RACA Journal August 2020 | Page 23

Feature Continued from page 19 A diffuser converts this energy into pressure energy and discharges into the condenser. The pumping efficiency increases with speed; hence this type of compressor is designed to operate at high speed. The main advantage of a centrifugal compressor is that there are no valves, pistons or cylinders. The wearing parts that need monitoring are the main bearings. Centrifugal compressors are available in larger capacities needed for larger air conditioning systems like district cooling, hotels, shopping malls, larger office complex, etc. “Digital Scroll and Semi hermetic compressors, Variable speed compressors are being considered by many end users,” says Vadivelan Kannan, director technical services at Emerson. CHOOSING THE RIGHT COMPRESSORS Engineers and HVAC contractors have a few options to choose from. Kannan says, “The application plays a major role in selecting the type of compressor. The type of compressor depends on the specific application and the capacity.” Kannan unpacks each application and type: Residential Application: Hermetic compressors are generally used for residential applications like refrigerators and airconditioners as the load variations and installation complications of these type of applications are limited enabling the use standard hermetic compressors. Commercial Application: Though Semi Hermetic compressors were popular in these applications due to their perceived repairability and reliability, of late, more and more customers are shifting to hermetic scroll compressors which have advantages of better efficiency, competitive prices, simplicity and higher reliability. Industrial Application: Depending on the capacity, many types of compressors are being used for this application. Screw compressors are used for larger capacity whereas Semi hermetic compressors are used for medium capacity and Hermetic Scroll compressors are being used for smaller capacity requirements. “Compressor capacity modulation has become popular as its saves lot of energy and at the same time reduces temperature fluctuation inside the conditioned space,” Kannan says. Additionally, most of the compressors used for commercial and industrial applications now come with sophisticated electronics which protects the compressor by using the compressor itself as a sensor. “This helps the service engineers to trouble shoot better and quicker. This even enables remote trouble shooting possibilities,” Kannan explains. Bell adds that “several design criteria would also need to be considered to ensure that a compressor runs optimally. These IMD include the compressor selection, refrigerant type, system components and evaporator and condenser selection. An engineer would be looking at information like the isentropic efficiency and volumetric efficiency of a refrigeration compressor which are very important for system operations including the COP.” Additionally, most of the compressors used for commercial and industrial applications now come with sophisticated electronics. ENSURING LONGEVITY “Matching of system components with correct operating condition are essential to get the maximum capacity and efficiency from the compressors. Periodic maintenance of heat exchangers is very critical for maintaining the capacity and efficiency of the compressors and the system. Customers should consider total lifetime cost rather than focusing on initial Continued on page 23 The reliability of the system is drastically improved with minimum shutdown time. www.hvacronline.co.za RACA Journal I August 2020 21