Friday, September 7, 2012

Water Cooled Chiller vs Air Cooled Chiller in HVAC System

  • Initial cost of air cooled chillers is lower than water cooled chillers as air cooled systems don’t require cooling towers,condenser water pumps and associated HVAC components.
  • Small systems are almost always air-cooled since the extra requirements of water treatment and maintenance are capital intensive and will not be offset quickly by the lower operating costs.  Larger HVAC water chillers are almost always water cooled .HVAC engineers and technicians feel easy with maintenance free or less maintenance equipment.
  • Water cooled chillers may be preferred because of  better heat transfer properties of water as compared to air cooled chillers.The primary advantage of water is that the film coefficient is 10-100 times better with water than air.Film coefficient controls heat transfer with given constant area and temperature difference.
  • Water-cooled HVAC units make less noise and give more cooling per square foot.While air cooled chillers have comparatively less cooling per square foot and are more noisy HVAC machines.Noisy machines are annoying especially for occupants of nearest offices or zones.
  • While comparing  air and water cooled chillers, an overall system cost (both running and installed) has to be worked out. Size of HVAC unit matters when selecting the water cooled chiller or air cooled chiller.Smaller HVAC units are usually air cooled, larger are water cooled.
  • HVAC engineers have to evaluate air cooled and water cooled chillers keeping in mind various factors.Refrigeration engineer has to consider the ambient conditions (wet bulb and dry bulb), number of hours of operation, part load pattern and cost of maintenance etc.
  • HVAC experts have no single answer to when asked “water cooled chiller is better or air cooled chiller?”. The fact is that “It all depends”. Water-cooled chillers will normally condense at an average lower pressure than air-cooled chillers.  This is because the water temperature will usually be less than the air temperature.If your condenser is operating at lower pressure then it is directly reflected and translated into lower operating costs or KW /TR(Ton of refrigeration).
  • The difference between the wet-bulb temperature and the dry-bulb temperature gives the available difference between the condensing temperatures of water-cooled and air-cooled condensers respectively.  
  • Cooling towers and/or evaporative condensers have main disadvantage of maintenance and water treatment costs. This running cost is not present in air cooled chillers.
  • Despite the advantages of lower IKW/TR of a water-cooled system or water cooled chillers, sometimes, air-cooled systems are preferred for various applications.
  • For example in 24 hour application of HVAC chillers,the night time dry bulb temperature are generally much lower than the day time dry bulb temperature, as compared to the difference in wet bulb temperature between day and night.  This advantage is best capitalized by an air-cooled system or air cooled chillers.
  • Other examples could be where the wet bulb temperatures are very high, especially coastal areas. 
  • When comparing water cooled and air cooled chiller,scarcity of water and hence cost of water is another issue which is a point to be considered while deciding on the system.  This happens in many countries .
  • Space is also of importance in some applications. Air cooled chillers can utilize open space, such as a parking lot, roof or ground level area. Although smaller in size but Water cooled chillers require a mechanical room and cooling towers.
  • Air recirculation is very important for air cooled chillers.Any obstruction in the circulation of air near water chillers would adversely affect the performance of air cooled chillers.Air cooled chillers can have problems with recirculation if there are walls higher than the unit or too many units to close together. If there is a situation where outside walls are required for building code and the unit will not be receiving enough fresh air, then a water cooled application may be the best option.
  • Water cooled chillers have longer life,higher efficiency,large capacity and also refrigerant containment.While looking on disadvantatges of air cooled chillers,they have lower efficiency,comparatively short life span and available in low capacity range.
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