Capillary Tube. - The capillary tube consists of a long tube of small diameter. It acts as a constant throttle on the refrigerant. The length and diameter of the tube are important; any restrictions cause trouble in the system. It feeds refrigerant to the evaporator as fast as it is produced by the condenser. When the quantity of refrigerant in the system is correct or the charge is balanced, the flow of refrigerant from the condenser to the evaporator stops when the compressor unit stops. When the condensing unit is running, the operating characteristics of the capillary tube equipped evaporator are the same as if it were equippedwith a high-side float.

The capillary tube is best suited for household boxes, such as freezers and window air-conditioners, where the refrigeration load is reasonably constant and small horsepower motors are used.

Accessory Devices

The four basic or major components of a refrigeration system just described are enough for a refrigeration unit to function. However, additional devices, such as the receiver already described, make for a smoother and more controlled cycle. Some of the accessory devices used on a refrigeration unit are described in this section. Before proceeding, take a close look at figure 6-25 that shows one type of refrigeration system with additional devices installed. Some of the devices and their functions are explained to help you understand installation and troubleshooting of a refrigeration unit.

RELIEF VALVE. - A refrigeration system is a sealed system in which pressures vary. Excessive pressures can cause a component of the system to explode. The National Refrigeration Code makes the installation of a relief valve mandatory. A spring-loaded relief valve is most often used and it is installed in the compressor discharge line between the compressor discharge connection and the discharge line stop valve to protect the high-pressure side of the system. No valves can be installed between the compressor and the relief valve. The discharge from the relief valve is led to the compressor suction line.

DISCHARGE PRESSURE GAUGE AND THERMOMETER. - A discharge pressure gauge and thermometer are installed in the compressor discharge line (liquid line) to show the pressure and temperature of the compressed refrigerant gas. The temperature indicated on the gauge is always higher than that corresponding to the pressure when the compressor is operating.

COMPRESSOR MOTOR CONTROLS. - The starting and stopping of the compressor motor is usually controlled by either a pressure-actuated or temperature-actuated motor control. The operation of the pressure motor control depends on the relationship

Figure 6-25. - A basic refrigeration system.