Refrigeration and Air Conditioning

Refrigeration and air-conditioning has become necessary in every field of human utility in houses, schools, cinema houses, restaurants, cars, buses, railways. It is also used in industries for various processes. Basically it is employed for the comfort of human beings in domestic appliances and industrial equipment. In the industry it is employed to maintain the temperature of factories. working place so that workers can do their duties efficiently. It is also used for producing ice, air conditioning of buildings, coaches and milk vanes.

REFRIGERATION

Some Commonly Used Terms :

1. Refrigeration: It is the process of removing heat from substances under controlled conditions. It also includes the process of reducing and maintaining the temperature of the body below the general temperature of its surroundings.

2. Refrigerator: This is a machine by which the temperature of substances is lowered by storing them in this machine.

3. Refrigerator system: The mechanism used for producing low temperature in a body whose temperature is already below the surroundings temperature is called a refrigeration system.

4. Air Conditioning: It involves the control of the temperature about 25°C and 50% humidity and motion of air in an enclosure. Air conditioner consists of filters for removing lint and dust and also having equipment for removing odours etc.

5. Refrigerant: It is a substance (generally a gas) which is circulated in a refrigeration system. It absorbs heat from a cold body and delivers it to a hot body. Freon, ammonia carbon dioxide, sulpher dioxide are the commonly used refrigerants.

Refrigerating Effect

The heat carrying ability of the refrigerant is called the refrigerating effect. The refrigerating fect may be found by determining the amount of heat that one kg of the refrigerant is capable of absorbing as it passes through the evaporator.

Properties of Refrigerants

The refrigerants used should possess the following properties:

1. It should be non-corrosive.

2. It should be non-inflammable,

3. It should be non-toxic.

4. It should be free from objectionable odour.

5. It should get liquified at a reasonable low pressure. 

6. Economical in cost in the quantity to be used and easy availability.

7. No action with lubricating oil.

Refrigeration Cycle

In order to lower the temperature of a body below that of its surroundings and then maintain it, a continuous extraction of the heat from the body is required. This is done by a mechanism called the refrigeration system. The total amount of heat being rejected to the outside body consists of two parts i.e. heat extracted from the body to be cooled and the heat equivalent to the mechanical work required for extracting it.

The refrigeration cycle starts from the evaporator inlet (1) in which low pressure liquid undergoes expansion, absorbs hent and evaporator changing to a low pressure vapour at the outlet of the evaporation (2). During the process of expansion, about 20-25% of the refrigerant changes to vapour.

The compressor (3) sucks this vapour from the evaporator and increases in pressure. It is then discharged to the condenser (4) where the heat is removed from the vapour and condensed into liquid. Now the refrigerant moves into the drier, which prevents the plugging of the flow control device by dirt and moisture etc., and then refrigerant flow into the evaporator continues and this process continues till a temperature control device i.e., thermostat stops the compressor. When the temperature again increases, the compressor starts once again and the cycle continued.

Refrigeration System

There are three refrigeration systems:

(i) Vapour compression refrigeration system. 

(ii) Vapour absorption refrigeration system.

(iii) Thermo-electric refrigeration system.

Vapour Compression Refrigeration System

This is the most popular system of refrigeration employed in domestic and industrial refrigeration and air conditioning plants. It works on the principle that when liquid evaporates, it absorbs heat. Once the refrigerant is charged in the system, the same refrigerant is used again and again.

The vapour compression cycle is chiefly composed of four stages i.e., compression, condensation, expansion and vaporisation. The purpose of the refrigeration cycle is to remove heat from refrigerant by again putting it into liquid form so that it may be used again and again.

The four main stages of vapour compression cycle are discussed as below:

(i) Vapourisation: From the expansion valve, the low pressure refrigerant at low temperature enters the evaporator which is placed near or around the material to be cooled down. Before entering the evaporator, the refrigerant is in the liquid state and when it absorbs heat from the material to be refrigerated it is transformed from a liquid to a vapour state. The process is called evaporation.

(ii) Compression: The compressor draws the vapours from the evaporator through the return piping and then compresses these vapours until their temperature is raised above that of the condensing medium. The compressor is driven by external mechanical energy which may be supplied by an engine or electric motor. Generally, electric motors are employed for this purpose. This process is called compression.

