Cathy what I believe you are asking is closed loop verse open loop on the cooling tower selection. If so I can help by explaining the difference between them. An open loop cooling tower means the condenser water is open to the atmosphere and a closed loop the condenser water is run through a heat exchanger built into the bottom of the cooling tower. If you're mechanical equipment is difficult to service or highly susceptible to debris like water source heat pumps use a closed loop tower. For a system with a water cooled chiller use a that is easily serviced-higher capacity use an open loop tower.
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