Refrigeration Compressors Decoded

A refrigerator is by far one of the most remarkable inventions in the history of humanity. The process of refrigeration has not just improved the longevity of our food items, but has facilitated storage, preservation and availability of food throughout the year. It has made sure no fruit or vegetable really goes out of season, and can be accessed as and when the need arises. Given that refrigeration lowers the reproduction rate of bacteria, the rate of spoilage drops down drastically.

Working Process of a refrigeration compressor

Having started out as an ice-making machine in the late 1800s, the mechanics of refrigeration has grown by leaps and bounds over the years. One of the most important principles of refrigeration is that of vapour-compression. It involves absorbing heat from one space, removing it and subsequently cooling that space, eventually rejecting the heat elsewhere. Refrigeration compressor thrives on four components: a compressor, a condenser, a thermal expansion valve and an evaporator.

The refrigerant enters the compressor, is compressed to a higher pressure, which raises its temperature. This hot, compressed vapour reaches a temperature and pressure at which it can be condensed with either cooling water or air. The hot vapour is then made to pass through a condenser, where it is cooled and condensed into a liquid after putting it through a coil or tubes, with cool water/air flowing across. This is where system heat is rejected by the circulating refrigerant, which is lugged away either by water or air.

Different Types of Refrigeration Compressors

There are many types of compressors used for refrigeration. They are divided in either the positive displacement or non-positive displacement category. Positive displacement compressors house chambers that reduce in volume when compressed; the non-positive compressors, on the other hand, have fixed-volume chambers.

The five main compressor types are piston, screw, rotary, centrifugal and scroll.

Piston: It uses a piston and cylinder arrangement to provide compressive force. The reciprocating motion of the piston due to external power compresses the refrigerant inside the cylinder.

Screw: This uses a helical screw or rotor pair, which mesh together to compress the refrigerant between them. It produces high pressure for a small quantity of gas and consumes less power.

Rotary: It has two rotating elements, like gears, between which the refrigerant is compressed. Rotary compressors are considered to be the most efficient.

Centrifugal: This uses the rotating action of an impeller wheel to exert centrifugal force on refrigerants inside a round chamber.

Scroll: It is a quiet, smooth-operating unit with few moving parts and the highest efficiency ratio of all compressor types. It is also more flexible for handling refrigerants in the liquid.

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Hydrocarbons - Definition, classes and chain lengths

A hydrocarbon is any chemical compound that is made of just carbon atoms and hydrogen atoms. They are obtained from crude oil by fractional distillation and can be classified in different groups. The four general classes of hydrocarbons are alkanes, alkenes, alkynes and aromatics. The first two groups are the most commonly known as they are used to obtain regular petrol or natural gas and in the production of plastic respectively.

Crude oil is a mixture of hydrocarbons with different carbon chain lengths. The length of these chains will have an effect in the ways how hydrocarbons can be obtained. Smaller hydrocarbons have a low boiling point and are quite volatile, making them ideal for ignition and they are not as useful. In order to obtain shorter chains, long hydrocarbon chains need to be cracked.

Cracking methods

There are two different methods to obtain shorter hydrocarbon chains:

Through thermal cracking. Hydrocarbon thermal cracking involves using high temperatures and low pressure. Here, long hydrocarbon chains are heated to temperatures up to 750 degrees Celsius and 70 atmospheres.

Catalytic cracking. This method is widely used by industries and it involves heating hydrocarbons at a low temperature, usually of 500 degrees Celsius and are passed over a zeolitic catalyst. This catalyst breaks the long chains and it provides a useful hydrocarbon chain.

Transporting short-chain hydrocarbons

As explained above, cracking methods allow for shorter hydrocarbons to be formed. Once these shorter chains have been obtained, due to their volatility, they need to be pressurised regularly, specially if the end product is natural gas. Compressors play an essential role in keeping this hydrocarbons pressurised as they will be transported from compressor stations to other locations.

A compressor stations is, therefore, a site where natural gas or hydrocarbons with very short chains are pressurised and transported. In order to do so, there must be compressor stations every 40 to 100 miles of distance. This will vary depending on terrain and the of wells in the vicinity. There are several types of compressors such as centrifugal compressors, reciprocating compressors, or diaphragm compressors.

