Working principle of vapour compression refrigeration system

A chiller is simply a device that used to remove heat from something. For industrial purposes, chillers can be thought of as a component within a complex mechanical system that is used to remove heat from a process or substance.

To really understand what a chiller is, a fundamental knowledge of the principles of basic refrigeration is required. Welcome to Berg's School of Cool. Before getting into the fundamentals of refrigeration, a few basic definitions should be considered:.

Heat is a form of energy transferred by virtue of a difference in temperature. Heat exists everywhere to a greater or lesser degree.

As a form of energy it can be neither created or destroyed, although other forms of energy may be converted into heat, and vice versa.

It is important to remember that heat energy travels in only one direction; from a warmer to a cooler object, substance, or area. Cold is a relative term referring to the lack of heat in an object, substance, or area. Another definition describes it as the absence of heat, no process yet has been devised of achieving "absolute zero," the state in which all heat has been removed from any object, substance, or area.

Theoretically this zero point would be Refrigerationor cooling process, is the removal of unwanted heat from a selected object, substance, or space and its transfer to another object, substance, or space.

Removal of heat lowers the temperature and may be accomplished by use of ice, snow, chilled water or mechanical refrigeration. Mechanical refrigerationis the utilization of mechanical components arranged in a " refrigeration system " for the purpose of transferring heat. Refrigerantsare chemical compounds that are alternately compressed and condensed into a liquid and then permitted to expand into a vapor or gas as they are pumped through the mechanical refrigeration system to cycle.

The refrigeration cycle is based on the long known physical principle that a liquid expanding into a gas extracts heat from the surrounding substance or area. You can test this principle by simply wetting your finger and holding it up. It immediately begins to feel cooler than the others, particularly if exposed to some air movement. That's because the liquid in which you dipped it is evaporating, and as it does, it extracts heat from the skin of the finger and air around it.

Refrigerants evaporate or "boil" at much lower temperatures than water, which permits them to extract heat at a more rapid rate than the water on your finger.Refrigeration is a process of cooling or remove heat to lower the temperature of a space, or items to be stored in the space, to a temperature below the temperature of the surrounding area.

This is accomplished by using a mechanical refrigeration process in which a refrigerant is used to alternately absorb heat from the space or object to be cooled and reject the heat to atmosphere or seawater. The basic components of any refrigeration system working on the vapour compression cycle, are the compressor, condenser, expansion valve, evaporator and the refrigerant fluid which is alternatively vapourized and liquefied during the refrigeration cycle.

The temperature at which a fluid boils or condenses is known as the saturation temperature and it varies with pressure. The compressor in a refrigeration system in raising the pressure of the vapourized refrigerant causes its saturation temperature to rise so hat it is higher than that of, the surrounding air or cooling seawater, whatever used in the condenser. The compressor also promotes circulation of the refrigerant by pumping it around the system. In the condenser the refrigerant is liquefied by being sub cooled to below the saturation temperature relating to the compressor delivery pressure, by circulating seawater or air for domestic refrigerators.

Latent heat, originally from the evaporator, is thus transferred to the cooling medium. The liquid refrigerant, still at the pressure produced by the compressor, passes to the receiver and then to the expansion valve. The expansion valve is the regulator through which the refrigerant flows from the high pressure side of the system to he low pressure side.

Its throttling effect dictates the compressor delivery pressure which must be sufficient to give the refrigerant a saturation temperature which is higher than the temperature of the cooling medium. The pressure drop through the regulator causes the saturation temperature of the refrigerant to fall, so that it will boil at the low temperature of the evaporator. In fact, as the liquid passes through the expansion valve the pressure drop makes its saturation temperature fall below its actual temperature.

Some of the liquid boils off at the expansion valve, taking latent heat from the remainder and causing its temperature to drop. The expansion valve throttles the liquid refrigerant and maintains the pressure difference between the condenser and evaporator, while supplying refrigerant to the evaporator at the correct rate.

It is thermostatically controlled in most of the systems. The refrigerant entering the evaporator coil, at a temperature lower than that of the surrounding secondary coolant air or brine receives latent heat and evaporates. Later the heat is given up in he condenser, when the refrigerant is again compressed and liquefied. For a small refrigerator the evaporator cools without forced circulation of a secondary coolant.

In larger installations, the evaporator cools air or brine which are circulated as secondary refrigerants. Your email address will not be published.

