If reducing the capacity using a slide valve results in a decline of compression efficiency, it is reasonable to explore other means of capacity control. The most attractive alternative is the use of variable-speed drive, provided either by a two-speed motor or by a frequency inverter that furnishes infinite variations of speed. The speed boundaries within which either of these concepts must operate is generally between 800 and perhaps 5000 or 6000 rpm. At low speeds the ratio of leakage gas to that pumped increases, so both the volumetric and the compression efficiency decrease. At high speeds the high pressure drop through the passages of the compressor reduces the compression efficiency. Furthermore, the noise level increases as the speed increases, and mechanical limits of the moving parts come into play. The rotor tip speed is the best criterion to guide the choice of speed, and the range of recommended tip speed is between about 20 and 50 m/s (4000 to 10000 fpm). Thus, higher rotative speeds are possible for compressors equipped with small-diameter rotors.
Screw compressors driven by a two-speed motors are available for certain applications. Such a compressor has the potential of a percent power versus percent capacity relationship as shown in Figure 5.15. The characteristics shown in Figure 5.15 assume that the compressor is equipped with a slide valve. Dropping the compressor speed from 3600 to 1800 rpm will normally reduce the volumetric efficiency only several percent. A further consideration is that two-speed motors are slightly less efficient than single-speed ones.
The variable-frequency inverter receives power at 60 Hz and converts the frequency to a different value which drives the motor with the corresponding speed change. With a variable-frequency drive no slide valve whatsoever is needed, which reduces the cost of the unit slightly and eliminates the occasional replacement of the slide valve due to its wear. The critical speed of the compressor may lie in the desired range of oper ation, but a standard capability of variable frequency inverters is to skip over the narrow band of frequencies associated with critical speed of the motor. An additional benefit of a variable frequency inverters is that a frequency higher than the power-line frequency of 60 Hz can also be developed. This capability permits the handling of peak loads with a compressor slightly smaller than would normally have been necessary.
Even though the listing of the advantages of variable-speed drive suggests that the method of capacity control would be in wide use, such is not the case in industrial refrigeration. The economic calculations for justifying the additional cost of the inverter are complex because of the various efficiencies that must be considered, including those of the inverter, the motor when operating at offdesign speed and the efficiency of the compressor equipped with the slide valve.
Furthermore, the load profile expected for the compressor must be evaluated. A compressor that operates nearly fully loaded most of the time is probably more efficient with a slide valve for capacity control. On the other hand, if the first cost of inverters continues to drop, marginal cases will fall toward the choice of the variable-speed inverter.