period three
System Variations
notes
waterside economizer
Refrigerant Migration
from | | | | | | |
compressor | | | | | | |
condenser | | | | | | shutoff |
| | | | | | |
| | | | | | | valve |
| | | | | | | vapor |
| | | | | | |
to | | | | | | migration |
compressor | shutoff | |
| | |
liquid | valve | evaporator |
| | | | | |
flow | | | | | | |
Figure 85
The final 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. 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 shutoff valves inside the chiller refrigerant circuit are opened. This allows refrigerant to circulate between the evaporator and condenser without the need to operate the compressor. Because refrigerant vapor migrates to the area with the lowest temperature, refrigerant boils in the evaporator and the vapor migrates to the cooler condenser. After the refrigerant condenses into a liquid, it flows by gravity back into the evaporator.
There are no additional fouling concerns because the cooling-tower water flows through the chiller condenser and is separated from the chilled-water loop.
Although not as effective as a plate-and-frame heat exchanger, it is possible for refrigerant migration in a centrifugal chiller to satisfy many cooling load requirements (up to 40 percent of the chiller’s design capacity) without operating the compressor. This can increase further if the system can accommodate warmer chilled-water temperatures at part-load conditions.