Search for:.Reference of steam jet refrigeration system diagram and P -H and T — S diagrams. Theory:. The steam jet refrigeration system also known as ejector system refrigeration system is one of the oldest methods of producing refrigeration effect. The basic components of this system are an evaporator, a compressor device, a condenser and a refrigerant control device.

This system employs a steam injector or booster instead of mechanical compressor to compress the refrigerant to the required condenser pressure level. In this system, water is used as the refrigerant. The steam jet refrigeration system is widely used in food processing plants for pre-cooling of vegetables and concentrating fruit juices, gas plants, paper mills, breweries etc. Principle of steam jet refrigeration system:. The boiling point of a liquid changes with change in external pressure.

In normal conditions, pressure exerted on the surface of a liquid is the atmospheric pressure. If this atmospheric pressure is reduced on the surface of a liquid, by some means, then the liquid will start boiling at lower temperature, because of reduced pressure. This basic principal of boiling of liquid at lower temperature by reducing the pressure on its surface is used in steam jet refrigeration system. The reduced pressure on the surface of water is maintained by throttling the steam through the jets or nozzles.

Working of steam jet refrigeration system:. The flash chamber or evaporator is a large vessel and is heavily insulated to avoid the rise in temperature of water due to high ambient temp. It is fitted with perforated pipes for spraying water. The warm water coming out of the refrigerated space is sprayed into the flash water chamber where some of which is converted into vapours after absorbing the latent heat, thereby cooling the rest of water.

The high pressure steam from the boiler is passed through the steam nozzle thereby increase its velocity. The high velocity steam in the ejector would entrain the water vapours from the flash chamber which would result in further information of vapour.

The mixture of steam and water vapour passes through the ventilate-tube of the ejector and gets compressed.

working principle of vapour compression refrigeration system

The temperature and pressure rises considerably and fed to the water cooled condenser where it gets condensed. A constant water level is maintained in the flash chamber and any loss of water due to evaporation is made up from the make- up water line.

The Science Behind Refrigeration

Steam Ejector:. The steam ejector is one of the important components of a steam jet refrigeration system.A refrigeration system can also be used as a heat pump, in which the useful output is the high temperature heat rejected at the condenser.

Most of the modern refrigerators work on this cycle, in Its simplest form there are four fundamental operations required to complete one cycle. During compression Stroke the pressure and temperature increase until vapour temperature is greater than the temperature of condenser cooling medium air or water.

At point 1 in the diagram, the circulating refrigerant enters the compressor as a saturated vapor. From point 1 to point 2the vapor is isentropically compressed i. From point 2 to point 3, the vapor travels through part of the condenser which removes the superheat by cooling the vapor. Between point 3 and point 4the vapor travels through the remainder of the condenser and is condensed into a saturated liquid.

The condensation process occurs at essentially constant pressure. Between points 4 and 5the saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure.

That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid typically, less than half of the liquid flashes. The low pressure refrigerant vapour after expansion in the expansion valve enters the evaporator or refrigerated space where a considerable amount of heat IS absorbed by it and refrigeration is furnished.

Between points 5 and 1the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air from the space being refrigerated that a fan circulates across the coil or tubes in the evaporator.

The resulting refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle. Its function is to provide a heat transfer surface through which heat can pass from the refrigerated space into the vaporizing refrigerant.

Suction Line. It carries the low pressure vapour from the evaporator to suction in let of the compressor. The function of the compressor is to draw refrigerant vapour from the evaporator and to raise It temperature and pressure to such a print to that it may be easily condensed with normally available condensing media.

It also maintains a continuous flow of the refrigerant through the system. The most common compressors used in chillers are reciprocating, rotary screw, centrifugal, and scroll compressors. Each application prefers one or another due to size, noise, efficiency and pressure issues. Discharge Line. It conveys the high pressure and high temperature refrigerant from the compressor to the condenser.

working principle of vapour compression refrigeration system

The function of the condenser is to provide a heat transfer surface through which heat passes from the refrigerant to the condensing medium which is either water or air. Liquid Receiver. It acts as, a reservoir which stores the liquid refrigerant coming from the condenser and supplies it to the evaporator according to the requirement.

Liquid Line. It carries the liquid refrigerant from the receiver and conveys it to the expansion valve.The Vapor Compression Refrigeration Cycle is nearly years old, but it does not seem ready to leave the scene any time soon. While some people have viewed this method as environmentally harmful and inefficient, the cycle is still applicable in the industrial sphere.

What is its defining feature of these systems? The simplest explanation of this system is a heat engine working in reverse, technically referred to as reverse Carnot engine.

