CyberChiller Series Installation, Operation & Maintenance Manual

2.7.6Head Pressure Controls

2.7.6.1Air-Cooled Systems

2.7.6.1.1Condenser Fan Speed

Remote air-cooled condensers use variable speed condenser motor control to maintain head pressure. The variable speed motor is located closest to the header end of the condenser. The fan speed control is a continual modulation of the motor's speed. As the condenser discharge pressure rises, the fan speed increases, cooling the condenser which lowers the discharge pressure. The controller is mounted in an electrical box for field installation or could be pre- packaged with the outdoor condenser. Mount the condenser control box on the header end of the condenser. The fan speed controller requires no adjustments.

Ensure the wiring to the condenser is in accordance with appropriate codes and the electrical schematic. On systems with more than one fan on the condenser, the remaining motors cycle on and off through additional thermostat(s).

2.7.6.2Water/Glycol-Cooled Systems

2.7.6.2.1Head Pressure Regulating Valve

In a water/glycol condenser, condensing temperature is maintained by the liquid flowing through a regulating valve and then into a brazed plate liquid-cooled condenser. The regulating valve opens to increase liquid flow as the pressure rises (or closes as the refrigerant pressure falls). The regulating valve is factory set for the correct condensing temperature however, it can be adjusted to increase or decrease the condensing temperature as follows:

Head pressure regulating valves are available in 2-way or 3-way configurations. 3-way valves are available with pressure ratings of 150 and 300 psig. The location and method for adjusting the valves for condensing pressure differs with the valve types. To increase the condensing temperature, decrease water/glycol flow. To decrease the condensing temperature, increase the water/glycol flow. A directional arrow is stamped on the metal housing of the valve stem.

2.7.7Thermal Expansion Valve

CyberChiller units utilize a thermal expansion valve (TEV) to control the flow of refrigerant entering the evaporator in order to maintain a constant superheat of the refrigerant vapor at the outlet of the evaporator. Superheat is the difference between the refrigerant vapor temperature and its saturation temperature at that pressure. By controlling superheat, the TEV keeps nearly the entire evaporator surface active while not permitting liquid refrigerant to return to the com- pressor.

The standard superheat is factory set at 12-20°F and should not need adjustment. If adjustment should be required, remove the cap from the valve. Turn the adjusting stem clockwise to increase the superheat and counter clockwise to decrease the superheat.

2.7.8Hot Gas Bypass

The hot gas bypass system provides some modulated capacity control and freeze protection. The hot gas bypass system consists of a discharge bypass valve that allows some hot gas from the compressor discharge line to flow directly to the evaporator, in order to maintain the fluid leaving temperature.

The hot gas bypass system also provides freeze protection for the evaporator coil by limiting the minimum refrigerant pressure, thereby preventing the surface temperature of the evaporator coil from dropping into the freezing range.

The hot gas bypass valve is automatically controlled by the Chiller’s unit mounted controller. The hot gas bypass valve will begin to open, allowing hot gas refrigerant to enter the evaporator once the leaving water temperature falls to the leaving water temperature setpoint.

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(©October, 2006)