refrigetrator

Parts of a Refrigerator

If you pour a little rubbing alcohol on your skin, it'll feel cold -- really cold. It isn't refrigerated, so how does this happen? Well, alcohol evaporates at room temperature the way water evaporates at a low temperature in an oven. As it evaporates, it absorbs the heat on the surface of your skin, making your skin cooler. A special coolant called a refrigerant functions in a refrigerator the way alcohol works on your skin, except in a refrigerator, the coolant is trapped inside a series of coils. As it makes a circuit through them, it changes back and forth from a liquid to a gas.

To pull off this frosty feat, a refrigerator uses five major components:

• Compressor
• Heat-exchanging pipes (serpentine or coiled set of pipes outside the unit)
• Expansion valve
• Heat-exchanging pipes (serpentine or coiled set of pipes inside the unit)
• Refrigerant (liquid that evaporates inside the refrigerator to create the cold temperatures)

         

The Basic Principle

Basic principle of refrigeration is simple. Just pass a colder liquid continuously around the object which is to be cooled. This will take heat out from the object. In the example shown below a cold liquid is made to pass over an Apple, which is to be cooled. Due to the temperature difference apple rejects heat to the refrigerant liquid. The refrigerant is in turn gets heated up due to heat absorption.

Fig.1 Basic principle of refrigeration is illustrated in this figure

The whole refrigeration technology is after this, how to produce cold liquid continuously? .We will see how this is achieved in the following sections.

Components of Refrigerator & Working

An inside view of the refrigerator is shown below.

Fig.2 An inside view of a refrigerator

It has got 4 main components; compressor, condenser, evaporator and throttling device. Of these throttling device is the one which is responsible for the production of cold liquid. So we will first analyze throttling device in a detailed way and move to the other components.

Throttling Device

Throttling device is an obstruction to the flow; cold liquid is produced with help of this device. In this case throttling device is a capillary tube. Capillary tube has got an approximate length of 2m and inside diameter of around 0.6 mm. So it offers a great resistance to the flow.

Fig.3 A Capillary tube: This results in sudden drop in pressure and temperature

For effective throttling at inlet refrigerant should be high pressure liquid. Throttling device is an obstruction to the flow, which causes tremendous pressure drop. Due to drop in pressure boiling point of refrigerant comes down, and it starts evaporating. Heat required for evaporation comes within the refrigerant, so its temperature drops. If you check temperature across throttling device, you can notice this drop.

It is wrong to say that throttling is a process. Because we know only the end points of throttling , that is states before and after throttling. We don’t know what are the states in between, since this is a highly irreversible change. So it would be correct to call throttling as a phenomenon.

Evaporator - Heat Absorption Process

Next phase is simple; just pass this cold liquid over the body where you have to absorb the heat. During the heat absorption process refrigerant further evaporates and transforms into pure vapor. A proper heat exchanger is required to carry the cold refrigerant over the body. This heat exchanger is known as evaporator.

Fig.4 Cold liquid is passed through a heat exchanger know as evaporator for absorbing heat from the refrigerator

So we have produced the required refrigeration effect. If we can take this low pressure vapor refrigerant to the state before throttling process, that is high pressure liquid state; we will be able to repeat this process. So first step, let’s raise the pressure.

Compressor

A compressor is introduced for this purpose. Compressor will raise the pressure back to its initial value. But since it is compressing gas, along with pressure, temperature will also raise. It is unavoidable.

Fig.5 A compressor is used to raise pressure of the refrigerant

Now the refrigerant is high pressure vapor. To convert it to liquid state we are introducing another heat exchanger.

Condenser

This heat exchanger is fitted outside refrigerator and refrigerant temperature is above atmospheric. So it will liberate heat to the surroundings. Vapor will get condensed to liquid and temperature will come down to normal level.

Fig.6 Condenser heat exchanger is fitted outside the refrigerator so it will reject heat to the surroundings

So the refrigerant is back to its initial state again, high pressure liquid. We can repeat this cycle over and over for continuous refrigeration. This cycle is known as vapor compression cycle. Refrigeration technology based on vapor compression cycle is the most widely used one in domestic and industrial applications.

Refrigeration Accessories

You can note more details of refrigerator components here. Evaporator and condenser are having fins attached to it. Fins will increase contact area available for convective heat transfer and thus will enhance heat transfer by a great extent.

Fig.7 Fins attached to the condenser and evaporator

Since evaporator is cooling the surrounding air, it is common that water will get condensed over it forming frost. Frost so formed will act as an insulator between evaporator heat exchanger and the surrounding air. Thus it will reduce effectiveness of heat removal process. Frequent removal of frost is required to enhance the heat transfer. An automatic defrosting mechanism is employed in all modern refrigerators.

More on Compressor

Apart from raising pressure compressor also helps in maintaining flow in the refrigerant circuit. Usually hermetically sealed reciprocating type with proper sealing is used for this purpose. You might have noticed that refrigeration technology based on vapor compression cycle is a highly energy intensive affair. The reason is that compressing gas and raising pressure is a highly energy intensive process.

Coefficient of Performance

Heat and power transfer happening in a vapor compression refrigeration circuit is shown below.

Fig.8 Energy interaction happening in a refrigeration system

A simple energy balance of the system yields in following relationship.

It is often required to evaluate performance of a refrigerator or compare between different refrigeration technologies. A term called Coefficient of Performance (C.O.P) helps in doing this. To understand this term completely we need to know what is input and output for a refrigeration system. What we need from a refrigerator is the cooling effect. Or Q_ABSORBED is the output of refrigeration cycle. Input to the refrigerator is the power given to the compressor. So the term C.O.P can easily be defined as output by input and is as follows.