Methods of Refrigeration: Literature Review

REVIEW OF LITERATURE

2.1 Introduction

In study of applied thermo dynamics all the while we have been observing heat transfer from a system at higher temperature to that at lower temperature. Now in the study of refrigeration we will be observing various methods of cooling the objects and maintaining the temperature of bodies at values lower than surrounding temperature.

According to American society of Heating, Refrigeration and Air-conditioning Engineers (ASHARE) “Refrigeration is the science of providing and maintaining temperature below that of the surrounding (ambient) temperature”.

In the olden days around 2500 years B.C. Indians, Egyptians, etc., were producing ice by keeping water in the porous posts open to cold atmosphere during the night period. The evaporation of water in almost cool dry air accompanied with recitative heat transfer in the clear night caused the formation of ice even when the ambient temperature was above the freezing temperature. Further references are available which support the use of ice in China 1000 years BC. Nero, the emperor, was using ice for cooling beverages. Further, the East Indians were able to produce refrigeration by dissolving salt in water as early as 4^th century A.D., of course, on very small scale. The use of evaporative cooling is another application of refrigeration used olden days. The cooling of water in earthen pots for drinking purpose; is the most common example where the evaporation for water through the pores of earthen pot is accompanied with cooling of water.

The fore said methods of the production of cooling were not feasible for the commercial use due to very small amount of ice production. Availability of natural ice in limited regions and unavailability of good quality insulation confined the application of ice to those localities only. These all led to the development of artificial refrigeration side, a few would be presented here. Thomas Harris and John Long got the earliest British parent in 1790. Later on in 1834 Jacob Perkins developed hand operated refrigeration system using ether (volatile) as the working fluid. Ether vapor is sucked by the hand-operated compressor and then high temperature and pressure either vapor is condensed in the water cooled chamber (condenser). Liquid ether is finally throttled to the lower pressure, and thus evaporation of this liquid in chamber A lowers the temperature of water surrounding the vessel. Finally ice is formed. In this system, ether is used again and again in the cyclic process with negligible wastage.

In 1851, Dr. John Garrie of Florida, a physician obtained the first American patent of a cold air machine to produce ice in order to cure people suffering from the high fever. Instead of air or ether, sulphuric ether was used by Dr. James Harrison of Australia in 1860, the world’s first installation of refrigeration machine for brewery. The steam engine works as a power source which drives the compressor for the pressurization of sulphuric ether vapor, which is, in turn, condensed and is allowed to expand and evaporate in order to produce refrigeration. Dr. Alexander Kirk of England constructed a cold air machine is 1861 similar to that of Dr. Gorrie. The air was compressed by a reciprocating compressor driven by a steam engine running on coal. His actual machine consumed about 1 kg of coal to produce 4 kg of ice (approximately).

In the 19^th century, there was tremendous development of refrigeration systems to replace natural ice by artificial ice producing machines. Unfortunately steam engine, a very low speed power developing source, was used to drive the compressor, rendering very poor performance of the refrigeration system.

Some Recent Advancements of in Refrigeration

In the beginning of 20^th century, large sized refrigeration machines were under progress. By 1904 about 450 ton cooling system for air conditioning the New York Stock Exchange was installed. In Germany people used air conditioning in theatre for comfort purposes. In around 1911 the compressor speed was raised between 100 to 300 rpm. The first two-stage modem compressor was brought under use in 1915.

During the civil war there was an acute shortage of the supply of natural ice from the north. Hence, Ferdinand Care of the USA developed vapour-absorption refrigeration system ammonia as a refrigerant and water as a absorbent. The system consists of an evaporator, an absorber, a pump, a generator, a condenser and an expansion device. The evaporated vapor is absorbed by the weak ammonia-water mixture in the absorber yielding strong aqua ammonia. The pump delivers this strong solution into the generator where heat transfer from a burner separates ammonia vapor and the weak ammonia water returns to the absorber. On the other hand the ammonia vapor condenses in the condenser before being throttled. The throttled ammonia liquid enters the evaporator resulting in completion of the cyclic process.

