Liquid Injection vs. Thermosyphon Oil Cooling
To understand all the advantages of a particular design, it often requires a look at the “what ifs” side of the picture. The least expensive first cost of anything often becomes the standard by which other equipment or accessories are measured. The penalty in performance and efficiency of the least expensive often proves to be the most expensive purchase in the end. The same holds true for screw compressors with standard liquid refrigerant injected cooling and pumpless, pressure fed, oil circulation systems.
Compressors that rely on cooling from liquid ammonia injected into the side body of the screw machine and operating loaded at 100% capacity are between three to ten percent (3 to 10%) less efficient than compressors with thermosyphon oil cooling. What the manufacturers haven’t shown is that the efficiency suffers even more when these same compressors are run unloaded. During extremely light loads and at low suction temperatures, some compressors may stay on line just to cool themselves.
Liquid injected systems typically require a minimum of 100 to 125 psi pressure difference across the thermostatic expansion valve to control the liquid being fed into the machine. A quick fix some manufacturers use to lower this pressure difference is to relocate the injection porting to the suction of the machine. This allows it to operate at a differential of about 85 psi, a little better when trying to lower head pressures, but it decreases overall refrigeration capacity. To maintain the highest efficiency it is best to run the discharge pressure as low as possible and the suction as high as possible. The closer this pressure split the less work is need to compress the gas to the higher pressure level.
During low ambient temperatures the evaporative condenser could be used to drive the discharge down to well under a 100 psi gage pressure. At the same time, if the refrigeration load is light with a room temperature of 30 oF, the suction pressure could be raised to 40 psi and still maintain room temperatures. As an example: if a 110 psi differential is the lowest discharge pressure allowed that would still feed the injection system the discharge would have to be a minimum of 150 psi. Having the ability to drop it from 150 to 110 psi increases the compressor efficiency by forty-five percent (45%) and that is over and above the higher efficiency achieved by increasing the suction pressure or the fact that thermosyphon provides more tons of refrigeration per horsepower than liquid injection does.
Most of the lesser expensive screw compressor packages are not equipped with oil pumps and rely only on the pressure differential between the suction and discharge sides to force feed lubrication through and into the screw. This minimum pressure differential of 80 psi is generally a little less than required for liquid injection but can cause the compressors to go through an emergency shut down if not maintained. The main concern is for cold weather operation when the discharge pressures have a tendency to fall. Basically, the same efficiency arguments apply that we expounded on above. Equipping screw compressors with both thermosyphon cooling and oil pumps help to sustain maximum efficiency under all operating conditions.