During operation, optimum efficiency is achieved when the compressor’s built-in volume ratio matches the actual pressure ratio of the refrigeration system. This is accomplished by controlling the compressor’s discharge opening using a VVR mechanism, which allows the compressor to adjust its built-in volume ratio (Vi) so it can more effectively match the system’s actual pressure ratio.
A VVR mechanism prevents both over-compression and under-compression, improves energy efficiency, and enhances both capacity and overall system performance.
There are 2 VVR systems: The individual-drive and the simultaneous-drive capacity control.
The individual-driven mechanism consists of two sliding valves, two pistons, and two cylinders housing the pistons, positioned 180° apart. In this design, the pistons move independently but reach the same volume-ratio (VR) positions. Conversely, the simultaneous-driven system uses two sliding valves operated by a single piston and a single cylinder. The disadvantage of this system is that it is longer, occupies more space, and therefore increases the overall length of the compressor.
The VVR pistons are actuated by high-pressure gas or oil through solenoid valves, which regulate the supply pressure to the mechanism and position the slides as required. The full-load position of the VR slide is referred to as the low-VR position, where the slide exposes the entire discharge-port area. Moving the piston to a higher-VR position reduces the discharge opening and enables part-load operation.
When operating conditions change and the required pressure ratio increases (such as during higher ambient temperatures), the VVR mechanism shifts to a higher volume-ratio position. Conversely, when the required pressure ratio decreases (such as during light load or colder ambient conditions), it shifts to a lower volume ratio.
The figure 1 below illustrates the ideal compression in white, with under- and over-compression shown in the hatched areas. In the case of under-compression, the compressor does not reach the system pressure, causing backflow from the discharge side to be recompressed, which reduces efficiency. In contrast, during over-compression, the compressor provides more pressure than required, leading to unnecessary energy consumption and higher operating costs.
Figure 1 - Ideal Compression VVR Compressor
Finally, compressors exhibit their maximum efficiency at part load under different pressure ratios. To determine the highest efficiency for a given VR position, a PR–VR test is performed. This test is conducted at a fixed speed and fixed suction pressure while varying the discharge pressure, and all VR positions of the compressor are evaluated. In this example, the compressor has three VR positions, each with its own highest efficiency point at a specific operating condition. For instance, during operation with a volume ratio range of 1 to 4, the compressor’s default VR position will be 1. Once the most efficient PR–VR combination has been identified, the slides are positioned accordingly so that the compressor operates at the optimal VR during real-time conditions.
Add Figure 13, need to renumber when split across pages
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