From hermetic to open cold compressors
Based on their structure/technology, cold compressors are divided into hermetic, semi-hermetic and open compressors. The compressor is one of the main components of a refrigeration system. The other ones are the evaporator, condenser and expansion device, which is usually an expansion valve. Due to their mechanical nature, compressors require oil lubrication to reduce friction during operation. Read more about oils >>
Cold compressors operating principle
The compressor sucks in vaporised refrigerant and compresses it into a higher pressure. This process increases the temperature of the refrigerant. The vapour is then led into the condenser.
In other words, the purpose of the device is to increase the pressure of the refrigerant to make it transition from the evaporation temperature to the condensation temperature. The pressure differential ensures that the refrigerant travels from the condenser into the plant’s evaporator.
The various compressors are among the most commonly used refrigeration technologies. In building cooling, up to 90% of the applications are based on compressor cooling. Climate protection has primarily steered the development of the refrigerants used in these technologies in the last few decades. (Source: VTT, 2016)
Cold compressors by type
- reciprocating compressors
- compressors with a rotating piston, most common: screw compressor
- rotary piston compressors: scroll (or spiral) compressor and rotary compressor
Cold compressors and motor cooling
The motors of certain cold compressors is cooled with vapour entering the compressor = suction gas cooled compressor. Non-suction gas cooled compressors are cooled with a fan.
Cold compressors and lubrication
Splash lubrication: in splash lubrication, the compressor’s crankpins strike the oil in the compressor crankcase, splashing it around. The oil is collected into a pocket, from which it is led into a hollow crankshaft that features openings. Through these openings, the oil is distributed to the crankpin and motor bearings.
Pressure lubrication: in pressure lubrication, the oil in the compressor crankcase is sucked through a sieve into the oil pump (gear pump), from which it is led into bores inside the crankshaft and, further, to the bearings.
A hermetic compressor consists of an electric motor and compressor, which are located inside a sealed and welded casing and supported by springs. This reduces motion-induced vibration. The electric motor’s shaft functions as the compressor’s crankshaft, which is installed on shaft bearings in an eccentric position. This creates the piston pumping motion in the cylinder as the motor rotates.
Hermetic compressors are used in household refrigerators, freezers, small refrigeration equipment in shops, cold rooms, air coolers, water cooling equipment and heat pumps.
When a compressor of this type fails, it is usually replaced with a new one instead of repairing it, which can also be done for semi-hermetic compressors (opening cover).
Semi-hermetic compressors differ from the above in that they feature an opening cover, which makes them easy to repair. These compressors are used in commercial refrigeration equipment as well as water cooling systems for air conditioning and process purposes.
In open compressors, the actuating power is transmitted by a shaft through the compressor casing via a shaft coupling or belt. Open compressors are used in the cooling units of car air conditioning systems, refrigeration systems of cargo spaces and large food industry refrigeration systems, for example.
Cold compressors and their operating principles in brief
The reciprocating compressor is a compressor in which a piston moves back and forth in a pumping motion (e.g. trunk piston compressor).
Reciprocating compressors remain competitive in many applications, such as cooling systems that utilise carbon dioxide. The power adjustment is often based on on-off adjustment. Frequency converters can be used.
Rotary piston compressor, screw compressor
An example of this is a screw compressor in which a piston moves back and forth in a cylinder with butterfly valves in the cylinder head. The intake valves are below the head and let refrigerant vapour into the cylinder whenever the piston moves down. When it returns up, the pressure valves above the head release the vapour from the cylinder.
Screw compressors are commonly used in air conditioning – particularly in higher power categories. Frequency converters can be used for power adjustment, but they rarely are. Slide adjustment is the most common adjustment method. However, frequency converter adjustment could significantly improve the energy efficiency ratio of partial equipment load.
In the scroll compressor, which is also known as the spiral compressor, the compression takes place between two scrolls. One of them is fixed, while the other is orbiting. In three cycles, the vapour between the scrolls is pressed from the suction side to the pressure side. The pressure cools the compressor’s electric motor either fully or partially – the feature is model-specific.
Scroll compressors boast superior efficiency and output compared to reciprocating compressors. Their other appealing attributes include silence and adjustability.
Scroll compressors have largely supplanted reciprocating compressors in low power category applications.
The figure shows a simplified illustration of the operating principle of a scroll compressor.
Turbocompressors are common in refrigeration systems of the highest power category (> 1 MW). The most common refrigerants in these applications are R134a in AC cooling and R717 (ammonia) in industrial applications. As regards newer refrigerants, solutions that use R1234ze (E) are also available.
The development of the technology has been significant, and improvements have been made with regard to magnetic bearings, for example; the turbocompressor shaft floats supported by magnetic bearings. This enables an oil-free compressor as well as an increase in speed/power. All other compressor types require oil to ensure flawless operation. The reduced friction and improved heat transfer have improved energy efficiency. The simplicity of the system is also a substantial benefit.
The magnetic bearing arrangement extends the compressor’s life span and maintenance frequency (no oil changes), and reduces noise and vibration. Turbocompressors with magnetic bearings are more expensive than corresponding screw compressors but the maintenance and energy costs are lower. The calculated payback period for these devices is less than five years.
VTT, Jäähdytyksen teknologiset ratkaisut, Laitinen, Rämä, Airaksinen, 2016
Want to know more?
Refrigeration technology of the future, magnetic cooling. Already available in refrigerators, for example. Watch a promotional video on the topic. More information on other refrigeration technologies. (The linked page is under construction, 21 February 2019)
General information on compressors
Digital scroll compressors
Ecological, economic and energy conservation goals are also mirrored by compressors. There are currently scroll compressors on the market with optimised operation for each individual application. The input power in these compressors is optimised at partial load. In other words, the general goal is to ensure that the compression power in each application is as precisely accurate as possible, without any excess.
These developments have created the following results:
- hot gas bypass with two compressors
- digital compressors
Digital compressors control the compression digitally (on/off). In practice, this is achieved by allowing the two scrolls pressing against each other to move apart slightly, as necessary, but only enough that the closed scroll compression chambers begin to leak at the seams in the rest state. This means that compression force is no longer required although the compressor continues rotation at full speed but without resistance.
By dividing the compressor operation into work cycles of a particular length, the compression cycle and rest cycle can be programmed accurately based on specific demand. Even though the compressor is always operational in the work and rest phases during compression, a calculated average of levels over the course of an hour, for example, yields the actual non-incremental compression action. In digital control, the output can be adjusted freely within a range of 10–100%.
Since refrigeration and heating technology is in constant development and commercial applications are improved to meet current requirements and customer demands, more and more manufacturers have come to offer their own commercially branded versions of the various technologies. When selecting a solution for your specific purpose, it is advisable to examine the input, heating and cooling power tables and the derived coefficients of performance (COP) in the normal operating situations of a component.
History of the compressor
The compressor was patented in the late 19th century, and its first purpose of use was to provide ventilation in coal mines.
Refrigeration unit and compressor – consumer information
Operating principle of a refrigeration unit:“The operation of a refrigeration unit is based on changing the state of the refrigerant circulated in the piping. The fluid absorbs heat from inside the device as it evaporates into gas in the evaporator. The gas then expels the heat outside the unit as it condenses into liquid in the condenser. The thermostat, i.e. the temperature controller, stops the compressor and reactivates at the set temperature." (Online source in Finnish: Helen, Kylmälaitteet.pdf)