2020 RETA Breeze Jan-Feb

The Basic Refrigeration Cycle

GORDON RASCOE, CIRO, CRST, RAI CHAPTER PRESIDENT FORT WAYNE CHAPTER

VARIOUS METHODS OF cooling product and spaces have been around for a long time. However, it wasn’t until the development of the Closed-Loop Refrigeration System that cooling products and spaces became feasible, efficient, and practical. Most closed-loop systems work the same basic way, with the differences being the addition of extra components and controls to make the systemmore efficient and to accomplish the task at hand. This article discusses the four (4) basic components and their function, the state of refrigerant at each component, and the relative pressure in each section of the closed-loop system. The first component in this system is the Compressor. The Compressor provides the motive force by taking a low- pressure vapor, compressing it into a high-pressure vapor, and sending it to the Condenser. Compressors can be a Positive Displacement type, such as a Reciprocating Compressor, a Screw Compressor, or a Rotary Vane Compressor. A Positive Displacement Compressor traps a volume of vapor and moves it to the discharge side. If that volume of air is blocked, either by a shut off valve or some other obstruction, then the pressure keeps building until something gives; that could be a relief valve or something in the line that

breaks. A Centrifugal Compressor is a non-positive displacement Compressor. It uses an impeller to increase the velocity of the vapor, thus allowing it to be compressed a certain amount. If the discharge is blocked, then the pressure only increases to a certain point. The impeller still rotates but won’t move the vapor. This causes excessive heat to build up causing damage to the Compressor. The next component is the Condenser. The condenser rejects the heat that the refrigerant obtained from the evaporator, piping, and the Compressor, and transforms the vapor back into a high-pressure liquid. Condensers can be 1) Air-Cooled, which moves air across a coil to cool the refrigerant; 2) Evaporative, which uses air and the evaporation of water across a coil to cool the refrigerant; or 3) Water-Cooled, which uses water through tubes or plates to cool the refrigerant. As refrigerant moves through the condenser, it condenses into a liquid, but the pressure stays relatively constant. The liquid then travels through an Expansion Device that takes the high- pressure liquid and reduces it to a low-pressure low-temperature liquid. The expansion device can be as simple as an orifice or small-diameter tubing, or as complicated as a TXV (thermo

expansion valve) or a motorized expansion valve. During this process, some vapor is created while cooling the liquid. This vapor is called Flash Gas. The pressure of the system is now near that of the suction pressure of the Compressor. The expansion device also regulates flow through the evaporator. If the expansion device lets through too much liquid, then the evaporator won’t be capable of turning all the liquid to a vapor; therefore, liquid can flow back to the Compressor, which can cause damage to the Compressor. The final component is the Evaporator. The evaporator removes heat from the room through the process of evaporating liquid ammonia flowing through the evaporator tubes. If flow is set properly, then all the liquid should evaporate before it leaves the component. Fins are built on the coils to assist in heat transfer by increasing the surface area. The fins must be kept clean, straight, and unbroken to keep the unit working efficiently. The results of a dirty evaporator could be an area not cooled sufficiently or liquid flowing back to the Compressor. This is the basic operation of a simple refrigeration system seen in home refrigerators or a window AC unit. As the size of the system grows, more components and controls are added to increase efficiency, reliability, and safety.

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