Cold Link Africa July/August 2019 - Page 42

FEATURE INCORPORATING COLD CHAIN modern and, with fewer moving parts, » » Large industrial screw compressors maintenance work on recips is quite automatically adjust to the actual will typically be equipped with simple and can always be done on evaporating and condensing devices for adjustment of the site. In addition, recip vendors are What’s best for your application? Let’s pressures. The latest recip internal volume ratio to ensure that working hard to address this. For screw dig deeper. compressor designs are optimised efficient operation can be kept compressors, the challenge is that • What temperatures are you dealing for variable speed with single-beam within a certain range around the parts and assembly are exceptionally with and what is the size of the design, offering skip-free regulation design conditions. precise, and replacements and main application? For air conditioning and over the entire speed range. maintenance is less of an issue. » » All recips will, by nature, • 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 about 1 200kW. 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 The short answer it that each compressor type has its particular advantages, depending on the required operating conditions. to run at part load for many hours a overhauls usually require skilled OEM of refrigerant and compressor intervention. type is in many cases inextricably What’s the bottom line here? In linked (when comparing screw vs general, the recips will typically incur centrifugal compressors), as the about twice the maintenance costs ideal compressor type depends on of a screw, but seen over a 40 000– the refrigerant’s specific volume 50 000-hour-operation period, the and its latent heat of vaporisation. maintenance costs will even out and However, when comparing a screw the recips will typically incur 20–30% and a reciprocating compressor, higher maintenance costs. However, the refrigerant does not have much after the same number of operating influence on the optimum choice of hours, the energy savings with a recips the compressor type. There will only will typically be three to five times be a difference for high-pressure larger than the higher maintenance refrigerants, such as R410A and R774 costs. In addition, with longer (CO 2 ). Reciprocating compressors maintenance intervals on newer will typically have significantly lower recips models, the maintenance be the optimum choice. Part-load particular air-conditioning screw energy consumption than screw numbers will improve. efficiencies are very different for the compressors are typically made compressors with these refrigerants. two compressor types. Typically, with fixed internal volume ratios, In addition, there’s a growing trend compressors are much more the relative energy consumption at which means that the gas will to use solutions that rely on ammonia compact than reciprocating 25% part load will increase by 35% always be compressed at the same refrigerant (as recips do), as it is compressors when we talk large for recips and by 75% for screws. In pressure ratio, no matter what the considered a more naturally eco- capacities. Two recips may air-conditioning applications, the evaporating or condensing safe option. typically be replaced by one screw compressor will run at part load for pressures in the plant are. This Will maintenance be a challenge? compressor having double the most of the year. results in less energy efficiency for Reciprocating compressors have capacity. That said, some recip  Will the equipment need to operate screw compressors running at more moving parts than screw vendors are offering a smaller in off-design conditions? The basic off-design conditions. For air- compressors, and maintenance footprint, lower-cost solution that is working principles of the two conditioning applications, the requirements are more rigorous. configured without oil separator. compressor types affect their ability compressor will run off-design for The suction and discharge to operate efficiently under off- 99% of the time, according to valves, in particular, have to be Which solution is best for your design conditions. standards.  replaced frequently. That said, the application? year, reciprocating compressors will •  Is refrigerant an issue? The choice 42 » » Small screw compressors and in • •  Is space a challenge? Screw CLA COLD LINK AFRICA • July/August 2019