Automobile Air Conditioning "DWIBAHASA"

PENGHARGAAN

           

 Terlebih dahulu, saya Farmiezie Morni ingin merakamkan ribuan terima kasih kepada pensyarah saya, En Wright anak Asui kerana telah mengajar saya terutamanya cara-cara penyengaraan kenderaan. Sepanjang tempoh menyiapkan kerja kursus ini, banyak ilmu yang telah dicurahkan oleh beliau seperti merombak, memeriksa, menguji, melaras, memasang komponen-komponen kenderaan dan sebagainya.

Tidak lupa juga saya tujukan penghargaan ini kepada kedua-dua ibu bapa saya kerana telah banyak membantu saya dari segi kewangan dan dorongan yang amat berguna kepada saya sepanjang masa saya untuk menyiapkan kerja kursus ini.

Semoga kerja kursus ini dapat dijadikan rujukan dan panduan kepada mana-mana mahasiswa yang ingin merujuk bahan saya pada masa akan datang. Mohon maaf sekiranya terdapat sebarang kekurangan dalam kerja kursus ini.

Akhir kata, saya berasa amat bersyukur kerana saya telah menyiapkan karja kursus ini sebelum masa yang ditetapkan untuk menghantar kerja kursus ini. Sesungguhnya yang baik itu datang daripada Tuhan, manakala yang lemah itu atas kelemahan kita sebagai insani.

Sekian, terima kasih.      

 PENGENALAN

 

Sistem penghawa dingin kereta berfungsi menggunakan gas yang dimampatkan oleh kompresor. Gas ini akan mengalir melalui cooling coil yang berada di dalam kabin kereta dan blower akan meniup angin melalui cooling coil ini dan mengeluarkan angin yang sejuk. Selepas itu gas ini perlu disejukkan kembali dan akan melalui radiator kondenser yang akan menyejukkan gas tersebut. Begitulah secara asasnya bagaimana  penghawa dingin kereta berfungsi.

 Automobile Air Conditioning

Theory of Air Conditioning

In order to understand how air conditioning works, it is necessary to understand several basic laws about the flow of heat. While it may seem puzzling to talk about heat in the same breath as air conditioning, heat is your only concern. An air conditioner does not cool the air, but rather, removes the heat from a confined space.

The law of entropy states that all things must eventually come to the same temperature; there will always be a flow of heat between adjacent objects that are at different temperatures. When two objects at different temperatures are placed next to each other, heat will flow from the warmer of the two objects to the cooler one. The rate at which heat is transferred depends on how large the difference is between their temperatures. If the temperature difference is great, the transfer of heat will be great, and if the temperature difference lessens, the transfer of heat will be reduced until both objects reach the same temperature. At that point, heat transfer stops.

Because of entropy, the interior of an automobile tends to remain at approximately the same temperature as the outside air. To cool an automobile interior, you have to reverse the natural flow of heat, no matter how thoroughly insulated the compartment might be. The heat which the body metal and glass absorb from the outside must constantly be removed.

The refrigeration cycle of the air conditioning system removes the heat from a vehicle's interior by making use of another law of heat flow, the theory of latent heat. This theory says that during a change of state, a material can absorb or reject heat without changing its temperature. A material is changing its state when it is freezing, thawing, boiling or condensing. Changes of state differ from ordinary heating and cooling in that they occur without the temperature of the substance changing, although they cause a visible change in the form of the substance. While many materials can exist in solid, liquid, or gaseous form, the best example is plain water.

Water is a common material that can exist in all three states. Below 32°F (0°C), it exists as ice. Above 212°F (100°C), at sea level air pressure, it exists as steam, which is a gas. Between these two temperatures, it exists in its liquid form.

Since a change in state occurs at a constant temperature, it follows that a material can exist as both a liquid and a gas at the same temperature without any exchange of heat between the two states. As an example, when water boils, it absorbs heat without changing the temperature of the resulting gas (steam).

The change from a solid to a liquid and vice versa is always practically the same for a given substance-32°F (0°C) for water-but the temperature at which a liquid will boil or condense depends upon the pressure. For example, water will boil at 212°F (100°C), but only at sea level. The boiling point drops slightly at higher altitudes, where the atmospheric pressure is lower. We also know that raising the pressure 15 lbs. above normal air pressure in an automobile cooling system will keep the water from boiling until the temperature reaches about 260°F (127°C)

One additional aspect of the behavior of a liquid at its boiling point must be clarified to understand how a refrigeration cycle works. Since liquid and gas can exist at the same temperature, either the evaporation of liquid or the condensation of gas can occur at the same temperature and pressure conditions. It's just a matter of whether the material is being heated or cooled.