(iii) Condensation: When the temperature of the vapour is raised above that of the condensing medium by the compressor, then the heat of vaporization will flow from the vapour to the condensing medium to condense the refrigerant to a high pressure liquid. This high pressure liquid then flows to the receiver where it is stored until it is supplied to the cooling unit through the expansion valve. This process is called condensation.

(iv) Expansion: As the compressor withdraws the refrigerant vapour from the evaporator. The cooling unit must be supplied with a low temperature, low pressure refrigerant capable of absorbing heat. This is done by a liquid control valve known as an expansion valve. This valve reduces the pressure of the high pressure liquid from the receiver to a low pressure liquid capable of absorbing heat. This also maintains a constant supply of liquid in the evaporator and acts as a dividing point between high and low pressure sides of the system. The process is called expansion.

Vapour Absorption Refrigeration System

It is one of the oldest methods of producing refrigerating effects. The refrigerant, commonly used in this system is ammonia.

In the vapour absorption refrigeration system, the compressor is replaced by an absorber, a pump, a generator and a pressure reducing valve. In addition to condenser, receiver, expansion valve and evaporator as in the vapour compression refrigeration system. In this system the vapour refrigerant from the evaporator is drawn into an absorber where it is absorbed by the weak solution of the refrigerant forming a strong solution.

This strong solution is pumped to the generator where it is heated by some external source. During the heating process, the vapour refrigerant is driven off by the solution and enters into the condenser where it is liquified. The liquified refrigerant then flow into the evaporator and thus the cycle is completed.

AIR CONDITIONER

Air conditioning may be defined as the production of an artificial atmosphere specially adapted to particular requirements or air-conditioning means control of temperature, humidity, purity and movement of air. The working conditions and comfort of human beings is improved by air conditioning. The humidity is maintained at 60% being most suitable for human beings. By air conditioning we heat the rooms in winter and cool them in summer. The air is also made dust free. The air conditioner is based on the principle of vapour compression refrigeration system.

The various parts of a room air conditioner are:

1. Compressor unit 

2. Condenser 

3. Receiver

4. Evaporator

5. Capillary tube

6. Impeller fan

7. Centrifugal fan

8. Partition

9. Air filter

Working

In the case of a window type air-conditioner, the evaporator faces towards the inside of the room whereas the condenser faces towards the outside of the room.

The centrifugal fan sucks air from the room and passes it over the evaporator coils. As the room air passes over the evaporator coils (the surface temperature of the coils is very low) it is cooled and this cooled air is supplied to the room. An impeller fan sucks outside air through the sides and throws it over the condenser and cools down the refrigerant passing through the condenser tubes.

The evaporator side and the condenser side are completely separated from each other by means of a partition.

Electrical Circuit of Air Conditioner

The following electrical components are fitted with the air conditioner.

1. Motor compressor with overload protection. 

2. Starting relay

3. Starting and running capacitor

4. Fan motor with capacitor

5. Thermostat

6. On/Off switch with speed control

When the supply is 'ON", the high current flows in the motor starting winding, it starts  and also the relay contacts close and connect the starting capacitor to the starting winding. As the motor gradually reaches the normal speed, the current in the running winding decreases, making the relay open, thus disconnecting the supply to the starting winding of the motors.

The overload protector protects the motor against excessive overload. This protection is provided inside the compressor and its function is to open the compressor motor circuit. Basically it is a bimetallic dise with contacts. When excessive current flows in the motor, the temperature increases thus opening the circuit automatically to stop the motor. The electrical circuit comprises the following components:

1. Fan Motor: 1-phi split phase motor is used for fan motors. The fan motor is provided with an ris overload protection placed in winding so that in the event of overheating it is protected.

2. Starting Capacitor: The starting capacitor (C) is of large capacity. The rating of any capacitor is given in terms of voltage and KVAR, whereas its capacitance is in microfarad. It remains in the circuit for a short time, till the motor starts. 

3. Running Capacitor: The running capacitor (C) is of smaller capacitance but heavy duty and is oil filled. It remains in the circuit continuously and improves the starting torque and power factor. 

