending on the entering air and water temperatures the water may be cooled by sensible and latent cooling of the air or simply by latent cooling of the air In either case latent i e evaporative cooling is dominant For example consider the case in which the air enters at a lower temperature than the water Figure 3a The air will leave completely saturated and the cooling is part sensible and part latent The sensible portion occurs as the air temperature increases by absorbing heat from the water The latent portion occurs as some of the water evaporates which draws energy out of the water 2 If the air enters at the same wet bulb temperature as before but at a higher dry bulb temperature than the water then the air will cool as it saturates Figure 3b Thus the sensible cooling component is negative and the all the cooling is due to evaporation In general cooling is dominated by latent cooling Figure 3 Psychrometric process lines for air through a cooling tower if the entering air temperature is a less than the entering water temperature and b greater than the entering water temperature The total cooling ma ha2 ha1 is the same for both cases since enthalpy is a function of wet bulb temperature alone However the dry bulb temperature significantly influences the evaporation rate mwe ma wa2 wa1 The rate of evaporation increases as the dry bulb temperature increases for a given wet bulb temperature 3 Evaporation Rate As discussed in the previous section cooling in cooling towers is dominated by evaporation The evaporation rate can be calculated from the pyschrometric relations in the previous section if the inlet and exit conditions of the air are known For example consider the case in which the cooling load Ql mass flow rate of air ma which can be calculated based on the fan cfm and specific volume of the inlet air and inlet conditions of air are known The enthalpy of the exit air ha2 can be calculated from an energy balance Ql ma ha2 ha1 ha2 ha1 Ql ma The state of the exit air can be fixed by assuming that it is 100 saturated with an enthalpy ha2 The evaporation rate mwe can be determined by a water mass balance on the air mwe ma wa2 wa1 The fraction of water evaporated is mwe mw Using this method for entering air temperatures from 50 F to 90 F we determined that the fraction of water evaporated typically ranges from about 0 5 to 1 with an average value of about 0 75 Another way to estimate the fraction of water evaporated is to assume that all cooling Ql is from evaporation Qevap The cooling load Ql is the product of the water flow rate mw specific heat cp and temperature difference dT The evaporative cooling rate is the product of the water evaporated mwe and the latent heat of cooling hfg Ql Qevap mw cp dT mwe hfg Assuming the latent heat of evaporation of water hfg is 1 000 Btu lb and the temperature difference of water through the tower dT is 10 F the fraction of water evaporated is mwe mw cp dT hfg 1 Btu lb F x 10 F 1000 Btu lb 1 If on average 75 of the cooling were from evaporation and 25 from sensible cooling then the evaporation rate would be 75 x 1 0 75 4 Thus both methods suggest that 0 75 is a good estimate of for the rate of evaporation however we have seen manufacturer data indicating average evaporation rates as low as 0 30 Water lost to evaporation should not be subjected to sewer charges Typical sewer charges are about 2 20 per hundred cubic feet Some water may be lost as water droplets are blown from the tower by oversized fans or wind This type of water loss is called drift Drift rates are typically about 0 2 of flow ASHRAE Handbook HVAC Systems and Equipment 2000 however we generally assume that drift losses are included in the 0 75 evaporation rate Water Treatment and Blow Down Rate Cooling tower water must be treated to prevent bacterial growth and maintain the concentration of dissolved solids at acceptable levels to prevent scale and corrosion Bacterial Growth The typical method of controlling bacterial growth is to add biocides at prescribed intervals and to keep the cooling tower water circulating If the tower will not be operated for a sustained period of time then the cooling water should be drained Dissolved Solids Water evaporated from a cooling tower does not contain dissolved solids Thus the concentration of dissolved solids will increase over time if only enough water is added to the tower to
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