Trane SYS-APM001-EN manual Chilled-Water System Variations, Refrigerant migration

Chilled-Water System Variations

load. The details of operation are discussed in “Sidestream plate-and-frame heat exchanger” on page 74.

Plate-and-frame heat exchangers isolate the building loop from the water in the open cooling tower loop, but they must be cleaned, typically annually. The labor and parts for cleaning and reassembly (e.g., gasketing) is an expense that should be factored into the life-cycle cost of this option.

Refrigerant migration

Another method of “free” cooling is to transfer heat between the cooling tower water and the chilled water inside a centrifugal chiller through the use of refrigerant migration, also known as a thermosiphon. Figure 41 shows a centrifugal chiller equipped for refrigerant migration free cooling. When the temperature of the water from the cooling tower is colder than the desired chilled-water temperature, the compressor is turned off and automatic shut- off valves inside the chiller refrigerant circuit are opened, as shown in Figure 42. Because refrigerant vapor migrates to the area with the lowest temperature (and pressure), refrigerant boils in the evaporator and the vapor migrates to the cooler condenser. After the refrigerant condenses, it flows by gravity back through a shutoff valve to the evaporator. This allows refrigerant to circulate between the evaporator and condenser without the need to operate the compressor.

Depending on the application, it is possible for refrigerant migration in a centrifugal chiller to satisfy many hours of cooling load without operating the compressor. Free cooling chillers serving systems that can tolerate warmer chilled-water temperatures at part-load conditions can produce more than 60 percent of the rated capacity without compressor operation. There are no cooling coil fouling concerns because the cooling-tower water flows through the chiller condenser and is separate from the chilled-water loop. There is no additional expense for cleaning, as the condenser tubes are the same as those used for normal cooling mode and should already be on a maintenance schedule. In addition, fewer pipes, pumps, and fittings are required, and no additional heat exchanger is required.

Figure 41. Refrigerant migration chiller in compression cooling mode

Conditioner

Refrigerant Storage Tank

Economizers

Compressor

Evaporator

SYS-APM001-EN Chiller System Design and Control 71