The energy used to move a compressor is obtained from two sources:

- natural gas
- electric power

When the source is natural gas, it is the pipeline itself that provides it to the compressor. When it is electric power, this usually is provided by a nearby and highly reliable source. Moreover, this type of source may still require an air permit.

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The Variable Frequency Drive - Why it works for industry

Variable frequency drive technology (VFD) has been developed to allow speed and torque control of an AC motor. This is achieved by connecting a variable frequency controller to an AC motor. Three phase induction motors are the most common though it is possible to connect it a single phase unit. In simple terms the controller converts AC to DC and then back to 'quasi-sinusoidal' AC power. This allows both the speed and torque of the motor to be controlled by varying the voltage and frequency.

VFD uses and benefits

Initially, the variable frequency drive was developed to give control of speed in various industrial processes, and to avoid the power surge associated with starting a single speed AC motor. However the variable frequency drive has found another purpose in compressor systems used throughout industry to provide a high pressure air supply. It has been calculated that 10% to 12% of power produced in the UK goes on creating compressed air. An air compressor that utilises variable frequency drive technology is able to switch speeds automatically, dependent on demand. This amounts to a real saving in compressor life as well as energy costs. With air compressors demanding such a significant amount of energy, it is easy to see why any potential reduction is of value to industry. However, this is not always the case.

Variable Frequency Drive costs

The main advantage of utilising a variable frequency drive is in the name It ability to vary frequency and voltage, therefore speed. However, the drive unit is an expensive addition to a system and where, for example, the compressor is working at a high percentage of its capacity on a more or less permanent basis, there is little gained by the expense of having a variable speed capability. The cost of purchase and installation needs to be weighed against potential energy savings.

The end analysis

The concept and application of the variable frequency drive has found uses through industry in machine processes' allowing smooth start up sequences, variable speed control for different processes on the same equipment, control over production rate and generally, a more accurate degree of control over a myriad of industrial processes. With the high levels of energy required to power an air compressor, it could be argued that this singular process has most benefitted from the development of modern variable frequency drive systems; bringing both control and efficiency to industry.

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The Usefulness of Commercial Refrigeration

Commercial refrigeration is used in a range of industries and, as such, there is a variety of refrigeration choices available. Industries that often use compressor refrigeration include:

  • Catering
  • Food Processing
  • Food Services
  • Retail
  • Transport
  • Oil

Depending on the industry, the type of commercial refrigeration used will vary in size and form. For instance, supermarkets require not only standard refrigeration but also cold displays. In large-scale factories, the refrigeration requirements often extend to keeping cool large open areas such as for preserving the freshness of meat.

The Wide Use of the Compressor in Refrigeration

No matter the size or form of refrigeration, however, it is likely that most types of refrigeration use some type of compressor system. To refrigerate an enclosed space, a refrigerant is compressed and decompressed in a compressor to draw heat out from an enclosed area. This method is widely used for domestic chillers also.

The Types of Commercial Refrigeration

Regarding the larger types of refrigeration, the walk-in freezer is a particularly common option, which can store large quantities of stock in an organised manner while allowing workers to enter and navigate easily. Stock such as food for a restaurant can be kept at optimum temperatures for as long as necessary, whether frozen or chilled. These compressor units are designed from materials that allow easy access while retaining cold air. On a larger scale, warehouses and factories will often use commercial refrigeration to chill large rooms.

Commercial refrigeration is also seen on the small scale in the form of displays in shops and supermarkets. Used to keep drinks chilled or food frozen, they can come in the form of open displays or closed displays with a sliding door. Closed varieties usually keep stock frozen and have a glass display window. For chilled food, an open display offers an easily-accessible option. While the open nature of this type of display may seem inefficient, a lot of the cold air used to chill the produce will sink to the bottom of the unit.

Businesses that Use Commercial Refrigeration

Commercial refrigeration is used in many types of businesses. Restaurants and food stores are widely known to use refrigeration, but florists do also. To prolong the shelf life of their flowers, many florists use refrigerated flower displays.

In the food production industry, refrigeration is a vital tool for maintaining the freshness of products. Factories will often have different areas for specific purposes (distribution, processing, etc.) and, therefore, will refrigerate at different temperatures using various compressor methods.

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