By Russell Hattingh, Engineering Manager, Johnson Controls Systems & Service, South Africa
What would you choose for your air conditioning or refrigeration applications: screw or reciprocating compressors? It’s a decades-old debate and there’s no single answer to this question—it depends on the application, operating conditions and attitudes. New ‘recip’ advances may win these compressors a few more converts, however.
In South Africa, as elsewhere globally, there has been a move away from recip compressors in the HVAC industry for more than a decade, with scroll type chillers taking some market share. Demand for recips is still high for certain applications, such as cold stores and process cooling however. At Sabroe, producer of one of the best-known brands of reciprocating and screw compressors for over 100 years, sales of both types of compressors remain positive with sales in Europe and the Middle East indicating that this trend will continue. It’s backing its prediction with continued R&D and ongoing new releases of both recips and screw solutions.
The short answer it that each compressor type has its particular advantages, depending on the required operating conditions.
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.
Let’s take a look at the fundamental differences.
Reciprocating compressors — Recips are piston-driven compressors delivering stepped control with specific output per piston. They works well in high temperature applications, delivering energy savings. They typically use ammonia refrigerant, which is in demand as it is considered a more naturally eco-safe option. R&D by large manufacturers for recip 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.
Screw compressors—Screw compressors offer a higher capacity output, larger compression ratio’s 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.
What’s best for your application? Let’s dig deeper.
- What temperatures are you dealing with 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.
- What is the load profile? 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 recips and by 75% for screws. In air conditioning applications the compressor will run at part load for most of the year.
- Will the equipment need to operate in off-design conditions? The basic working principles of the two compressor types impact their ability to operate efficiently under off-design conditions.
- All recips will, by nature, automatically adjust to the actual evaporating and condensing pressures. The latest recip 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.
- Is refrigerant 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 recips do), as it is considered a more naturally eco-safe option.
- Will maintenance be a challenge? Reciprocating compressors have more moving parts than screw compressors, maintenance requirements are more rigorous. The suction and discharge valves, in particular, have to be replaced frequently. That said, the maintenance work on recips is quite simple and can always be done on site. In addition, recip vendors are working hard to address this. A key feature of Sabroe SMC Mk 5 is the extended service length (12,500 intervals) and the service life (25%) of the compressor over its entire operating range.
For screw compressors the challenge is that parts and assembly are exceptionally precise, and replacements and main overhauls usually require skilled OEM intervention.What’s the bottom line here? 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.
- Is space a challenge? 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 that is configured without oil separator.
Which solution is best for your application? Johnson Controls produces both screw compressors and reciprocating compressors and we are committed to enhancing the performance of both. We know that there is an optimum choice for every application for our customers. We hope this information will bring you a little closer to making the best choice for you.
About Johnson Controls
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