In the beginning of two decades of the twentieth century, the development in refrigeration system was confined to refinement in cold air machines and vapor compression thermoelectric, pulse tube refrigeration systems, etc. The developments are vortex tube, steam-jet refrigeration system, availability of materials of specific properties for thermoelectric materials. The possible use of waste heat or solar energy in case of vapor-absorption and thermoelectric systems has led to development of several commercial units these days especially due to the like hood of future energy crisis, the world is going to face.

A condenser is a heat exchanger is which de-superheating of high temperature vapor changes the phase from vapor to liquid and sub cooling of condensate occurs. The condenser is an important device used in the high pressure side of a refrigeration system. Its function is to remove heat of hot vapor refrigerant discharged from the compressor. The hot vapor refrigerant consists of the heat absorbed by the evaporator and the heat of compression added by the mechanical energy of the compressor motor. The heat from the hot vapor refrigerant in a condenser is removed first by transferring it to the walls of the condenser tubes and then from the tubes to the condensing or cooling medium. The cooling medium may be air or water or a combination of the two. An air cooled condenser is one in which the removal of heat is done by air. It consists of steel or copper tubing through which the refrigerant flows. The size of tube usually ranges from 6mm to 18mm outside diameter, depending upon the size of the condenser. Generally copper tubes are used because of its excellent heat transfer ability. The condensers with steel tubes are used in ammonia refrigerating systems. Majority of the domestic refrigerators uses the natural convection air cooled condenser. The present work refrigerator uses the natural convection air cooled condenser. In natural convection air cooled condenser, the heat transfer from the condenser coils to the air is by natural convection. As the air comes in contact with the warm condenser tubes, it absorbs heat from the refrigerant and thus the temperature of air increases. The warm air being lighter, rises up and cold air from below rises to take away the heat from the condenser. This cycle continues in natural convection air cooled condensers. This work is an experimental approach to increase the heat to be rejected in the condenser as well as increase the performance of the system. If the condenser is having more fins spacing then the number of fins available at the condenser are less. Due to this surface area decreases. Therefore less heat transfer occurs. On the other hand if the condenser is having less fins spacing then the number of fins available at the Condensers are more. Therefore more heat rejection takes place in the condenser. Because of more heat rejection sub cooling occurs at the exit of the condenser which in turn increases the performance of the system.

Decreasing energy consumption and increasing efficiency is one of the most important points in our era. Becoming a matter of primary importance in air conditioning, industrial and commercial cooling applications, supermarket cooling, blast freezing and process cooling applications, energy efficiency affects design of chillers (and its equipment such as condensers, compressors etc.) and urges manufacturers to develop high performance, energy-efficient, environment friendly, economic, and long life products.

Becoming a matter of primary importance in air conditioning, industrial cooling, supermarket cooling, commercial cooling, blast freezing and process cooling applications, etc., energy efficiency affects the design of chillers that account for a significant ratio of the energy consumption in plants. Legal legislations that for the short run limit, and in the long run prohibit the use of fluids of high global warming potential, that are detrimental to the ozone layer are also influential on designs. Since air cooled condensers are among the essential components of cooling groups, efforts to improve their energy efficiency are made incessantly, the related national and international standards are upgraded and limitations on their energy consumptions are always increasing.

Our statement will convey –in the light of the latest applicable standards- detailed and comparative information on applications aimed at improving the energy efficiency in air cooled condensers, highlighting the importance of energy efficiency in cooling installations.

2.2 The effect of condensation temperature on capacity

The condensation temperature of the refrigerant is considered to be 6°C – 20°C above air inlet temperature for general purposes. The condensation temperature varies according to the ambient temperature in which the system will operate. This said, the condensation temperature for applications is commonly taken as 30-60°C.

Factors taken into consideration for determining the condensation temperature

1. Ambient temperature,
2. Thermo physical properties of the refrigerant,
3. Properties of the selected compressor and
4. The dimensions of the condenser.

