Refrigeration

Refrigeration:

1 Vapour Compression Refrigeration System

2 Vapour Absorption Refrigeration System

3 Comparison Between Vapour Compression & Vapour Absorption Refrigeration Systems

REFRIGERATION:

^·         It is defined as the process of providing and maintaining a temperature well below that of surrounding atmosphere.

^·         In other words refrigeration is the process of cooling substance.

Refrigerators and heat pumps:

If the main purpose of the machine is to cool some object, the machine is named as refrigerator.

If the main purpose of machine is to heat a medium warmer than the surroundings, the machine is termed as heat pump.

Terminologies of Refrigeration:

Refrigerating Effect (N):

It is defined as the quantity of heat extracted from a cold body or space to be cooled in a given time.

N= Heat extracted from the cold space Time taken

Specific Heat of water and ice :

It is the quantity of heat required to raise or lower the temperature of one kg of water (or ice), through one kelvin or (1⁰ c) in one second.

Specific heat of water, C_pw  = 4.19 kJ/kg K

Specific heat of ice,  C_pice = 2.1 kJ/kg K.

Capacity of a Refrigeration Unit :

^·         Capacity of a refrigerating machines are expressed by their cooling capacity.

The standard unit used for expressing the capacity of refrigerating machine is ton of refrigeration.

· One ton of refrigeration is defined as, “the quantity of heat effect) to freeze one ton of water Heat extracted from at o^o c = latent heat of ice into one ton of ice in a duration of 24 hours at 0^o C”.

Latent heat of ice= 336 kJ/kg i.e., 336 kJ of heat should be extracted one kg of water at

0^o C to convert it into ice.

One ton of refrigeration

= 336x1000 kJ/24 hrs. = 336x1000 kJ/min

=24x60

One ton of refrigeration

= 233.333 kJ/min

 = 3.8889 kJ/sec

Co efficient of Performance

It is defined as the ratio of heat extracted in a given time (refrigerating effect) to the work input.

Co efficient of performance = Heat extracted in evaporator Work Input

Co efficient of performance =    Refrigerating Effect

Work Input

Co efficient of performance =    NW

The COP is always greater than 1 and known as theoretical coefficient of performance.

Applications of Refrigeration:

In chemical industries, for separating and liquefying the gases. In manufacturing and storing ice.

For the preservation of perishable food items in cold storages. For cooling water.

For controlling humidity of air manufacture and heat treatment of steels. For chilling the oil to remove wax in oil refineries.

For the preservation of tablets and medicines in pharmaceutical industries. For the preservation of blood tissues etc.,

For comfort air conditioning the hospitals, theatres, etc.,

Properties of Refrigeration:

A good refrigerant should have high latent heat of vapourisation. It should have low boiling and low freezing point.

It should be non toxic and should non corrosiveness It should be non flammable and non explosive.

It should have high thermal conductivity It should be easy to handle

It should have low specific volume of vapour. It should have high co efficient of performance

VAPOUR COMPRESSI ON REFRIGERATION SYSTEM

Construction:

This system consists o f a compressor, condenser, a receiver tank, a n expansion valve and an evaporato r.

Compressor : Recipro cating compressors generally used. For very big plants centrifugal compressor s directly coupled with high speed rotating engi nes (gas turbine) are used.

For  very  big  plantsC entrifugal  compressors  directly  coupled  with  high  speed rotating engines (gas turbine) are used

Condenser : It is a coil of tubes made of copper.

Receiver tank: It is the reservoir of liquid refrigerant.

Expansion Valve: Thi s is a throttle valve. High pressure refrigerant is made to flow at a controlled rate through this valve.

Evaporator : It is the actual cooler and kept in the space to be cooled. The evaporator is a coil of tubes made of copper

VAPOUR ABSORPTION  REFRIGERATION SYSTEM:

Construction:

The vapour absorption system consists of a condenser, an expansion valve and an evaporator.

