Document YjyqqmbYEMLJBnga2yVBjYoJE

858 CHAPTER 37 1955 Guide of refrigerant entering the evaporator remains unchanged, but the leaving enthalpy increases and, hence, the refrigerating effect increases. Further, compressor energy input is reduced not merely because of the greater enthalpy of the gas at suction, but also because of a reduction in the en thalpy of the superheated gas at discharge. Since the refrigerating effect is greater and the work less, it is obvious that there will be a substantial gain in the coefficient of the performance. -- The actual value of suction pressure on any system is obviously de termined by the required temperature which must be maintained in the conditioned space. For a direct expansion system the evaporator can be held at a temperature not much less than that of the conditioned enclosure, except in cases where lower temperatures may be needed in order to es tablish a desired ratio of dehumidifying to cooling load. When dehumidi fication requirements dictate the use of unusually low evaporator temperatures, the increased operating cost should properly be charged against the dehumidification rather than the sensible cooling. Influence of Discharge Pressure In contrast to the suction pressure, the compressor discharge pressure should be kept as low as operating conditions will allow. This pressure must be high enough to provide a saturation temperature of refrigerant within the condenser which is greater than the exit temperature of the cooling water. The discharge pressure therefore is a direct function of the temperature of the cooling fluid, and will automatically rise whenever the temperature of cooling water (or air) rises; it will also rise when the flow rate of the cooling medium is decreased. Increase in discharge pressure (for fixed suction pressure) raises the enthalpy of the gas leaving the compressor; hence, increases the work of compression. Further, as the enthalpy of saturated liquid leaving the con denser increases with pressure, the refrigerating effect must decrease. Thus the effect of such a pressure rise is to require more work per pound of refrigerant handled, and at the same time to necessitate an increase in the refrigerant flow rate. Influence of Water Jacket The preceding discussion has, in every case, assumed isentropic com pression. Where exact performance data are not available, this assumption is a desirable one since it leads to a conservatively large determination of the power required. In most actual systems, the compression process departs from isentropic due to irreversible heat transfers which occur between the vapor in the cylinder and the cylinder wall, and also because of intentional heat dissipation from the outside of the cylinder walls to the surroundings, or to a cooling fluid passing through a water jacket around the cylinder. Compressor cooling is highly desirable as a method of re ducing power consumption. Influence of Superheating and Subcooling The most common departure from conditions of the simple saturation cycle is that resulting from admission of superheated vapor to the com pressor. Thermodynamically, superheat is undesirable because the en thalpy increase required to compress a vapor through a given pressure range increases with superheat. Further, superheated vapor leaving an evapo rator is usually an indication that the suction pressure is lower than Refrigeration 859 necessary. Under practical operating conditions, however, superheat is almost universally used as a means of assuring complete vaporization of the refrigerant going to the compressor. With modern compressors operating at high speed, and with relatively small clearance space, it is particularly necessary to avoid admission through the suction valves of liquid refrigerant. Another common departure of actual systems from the simple satu ration cycle occurs because of subcooling of refrigerant in the condenser. Thermodynamically, such subcooling is advantageous since it increases the refrigerating effect without affecting the unit energy requirements of the compressor. Further, it can be shown that for a fixed ratio of con denser cooling water to refrigerant circulating rate, the total compressor power requirements will be greater when operating at simple saturation than when operating with maximum sub-coolirig. What is even more surprising is that condenser pressure may be lower for the sub-cooling cycle than for the saturation cycle. This condition results from the fact that, Fig. 5. Pressure-Enthalpy Diagram for Refrigeration Cycle with Subcooling and Superheating for the same capacity on a heavily loaded condenser, the refrigerant flow . rate is less when there is sub-cooling. Because of the advantages attendant upon the use of sub-cooling, many methods are in use for obtaining some sub-cooling effect outside of the condenser. One common procedure is to use the cold vapor leaving the evaporator to cool the liquid flowing from condenser to expansion valve. Another somewhat unusual sub-cooling cycle allows cold refrigerant from the downstream side of the expansion valve to cool liquid refrigerant from the condenser down to the evaporator temperature. Fig. 5 shows the pressure-enthalpy diagram for a typical refrigeration cycle operating with both sub-cooling of the refrigerant from the condenser and superheating of the refrigerant leaving the evaporator. Clearance and Volumetric Efficiency Clearance, like displacement, is a characteristic--usually fixed--of a given compressor. In some cases clearance pockets are provided which place within the operator's control the ability to alter the clearance of the machine, but most moderate size compressors are built with fixed clearance. By definition, the clearance is the percentage of the volume swept by the Piston, which is represented by spaces in the end of-the cylinder (including 'alve spaces, etc.) when the piston is at the end of its stroke. Because of the trapping of high pressure vapor in the clearance space, ncl its subsequent re-expansion, the suction valves of the compressor