The previous several sections have concentrated on the refrigerating capacity that a reciprocating compressor develops and how the evaporating and condensing temperatures governs it. The other major characteristic of any compressor is its power requirement. One way to approach the question of how the evaporating and condensing temperatures affect the power requirement is to apply the equation:
Equation 4.10 is semi-realistic, because the actual mass rate of flow, m, is available from Figure 4.6. On the other hand, the change in enthalpy during the compression, Δhideal, used in Equation 4.10 is ideal. This combination is effective, however, in showing the trend in the power requirement.
Explore first the effect of evaporating temperature on the work of compression, as shown in Figure 4.11. With a given condensing temperature, the ideal work of compression decreases as the evaporating temperature increases, until the work of compression shrinks to zero when the evaporating temperature reaches the same value as the condensing temperature.
Figure 4.12 shows the trends of m, Δhideal, and the power as the evaporating temperature changes while the condensing temperature remains constant. Starting at very low evaporating temperatures an increase in this temperature provides a progressive increase in the mass rate of flow. Figure 4.11 suggests that the Δhideal progresses from its high value at low evaporating temperatures to zero when the evaporating temperature reaches the condensing temperature. Equation 4.10 specifies that the compressor power is the product of these two terms, so the power would be expected to be low at both very low and very high evaporating temperatures. between those extremes the power requirement reaches a peak. The trends indicated by the power curve in Figure 4.12 are significant and may be surprising. Someone analyzing the power requirement of a reciprocating compressor for the first time may expect that raising the suction pressure will lighten the load on the compressor and lower the draw of power.
The range of pressure ratios against which reciprocating compressors operate is typically between about 2.5 and 8 or 9. As Figure 4.12 shows, in this range of pressure ratios the power required by the compressor increases as the suction pressure and temperature increase. This trend appears in the industrial refrigeration plant, for example, if the refrigeration load on the evaporator increases. The increase in refrigeration load almost certainly is precipitated by an increase in temperature of the product being cooled, which in turn raises the evaporating temperature. As a result, the power requirement of the compressor increases, often resulting in overload of the motor that drives the compressor.
Another motor that might be subject to overload is the one driving a highstage compressor in a two-stage system. If for some reason the intermediate pressure increases, the high-stage compressor feels the additional power requirement. Still another situation where the power curve of Figure 4.12 explains a potential motor overload is during a pull-down of temperature. If facility has been idle and then is brought into service, the evaporating temperature starts high and progressively drops. The power requirement of the compressor passes through the peak, and unless it does so quickly, the compressor motor might overload. Most reciprocating compressors in industrial applications are equipped with cylinder unloaders, as will be discussed in Sec. 4.18, which may need to be activated by special intervention during this peak power requirement. Normal activation of these cylinder unloaders occurs only when the evaporating temperature drops below the normal operating setting.
Figure 4.12 shows the power requirement of a compressor if the compression were ideal. Figure 4.13, on the other hand, presents the power requirements of the actual compressor that has been the subject of analysis in this chapter These trends derive directly from catalog data. One of the conclusions from an examination of Figure 4.13 is that the power increases toward a peak as the evaporating temperature increases. Curves for three different condensing temperatures appear on Figure 4.13, so the influence of the condensing temperature can be determined. The power requirement always increases with an increase in condensing temperature, at least within the normal range of operation.
Of possible interest, but of no practical value in industrial refrigeration, is the fact that the power requirement with a constant evaporating temperature reaches a peak as the condensing temperature increases. One of the safety tests of small window air conditioners is to stop the condenser fan, which results in extreme discharge pressures. The compressor continues to operate, although there is no additional compression of refrigerant because the volumetric efficiency has dropped to zero.
The influences of both evaporating temperature and condensing temperature on the refrigerating capacity and the power requirement have now been analyzed. The next two sections address byproducts of these trends.