One of the items we see in various advertisements of competing cooling technology is the notion that vapor compression systems do not have the ability to adjust to various operating conditions or meet a temperature tolerance requirement. While this position may have been true at one point in the distant past, modern vapor compression systems can hit a very tight temperature tolerance under a wide range of operating conditions. Modern vapor compression systems use variable-speed compressors with turn-down ratios that reach above 4:1. This means that a variable-speed compressor with a capacity of 1000 watts at maximum speed can also meet the requirement at any cooling load value down to 250 watts or less.
Compressors are sized based on their displacement. All vapor compression systems use refrigerant to extract heat at low temperatures and dissipate that heat to an ultimate heat sink, (usually ambient air) at the same or higher temperature than the system being cooled. All of the heat is “gathered” into the refrigerant in the evaporator and dissipated in the condenser. The mass flow rate of the refrigerant determines the capacity. Therefore, a modern rotary compressor operating at 6500 RPM will have significantly greater cooling capacity than that same compressor operating at 1500 RPM. A simple PID controller using a temperature measured at the important point in the system can control the cold side temperature to within ±0.05°C. These systems have the ability to meet this temperature tolerance over the entire capacity range of the compressor. The graph below illustrates the capacity range of Aspen’s LCM-1400. This unit utilizes a rolling piston design with a brushless DC motor.
The drive board for the compressor receives the PID signal and adjusts the speed of the compressor, thereby changing the temperature of the evaporator. Higher speed produces more refrigerant flow and increased capacity/lower temperature. The upper curve shows the capacity with the compressor operating at its maximum speed of 6500 RPM. The lower curve shows the compressor operating at its minimum speed of ~1500 RPM. By choosing the compressor/system carefully at the beginning of the design, a vapor compression system can be chosen that will cover all known operating conditions. The process is to look at the area between the curves to determine the suitable solution.
Next time, we will investigate the compact sizes that are available in Modern Vapor Compression Systems.
