Which is more energy-efficient for screw refrigeration compressors: energy slider regulation or variable-frequency motors?

I think variable frequency drives are more energy-efficient, but with a change in speed, factors such as internal leakage also change; it’s necessary to analyze whether this is beneficial for the machine

This post was last edited by danielzhan on 2017-5-23 at 14:03. I’m not very familiar with this, but I do know something about air compressors. Modern refrigeration systems all come equipped with sliders that automatically adjust the volume of exhaust gas in order to save energy. I’ve never heard of systems that use variable frequency technology to save power. For it to be necessary to install a variable frequency drive on an air compressor, the load rate must be below 80%; otherwise it’s pointless, as the variable frequency drive still consumes around 4% of the motor’s power

The theoretical volumetric flow rate of a screw compressor is primarily determined by factors such as the rotor diameter, rotor length, rotor speed, profile coefficient, and liquid injection volume； The suction volumetric flow rate of a screw compressor is also related to the inlet pressure and temperature of the compressor. The methods for adjusting the air volume in screw compressors generally include: 1. On/off control – this is achieved by opening or closing the inlet valve of the compressor, thereby allowing 100% air intake and exhaust, or 0% air intake and exhaust ； The principle is to adjust the intake pressure of the host. 2. Continuous adjustment – achieved by adjusting the opening degree of the main unit’s intake valve ； Principle: Adjust the intake pressure of the host; 3. Variable frequency adjustment – Adjust the speed of the host ； Based on the fluctuations in the exhaust pressure of the main engine, the engine speed is adjusted via PID control to maintain stable exhaust pressure, thereby achieving energy savings ； However, there are issues such as thermal losses from the inverter, reduced motor efficiency when the motor speed decreases, and a drop in the volumetric efficiency of the main unit ； 4. Slide valve adjustment – This adjusts the effective compression length of the air inlet and outlet ports in the compressor rotor’s compression chamber, thereby regulating the volumetric flow rate and internal pressure ratio of the compressor, achieving gas volume control and thus saving energy ； 5. Exhaust reflux regulation (bypass reflux) – backflow to the main unit’s inlet through the valve in the outlet pipeline of the main unit;

It cannot be considered solely from the perspective of power consumption; the energy slide valve has a wide range of adjustment possibilities, and this is one of the crucial functions for protecting the unit during startup. It operates quickly, provides accurate adjustments, and ensures a smooth output. Frequency conversion is also an option, but the output power does not change linearly with speed, so this factor needs to be taken into account. Operators also need to find the right settings. Additionally, the return flow of the energy slide valve differs from that of pumps and reciprocating machines. In my understanding, return flow refers to liquid-phase flow, so there is not much loss of energy in this process; however, it reduces the amount of material entering the system, thereby achieving the purpose of regulation. As a result, the slide valve is easier to operate and maintain, with similar energy-saving effects.

I don’t have much experience with refrigerators, but I’ve never heard of any that are energy-efficient due to variable frequency technology; it’s necessary to consider the cooling capacity in such cases