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In chilled water system, the primary loop consists of primary pump sized to circulate the chilled water through the chiller and the rest of the primary piping loop. The secondary chiller water loop, which consists of secondary pump, is a variable flow system. Chilled water is circulated by the secondary pump through the control valves and cooling loads connected to the circuit. However, most of the control valves in the circuit are not fully open because of part load in most of the time. Therefore, the chilled water flow in the secondary water loop can be regulated by VSD to handle the chilled water required by the cooling load, thus to save energy.Whereas Csd Is not used for energy saving puposes instead it is used to supply constant loads
VSD chiller is applicable to heat load with hi and low demand or variable heat load to save energy like commercial building application where as CSD chiller application is for steady heat load at any given time like industrial application. Selecting the brand of chiller or type of chiller will depend on your load demand. There are chiller manufacturers whose chillers are designed for Part load efficiency and some are full load efficiency. So as an engineer you should select a chiller according to their application and load efficiency.
CSD chiller is constant speed drive (i.e. constant speed compressor)
VSD chiller is a variable speed drive (i.e. variable speed compressor)
the major benefit for using VSD chiller is decreasing the running cost for HVAC system if you have a wide range of heat load MIN and MAX
if you have a small variation for heat load the its better to use CSD chiller this will decrease the initial cost
