REFRIGERATION- 4

Dehydrator

Systems opened for repairs or service are vulnerable to moisture contamination. The maximum level of moisture permitted in an operating refrigeration system is 15 parts per million.

 A dehydrator, or dryer, containing silica gel or activated alumina, is placed in the liquid refrigerant line between the receiver and the TXV. In older installations, bypass valves allow the dehydrator to be cut in or out of the system. In newer installations, the dehydrator is installed in the liquid refrigerant line without any bypass arrangement. A dehydrator is shown in upper left figure . A refrigeration system contaminated with moisture can be affected by, acid formation, sludge formation, ice in the expansion valve, and corrosion.

If a liquid drying agent is used in a refrigeration system already equipped with a solid drying agent, the liquid drying agent will release the moisture already trapped in the solid drying agent.

 

Moisture Indicator and Liquid Eye

A moisture indicator is located either in the liquid refrigerant line or built into the dehydrator. The moisture indicator contains a chemically treated element that changes color when there is an increase of moisture in the refrigerant. The color change is reversible and changes back to a DRY reading when the moisture is removed from the refrigerant. Excessive moisture or water will damage the moisture indicator element and turn it gray, which indicates it must be replaced. In an operating refrigeration system low on refrigerant, a liquid line sight glass will show bubbles.

Solenoid Valve and Thermostatic Control Switch

A solenoid valve is installed in the liquid line leading to each evaporator. Figure right shows a solenoid valve and the thermostatic control switch that operates it. The thermostatic control switch is connected by long flexible tubing to a thermal control bulb located in the refrigerated space. When the temperature in the refrigerated space drops to the desired point, the thermal control bulb causes the thermostatic control switch to open. This action closes the solenoid valve and shuts off all flow of liquid refrigerant to the TXV. When the temperature in the refrigerated space rises above the desired point, the thermostatic control switch closes, the solenoid valve opens, and liquid refrigerant once again flows to the TXV.  This is is an example of two position control.

The solenoid valve and its related thermostatic control switch maintain the proper temperature in the refrigerated space. You may wonder why the solenoid valve is necessary if the TXV controls the amount of refrigerant admitted to the evaporator. Actually, the solenoid valve is not necessary on units that have only one evaporator. In systems that have more than one evaporator and where there is wide variation in load, the solenoid valve provides additional control to prevent the spaces from becoming too cold at light loads.

In addition to the solenoid valve installed in the line to each evaporator, a large refrigeration plant usually has a main liquid line solenoid valve installed just after the receiver. If the compressor stops for any reason except normal suction pressure control, the main liquid solenoid valve closes. This prevents liquid refrigerant from flooding the evaporator and flowing to the compressor suction. Extensive damage to the compressor can result if liquid is allowed to enter the compressor suction.

Refrigeration Valve

Refrigeration valves are used to add and remove refrigerant from the system. Most systems have refrigeration valves installed in both, high and low pressure sides of the system.

Like most valves used in refrigeration they are double seating. When the valve stem is rotated counter-clockwise to the fully opened position the upper seat seals the valve from leakage. Double seating valves should be used either fully opened or closed. Double seating valves permit repacking under pressure. A cap is provided to reduce the possibility of loss of refrigerant from the system. The valve shown has a gauge connection.

Modulating Valves

Modulating valves are similar to solenoid valves, as they control the liquid refrigerant to the TXV. A liquid line solenoid valve is either completely opened or closed, whereas a modulation valve is positioned according to the strength of the applied electrical signal. The movement of the armature within a modulating valve is controlled by the electromagnetic force of the coil and opposed by spring pressure. This arrangement will cause the valve to the open position due to the spring pressure acting upon the armature if the coil fails.