Refrigerating System – Superheating and subcooling
Superheating (referring to superheating of the refrigerant vapor leaving evaporator) and subcooling (referring to subcooling of refrigerant liquid leaving the condenser) are apparently two significant processes in practical vapor-compression refrigeration systems and are applied to provide better efficiency (COP) and to avoid some technical problems, as will be explained below.
During the evaporation process the refrigerant is completely vaporized partway through the evaporator. As the cool refrigerant vapor continues through the evaporator, additional heat is absorbed to superheat the vapor. Under some conditions such pressure losses caused by friction increase the amount of superheat. If the superheating takes place in the evaporator, the enthalpy of the refrigerant is raised, extracting additional heat and increasing the refrigeration effect of the evaporator. If it is provided in the compressor suction piping, no useful cooling occurs. In some refrigeration systems, liquid-vapor heat exchangers can be employed to superheat the saturated refrigerant vapor from the evaporator with the refrigerant liquid coming from the condenser (Figure 3.32). As can be seen from Figure 3.32, the heat exchanger can provide high system COP. Refrigerant superheating can also be obtained in the compressor. In this case, the saturated refrigerant vapor enters the compressor and is superheated by increasing the pressure, leading to the temperature increase. Superheating obtained from the compression process does not improve the cycle efficiency, but results in larger condensing equipment and large compressor discharge piping. The increase in the refrigeration effect obtained by superheating in the evaporator is usually offset by a decrease in the refrigeration effect in the compressor. Because the volumetric flow rate of a compressor is constant, the mass flow rate and the refrigeration effect are reduced by decreases in the refrigerant density caused by the superheating. In practice, it is well known that there is a loss in the refrigerating capacity of 1% for every 2.5°C of superheating in the suction line. Insulation of the suction lines is a solution to minimize undesirable heat gain. The desuperheating is a process to remove excess heat from superheated refrigerant vapor, and if accomplished by using an external effect it will be more useful to the COP. Desuperheating is often considered impractical, owing to the low temperatures (less than 10°C) and small amount of available energy.
This is a process of cooling the refrigerant liquid below its condensing temperature at a given pressure (Figure 3.32). Subcooling provides 100% refrigerant liquid to enter the expansion device, preventing vapor bubbles from impeding the flow of refrigerant through the expansion valve. If the subcooling is caused by a heat transfer method external to the refrigeration cycle, the refrigerant effect of the system is increased, because the subcooled liquid has less enthalpy than the saturated liquid. Subcooling is accomplished by refrigerating the liquid line of the system, using a higher temperature system. Simply we can state, subcooling cools the refrigerant more and provides the following accordingly:
• increase in energy loading,
• decrease in electrical usage,
• reducing pulldown time,
• more uniform refrigerating temperatures, and
• reduction in the initial cost.
Note that the performance of a simple vapor-compression refrigeration system can be significantly improved by further cooling the liquid refrigerant leaving the condenser coil. This subcooling of the liquid refrigerant can be accomplished by adding a mechanicalsubcooling loop in a conventional vapor compression cycle. The subcooling system can be either a dedicated mechanical-subcooling system or an integrated mechanical-subcooling system (Khan and Zubair, 2000). In a dedicated mechanical-subcooling system, there are two condensers, one for each of the main cycle and the subcooler cycle, whereas for an integrated mechanical-subcooling system, there is only one condenser serving both the main cycle and the subcooler cycle.
For example, subcooling of R-22 by 13°C increases the refrigeration effect by about 11%. If subcooling is obtained from outside the cycle, each degree increment in subcooling will improve the system capacity (approximately 1%). Subcooling from within the cycle may not be as effective because of offsetting effects in other parts of the cycle. Mechanical subcooling can be added to existing systems or designed into new ones. It is ideal for any refrigeration process in which more capacity may be necessary or operating costs must be lowered. It has proved cost efficient in a variety of applications and is recommended for large supermarkets, warehouses, plants, etc. Figure 3.33 shows a typical subcooler for commercial refrigeration applications.