While providing the nominal condenser capacity in condensers as per the Euro vent Standard, the air inlet temperature and condensation temperature are taken as 25°C and 40°C respectively. In other words, T = 15 °C.

A low value should be selected for the temperature differential T, in places of high ambient temperature. For instance, while designing for the conditions of Antalya the T value should be selected within the range of 7° – 10° C. For systems that will operate in outdoor environments in conditions of Turkey, the temperature differential should be lowered as one goes from the north to the south and selections should be made accordingly. It must always be considered that High Compression Temperature creates a load on the compressor that reduces efficiency and shortens its useful life. It will be quite beneficial in the design for the condensation temperature to be specified as low as possible. However, in some conditions it is not possible to take a low value for the condensation temperature. For example in Middle East countries where the outdoor temperature is 50°C-55°C, high condenser temperature is unavoidable.

The following example shows the compressor absorbed power at various condensation temperatures and the achieved cooling capacities for a semi-hermetic type HGX4/555-4 model compressor manufactured by BOCK, which uses R134A gas. The cooling gas selected for the example is R134A. As seen from the table, when the condensation temperature rises from 30°C to 60°C, the compressor draws 25% more power while the cooling capacity drops by 38.5%, the COP value decreases by 51% and the a unit that is 38.5% larger must be used to achieve the cooling capacity declared in the design.

2.3 Tried and True for Millennia

Copper tubing has been in use by civilization for thousands of years, and it has been found present in the water lines of Ancient Egypt.

Modern-day houses having copper pipes installed 70 years ago are still working well in the present day and require far less to fix and modify than alternative methods. Should you choose copper, you will have very little maintenance, whereas even stainless steel begins to rust over time, especially if it is not painted or coated.

There are a great variety of uses for copper tubing. If you invest a high degree into copper, you may be able to easily reclaim your investment with high rates of return for reuse and recycling for example. Copper is a fantastic renewable resource because it is easily recycled.

Beneficial Qualities of Copper Tubing

1. Proven Longevity
2. Comparably Lightweight
3. Resistant to Corrosion
4. Can Be Formed to Shape
5. Largely Nontoxic and Non Carcinogenic

2.4 Copper Condenser Coils

Condensers are one of the prominent uses of copper tubing. Changing gases into liquids occur at appropriate temperature dew point. Copper’s fantastic heat transfer rates are one of the best choices for achieving this goal, exceeding other heat transfer tubing materials (such as aluminum in this case) by nearly 800% while maintaining a strong, lightweight structure. Having a higher heat transfer rate renders less total material necessary for a given space, allowing for more compact, lightweight, and simple condenser applications that minimize back pressure. CTCG builds condensers with copper tubing, aluminum tubing, and stainless steel tubing, to customer prints–but we highly recommend copper tubing for condenser applications.

2.5 All Designs Accepted for Review

Whether your coil is, serpentine, coaxial, or completely customized, all designs are accepted for review. Both hard drawn and annealed, flexible copper tubing acceptable for forming and coiling. Easily outsource production runs of custom copper tubing coils or buy bulk loads of custom and non-custom length copper tubing, including level wound coils, water tubing and piping, refrigeration tubing, DWV and capillary tubing.

Providing Tubing Coils for Most Industries

Copper meets many industries’ highest standards for excellence; thus we recommend the use of copper for your applications in

1. The Aerospace Industry,
2. Construction Industry,
3. Freon Recycling Industry,
4. Medical Industry, and the
5. AC & Refrigeration Industry

Most projects can be easily manufactured, including such items as gas turbines, artistic projects, diagnostic hardware, in one to four weeks (and sometimes faster). Check our present inventory to see what tube types can be serviced this month

2.6 Why Copper

Copper tubing is the ideal material for many applications. Its strength, the ease with which it is fabricated and soldered, and its high conductivity to heat is invaluable in the process of building homes and industrial buildings, particularly for the refrigeration and air conditioning equipment installed.

Tubing made from copper is one of the few naturally-sourced materials that are 100% recyclable, so no metal is lost in reclaiming copper from pipes no longer needed, making it a good choice for the future.