More On The Basic Refrigeration System

June 30, 2005
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This is another in a series of columns addressing advanced basics in the understanding of the refrigeration cycle. In this column, we continue our look at components. The June 6 column ("The Basic Refrigeration System") dealt with the compressor, discharge line, and condenser. This column will focus on the receiver, liquid line, metering device, evaporator, and suction line. In each case, details will be given as to the component's function.

Mastering the function of each individual component can assist the technician with analytical troubleshooting skills. In the long run, this will save time and money for both technician and customer.

Receiver

Stores liquid: Once the subcooled liquid exits the condenser, the receiver receives and stores the liquid. The liquid level in the receiver will vary if the metering device is throttling open or closed.

Usually receivers are on systems when thermostatic expansion valves are used as metering devices. The subcooled liquid in the receiver may lose or gain subcooling depending on the temperature of the area surrounding the receiver. If the subcooled liquid is warmer than the receiver's surroundings, then the liquid will reject heat to the surroundings and subcool even more.

Figure 1. A receiver with a thermostatically controlled liquid bypass. (Courtesy of Esco Press.)
However, if the subcooled liquid is cooler than the receiver's surroundings, heat will be gained by the liquid and subcooling will be lost. Often a receiver bypass will be used so the subcooled liquid will not sit in the receiver and lose its subcooling.

This bypass simply bypasses liquid around the receiver and directly to the liquid line and filter-drier. A thermostat with a sensing bulb on the condenser outlet senses liquid temperature coming to the receiver to control the bypass solenoid valve. (See Figure 1.) If the liquid is subcooled to a predetermined temperature, it will be bypassed around the receiver to the filter-drier.

Liquid Line

Transports liquid: The liquid line simply transports high-pressure, subcooled liquid to the metering device. In transport, the liquid may either lose or gain subcooling, depending on the surrounding temperature. Liquid lines may be wrapped around suction lines to help them gain more subcooling. (See Figure 2.) Liquid/suction line heat exchangers can be purchased and installed in existing systems to gain subcooling of the liquid.

Metering Device

Meters liquid refrigerant: The metering device meters liquid refrigerant from the liquid line to the evaporator. There are several different styles and kinds of metering devices on the market with different functions, such as control of evaporator superheat and pressure. Some metering devices even have pressure-limiting devices to protect compressors at heavy loads.

Restricts: The metering device is a restriction that separates high pressure from the low pressure in the refrigeration system. The compressor and the metering device are the two components that separate pressures in the refrigeration system. This restriction in the metering device causes the liquid refrigerant to flash to a lower temperature in the evaporator because of the evaporator being at a lower pressure and temperature.

Figure 2. A liquid/suction line heat exchanger. (Courtesy of Esco Press.)

Evaporator

Heat exchange: The evaporator, like the condenser, acts as a heat exchanger. Heat gains from the product load and outside ambient will travel through the sidewalls of the evaporator to vaporize any liquid refrigerant. The pressure drop through the metering device causes vaporization of some of the refrigerant. The pressure drop causes a lower saturation temperature in the evaporator. This temperature difference between the lower pressure refrigerant and the product load is the driving potential for heat transfer to take place.

Superheats: The last pass of the evaporator coil acts as a superheater to make sure that the entire liquid refrigerant has been vaporized. This will protect the compressor from any liquid slop over that may result from valve damage or diluted oil in the crankcase. A thermostatic expansion type of metering device usually controls the amount of superheat in the evaporator.

Suction Line

Transports vapor: The suction line transports low-pressure, superheated vapor from the evaporator to the compressor for compression. There may be other components in the suction line, such as suction accumulators, crankcase pressure regulators, P-traps, filters, and screens. Liquid/suction line heat exchangers are often mounted in the suction line to transfer heat away from the liquid line (subcool) and into the suction line (superheat).

Superheats: Another function of the suction line is to superheat the vapors as they approach the compressor. Even though the suction lines are usually insulated, sensible heat still enters the refrigerant vapors and adds more superheat.

This additional superheat de-creases the density of the refrigerant vapors to prevent overloading of the compressor. Lower amp draws can now be realized. This additional superheat also helps assure that the compressor will see only vapors under low loading conditions.

Many metering devices have a tendency to lose control of evaporator superheat at low loads. It is recommended that systems should have at least 20 degrees of total superheat at the compressor to prevent liquid slugging and/or flooding the compressor at low loadings.

John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich., and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached by e-mail at tomczykj@tucker-usa.com.

Publication date: 07/04/2005

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