Refrigeration onboard Ships

In other words, it is the transfer of heat from a cold reservoir to a hot one. Clausius Statement of the Second Law of thermodynamics states:.

Since the vapor compression cycle is against the Second Law of Thermodynamics, some work is necessary for the transfer to take place. It is a compression process, whose aim is to raise the refrigerant pressure, as it flows from an evaporator. A more detailed explanation of the steps is as explained below. The refrigerant for example R enters the compressor at low temperature and low pressure. It is in a gaseous state.

Here, compression takes place to raise the temperature and refrigerant pressure. The refrigerant leaves the compressor and enters to the condenser. Since this process requires work, an electric motor may be used. Compressors themselves can be scroll, screw, centrifugal or reciprocating types. The condenser is essentially a heat exchanger. Heat is transferred from the refrigerant to a flow of water.

Basics of Vapour Compression refrigeration system (VCRS)- by Mona Yadav

This water goes to a cooling tower for cooling in the case of water-cooled condensation. Note that seawater and air-cooling methods may also play this role. As the refrigerant flows through the condenser, it is in a constant pressure. One cannot afford to ignore condenser safety and performance. Specifically, pressure control is paramount for safety and efficiency reasons.

There are several pressure-controlling devices to take care of this requirement. When the refrigerant enters the throttling valve, it expands and releases pressure. Consequently, the temperature drops at this stage. Throttling valves play two crucial roles in the vapor compression cycle. First, they maintain a pressure differential between low- and high-pressure sides.In fact, conventional refrigerators use this method of cooling to keep your leftovers and drinks chilled!

Air conditioners also employ a vapor-compression cycle to cool the ambient air temperature in a room. Basically, vapor-compression refrigeration employs a heat engine run backwards, so heat energy is taken from a cold reservoir and deposited into a hot reservoir.

By the Second Law of Thermodynamics, heat energy does not spontaneously transfer from a cold to a hot reservoir. In order to have heat transfer in that direction and not from from hot to cold, as the system is naturally inclined to doit is necessary to do work on the system. A working fluid often called the refrigerant is pushed through the system and undergoes state changes from liquid to gas and back.

The latent heat of vaporization of the refrigerant is used to transfer large amounts of heat energy, and changes in pressure are used to control when the refrigerant expels or absorbs heat energy. Thus, various fluids have been identified as practical refrigerants. The most common include ammonia, Freon and other chlorofluorocarbon refrigerants, aka CFCsand HFCa a non-toxic hydrofluorocarbon.

The conceptual figure of the process shows the PV changes during each part. Vapor-Compression Refrigeration. The Peltier Effect and Thermoelectric Cooling. Laser Cooling and Optical Molasses. Magnetic Cooling.Vapour-compression refrigeration or vapor-compression refrigeration system VCRS[1] in which the refrigerant undergoes phase changesis one of the many refrigeration cycles and is the most widely used method for air-conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services.

Oil refineriespetrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems. Cascade refrigeration systems may also be implemented using 2 compressors.

Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called an air conditionerrefrigeratorair source heat pumpgeothermal heat pumpor chiller heat pump.

Vapor-compression uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. All such systems have four components: a compressora condensera thermal expansion valve also called a throttle valve or metering deviceand an evaporator.

Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor [2] and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air flowing across the coil or tubes.

This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air whichever may be the case. The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquidis next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant.

working principle of vapour compression refrigeration system

The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated. The cold mixture is then routed through the coil or tubes in the evaporator.

A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.

To complete the refrigeration cyclethe refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor. Over time, the evaporator may collect ice or water from ambient humidity. The ice is melted through defrosting.

The water from the melted ice or the evaporator then drips into a drip pan, and the water is carried away by gravity or by a pump.

The selection of working fluid has a significant impact on the performance of the refrigeration cycles and as such it plays a key role when it comes to designing or simply choosing an ideal machine for a certain task.

One of the most widespread refrigerant is " Freon ". Freon is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable at room temperature and atmospheric pressure, nor obviously toxic as were the fluids they replaced, such as sulfur dioxide. Haloalkanes are also an order s of magnitude more expensive than petroleum derived flammable alkanes of similar or better cooling performance.

Unfortunately, chlorine- and fluorine-bearing refrigerants reach the upper atmosphere when they escape. These chlorine free radicals act as catalysts in the breakdown of ozone through chain reactions. One CFC molecule can cause thousands of ozone molecules to break down. This causes severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation, and has been shown to lead to increased rates of skin cancer.

The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule.


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