Looking at pressure relief, decompression events, and how to manage them safely.

Following last month’s thoughts about oil leads me to an even deeper question that was prompted by the plenary panel to the Herrick compressor conference: do we need compressors at all?

The discussion emphasized that the three main types of positive displacement compressors have total dominance of the market. The reciprocating compressor has been in use in refrigerating systems since Jacob Perkins built his ice making machine using ethyl ether as the refrigerant in 1834. It is, at face value, an extremely simple device. It draws in a volume of gas at low pressure and then reduces the volume by the linear motion of a piston, producing a flow of the gas at higher pressure and temperature. So far, so good, but several levels of complexity need to be added in order to create a practical, working machine.

Some form of valve arrangement is required to admit gas into the compression chamber and then to hold it until it is sufficiently reduced in volume to be delivered to the discharge side of the system. Fortunately the pressures created in the cylinder act to open suction valves when the pressure is low enough and discharge valves when it is high enough. In this respect a refrigeration compressor is different to an internal combustion engine, which requires a complex arrangement of cams and levers to open the valves at the right time, or a carefully positioned set of openings in the cylinder wall in the case of a two-stroke engine. BSRIA (“Quick Recovery in the Global Compressor Market: Air Conditioning, Heat Pumps and Refrigeration Applications Surge,” ASHRAE Journal, July 2022) estimate that of the 486 million refrigeration compressors sold worldwide in 2021, 57% were reciprocating machines.

Screw and scroll compressors have come to dominate the rest of the compressor market due to their relative simplicity compared to the piston compressor. They have both been known since the early 20th century, but only became commercially viable when production engineering enabled their complex forms to be machined cheaply and accurately. Screw and scroll compressors use ports, like the two-stroke engine, rather than valves to control the influx and discharge of the compressed refrigerant and they avoid the Achilles heel of the reciprocating mechanism, the re-expansion of compressed gas, by ensuring that the compressed volume reduces to zero as the rotating element sweeps past the discharge port.

Re-expansion is why the volumetric efficiency of a reciprocating compressor reduces to zero if the pressure ratio is high enough. There will always be a small amount of high pressure gas in the compression space when the discharge valve closes, and if the pressure ratio is high enough this is sufficient to fill the space when the suction valve opens, meaning that there is no room for any additional gas to be drawn in.

There were many papers, as there have been at many previous Herrick conferences, about novel compressors. This got me thinking: if the established machines are so good, why all the interest in superseding them with something else? Furthermore, if there is a desire for improvement, why not just improve the established machines? Are they as good as they could possibly be, or is there still scope to make them better? If there is then why have we not already taken that step?

Despite all the pressure on traditional technologies it was clear from the discussion at the plenary panel, and in the conversations in the break rooms and hallways, that the majority of attendees reckoned that vapor compression is here to stay and is unlikely to be surpassed for efficiency, reliability or economy any time soon.