Understanding of the three basic types of piping systems: closed-loop, open-loop, and once-thru
Closed-Loop (Evaporator)
In a closed-loop piping system, the water is contained within a closed piping system, or loop, through which it circulates. While there may be some nominal contact with the air depending on the type of tank used, the system is considered closed to the environment. Typically, closed-loop systems are chemically treated to control corrosion, scale, slime, and algae within the piping but their chemical treatment requirements typically are not as extensive as an open-loop.
Open-Loop (Condenser)
In an open-loop piping system, the water is in constant contact with the air and the system is therefore open to the atmosphere. A typical example of an open-loop system is a recirculating condenser water system with a cooling tower where the water is circulated through the cooling tower, sprayed over the tower media surface, collected into the tower basin, circulated through the condenser, and then sent back through the cooling tower.
Once-Thru
In this type of system, water passes through the system once and is then discharged. An example of a once-thru system would be a chiller with river water piped into its water-cooled condenser. The rejected heat from the condenser is introduced back into the river, which is not always acceptable from an environmental perspective. In general, once-thru systems that use “city” water are not allowed because they use excessive amounts of water.