They perform the same as t hey do in vapour compression method.

In addition to these, this system has an absorber, a heat exchanger, an analyser and a rectifier.

Working:

•         Dry ammonia vapour at low pressure passes in to the absorber from the evaporator.

•         In the absorber the d ry ammonia vapour is dissolved in cold water and strong solution of ammonia is formed.

•         Heat evolved during the absorption of ammonia is removed by cir culating cold water through the coils kept in the absorber.

The highly concentrated ammonia (known as Aqua Ammonia) is then pumped by a pump to generator through a heat exchanger.

•         In the heat exchanger the strong ammonia solution is heated by the h ot weak solution returning from the generator to the absorber.

•         In the generator the w arm solution is further heated by steam coils, gas or electricity

and the ammonia vapour is driven out of solution.

•   The boiling point of a mmonia is less than that of water.

•         Hence the vapours le aving the generator are mainly of ammonia.

•         The weak ammonia solution is left in the generator is called weak aq ua.

•      This weak solution is returned to the absorber through the heat exchan ger.

•         Ammonia vapours le aving the generator may contain some water vap our.

•         If this water vapour i s allowed to the condenser and expansion valv e, it may freeze resulting in chocked f low.

•         Analyser and rectifier s are incorporated in the system before condenser.

•         The ammonia vapour from the generator passes through a series of trays in the analyser and ammonia is separated from water vapour.

•         The separated water v apour returned to generator.

•   Then the ammonia va pour passes through a rectifier.

•   The  rectifier  resembles  a  condenser  and  water  vapour  still  prese nt  in  ammonia

vapour condenses andd the condensate is returned to analyser.

•         The virtually pure am monia vapour then passes through the condense r.

•         The  latent  heat  of  ammonia  vapour  is  rejected  to  the  cooling  water  circulated

through the condenser and the ammonia vapour is condensed to liquid ammonia.

•        The high pressure liquid ammonia is throttled by an expansion v alve or throttle valve.

•         This reduces the high temperature of the liquid ammonia to a low value and liquid ammonia partly evaporates.

•         Then this is led to the evaporator.

•         In the evaporator the liquid fully vaporizes.

•         The latent heat of evaporation is obtained from the brine or other body which is being cooled.

•         The low pressure ammonia vapour leaving the evaporator again enters the absorber and the cycle is completed.

•         This cycle is repeated again to provide the refrigerating effect.

Applications of refrigeration system:

^·         Preservation of food items like vegetables, milk and eggs.

^·         Preservation of medicines.

^·         Preservation of blood, tissues, etc.,

^·         Preservation and cooling of cool drinks.

^·         Preservation of chemicals (Chemical industries)

^·         Cooling of water.

^·         Industrial and comfort airconditioning.

^·         Processing of dairy products.

COMPARISON BETWEEN VAPOUR COMPRESSION & VAPOUR ABSORPTION REFRIGERATION SYSTEMS:

Vapour Compression System

1. This system has more wear and tear  and produces more noise due to the moving parts of the compressor.

2. Electric power is needed to drive the system

3. Capacity of the system drops rapidly with lowered evaporator pressure

4. At partial loads performance is poor.

5. Mechanical energy is supplied through compressor

6. Energy supplied is ¼ to ½ of the refrigerating effect

7. Charging of the refrigerating to the system is easy

8. Preventive measure is needed, since liquid refrigerant accumulated in the cylinder may damage to the cylinder

Vapour Absorption System

1. Only moving part in this system is an aqua pump. Hence the quieter in operation and less wear and tear

2. Waste of exhaust steam may be used. No need of electric power

3. Capacity of the system decreases with the lowered evaporative pressure, by increasing the steam pressure in generator.

4. At partial loads performance is not affected.

5. Heat energy is utilized

6. Energy supplied is about one and half times the refrigerating effect

7. Charging of refrigerant is difficult

8. Liquid refrigerant has no bad effect on the system.