As an example, when a pan of water is placed on a hot stove, the heat travels from the hot burner to the relatively cool pan and water. When the water reaches it's boiling point, its temperature will stop rising, and all the additional heat forced into it by the hot burner will be used to turn the liquid material into a gas (steam). The gas thus contains slightly more heat than the liquid material.

If the top of the pan were now to be held a couple of inches above the boiling water, two things would happen. First, droplets of liquid would form on the lower surface of the lid. Second, the top would get hot very quickly. The top becomes hot because the heat originally used to turn the water into steam is being recovered. As the vapor encounters the cooler surface of the metal, heat is removed from it and transferred to the metal. This heat is the same heat that was originally required to change the water into a vapor, and so it again becomes a liquid.

Since water will boil only at 212°F (100°C) and above, it follows that the steam must have been 212°F (100°C) when it reached the top and must have remained that hot until it became a liquid. The cooling effect of the top (which started out at room temperature) caused the steam to condense, but both the boiling and the condensation took place at the same temperature.

To sum up, refrigeration is the removal of heat from a confined space and is based on three assumptions:

1.    Heat will only flow from a warm substance to a colder substance.

2.    A refrigerant can exist as both a liquid and a gas at the same temperature if it is at its "boiling point." A refrigerant at its boiling point will boil and absorb heat from its surroundings if the surroundings are warmer than the refrigerant. A refrigerant at its boiling point will condense and become liquid, losing heat to its surroundings, if they are cooler than the refrigerant.

3.    The boiling point of the refrigerant depends upon the pressure of the refrigerant, rising as the pressure rises and falling as the pressure falls. The operation of the refrigeration cycle illustrates how these three laws are put to use.

Fig. 1: The same substances can exist in three states, depending on the temperature

How the Air Conditioner Works

REFRIGERATION CYCLE

Any automotive air conditioning system employs four basic parts-a mechanical compressor, driven by the vehicle's engine; an expansion valve, which is a restriction the compressor pumps against; and two heat exchangers, the evaporator and the condenser. In addition, there is the refrigerant that flows through this system.

The belt-driven compressor uses engine power to compress and circulate the refrigerant gas throughout the system. The refrigerant passes through the condenser on its way from the compressor outlet to the expansion valve. The condenser is located outside the passenger compartment, usually in front of the vehicle's radiator. The refrigerant passes from the expansion valve to the evaporator, and after passing through the evaporator tubing, it is returned to the compressor through its inlet. The evaporator is located inside the vehicle's passenger compartment.

When the compressor starts running, it pulls refrigerant from the evaporator coil and forces it into the condenser coil, thus lowering the evaporator pressure and increasing the condenser pressure. When proper operating pressures have been established, the expansion valve will open and allow refrigerant to return to the evaporator as fast as the compressor is removing it. Under these conditions, the pressure at each point in the system will reach a constant level, but the condenser pressure will be much higher than the evaporator pressure.

The pressure in the evaporator is low enough for the boiling point of the refrigerant to be well below the temperature of the vehicle's interior. Therefore, the liquid will boil, remove heat from the interior, and pass from the evaporator as a gas. The heating effect produced as the refrigerant passes through the compressor keeps the gas from liquefying and causes it to be discharged from the compressor at very high temperatures. This hot gas passes into the condenser. The pressure on this side of the system is high enough so that the boiling point of the refrigerant is well beyond the outside temperature. The gas will cool until it reaches its boiling point, and then condense to a liquid as heat is absorbed by the outside air. The liquid refrigerant is then forced back through the expansion valve by the condenser pressure.

Fig. 1: Basic components of an air conditioning system and the flow of refrigerant

 

Refrigerant

A liquid with a low boiling point must be used to make practical use of the heat transfer that occurs when a liquid boils. Refrigerant-12 (R-12) is the refrigerant that was universally used in automotive air conditioning systems. At normal temperatures, it is a colorless, odorless gas that is slightly heavier than air. Its boiling point at atmospheric pressure is -21.7°F (minus 6°C). If liquid R-12 is spilled into the open air, it would be seen for a brief period as a rapidly boiling, clear liquid.

R-12 was nearly an ideal refrigerant. It operated at low pressure and condenses easily at the temperature ranges found in automotive air conditioning systems. It is also non-corrosive, non-toxic (except when exposed to an open flame), and nonflammable. However, due to its low boiling point and the fact that it is stored under pressure, certain safety measures must be observed when working around the air conditioning system. Unfortunately it was discovered the carbo-floro-carbons (CFC's) which were chemicals in the same group as dichlorodifluoromenthane, which you know as R-12 or Freon were depleting the ozone layer of the atmosphere.

