Written by Brad Crawford
Air conditioning technology has come a long way since the latter part of the 1950s when there were just two types of compressors available in the United States for refrigeration and air conditioning units – a choice of dynamic centrifugal refrigeration machines or positive displacement machines. The potential of the ranges overlapped, but many experts recognized that there would be plenty of room for improvement.
Air conditioning development soon became focused on bridging higher speed compressors, which aimed to combine the advantages of both types of design – the centrifugal and the reciprocating models. This quickly led to a reduction in surging which was recognized to be useful in many different applications of refrigerants under high pressure.
The trend was definitely going the way of portable air conditioning units which were engine-driven,replacing the traditional reciprocating function with oil cooled rotary machines, initially utilizing the sliding vane design before being replaced by the screw design. The capacity control was effectively achieved by a variation of the RPM without unloading the compressor device.
This screw compressor design had been patented in 1934 in Sweden and landed in the USA in the 1950s at a laboratory belonging to the Worthington Pump Corporation. After numerous tests and evaluation procedures for its potential in refrigeration and air conditioning applications, the design got a resoundingly negative response. The major problem was the intense amount of noise which was produced from the high speed, oil-free, compression based machine. The device also needed mechanical speed increases even if it was directly attached to two electric motors measuring around 3,600 RPMs. This machine was noisy, inefficient and would prove to be difficult for air conditioning manufacturing purposes — but it was a start.
The Swedish company which owned the rights to the 1934 design (Svenska Rotor Maskiner AB) began development of an oil flooded device with a much simpler manufacturing procedure – there were no timing gears, and this machine could be coupled directly to the two-pole motors simultaneously. Other improvements were also made to the design, including fixed RPM drives. With these improvements the technology suddenly looked a whole lot more viable in the world of air conditioning and large scale refrigeration.
By the 1960s, three companies had successfully pioneered the latest technology and developed machines specifically designed for refrigeration units – plus some large scale industrial type air conditioning units. All three manufacturers initially used the standard rotors design, with rotors of similar lengths and diameters and ratios ranging from around 1.0 up to 1.7. Changes were needed,however, to really make an impact on the air conditioning market for which it was necessary to develop hermetic screw type compressors.
The US manufacturing company D/B – Dunham-Bush seized the new technology and ran with it having the foresight and the sheer guts to enter the air conditioning marketplace using hermetic screw-type compressors, meeting the traditional centrifugal system developers head-on in the fight for supremacy. 1967 saw the official launch of the DBX line of machines utilizing the screw compressor functions, and a large display of hermetic screw compressors and chillers were unveiled at the winter trade show.
Fast forward to the 1980s, and although a handful of manufacturers had embraced the screw compressor function in the 1960s, 1988 saw more than 20 different manufacturing organizations, including five in the United States, realizing the commercial viability of the venture.
There are a number of advantages for utilizing screw compressor technology in air conditioning and refrigeration units. For a start they are generally smaller in size, lighter in weight and a much more affordable option than those utilizing hermetic motors or basic compressors. They can also operate both reliably and efficiently while coupled directly to 2-pole sixty or fifty cycle electrical motors with increased RPM’s, VFD’s (variable frequency drives) and step up gears. This leads to the flattening of the volumetric efficiency curves against the compression ratio, which leads to a reduction in the compressor displacement. This is required as compared to any reciprocating model with a higher CR used typically in low or medium temperature air conditioning, heat pump and refrigeration applications.
The technological advancements have really moved forward at an incredible rate during the last 50 years within the world of air conditioning and refrigeration. We’ve all come to rely on this technology so much in our everyday lives; it’s difficult to imagine how we would ever manage without it.