Along with the power requirement, the refrigerating capacity is a key characteristic of a compressor. The meaning of refrigerating capacity of a compressor should first be explained, because no refrigeration takes place at the compressor. For a compressor to possess a certain refrigerating capacity means that the compressor is capable of compressing the flow rate of refrigerant from its suction pressure to its discharge pressure that will provide the specified heat-transfer rate at the evaporator.
The overall equation that expresses the refrigeration rate is:
Equation 4.7 expresses in compact form what was executed in stepwise fashion in Example 4.1, and will now be used to show the trend in performance variables as influenced by the evaporating temperature with a constant condensing temperature. We arbitrarily choose a condensing temperature of 30°C (86°F) and predict the refrigeration capacity over a range of evaporating temperatures. Furthermore, calculations will be performed for the compressor whose volumetric efficiency is shown in Figure 4.2 and which has a displacement rate d of 0.123 m3/s (260 cfm).
The volume rate of flow is available from a part of Eq. 4.7:
This trend is shown in Figure 4.4, where as the evaporating temperature drops the pressure ratio increases and the volumetric efficiency drops off.
The next objective will be to show the trend in the mass rate of flow m, which can be done by introducing the specific volume of the suction gas vs:
The magnitudes of specific volumes are unique for each refrigerant, so we will arbitrarily illustrate the principles using ammonia from this point on. Figure 4.5 shows the variation in specific volume of saturated vapor leaving the evaporator and entering the compressor. As the evaporating temperature drops, the volume flow rate decreases and the specific volume increases, both effects causing a decline in the mass flow rate. This trend is shown in Figure 4.6.
The final term to be brought into Equation 4.7 is the refrigerating effect, Δhev, which is the change in enthalpy experienced by the refrigerant as it passes through the evaporator. Because the catalog data on which the volumetric efficiencies of Figure 4.2 are based specifies 5.6°C (10°F) subcooling of the liquid entering the expansion device and 5.6°C (10°F) of useful superheat leaving the evaporator those additions to the refrigerating effect are incorporated in the graph of Figure 4.7. The refrigerating effect is only mildly influenced by the evaporating temperature.
This foregoing sequence of graphs culminates in the goal of showing the effect of evaporating temperature on the refrigeration capacity, Figure 4.8. The values shown in the figure are realistic and match catalog data. This fact is not surprising, because the volumetric efficiency (Figure 4.2) used to develop Figure 4.8 was actually calculated from a revised form of Equation 4.7 using catalog data for the refrigerating capacity.
An immediate observation from Figure 4.8 is that the refrigerating capacity always decreases as the evaporating temperature drops. At high evaporating temperatures the decrease in refrigeration capacity is approximately 4% per °C (2.2% per °F) and at low evaporating temperatures, near the maximum pressure ratios of reciprocating compressors, the decrease in refrigerating capacity is approximately 9% per °C (5% per °F). Since the usual objective is to achieve high refrigerating capacities, what can be done to maintain a high evaporating temperature? The temperature of the product or process is usually imposed on the designer and operator, which fixes the maximum ideal evaporating temperature. But the evaporating temperature must be lower than the temperature of the product or process in order to provide the temperature difference that motivates the flow of heat. It is in the potential reduction of this temperature difference that some latitude might exist to elevate the evaporating temperatures. Choosing a larger heat-transfer area for the evaporator or improving the heat-transfer coefficients by such measures as higher fluid velocities are possibilities under the control of the designer and operator.