4. Thermostat: The function of the thermostat is to monitor the room temperature and start or stop the motor. When the room temperature increases, the contacts of the thermostat close and start the compressor and when the temperature decreases the contacts open up and the compressor of the air conditioner stops.

Advantages of Room Air Conditioners

1. Saving in installation.

2. Only those rooms which need cooling will have their units running.

3. Low initial cost.

4. Flexibility of operation. 

5. Duct work is eliminated.

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Electric Heating | Advantages and Methods of Electric Heating

ELECTRIC HEATING

Heating by means of converting electricity into heat is called electric heating. Heating is required for domestic purposes such as cooking and heating of buildings, as well as for industrial purposes such as melting of metals, hardening and tempering, drying and welding. Practically all the heating requirements can be met by some form of electric heating equipment. The main advantages of electric heating over other systems of heating i.e., gas, coal, oil heating are given below.

ADVANTAGES OF ELECTRIC HEATING OVER OTHER SYSTEM OF HEATING 

The main advantages of electric heating over other system of heating such as coal, gas or oil heating are given below.

1. Economical: Electric energy is cheaper to use as it is produced on a large scale. The electric furnaces run by electric energy are cheaper in initial cost. The maintenance cost is also low as compared to other system. 

2. Cleanliness: It is free from ash, dust and soot, so it is clean system of heating.

3. Ease of temperature control: Desired temperature can be controlled either by hand or by fully automatic switches such as thermostat in electric system. 4. Absence of flue gases: When we use electrical energy to generate heat, no flue gas is produced. So the surrounding atmosphere is not polluted.

5. Saving in space: With electrical heating, no storage of fuel is required as may be the case with coal, oil or gas heating. 

6. Uniform heating: In electric heating, the heat can be generated inside the core of the material which achieves uniform heating. 

7. Maintenance cost: The maintenance costs are lower for electric equipments used for heating.

8. Less attention required: No attention is required in general as the process can be made fully automatic, whereas for use of coal, or gas etc., an attendant is required.

9. Location of heat: If necessary, the heat can be localised in a particular portion of the article to the heated.

10. Safety: Electric heating is quite safe and respond quickly.

11. Absence of Noise: Noise is entirely eliminated in case of electric heating. 

12. Quick operation: Heat by electricity can be produced more quickly than by any other process.

13. Efficiency: The overall efficiency of electrically produced heat is comparator higher, because the source can be brought directly to the point where heat is required. Hence losses can be reduced.

14. Automatic Protection: Automatic protection against over current or over heating can be provided through suitable switch gears in the electric heating system.

Methods of Transfer of Heat

Heat is transferred from a hot body to cold body by the following three methods normally. 

1. Conduction: Heat is transferred from one surface area to the other by way of conduction.

Solid get heated according to this mode heating.

2. Convection: In this mode of heating, heat is transferred by actual motion of the molecules. Liquids are usually heated due to convection. Immersion type water heater is the application of this process.

3. Radiation: In this mode of heat, the heat reaches the substances to be heated from the source without heating the medium in between.

METHODS OF ELECTRIC HEATING

The various methods of electric heating are enlisted below: Considering that it is both economical and desirable to produce heat with the help of electricity, again there are various methods of heating. The merits of each method depend upon the degree of temperature required as well as the purpose of it. For example, resistance type heating element is used in a room heater, in an oven or in a press. For melting charge of a metal, an induction or arc furnace is used. The various methods of producing heat are classified as below:

(i) Resistance Heating:

(a) Direct resistance heating

(b) Indirect resistance heating

(ii) Induction Heating or Eddy Current Heating:

(a) Direct induction heating

(b) Indirect induction heating

(iii) Electric Arc:

(a) Direct arc furnaces.

(b) Indirect arc furnaces

(iv) Dielectric or Capacitive Heating:

The comparative study of each type is given as below:

RESISTANCE HEATING

Direct Resistance Heating

In case of direct resistance heating, the material or charge to be heated is taken as resistance and current is passed through it. This current produces FR losses in the form of heat within the body itself. The charge may be in the form of powder, pieces or liquid.