The function of each part of Absorbtion Refrigeration System.

o   In simple absorbtion system, used part are shown in Figure. These are mainly evaporator, absorber, compressor and condensor. In complex system to improve the performance and high efficiency of the plant, a heat exchager, an analyzer anc rectifier are used. The other part of expansion valve, pressure reducing valve and pump ae also fitted.

o   Evaporator and absorber are used in the side of low pressure side but compressor and condensor are used in the side of the high pressure side. The pressure of low side is according to the temperture of evaporator and high side pressure is according to the condensing temperature.

The Part Of Air Conditioning

-The part of automobile air onditoning consist of these 10 item:

1) Compressor

2) Condensor

3) Analyzer and rectifier

4) Expansion valve

5) Evaporator

6) Receiver

7) Absorber

8) Heat exchanger

9) Pump

10) Liquid pre-cooler

-All these parts are played a very inportant role in Car, Zeep, trucks, bus, van good courier who transport milk-ice cream an so many other eatible thing. Modern world without A.C and heating cannot be run smoothly. So both systems ar e run side by side in automobile industry. According to the weather, each system have a play good efficient role.

Function Of all Air Conditoning the Part

The function of each part are described below:

            1.Compressor:

Commonly referred to as the heart os the system,the compressor is the belt driven pump that is fastened to the engine.It is responsible for compressing and transferring refrigerant gas.The A.C system is split into two sides,a high pressure side and a low pressure side;defined as discharge and suction.Since the compressor is basiccally a pump;it must have an intake side and a discharge side.The intake,or suction side,drawsa in refrigerant gas from the outlet of the evaorator.In some cases it doe this via the accumulator.Once the refrigerant is drawn itto the suction side,it is compressed and sent to the ondensor,where it can then transfer the heat that is absorbed from the inside of the vihicle.

            2.Condensor:

This is the area in which heat dissipation occurs. The condensor,in many cases,will have much the same appearence as the radiator in ypur car as the two have very similar functions.The condensor is designed to radiated heat.Its location is usually in front of the radiator,but in the same cases,due to aerodynamic inprovements to the body of the vehicle;it locations may differ.Condensor must have good air flow  airtime in the system is in operation.On rear wheel drive vehicles,thiis usually accomplished by taking advantage of your engines cooling fan.On front wheels drive vehicles,condensors arir flow is suplemented with one or more electric cooling fans.As hot compressed gasses are introduced into the top the condensor,they are cooled off.As the gas cools,it condenses and exits the bottom od the condensor as a high pressure liquid.

            3.Analyzer and rectifier:

 They are used to function as an automatic flow control system.They control the flow of the refrigerant into the condensor and stores the liquid refrigerant in a reservoir incorporated into the condensor.

4.Expansion Valve:

It is used to reduce the pressure of ammonia liquid.It is placed bettween evaporator and receiver but in some equipments it is placed bettween condensor and evaporator.In some equipments the expansion vave is removed and used hydrogen gas which is called flasg gas,for reducing the pressure of ammonia refrigerant.

      i

5.Evaporator:

This is called the Brine Cooler,in which there is a welded cylinder shaped steel-spot.There are many tubes on its top in which the refrigerant liquid gets transformed into vapour owing to less pressure.Under reduced pressure pure ammonia enters in the evaporator and evaporates by taking the latent heat fom adjoinings.In this way it produces cooling.After that,it flows toward the absorber.

6.Receiver

This storage tank for the refrigerant which is placed bettween condensor and expansion valve.This is filled with liquid ammonia under the high pressure.

7.Absorber

The absorber is the cylinderical vessel or shell in which the vapour of ammonia is lead as it comes from the refrigeration coils and where it is mixed with the weak liquid which comes from the lower part of the generator.It may be of the shell and coil.straight tube,double pipe or atmospheric type.This is used for condensing the refrigerant vapour at low pressure.Due to this the weak solution gets transfromed into strn solution and the vapuor pressure of amonia becomes less that of the solution.

Following are the main functions of it: 

a.    Weak solution changes into strong solution.

b.    The latent heat of condesation increases with the temperature of solution.

c.    The heat of water mixture increases the temperature of the solution.

d.    Vapour pressure of ammonia reduced the pressure of the solution.

The cooling water used in the absorber is to remove the heat of condensation and water mixture.the quatity of ammonia increased to be reduced absorber temperature which goes in solution.The vapour pressure of solution is reduced that pressure from ammonia vapuor in evaporator while pressure of both ambient pressure are equal bywhich stream is absorber from evaporator.Thus the absorbtion function is established abd the steam of refrogreant by the system.Removed heat increase haste in absorber by which absorbtions maintains temperature.The rate of flowing isincreased in evaporator and it increase refrigerating capacity.Therefore this is useful for cooling water of least temperature absorber.