Tuesday, December 2, 2014

System control valves

 System control valves

Crankcase pressure regulator

As a compressor starts, a heavy load is imposed on the drive motor. The crankcase pressure will be at its highest against normal or abnormal loading.

The function of the regulator is to keep the pressure in the crankcase to a reasonable level to protect the motor from overload. This applies when the evaporator pressure is above normal operating pressure, for example:

1 During the initial start-up after defrosting.

2 During periods of high starting loads.

3 Under high suction pressure caused by hot gas defrosting.

4 During surges in suction pressure.

5 Under prolonged high suction pressure.

The regulator is installed in the suction line as shown in Figure 83.

imageAs a guide, the control should be set to the following crankcase pressures:

Hermetic and semi-hermetic compressors: 2.5 bar

Open-type compressors with standard motor: 3.0 bar

Open-type compressors with oversized motor: 4.5 bar.

Final adjustments are made to make the running current conform to that specified on the motor nameplate (running current rating).

Evaporator pressure regulator

This valve is fitted in the suction line to control the pressure in the evaporator, to prevent it dropping below a predetermined pressure. It is used to control the evaporating temperatures of systems such as drinking water fountains and beverage coolers, to maintain a constant evaporator pressure/temperature and prevent freezing.

It is also used in multi-evaporator systems, when it is installed in the suction line of the warmest evaporator. Check valves or one-way valves are necessary to prevent migration of refrigerant from the warmer to the cooler evaporators during off cycles (see Figure 84).

Water regulating valve

The pressure operated type is the most popular. It is employed in water cooled systems to control the flow rate through the condenser, modulating in response to changes in the condensing or operating head pressure.

imageIt is designed to stop the flow of water to the condenser when the plant is at rest. An increase in the head pressure will open the valve to allow a greater volume of water through the condenser. A decrease in head pressure will automatically reduce the volume of water flowing through the condenser.

The location of the valve may vary; it may be installed at the inlet to the condenser or at the outlet. Current practice is to install it at the condenser outlet, to ensure that the condenser does not drain when throttling takes place during operation and normal modulation of the valve. Figure 85 shows the valve installed in both positions.

imageclip_image009clip_image009[1]The valves are adjustable. As a guide to operating conditions, the valves should be adjusted to maintain a temperature difference of 7 9 °C (15 18 °F) between the inlet and the outlet water.

Reversing valve

This type of valve is in common use for hot gas defrost systems and room air conditioners. It effectively reverses the flow of refrigerant to the evaporator and condenser when a defrost period is initiated. In room air conditioners, it reverses the flow when heating is required rather than cooling.

The valve is an electromagnetic type and operates by pressure when the solenoid is energized. Figure 86a shows how the valve is installed. When installation of the valve requires brazing, always remove the solenoid coil (which is detachable) and wrap a wet cloth around the valve body to avoid damaging the valve mechanism when intense heat is applied.

imageFusible plugs

Note: Before charging a system with a new or replacement refrigerant check the fusible plug rating.

Condensers and receivers are fitted with fusible plugs. These are safety devices against excessive high side pressures developing in the event of a fire. (Excessive operating pressures are controlled by the high pressure cut-out switch: see Chapter 6.) The plugs are normally brass studs with a 3 mm hole drilled through the centre, which is filled with a low melting point solder.

It is important that, when a replacement is fitted, the correct plug is selected. The following are the required melting points for various systems:

R12 100 °C (210 °F)

R22 76 °C (170 °F)

R502 76 °C (170 °F)

R717 68 °C (155 °F)