Which compressor type to use – screw or reciprocating?
Whether to use screw or reciprocating compressors in air conditioning or refrigeration applications is a decades-old question with no simple answer.
The fact is, the correct choice depends on a range of factors such as application and operating conditions.
Advances in reciprocating (or “recip”) compressors, however, may well add weight to the argument in their favour.
Although demand for reciprocating is still high for applications such as cold stores and process cooling, there has been a global move away from reciprocating compressors in the HVAC industry for more than a decade, with scroll-type chillers gaining market share.
To make the right selection, the actual application conditions must be analysed and certain criteria be considered. These include required capacity; operating conditions; part load; energy consumption; temperature levels; refrigerant; ease of maintenance and available space.
Fig. 1: Reciprocating compressor.
Reciprocating vs screw compressors: main differences
Reciprocating compressors
Reciprocating compressors are piston-driven and deliver stepped control with specific outputs per piston. They work well in applications under high temperature and are energy-efficient. They typically use ammonia refrigerant, which is in high demand as it is considered a more eco-safe option. Research and development by major manufacturers of reciprocating compressors continue to deliver enhancements and new features that keep these compressors “evergreen”.
Screw compressors
Screw compressors offer higher capacity output; larger compression ratios and smoother control to deliver energy savings in large applications. As an evolution of the compressor, they are more modern and, with fewer moving parts, require less maintenance.
Factors to consider
Temperature, application size
For air conditioning and other “high”-temperature applications (evaporating temperatures higher than -15°C) reciprocating compressors will typically have between 5 and 15% lower energy consumption than small screw compressors with capacities lower than around 1200 kW. On the other hand, screw compressors will have lower energy consumption in larger applications and in lower temperature applications.
Load profile
The load profile for the application is another important criterion. If the compressor has to run at part load for many hours per year, reciprocating compressors will be the right 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 and by 75% for screw-types.
Compressors in air-conditioning applications will run at part load for the greater part of the year.
Off-design conditions
The basic working principles of the respective compressor types impact their ability to operate efficiently under off-design conditions. Reciprocating compressors will 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 particularly air conditioning screw compressors are typically made with fixed internal volume ratios, which means the gas will always be compressed at the same pressure ratio regardless of the evaporating or condensing pressures in the plant. This results in reduced energy efficiency for screw compressors running in off-design conditions. According to ARI standards, compressors in air conditioning applications will run off-design for 99% of their lifetime.
Large industrial screw compressors are typically equipped with devices which adjust the internal volume ratio to ensure that operation is kept within the design conditions.
Refrigerants
When comparing screw and centrifugal compressors, the choice of refrigerant often depends on the compressor type as the ideal compressor for the application depends on the refrigerant’s specific volume and its latent heat of vaporisation.
Fig. 2: Screw compressor.
When comparing screw with reciprocating compressors, however, the refrigerant does not have much influence on the correct choice of compressor type. The only difference lies with high-pressure refrigerants such as R410A and R774 (CO2). With these refrigerants, reciprocating compressors typically have significantly lower energy consumption than screw compressors. There is also a growing trend to use solutions which rely on ammonia as a refrigerant (as do reciprocating compressors) because this refrigerant is considered more eco-safe.
Maintenance
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, maintenance work on reciprocating compressors is quite simple and can always be done on site. Vendors are currently addressing the higher maintenance requirement of reciprocal compressors and models such as Sabroe’s SMC Mk 5 have extended service lengths of 12 500 intervals and service lifetimes extended by 25%.
The challenge for screw compressors is that assembly is exceptionally detailed and replacements and main overhauls usually require skilled OEM intervention.
Generally, reciprocating compressors typically incur about twice the maintenance costs screw-types do but, over a 40 000 – 50 000-hour operation period, these expenses are recovered to the point where maintenance for recip compressors amounts to between 20 and 30% that of screw types.
However, after the same number of operating hours, the energy savings incurred with reciprocating compressors are typically three to five times greater than the higher maintenance costs.
Footprint
Large-capacity screw compressors are much more compact than reciprocating compressors, two of which generally occupy the same space as a single screw compressor with double the capacity. That said, some reciprocating-type vendors offer smaller-footprint, lower-cost solutions configured without oil separators.
Contact Russell Hattingh, Johnson Controls, Tel 011 921-7129, russell.hattingh@jci.com
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