This principle is made use of in resistance welding and in heating water by means of an electrode Boiler. In case of electrode boiler, the electrodes are lowered into the tank filled with water. The current flows through electrodes into the water and the water gets heated up by FR losses. The current must be adjusted from time to time since the resistance of water lowers down as its temperature rises. A water load may also be prepared in the lab, that works on the same principle. It is important to note that only AC is suitable for this purpose as DC will cause electrolysis of water.

AC supply is used having voltage varies from 2 to 20 volts and currents upto 2500 amps. Automatic stirring action is produced in the charge to be heated and no external method of stirring is required to get uniform heating.

Indirect Resistance Heating

So many domestic appliances work on the principle of Indirect Resistance Heating. In this case the current does not flow through the body to be heated but it flows through resistance elements which get heated up. The heat is then carried to the body by convection or radiation. The appliances that work on this principle include room heater, cooking ovens, immersion rod, electric kettles, electric iron and various types of resistance ovens used for bright annealing and Salt Bath heating etc.

INDUCTION HEATING

In induction heating effect of currents induced by electromagnetic action in the charge is employed.The heat developed depends upon the voltage and resistance of the charge, because power drawn is equal to V2/R so to develop heat sufficient of melt the charge, the resistance of the charge must be low which is possible only with metals and voltage must be higher, which is obtained by employing higher flux and higher frequency.

Types of Induction Heating 

(i) Direct induction heating

(ii) Indirect induction heating

Direct Induction Heating

The induction heating works on the transformer principle. It is also known as eddy current heating The currents are induced by the principle of Electromagnetic Induction. The induction heating may be low frequency as in case of core type induction furnace of high frequency as is the case with coreless induction furnace or various processes utilising high frequency eddy current heating.

Again the induction heating may be Direct Induction heating when the eddy currents are produced within the material itself that is to be heated or it may be indirect induction heating in which case the currents are produced in a coil which gets heated up and the heat is carried to the charge through convection and chiefly by Radiation.

The examples of Direct Induction heating are the high frequency eddy current heating used for case hardening or tempering of various machine parts, annealing of stell strip and soldering. The core type induction furnace used for melting non-ferrous metals such as Copper, Zine, Brass and the coreless induction furnace used for preparing various high grade steels also work on the same principle.

Indirect Induction Heating

The example of indirect induction heating is the indirect induction oven which is indirect competition with resistance oven and is preferred over it due to its fine temperature control. It is used for the same purpose as the resistance ovens.

Moreover in the indirect induction heating method eddy current are induced in the heating elements by electromagnetic induction which produces heat in the heating elements. The heat thus produced is transferred to the body to be heated by radiation.

EDDY CURRENT HEATING

This is also known as the induction heating, The material to be heated is placed inside the coil. The heat in the material to be heated is produced by eddy currents. Power loss due to eddy currents is eddy current loss and appears in the form of heat. The metal to be heated is placed within a high frequency current carrying coil. By doing so an alternating magnetic field is set up, eddy currents are induced in the metal piece and heating is affected. The power loss due to eddy current in the metal piece depends upon the power drawn by the metal piece.

The frequency may vary from 50 Hz to 8 MHz depending upon the type of work done. This method is frequently used for forging, annealing. The process is economical for continuous heating. It may also be used for welding, brazing and soldering.

Advantages of Eddy Current Heating

1. It is quick, and clear.

2. There is little wastage of heat as heat is produced in the body to be heated up directly. 

3. Temperature control is easy i.e., by controlling the supply frequency and flux density. 

4. The heat can be made to penetrate into the metal surface to any desired depth.

5. The equipment can be operated even by unskilled worker.

Disadvantage of Eddy Current Heating

1. It is a costly method for the production of heat.

2. Low efficiency.

3. Initial cost of the apparatus is high.

Uses: Eddy current heating is used for the heat treatment of metals i.e., annealing, tempering. surface hardening etc.

ELECTRIC ARC HEATING

The arc furnaces depend upon the principle of heat generated by electric arc. The electric arc heating may be used in the following different ways.

(i) By striking the arc between the charge and electrode. In this method the heat is directly conducted and taken by the charge. The furnaces operating on this principle are known as direct arc furnaces. These furnaces are used for production and refining of various grade of steel. 

(ii) By striking the arc between two electrodes. In this method the heat is transferred to the charge by radiation. The furnaces operating on this principle are known as indirect arc furnaces. These types of furnaces are used for melting of non-ferrous metals such as brass, copper and zinc.

