How much water is lost to evaporation in a chiller system used for a hydraulic power unit (HPU)?



How much water is lost to evaporation in a chiller system used for a hydraulic power unit (HPU)?





A traditional chiller system is a sealed so no water should be lost.  Heat is exchanged between the chiller water and condenser water.  The condenser water is then pumped to a cooling tower where it is sprayed into a collecting tank while outside air is blown over it to remove heat and the cooled water is pumped back to the condenser where it is cooled again.   The heated water from the HPU’s heat exchanger enters the chiller where the heat is exchanged again between it and the cooled water in the condenser.  This is a continuous cycle where water in the condenser/cooling tower is lost to evaporation but not in the sealed chiller system.


Because chiller water is sealed from the condenser/cooling tower there should be no appreciable loss of water in a sealed chiller system.   If there is, you probably have a leak somewhere.   


Water will be lost in the cooling tower as heat is removed to the outside ambient air.  The amount of evaporation will depend on several variables but primarily determined by the number of BTUs that must be removed from the HPU’s oil during operation. 


Below are the BTU specifications for different model HPUs (Table 1).  We cannot tell you how much evaporation should be expected but the manufacturer or installer of your chiller system should be able to tell you the evaporation rate based on the BTU removal capacity. 


Table 1 – Heat Exchanger heat removal capacities for various model HPUs


For comparison, an open water surface of 150 square feet with 15 square feet of splash activity at 80 degrees can lose roughly 70 gallons per day to evaporation (3/4” drop in water level).


Many variables can determine the evaporation rate from a water reservoir, relative humidity, splash activity, air temperature, air flow at surface, surface area, vapor pressure, and water temperature.  In a closed system any evaporation will re-condense back into the system so water loss should be minimal.  


Keary is the author of this solution article.

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