VAPOUR COMPRESSION CYCLE

VAPOUR COMPRESSION CYCLE 

Introduction

The  vapour   compression   cycle   is  the  mostly   widely   used   method   of refrigeration in the modern  application.  Your household  refrigerator,  water cooler, deep  freezer, air-conditioner etc, all run on vapour compression cycle. The cycle is called   as   vapour   compression   cycle,   because   the   vapours   of   refrigerant   are compressed in the compressor of the system to develop the cooling effect.

Working

Here are the various process of vapour compression cycle (refer the figure).

1)  Compression:  The vapours of refrigerants enter the compressor and get compressed to high pressure and high temperature. During this process the entropy   of  the  refrigerant  ideally  remains  constant  and  it  leaves  in super heated state.

           

        2)  Condensation:  The  superheated  refrigerant  then  enters  the  condenser where  it  is cooled  either  by air or water  due to which  its temperature reduces, but  pressure  remains constant  and it gets converted  into liquid state.

             3) Expansion:  The  liquid  refrigerant  then  enters  the  expansion  valve  or throttling valve or capillary tube when sudden expansion of  the refrigerant occurs,   due  to  which  its  temperature  and  pressure  falls  down.  The refrigerant leaves expansion valve or capillary tube in partially liquid state and partially in gaseous state.

             4) Evaporation   or  cooling:  The  partially  liquid  and  partially  gaseous refrigerant  at  very  low  temperature  enters  the  evaporator  where  the substance to be cooled is kept. It is here where the refrigeration effect is produced.  The  refrigerants  absorbs  the  heat  from the  substance to be cooled and gets converted into vapour state.

                                       Fig : Simple VCR System

 Advantages

» Capable  of  large  refrigerating  loads  at  lower  initial purchase and operating cost.

» Very efficient

» Very compact system for small to very large heat loads.

» Cycle can be reversed for heat pump operation.

 Disadvantages

» Parts can wear out.

» Noise

» Potential refrigerant leaks.

» Operates in limited orientation.

REFRIGERATION PROCESS

  

 REFRIGERATOR

Refrigerator   keep   things   cold   because   of   the   nature   of   the  heat. Thermodynamics essentially starts that if a cold object is placed to a next to a hot object, the cold object will become warmer and the hot object will become cooler. A refrigerator  does  not cool items by lowering their original temperature; instead, an evaporating gas  called a refrigerant draws heat away, leaving the surrounding area much colder. Refrigerators and air conditioners both work on the principle of cooling through evaporation.

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A refrigerator consists of two storage compartment – one for frozen items and the  other  for items requiring refrigeration  but no freezing. These compartment  are surrounded by a series of heat-exchanging pipes. Near the bottom of the refrigerator unit is a heavy metal device called a compressor. The compressor is powered by an electric  motor.  More  heat-exchanging   pipes  are  coiled  behind  the  refrigerator. Running through the entire system is pure ammonia, which evaporates at -27 ºF. this system is closed, which means nothing is lost or added while it is operating. Because liquid ammonia is a powerful chemical, a leaking refrigerator should be repaired or replaced immediately.

The refrigeration process begins with the compressor. Ammonia compressed until it becomes very hot from the increased pressure. This heated gas flows through the coils  behind  the refrigerator,  which allows excess  heat to be released  into the surrounding air.  This is why users sometimes fill warm air circulating  around the fridge. Eventually the  ammonia cools down to the point where it become a liquid. This liquid form of ammonia  is then forced through a device called an expansion valve or capillary tube. Essentially, the  expansion valve has a small opening or the capillary tube has a very small diameter of  copper tube that the liquid ammonia is turned into a very cold, fast-moving mist, evaporating as it travels through the coils in the freezer.

As the evaporating ammonia gas absorbs more heat, its temperature rises. Coils surroundings the lower refrigerator compartment are not as compact. The cool ammonia still draws heat from the warmer objects in thefridge, but not as much as the freezer section. The ammonia gas is drawn back into the compressor, where the entire cycle of pressurization, cooling and evaporation begins anew.

