90 Modern Refrigeration and Air Conditioning Copyright Goodheart-Willcox Co., Inc. 6.1 Compression Refrigeration Cycle From residential comfort cooling systems to com- mercial freezers, all compression refrigeration equip- ment removes heat using evaporation. Evaporation is one method the human body uses to cool itself. When our bodies detect that we are overheating, we sweat. The perspiration absorbs heat from our skin and evap- orates. This transfer of heat by evaporation cools us. The refrigerant in a refrigeration system works like perspiration for buildings. It cools by absorbing heat. In refrigeration systems, refrigerants are fluids that absorb heat inside the refrigerated space and release the heat outside. Removing heat from inside a refrigerated space is comparable to using a sponge to remove water from a leaking canoe. After the sponge soaks up some water in the canoe, it is held over the side of the canoe and squeezed, releasing the water overboard. This pro- cess transfers the water from the canoe into the lake, where the addition of more water makes little differ- ence. Refrigeration systems transfer heat instead of water. In a refrigeration system, an evaporator is where the refrigerant “soaks up” heat. A compressor then “squeezes” the refrigerant into a condenser. In the con- denser, the absorbed heat is released “overboard” out- side the system, Figure 6-1. In a refrigeration system, the refrigerant repeat- edly changes phase from liquid to vapor (gas) and back again. These liquid and vapor phase changes are due to changes in pressure and temperature. Low-pressure liquid refrigerant can be compared to a dry sponge. A dry sponge has the potential to soak up a lot of water, and low-pressure liquid refrigerant has the potential to soak up and remove a lot of heat. When the sponge is dipped in the canoe water, it is similar to refrigerant entering the evaporator. Both the canoe and the evaporator are the places that we want to modify and control. In the evaporator, low-pressure liquid refrigerant soaks up heat and changes into low- pressure vapor refrigerant. The absorbing or soaking up of heat by the liquid refrigerant in the evaporator occurs for two reasons: a decrease in pressure and a difference in temperature. First, as the refrigerant enters the evaporator, a decrease in pressure causes the refrigerant’s boiling point to decrease. A lower boiling point allows some of the liquid refrigerant to evaporate, which is a pro- cess that absorbs heat. Second, the rest of the cool liquid refrigerant absorbs the heat of the warm air sur- rounding the evaporator because of the temperature difference. Remember that heat naturally flows from a warmer to a cooler place. In this case, heat flows from the air around the evaporator to the refrigerant inside the evaporator. Low-pressure liquid refrigerant is like a dry sponge, but low-pressure vapor refrigerant is like a wet sponge that has sopped up water. The compressor is a pump that draws the evapo- rator’s low-pressure vapor refrigerant into it. By com- pressing the refrigerant into an increasingly smaller space, the compressor increases pressure and liter- ally squeezes the low-pressure vapor refrigerant into a high-pressure vapor refrigerant and expels it into the condenser. High-pressure vapor refrigerant is like a soaking wet sponge that cannot absorb more water. The compressor’s squeezing causes an increase in both the pressure and the temperature of the refrigerant. In the condenser, heat is released from the high- pressure, heat-soaked vapor refrigerant as a result of the temperature difference between the hot refriger- ant and the cooler air surrounding the condenser. The added heat content from the compressor increases the condenser’s potential for releasing heat by raising the temperature difference between the refrigerant and the air surrounding the condenser. This increased dif- ference allows the heat to escape more quickly. The condenser is like the space beside the boat where the soaked sponge is wrung out into the lake. Refrigerant leaves the condenser as a high-pressure liquid. Enough heat is released in the condenser to change the vapor back into liquid. This high-pressure liquid on its way back to the evaporator is comparable to the wrung-out sponge being brought back into the canoe for more water. In a compression refrigeration system, absorp- tion and rejection of heat occurs as often as necessary. Goodheart-Willcox Publisher Figure 6-1. Using a sponge to remove water from a canoe is an analogy for how a compression refrigeration system removes heat. Heat is absorbed and released due to phase changes and changes in pressure. Evaporator Heat is absorbed Metering device Heat is released Condenser Compressor Pressure is increased Low Pressure High Pressure Pressure is decreased