A Look At Head Pressure Controls
However, there is a downside to this added benefit: If the condensing pressure falls too low, it will cause some system problems. One major problem that could develop is that the metering devices at the refrigerated cases will not be fed properly. Typically, thermostatic expansion valves (TXVs) are used as metering devices. A TXV should have a full column of liquid at its inlet.
At low condensing pressures, it is possible for some of the refrigerant to flash off before reaching the TXV. This is due to the fact that at low condensing pressures the refrigerant in the liquid line may pass through areas where the refrigerant can be warmed enough to raise the refrigerant to its saturate temperature. The refrigerant can then flash off before reaching the TXV. There will be both liquid and vapor being fed to the TXV. This will cause irregular flow through the valve. This can result in the valve seats eroding prematurely, and refrigeration capacity can be reduced.
Problem SolversThere is a solution to this potential problem. With the use of pressure regulators, the system could be designed to maintain a minimum discharge pressure during low ambient conditions. One popular method of maintaining a minimum condensing pressure is the use of an inlet pressure regulator, outlet pressure regulator, and check valve. The inlet pressure regulator and the check valve are installed at the outlet of the condenser with the check valve installed downstream of the inlet pressure regulator.
This inlet pressure regulator is referred to as a liquid drain regulator. The outlet pressure regulator is installed in a bypassed line from the discharge line to the liquid receiver. It is referred to as the condenser bypass regulator.
As the condensing pressure falls below a preset minimum, the liquid drain regulator will begin to close down, causing the refrigerant to back up in the condenser. As the refrigerant begins to back up in the condenser, its condensing surface will be reduced, causing the condensing pressure to elevate. If the condensing pressure still continues to fall, the bypass regulator will open, introducing discharge vapor directly into the receiver.
The check valve installed downstream of the liquid drain regulator will prevent the discharge vapor from entering the condenser and will direct it into the liquid receiver. The two regulators working together will maintain a minimum condensing pressure and allow the system to function properly during low ambient conditions.
Joe Marchese is owner of Coldtronics, Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or firstname.lastname@example.org.
Publication date: 01/12/2004