Examining economizer strategies to cut energy use without sacrificing performance.

The original “free lunch” was served in western saloons in the nineteenth century to anyone who had bought at least one drink. The food was plentiful and without charge, but tended to be salt beef, salted ham, salty peanuts and so on. Hence the phrase, popularized by Milton Friedman in the 1970s but at least 50 years older than that, “there’s no such thing as a free lunch.”

Economizers fitted to refrigeration plants seem like a free lunch. More cooling capacity from the same size of compressor; what’s not to like? I am a fan of the concept, but without a clear understanding of what’s going on it is easy to make expensive mistakes. Some of the high pressure liquid coming out of the condenser is separated from the main flow and expanded to an intermediate pressure to be used to refrigerate the remaining liquid. This achieves a greater cooling effect by the main flow when it is expanded and evaporated and increases the capacity of the compressor (although it handles the same suction volume with or without an economizer).

This technique is regularly used with screw compressors, which can be equipped with a side port to enable the intermediate pressure gas coming from the economizer to be added to the compressor flow along the compression path. This means that less work is required to compress that portion of the total compressor flow which increases efficiency.

The economizer can be “open flash,” meaning that all of the condenser outlet is expanded to intermediate pressure in a receiver and the resulting flash gas is diverted to the economizer port, or “closed flash,” meaning that only the economizer port flow is expanded initially and is used in a heat exchanger to refrigerate the high-pressure liquid stream. There are merits and disadvantages with each system, but the concept is often thought to be only applicable to screw compressors.

However, the same principle applied to reciprocating compressors has been patented several times over the last 120 years and delivers the same capacity and efficiency benefits. It’s often called a Voorhees system, after G.T. Voorhees, who was granted a patent on a ported reciprocating compressor in 1905. An equivalent arrangement was patented in the early 1920s by the Haslam Company in England for carbon dioxide compressors discharging at or above the critical pressure.

My father was granted a patent (now expired) on a similar concept where the inlet of one of the cylinders of a multicylinder reciprocating compressor was connected to the intermediate pressure source and the rest were connected to the suction chamber. This allowed the economizer gas to be compressed in parallel with the main gas flow, with the two gas streams only meeting in the discharge gallery of the compressor. The concept, named “parallel compression,” was intended for use in single compressor transcritical CO2 systems such as heat pump water heaters. It didn’t take off in the single compressor variant but the name stuck and the technique is now frequently applied to multiple compressor systems with one or more machines on “economizer duty” and the rest on the main stream.

A quirk of this arrangement is that the cooling effect can be increased by up to 20% when the suction mass flow is reduced by 25% because the extra subcooling of the transcritical gas cooler outlet is so useful.

However, this is not really a free lunch. The capacity is increased but so is the system complexity, with the requirement for additional heat exchangers, additional control valves and additional pipework. Rising energy costs might make it easier to justify that capital expenditure, but the complexity also makes it harder to monitor performance and ensure that it is paying back fast enough.