1 Low-Pressure Appliances and Chiller Fundamentals
Learn how low-pressure chillers use refrigerant pressure and temperature to move heat, how their components and water circuits work, and what operating conditions matter during service.
Low-pressure classification
In U.S. EPA terminology, a uses a refrigerant with a boiling point above () at standard atmospheric pressure. This classification describes the refrigerant’s properties; it does not mean every part of the operating system is always below atmospheric pressure.
Many low-pressure chillers, especially centrifugal chillers using refrigerants such as R-11 or R-123, operate with subatmospheric pressure in the .
Pressure, temperature, and pressure readings
A refrigerant’s describes its saturation temperature at a given pressure. For a particular refrigerant, lowering the pressure lowers its boiling temperature, while raising the pressure raises it. Technicians use the correct refrigerant P–T chart or digital equivalent to compare measured pressure with saturation temperature.
is measured relative to local atmospheric pressure, whereas is measured from a perfect vacuum. A gauge may show a vacuum when is below atmospheric pressure. Since local atmospheric pressure varies, a gauge-vacuum reading should not be treated as interchangeable with an absolute-pressure value.
How the chiller moves heat
In the , refrigerant is held at a pressure low enough to boil at a temperature below that of the water or fluid being cooled. As it boils, the refrigerant absorbs heat through the heat-exchanger surface. The raises the vapor’s pressure and temperature so that it can reject heat and condense in the condenser. The expansion device then meters liquid refrigerant into the and reduces its pressure, lowering its saturation temperature.
Chiller components
The transfers heat from the chilled-water or process-fluid loop to the refrigerant. The draws vapor from the and raises its pressure and temperature; centrifugal, screw, and reciprocating designs are common in chillers. The condenser transfers heat from the refrigerant to condenser water or air, allowing the refrigerant to reject heat and condense into a liquid. The expansion device meters liquid refrigerant into the and reduces its pressure.
A , where fitted, removes air and other noncondensable gases that collect in low-pressure equipment. A need for unusually frequent purging can indicate air entering through a leak or another system problem.
The two water circuits
A typical water-cooled chiller connects to two separate water circuits. Chilled water carries cooling to the building or process, while condenser water carries rejected heat to a cooling tower or another heat sink.
Operating conditions and diagnostic clues
Because parts of a low-pressure chiller may operate below atmospheric pressure, a leak can draw air and moisture into the system rather than simply push refrigerant outward. Noncondensables can impair heat transfer and increase condenser pressure. Moisture can contribute to corrosion, contamination, or ice-related restrictions.
Purge-unit activity, pressure and temperature readings, water temperatures, and operating trends are useful diagnostic clues, but they do not substitute for locating and confirming a leak.
Recovery, evacuation, and tube protection
During refrigerant recovery or evacuation, water-side conditions matter: water left in tubes can freeze as refrigerant is removed and temperatures fall. Follow the equipment manufacturer’s service instructions and applicable refrigerant-handling requirements.
The EPA service-practice table specifies an evacuation level of absolute for low-pressure appliances. This is an absolute-pressure value, not inches of mercury vacuum.