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    Tuesday
    Apr032012

    Warmer Chilled Water on Existing Systems

    So we are looking to save energy, and one of those ways is increasing the chilled water temperature.  But how does that save anything?  Increasing the chilled water temperature brings it closer to the condenser water temperature and thereby reduces the amount of work (lift) a chiller has to perform.  Since the chiller usually has the largest energy demand in a chilled water plant, any reduction is almost a direct savings of energy.  And increasing the chilled water temperature is part of that plan. 

    But what are the implications?  As long as you are looking to keep the same temperature difference between the supply and return water, the amount flowing will stay the same, which means your pumps won't miss a beat.  Downstream, you will need to realize how your equipment will be affected.  The computer room air handlers (CRAH) units and other air handling will need to be reviewed to ensure that they can meet the cooling requirements, but also the dehumidification needed for the spaces served.  The CRAH units and air handlers are designed around a supply and return water temperature, and deviating from that may derate the unit, reducing its overall cooling capacity.  More importantly, air handlers aimed to dehumidify outside air for use inside will take less moisture out of the air than before.  Often this becomes the more important limiting factor for how high you can take that chilled water temperature.

    How should it be done?  Often this question should involve the managers of the chilled water equipment, as they may be able to tell you immediately if certain areas will likely see spikes in temperatures during peak performance days.  Stepping up by one or two degrees F for a week at a time while continuously monitoring the system provides the least risk.  As the chilled water supply creeps upward, outside air, building usage, and any other factors affecting the load need to be meticulously tracked.  Missing part of the load due to renovations, office swapping, or new added equipment can be detrimental to comfort. 

    Beyond the comfort level for the occupants, you will need to look at your process equipment limits: 

    • For water cooled equipment: if equipment has an intermediate heat exchanger, external or internal, the rise in temperature may not yield the best performance for the equipment.  Once the temperature starts to rise, the process equipment may not notice until it becomes 100% utilized - and at peak cooling demand.  If the equipment has to reduce the speed of its processes because it is too hot, then an increase on chilled water will exacerbate that problem.  However, it should be noted that most equipment will have a rating with limitations for water cooling. 
    • For air cooled equipment: ASHRAE has recommended and allowable operating limits, and in most cases this follows the Class 1 & 2 operating environments.  The CRAH units will need to be reviewed to see the impact to the air cooled equipment, but more importantly the data center rooms will need to be studied to ensure there are no hot areas that cannot meet the ASHRAE conditions for that data center. 

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