Arc Furnaces

It has already been mentioned that the arc furnaces are of two types viz., the direct are furnace and indirect are furnace. These are usually operated from 3 phase supply.

Direct Arc Furnace

The Arc is struck between the electrode and the charge. Three electrodes are used for three phase supply. In each case the arc is between tip to electrode and the charge which forms the star point. 

Since the arc is struck on the charge itself, it is possible to produce the highest temperatures by this method.

The Refractory lining used in the inside of arc chamber is for high quality fire clay bricks which do not soften or melt at the melting temperatures of steel for which purpose this furnace is mostly used.

The electrodes may be of carbon or graphite. The graphite electrodes are of about half the size of carbon electrodes for the same power, Graphite electrodes permit production of higher temperatures.

The arc has a-ve resistance characteristic i.e., the resistance falls with increase in temperature. Thus some sort of current limiting device is essential in the circuit in order to prevent short circuits. This may be in the form of a Reactor. Alternatively the supply transformer may be so wound that the voltage is adjusted from it. This limits the current also. 

A more uniform product is obtained by this method since the automatic string action is produced when the arc is focused on the charge itself. 

This type of furnace is extensively used for melting of various types of ferrous alloys, in production of high quality steels and for refining purposes when the steel is produced in cupola and refined in arc furnace.

There is a charging door from where the charge is supplied and also there is an outlet for molten metal.

The sizes of furnaces in common use are between 5 to 10 tons.

Indirect Arc Furnace

In this case the arc is struck between two electrodes. The chief mode of transfer of heat is through radiation. The temperature attained is lower than the direct arc furnace. So these furnaces are suitable for melting metals having lower melting points e.g., Non ferrous metals such as brass, copper, zinc, bronze etc. Furnace supported by steel frame work and lined with refractory material.

The arc is struck between the electrodes so only two electrodes are required. The supply is therefore single phase.

Since during the process of heating the electrodes are consumed, so the feeding of electrodes to the furnace is automatic. The furnace is cylindrical in shape.

Since the arc does not come in contact with the charge so the automatic stirring action which is present in direct arc furnace is absent. The furnace may be equipped with automatic rocking equipment. The power factor varies from 0.7 to 0.8.

The electrode material and the electrical equipment is similar to that used for direct arc furnaces.

DIELECTRIC HEATING

When non-metallic parts such as wood, plastics, bones ceramics are subjected to an alternating electrostatic field, dielectric loss occurs. These losses are used in dielectric heating which appears in the form of heat. The material to be heated is placed as a slab between two metallic electrodes across which high frequency voltage is applied.

To ensure sufficient loss and to give an adequate amount of heating, frequencies between 10 to 30 mega cycle per second must be used and the voltage needed may be as high as 20 kV. The necessary high-frequency supply voltage is obtained from a valve oscillator.

The current drawn by the capacitor, when an ac supply voltage is applied across its two plates, does not lead the supply voltage by 90° exactly. Due to this component of current, heat is always produced in dielectric material placed in between the two plates of the capacitor.

The electric energy dissipated in the form of heat energy in the dielectric material is known as dielectric loss. The dielectric loss is directly proportional to the frequency of ac supply. 

This method of heating is also employed for drying of textiles, manufacture of plywood, paper etc. The overall efficiency in case of dielectric heating is about 50%.

Advantages of Dielectric Heating

1. Since the heat is produced throughout the whole mass of material, we get uniform heating. By conventional method of heating, it is not possible to achieve this.

2. Short time is required to complete the process as compared to other methods.

3. Materials heated by this method are non-conducting, so by other methods heat cannot be conducted inside so easily.

Disadvantages of Dielectric Heating

1. Only those materials can be heated which have high dielectric loss.

2. The cost of equipment required for dielectric heating is so high that it is employed only where other methods are impracticable.

Applications of Dielectric Heating

1. It is used in drying tobacco, paper, wood, gluing and bonding of wood.

2. Welding of PVC.

3. Sterilization of medical supplies.

4. For producing artificial fibers, heating of bones and tissues etc.

5. Food processing.

6. It is employed for dyeing textiles.

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