Introduction to Energy Management

Week/Lesson 9 part a Evaporative Cooling and Cooling Towers

After completing this chapter, you will be able to:  Distinguish between direct and indirect evaporative cooling processes  Understand the psychrometric processes that occur during evaporative cooling processes  Define range and approach and how they relate to cooling towers and evaporative coolers Evaporative Cooling and Cooling Towers

 Recognize the different arrangements of airflows, fans and sprays within a cooling tower  Understand the design and operational differences between cooling towers and evaporative condensers Evaporative Cooling and Cooling Towers

Theory of evaporative cooling  Evaporative cooling Heat exchanges between water and air Sensible heat is exchanged for latent heat Direct contact or indirect contact Usually both the air and water are cooled Evaporative Cooling and Cooling Towers

 Adiabatic saturation No external heat is involved Sensible and latent heat are exchanged Air and water temperatures approach each other No heat transfer occurs when air is saturated Evaporative Cooling and Cooling Towers

Cooling 7 Pictures from

Typical system  Water returning to cooling tower = 95ºF  Water supplied by cooling tower = 85ºF  The condenser adds about 10ºF to the water  The cooling tower removes about 10ºF from the water Evaporative Cooling and Cooling Towers

Cooling tower design  Heat rejected = system heat  System heat comes form two sources Heat absorbed by the evaporator Heat produced by the compression process  Water flow is about 3 gpm/ton Evaporative Cooling and Cooling Towers

Analysis of a cooling tower’s performance  Performance affected by WB temperature of entering air  Range – change in tower water temperature  Approach = leaving water temp – WB of air  Normal approach temperature = 7 to 9ºF Evaporative Cooling and Cooling Towers

Example 11-1  Design approach temperature = 7ºF  WB of the entering air = 68ºF  Water temperature = WB + Approach  Water temperature = 68ºF + 7ºF  Water temperature = 75ºF Evaporative Cooling and Cooling Towers

Cooling tower classification and construction  Parallel flow Air and water flow in the same direction Air velocity is low  Cross flow Water flows from top to bottom Air flows from side to side Evaporative Cooling and Cooling Towers

 Counter flow Air flows from bottom to top Water flows from top to bottom  Atmospheric (natural) draft tower – No fan  Forced draft – air pulled through the tower  Induced draft – air pushed through the tower Evaporative Cooling and Cooling Towers

Other tower components  Distribution basin – top of the tower  Water flows through orifices and over fill  Spray nozzles can replace the orifices  Eliminators – limit water loss by blow-by  Sump – reservoir at the bottom of the tower Evaporative Cooling and Cooling Towers

 Water treatment Combats bacteria growth Reduces corrosion and mineral deposits  Blowdown cycle Drains a portion of tower water Reduces mineral concentrations Evaporative Cooling and Cooling Towers

 Inhibitors Usually painted on the tower surfaces Reduces oxidation Reduces scale formation and buildup  Strainers Catch foreign matter in the water Prevent clogging of the water loop Evaporative Cooling and Cooling Towers

Tower control  Low ambient conditions  Maintains a minimum water temperature Fan control  Fan cycles on when water temp is high  Fan cycles in order to maintain desired water temperature Evaporative Cooling and Cooling Towers

Valve Control – water flow through the tower  Low ambient conditions More water bypasses the tower Water temperature is increased  High ambient temperatures Little or no water bypasses the tower Water temperature is decreased Evaporative Cooling and Cooling Towers

Damper control – air flow through the tower  Low ambient conditions Air flow is reduced Water temperature is increased  High ambient temperatures Air flow is increased Water temperature is reduced Evaporative Cooling and Cooling Towers

Evaporative condensers  Uses both air and water  Air flows over the refrigerant coil  Water flows over the coil when needed  Water remains in the condenser  Indirect contact evaporative cooling Evaporative Cooling and Cooling Towers