REFRIGERATION CYCLE

The refrigeration cycle uses a fluid, a called a refrigerant, to move heat from one place to other. We will begin with the cool, liquid refrigerant entering the indoor coil, operating as the evaporating during cooling. As the name implies, refrigerant in the  evaporator  “evaporator”.  Upon entering the evaporator,  the liquid refrigerant’s temperature is between 40 and 50 ºF and without changing its temperature, it absorbs heat as it changes  state from a liquid to a vapour. The heat comes from the warm, moist room air blown across the evaporator coil. As it passes over the cool coil, it gives up some of its heat and moisture may condense from it. The cooler, drier room air is re-circulated by a blower into the space to be cooled.

The vapour refrigerant  now moves into the compressor,  which is basically a pump  that  raises  the  pressure  so it will  move  through the  system.  The  increased pressure from the compressor causes the temperature of the refrigerant to rise. As it leaves the compressor, the refrigerant is a hot vapour, roughly 120 to 140 º F.

It now flows into the refrigerant-to-water heat exchanger, operating as the condenser during the cooling. As it condenses, it gives up heat to the loop, which is circulated by a pump

                    

  Fig:  Refrigeration cycle.

As the refrigerant leaves the condenser, it is cooler, but still under pressure provided  by the compressor.  It then reaches the expansion valve or capillary tube. That  the  high  pressure  refrigerant  to  “flash”  through  becoming  a  lower  pressure, cooled liquid.  When pressure is reduced, as with spraying an aerosol can or a fire extinguisher, it cools.  The cycle is complete as the cool, liquid refrigerant re-enters evaporator to pick up room heat.

2.3 HOW REFRIGERATOR WORKS

In the summertime, have you ever gotten out of a swimming pool and then felt very cold standing in the sun? that’s because the water on your skin is evaporating. The air carries off the water vapour, and with it being taken away from your skin.

This is similar to what happens inside older refrigerators. Instead of eater, through, the refrigerator uses chemicals to do the cooling.

There are two things that need to be known for refrigeration.

1.        A gas cools on expansion.

2.      When you have two things that are difference temperature that touch or are near each other, the hotter surface cools and the colder surface warms up. This is a law of physics called the Second Law of Thermodynamics.

TYPES OF DOMESTIC REFRIGERATOR

There are two types of domestic refrigerator.

1.      Single door fresh food refrigerator

2.      Double-door refrigerator-freezer

Most domestic refrigerator are of two types – either a single door fresh food refrigerator   or   a   two-door   refrigerator-freezer   combination,   with   the   freezer compartment on the top portion of the cabinet, or a vertically split cabinet (side-by- side),  with  the  freezer  compartment  on  the  left  side  of  the  cabinet.  They  are completely  self-contained units and are easy to install. Most refrigerators use R-22 refrigerant, normally maintaining temperatures of 0 ºF in the freezer compartment and about 35 ºF to 45 ºF in the refrigerator compartment.

    TEMPERATURTE ZONE AND RATING

Some refrigerators are now divided into four zones to store different types of food:

»           -18 ºC (0 ºF) (freezer)

»           0 ºC (32 ºF) (meats)

»           5 ºC (49 ºF) (refrigerator)

»           10 ºC (50 ºF) (vegetables)

The capacity of a refrigerator is measured in either litres or cubic feet (US). Typically the volume of a combined fridge-freezer is split to 100 litres (3.53 cubic feet) for the freezer and 140 litres (4.94 cubic feet) for the refrigerator, although these values are highly variable.

Temperature settings for refrigerator and freezer compartments are often given arbitrary numbers (for example, 1 through 9, warmest to coldest) by manufacturers, but generally 2 to 8 ºC (36 to 46 ºF) is idle for the refrigerator compartment and -18ºC (0 ºF) for the freezer. Some refrigerators require a certain external temperature 16ºC (60 ºF) to run properly. Thus can be an issue when placing a refrigerator in an unfinished area such as a garage.

    REFRIGERANT

Refrigeratin application	Short description	Typical HFCs used
Domestic Refrigeration	Appliances used for keeping food in dwelling units.	HFC-134a
Commercial Refrigeration	Holding and displaying frozen and fresh food in retail outlets	R 404A, R 507, HFC-234a
Food processing and cold storage	Equipment to preserve, process and store food from its source to the wholesale and cooling	R410A, R407C,R 507, HFC-134a
Industrial Refrigeration	Large equipment, typically 25 kW to 30 MW, used for chemical processing, cold storage, food processing and district heating and cooling	HFC-134a, R- 404A, R-507
Transport refrigeration	Equipment to preserve and store goods, primarily foodstuffs, during transport by road, rail, air and sea	R410A, R407C, HFC- 134A