Refrigeration compressors

The compressor, or refrigeration compressor, is the basic element of the entire refrigeration system. Thanks to it, the refrigerant can circulate in the system. The efficiency of the installation depends on the operation of the compressor. It affects every other element in it. In this entry, we have tried to take information that will explain in detail why this aggregate plays such a key role. We will also clarify issues regarding the selection of a compressor for a refrigeration system.

How a refrigeration compressor works

The compressor forces refrigerant through each of the other refrigeration components, including heat exchangers and expansion valves. As a refrigeration unit, however, it is used not only to move the medium, but above all to compress it, i.e. to increase the pressure. The compressor compresses the refrigerant when it is in a gaseous state. This device, by sucking in a low-temperature and low-pressure medium, compresses it to high pressure through the work done on it, while increasing its temperature. The type of gas transformation that takes place in the compressor is called polytorpe, i.e. a transformation in which the pressure, volume and temperature of the gas change. The specificity of this transformation says that with an increase in pressure and a decrease in volume, there will always be an increase in temperature.

Types of compressors and their properties

Each of the compressors has one, the same purpose - to increase the pressure of the refrigerant. However, different types of compressors achieve it in different ways. Below we have described how the compressors work, divided according to the principle of operation.

Positive displacement compressors are those whose operation is based on cyclic pumping of the refrigerant in a closed, shrinking working space. We divide them into: piston and rotary.

• Reciprocating compressors - this is the type of compressors that are most common in refrigeration. Their work is carried out in cycles and is carried out thanks to the movement of the piston. The movement begins with a suction cycle (piston moves down), as a result of which refrigerant gas enters the working space. When the entire working space is filled, the piston changes direction (moves up) and begins to compress the gas. When a certain compression pressure is reached, the discharge valve opens and discharge takes place. Then the piston returns to its previous position and the whole cycle starts again.
• Rotary compressors - these are refrigeration devices in which the working body does not perform reciprocating motion, but rotation. This is, for example, a vane compressor in which there is a shaft equipped with blades. It is placed in the cylinder, with the axis of the shaft and cylinder not coinciding. The shaft pumps the refrigerant through rotation, and the space in which it is located is reduced due to the misalignment of the components. There is a decrease in volume and an increase in pressure. Another example is scroll compressors where the refrigerant is compressed between a moving and a stationary scroll. We will also often meet with screw compressors. Their operation is based on the work of two screws that are located next to each other. On the inlet side, the space between their strands is large, while as the screws turn, the space decreases, compressing the refrigerant.

Flow compressors are those in which the refrigerant is accelerated in the inter-blade channels of the rotor, which causes its increase in kinetic energy (including speed). The increase in energy is converted into an increase in pressure by reducing the diameter of the cylinder. These types of compressors are used only in high-power devices, e.g. jet aircraft engines

Compressors for the refrigeration industry

Positive displacement compressors are the most common in refrigeration. They are used in the refrigeration industry practically from the beginning of its existence. Thanks to the simple structure and simple principle of operation, they are cheaper, and at the same time they are characterized by high efficiency. These are compressors adapted to work with many refrigerants, and the availability of many variants of such compressors guarantees the possibility of matching them to any refrigeration system, regardless of its size or the requirements it poses. Due to the uncomplicated design, operating costs are lower, and in the event of a failure - service costs. These devices can be used as compressors for refrigerated counters, refrigerators, freezers, ice cube makers and much more.

Hermetic, semi-hermetic and open compressors

In addition, there are three types of construction among reciprocating compressors, which determine the way of connecting their engine with the working body (piston, compressor) and accessibility to other components of the refrigeration unit:

• Hermetic compressors have a welded housing in which both the motor and the compressor are located. This type of construction of the compressor brings advantages such as quiet operation and high tightness, which effectively blocks dirt from getting inside. They are compact devices, which makes their installation in the system very easy. An example of such a compressor is the hermetic compressor Comptek MAQQD142UHW available in our offer with a displacement of 14.2 cm3. At the same time, the repair of a hermetic compressor can be problematic and such work is generally not performed, as the compressor housing would have to be cut open.
• Open compressors are those in which the piston and the motor are connected by a coupling element, they have an open structure, i.e. direct access to all components.
• Semi-hermetic compressors, on the other hand, are structurally similar to open compressors, and the difference is that the piston and the engine have a common crankshaft, the engine and compressor bodies are connected to each other, or the compressor housing is a single casting with a cover bolted to the engine side. The use of semi-hermetic compressors brings advantages such as: compact design, facilitating installation, low level of vibration during operation, well-distributed weight of the entire unit, guaranteeing a large balance, low value of pulsation during the outlet of the medium and long life of the device. An example of this type of compressor is the two-cylinder semi-hermatic compressor RFC-2DC-3.2Y, generating a flow of 13.4 m3/h.

Refrigeration compressor service

A very important operating element is the lubrication of the compressor. The oil in the installation is to ensure good lubrication of all rotating and sliding elements, and it should also dissipate heat from thermally loaded elements. Oil and refrigerants are mutually soluble, so oil should be continuously fed back to the compressor from the refrigerant. For this purpose, e.g. appropriate inclination of the lines, or oil separators. Blue Refrigeration's offer includes a wide selection of oil separators that will perfectly ensure the proper operation of the refrigeration compressor. For example, there is a hermetic oil separator BLR/A-W55833 (3/8”ODF) adapted to work in temperatures from -10oC to -130oC, with a working pressure of 33 bar.

How to choose a refrigeration compressor

When selecting a compressor for a refrigeration system, we may encounter various parameters describing their operation. First of all, these are the way of operation (e.g. reciprocating) and the construction of the compressor (e.g. hermetic). Each technical specification of a refrigeration compressor will also have a lot of additional information. A list of the most important parameters to consider when choosing a compressor can be found below (discussed based on reciprocating compressors):

• compressor displacement [cm3] tells us about the volume of the cylinder in which the piston moves, and more precisely about the volume created in the cylinder between the top dead center position of the piston and its bottom dead center position. The higher this value is, the more refrigerant will be compressed to the set pressure in a given time interval
• volumetric efficiency of the compressor [m3/h] allows us to visualize how many cubic meters of refrigerant will be compressed during 1 hour of operation
• power [HP] is the power of the compressor given in horsepower (Horse Power)
• cooling [S or F] – describes the type of cooling that our refrigeration unit must cover (compressors heat up during operation). We can meet with two methods: S - static Comptek LAFQD75HHP hermetic compressor and F - fan (fan) Comptek LAQQD158YHP hermetic compressor, sometimes the S/F method is also allowed, i.e. freedom is left here.
• static pressure [-] is the ratio of the discharge pressure to the suction pressure. This may be information for us about the specificity of the refrigeration unit's operation
• start [RSIR, RSCR, CSIR, CSR] – informs us about the method of starting and working of the compressor

In addition, important information is: what refrigerant the compressor works with and at what temperatures it can work. It is worth remembering that at different operating temperatures, compressors, like any device, have different capacities.