The choice of screw or reciprocating compressors for air conditioning or refrigeration applications is a decades-old debate. According to Russell Hattingh, Engineering Manager at Johnson Controls Systems & Service, there’s no single answer to this question—it depends on the application, operating conditions and attitudes.

Russell Hattingh smaller“New ‘recip’ advances may win these compressors a few more converts, however,” he says. “The short answer it that each compressor type has its particular advantages, depending on the required operating conditions.”

Reciprocating compressors are piston-driven compressors delivering stepped control with specific output per piston. They work well in high temperature applications, delivering energy savings. They typically use ammonia refrigerant, which is considered a more naturally eco-safe option. “R&D by large manufacturers for reciprocating compressors continue to deliver enhancements and new features that keep the compressor evergreen. The latest iteration of the Sabroe recip, the Sabroe SMC Mk 5, delivers proof positive,” Hattingh says.

Screw compressors offer a higher capacity output, larger compression ratios and smoother control to deliver energy savings in large applications. A later evolution of the compressor, they are considered more modern and, with fewer moving parts, maintenance is less of an issue.

Hattingh says that to make an optimum selection, the actual application conditions must be analysed. Relevant criteria, such as required capacity, operating conditions, part load, energy consumption, temperature levels, refrigerant, ease of maintenance, investment and available space should be taken into consideration.

One of the first questions to ask, he explains, is what are the temperatures and what is the size of the application? For air conditioning and other “high” temperature applications (evaporating temperatures higher than -15°C) reciprocating compressors will typically have 5-15% lower energy consumption than small screw compressors with capacities lower than approximately 1200kW. On the other hand, screw compressors will have lower energy consumption in larger applications and in lower temperature applications.

Another important issue is the load profile for the application. If the compressor has to run at part load for many hours a year, reciprocating compressors will be the optimum choice. Part load efficiencies are very different for the two compressor types. Typically, the relative energy consumption at 25% part load will increase by 35% for reciprocating compressors and by 75% for screws. In air conditioning applications the compressor will run at part load for most of the year.

The basic working principles of the two compressor types impact their ability to operate efficiently under off-design conditions. All reciprocating compressors will, by nature, automatically adjust to the actual evaporating and condensing pressures. The latest reciprocating compressor designs are optimised for variable speed with single-beam design, offering skip-free regulation over the entire speed range.

Small screw compressors and, in particular, air conditioning screw compressors are typically made with fixed internal volume ratios, which means that the gas will always be compressed at the same pressure ratio, no matter what the evaporating or condensing pressures in the plant are. This results in less energy efficiency for screw compressors running at off-design conditions. For air conditioning applications the compressor will run off-design 99% of the time according to ARI Standards. 

Large industrial screw compressors will typically be equipped with devices for adjustment of the internal volume ratio to ensure that efficient operation can be kept within a certain range around the design conditions.

Another question to ask, Hattingh says, is whether refrigerant is an issue. The choice of refrigerant and compressor type are in many cases inextricably linked (when comparing screw vs centrifugal compressors) as the ideal compressor type depends on the refrigerants specific volume and its latent heat of vaporisation. However when comparing a screw and a reciprocating compressor the refrigerant does not have much influence on the optimum choice of the compressor type. There will only be a difference for high-pressure refrigerants, such as R410A and R774 (CO2). Reciprocating compressors will typically have significantly lower energy consumption than screw compressors with these refrigerants. In addition, there’s a growing trend to use solutions that rely on ammonia refrigerant (as reciprocating compressors do), as it is considered a more naturally eco-safe option.

Hattingh adds that reciprocating compressors have more moving parts than screw compressors, making maintenance requirements more rigorous. The suction and discharge valves, in particular, have to be replaced frequently. That said, the maintenance work on reciprocating compressors is quite simple and can always be done on site. For screw compressors the challenge is that parts and assembly are exceptionally precise, and replacements and main overhauls usually require skilled OEM intervention.

“In general, the recips will typically incur about twice the maintenance costs of a screw but seen over a 40,000 – 50,000 hour operation period, the maintenance costs will even out and the recips will typically incur 20-30% higher maintenance costs. However, after the same number of operating hours the energy savings with a recips will typically be 3–5 times larger than the higher maintenance costs. In addition, with longer maintenance intervals on newer recips models, the maintenance numbers will improve,” he says.

He adds that screw compressors are much more compact than reciprocating compressors when we talk large capacities. “Two recips may typically be replaced by one screw compressor having the double capacity. That said, some recip vendors are offering a smaller footprint, lower cost solution."