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By Ntsako Khosa
Compressors play a crucial role in ensuring an HVAC unit works effectively.
An air conditioner compressor is used to remove the heat-laden vapour refrigerant from the evaporator of the air conditioning system. In laymen’s terms, the compressor compresses or squeezes the vapour into a smaller volume at a higher temperature.
The external-drive compressor has a crankshaft that is driven by a pulley and belt system. An electric motor can also be used to drive it directly. The hermetic compressor has a motor which is sealed inside a housing with the compressor, hence a crankshaft seal is not required. The motor rotor is located either at the top or bottom of the unit.
“Selecting the correct compressor for an application will depend entirely on the design application criteria, however, the type of compressor may vary between residential, commercial and industrial applications,” says Alistair Bell, Technical Sales Engineer at Danfoss.
There are five types of air conditioner compressor that are commonly used in the HVAC industry as Bell explains:
- Reciprocating: The most commonly used can be used in appliances, residential A/C, light commercial A/C, commercial A/C, refrigeration, and industrial applications
- Rotary: Can be used in appliances and residential A/C units.
- Scroll: Can be used in residential, light commercial A/C, commercial A/C, and refrigeration applications.
- Screw: Can be used in commercial A/C, refrigeration, and industrial applications.
- Centrifugal: Can be used in commercial A/C systems.
The reliability of the system is drastically improved with minimum shutdown time.The reliability of the system is drastically improved with minimum shutdown time. Image credit: IMD
“Compressor technology has undergone significant developments in the last hundred years. Almost all the compressors mentioned in this article have reached a high level of perfection. Today, different compressors are available for different applications, starting from small hermetic reciprocating and rotary compressors for domestic refrigerators to very large screw and centrifugal compressors for huge industrial and commercial refrigeration and air conditioning applications. However, development is a never-ending process, and efforts are going on to develop more efficient compact, reliable and quiet compressors. New types such as linear compressors, trochoidal compressors, acoustic compressors, digital scroll, inverter compressors and oil less compressors are being introduced in refrigeration and air conditioning applications,” says Bell.
Reciprocating air conditioner compressor
The reciprocating compressor uses a piston driven by a crankshaft in a straight line back and forth motion to compress the refrigerant. This rotary motion is achieved using an electric motor and the construction is quite similar to that of an automobile engine.
The piston moves up and down inside a cylinder. Vapour from the suction line is moved through the intake valve as the pistons move downward. As the piston moves upward, it compresses the vapour refrigerant which is then pushed through the exhaust valve into the condenser.
The compressor may have more than one cylinder which is also known as a multi-cylinder compressor. The common ones are the two-cylinder, four-cylinder and eight-cylinder compressors.
The compressor is available in semi hermetic and hermetic. Semihermetic reciprocating compressors are used for commercial and industrial refrigeration applications, while hermetic reciprocating compressors are used for residential refrigeration and commercial refrigeration applications.
“These are the most widely used in the commercial refrigeration sector. A ‘hermetic’ compressor essentially means that the housing has been sealed airtight,” adds Bell.
Scroll
A scroll compressor has one fixed scroll which remains stationary and another moving or orbiting scroll that rotates through the use of a swing link. When this happens, the pockets of refrigerant gas between the two scrolls are slowly pushed to the centre of the two scrolls causing the reduction of the volume of the gas. It is then discharged through the centre port to the condenser.
Additionally, most of the compressors used for commercial and industrial applications now come with sophisticated electronics.
The advantage of a scroll compressor is that it has fewer moving parts and less torque variation compared to the reciprocating compressor. This advantage is translated to a smooth and quiet operation.
The scroll compressor is also known as a scroll pump or scroll vacuum pump. Scroll compressors are mainly used for residential and commercial air conditioning as well as commercial refrigeration applications.
Screw
The screw compressor uses a pair of helical rotors where it traps and compresses the refrigerant gas as the rotors revolve in a cylinder. In HVAC, they are usually used in systems with 20-ton capacity and above. The male rotor and the female rotor are built inside the cylinder. The low-pressure refrigerant enters one end of the compressor and the resultant high-pressure refrigerant is discharged from the opposite end to the condenser.
Screw Compressors are used for larger industrial refrigeration systems and medium capacity air conditioning / process cooling chillers.
Rotary
The rotary compressor can be divided into two types. One has blades or vanes that rotate with the shaft. The other type has the blade which remains stationary and is part of the compressor housing assembly. In both types, the vapour from the suction line is drawn into the cylinder through the suction port.
Rotary vane compressors consist of a cylindrical casing, two openings – one suction and one discharge – and a rotor positioned eccentrically with respect to the casing. Compression occurs by refrigerant flowing into the chamber where, due to eccentric rotation, there is a reduction in the desired volume.
As the blade rotates, trapped vapour in the space ahead of the blade is compressed into high pressure gas after which it is discharged to the condenser through the exhaust port. The number of blades can range from two to eight in a single system. These are mainly found in residential air conditioning applications.
Centrifugal
Centrifugal compressors are usually used in large capacity refrigerating systems. In this type of compressor, the refrigerant vapour or gas is moved in a circular motion known as centrifugal force. An impeller which is a disk with radial blades spins rapidly inside this housing causing the gas to gain velocity.
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.
A compressor is used in different applications. Image credit: IMD
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 air-conditioners 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 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.”
Compressor technology has undergone significant developments in the last hundred years. Image credit: Danfoss
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 investment while selecting the compressor,” advises Kannan.
From an engineer’s perspective, he recommends that the engineer needs to observe the operating parameters like suction pressure, discharge pressure, suction temperature, discharge temperature, superheat, subcooling and power consumption. Feed these parameters to the software to compare the capacity and the power consumption. He needs to also look at the possibilities of increasing the capacity by changing these parameters. He needs to also observe vibration, abnormal noise, oil level in the compressor sight glass and the net oil pressure to understand the health of the compressor.
“Proper usage of mounting grommets and providing discharge / suction loops/vibrobsorbers are necessary to avoid leakages, reduce sound and vibration from the condensing unit.
The cleanliness of the system plays vital role for the reliability of the compressor and the system. All the contaminants like air, moisture and other solid contaminants should be completely removed from the system during the installation process. Also, there is a need to ensure that refrigerant piping recommendations are followed for the proper oil return,” he adds.
How to get the most out of your compressor:
- Application
- Refrigerant
- Capacity required at operating conditions
- Power supply
- Modulation requirements
- Always follow manufacturer’s recommendations.
- Use only authorised refrigerant and oil for the given compressor.
Source: Emerson
References:
- Airconditioning Systems
- Contributions from Emerson and Danfoss
