Automobile Airconditioning

INTRODUCTION


Keeping comfortable when we are driving is very important especially when we spend a god deal of time in our vehicle. Automotive air conditioning helps us stay nice & cool when we are driving and comes in handy when we have to travel for work.
There are 4 important factors which affect the human comfort such as temperature, humidity, purity & motion of air. The system which effectively controls these conditions to produce the desired effects upon the occupants of the space known as an air conditioning system.
An automobile air condition system involves three main processes of heating, cooling & dehumidification. The heat needed to warm the automobile is derived from engine coolant or circulating warm water after passing through a heating coil.
For producing a cooling effect an evaporator coil operated by a refrigerating system is placed inside the chamber of automobile. The air to be circulated inside the passenger comfort is cooled by the evaporator coil. Depending upon the size of vehicle, frequency or shops atmosphere and traffic condition. The air conditioner loads vary from instant to instant place to place. Due to moving nature of vehicle, the equipment used should be light, contact, quickly serviceable and unaffected by vibration.
The dust particles are entrapped by the wet surface of the evaporator coil and drained out with the condensed moisture. This provides clear & pure air conditioners.
In comparison to domestic air conditioner a very large amount of cooling capacity is required for a car. This is due to metal construction of a car. The flow of air around a moving car and relatively large glass areas in the passenger compartment. Typically a car air conditioning system may be in between 1-4 tons.
A refrigerating machine driven by car engine is used in the automobile air conditioning to maintain the desired temperature level. For heating purposes the warm water from engine cooling system is used. The heat required to warm the automobile is generally provided. Circulating water then a heating coil.

BASIC PARTS & THEIR FUNCTIONS
The automobile air conditioner consists of following basic parts. They are
1. Compressor
2. Condenser
3. Evaporator
4. Pressure Regulating Devices
a. Orifice Tube
b. Thermal Expansion Valve
c. Receiver Drier
d. Accumulator

COMPRESSOR
Commonly refereed to heart of a system. The compressor is a belt driven pump. That is fastened to engine. It is responsible for compressing & transferring refrigerant gas. The A.C system is split into 2 sides, a high pressure side and low pressure side, defined as discharge and suction. Since the compressor is basically a pump it must have an intake side and a discharge side. The intake or suction side draws in refrigerant gas from the outlet of the evaporator.
Once the refrigerant is drain into the suction side, it is compressed and sent to condenser where it transfers the heat that is absorbed from inside of the vehicle.

CONDENSER
This is the area where heat dissipation occurs. The condenser in many cases will have much the same appearance as the radiator in our car as the 2 have very similar functions. The condenser is designed to radiate heat. Its location is usually in front of radiator. But in some cases due to arrangement improvements to the body of a vehicle its location may differ. Condensers must have good air flow anytime. The system is in operation on rear wheel drive vehicles. This is usually accomplished by taking advantage of our existing engine cooling fan. On front wheel drive vehicles, condenser air flow is supplemented with one or more electric cooling fans.
As hot compressed gases are introduced to the top of condenser they are cooled off. It is the gas cools; it condenses and exits the bottom of condenser as a high pressure liquid.

EVAPORATOR
Located inside the vehicle, the evaporator serves as the heat absorption component. The evaporator provides several functions. Its primary duty is to remove heat from inside of our vehicle. A secondary benefit is dehumidification. A warm air travels through aluminium fins of the cooler evaporator coil; moisture contained in the air condenses on its surface. Dust & pollen passing through stick to its wet surfaces and drain off to outside. On humid days we may have seen this as water dripping from our vehicle. Rest assured this is perfectly normal.
The ideal temperature of the evaporator is 32 Fahrenheit or Celsius. Refrigerant enters to the bottom of the evaporator as a low pressure liquid. The warm air passing through evaporator fins causes the refrigerant to boil, it can absorb large amount of heat. This heat is then carried off with the refrigerant to outside of vehicle. Several other components work in conjunction with the evaporator. As mentioned above the ideal temperature for an evaporator coil is 32 F. temperature & pressure regulating devices may be used to control its temperature, while there are many variations of devices used, their main functions are the same, keeping pressure in the evaporator low and keeping the evaporator from freezing. A frozen evaporator coil will not absorb as much as heat.

PRESSURE REGULATING DEVICES
Controlling the evaporator temperature can be accomplished by controlling refrigerant pressure and flow into evaporator many variables of pressure regulator have been introduced since 1940s. Listed below are most commonly found.

Orifice Tube
The orifice tube probably the most commonly used can be found in most ford models. It is located in the inlet tube of the evaporator or the liquid line somewhere between the outlet of the condenser and inlet of evaporator, that suddenly makes hot to cold we should then see small dimples placed in the line that keeps the orifice tube from moving most of orifice tube in use today measure approximately 3 inches in length and consists of small brass tube surrounded by plastic and covered with a filter screen at each end. It is not uncommon for these tubes to become clogged with small debris while inexpensive usually between 3 to 5 dollars. The labour to replace one involves recovering the refrigerant opening the system up replacing the orifice tube. Evacuating and then recharging it might male sense to install a larger prefilter in front of orifice tube to minimize the risk of problem reoccurring. Some ford models have a prominently fixed orifice tube in liquid line. This tube can be out and replaced with a combination filter orifice assembly.

Thermal Expansion Valve (TXV)
It is commonly used in import and after market system. This can sense both temperature and pressure. It is very efficient at regularly refrigerant flow to evaporator. Several variables of this valve are commonly found. Another example of TXV is chryster’s block type. This type of valve is usually located at that firewall between the evaporator inlet and outlet tubes end liquid at suction times. These types of valves although efficient have some disadvantages over orifice tube systems. Like orifice tube these valves can become clogged with debris, but also have some moving parts that may stick and malfunction due to corrosion.

Receiver Drier
This receiver drier is used on the high side of systems that use a TXV. This type of malfunctioning valves regulates liquid refrigerant to ensure that valve gets liquid refrigerant a receiver is used. The primary function of receiver drier usually has a sight glass on the top. This sight glass is often used to charge the system under normal operating conditions vapour bubbles. This type of mistake can lead to dangerous changed condition.

Accumulator
The accumulator accommodates an orifice tube to meter refrigerants into evaporator. It is connected directly to evaporator outlet and stores excess liquid refrigerant. Introduction of liquid refrigerant into a compressor can do a serious damage. Compressors are designed to compress gas not liquid. The chief rule of accumulator is to isolate the compressor from any damaging liquid refrigerant. Accumulator like receiver drier also removes debris and moistures from a system. It is a good idea to replace accumulator each time in a system is opened up. For major repair & anytime moisture and or debris is of concern. Moisture is enemy number one for you’re a/c system. Moisture in a system mixes with refrigerant and forms a corrosive acid. When in doubt it may be to your advantage to change the accumulators or receiver in your system, while this may be a temporary discomfort for your wallet. It is of long term benefit to your a/c system.

CONCLUSION
Air conditioning a luxury that is becoming a necessity for commercial as well as domestic use in a system of controlling temperature, humidity, cleanliness and movements of air. It has a positive effect on personal performances, health and indoor equipment by the very virtue of its ever refining feature and the benefits offered to consumers. The industry is expanding rapidly and having its presence everywhere.

REFERENCES
Automobile engineering by R.B Gupta
Internet (www.familycar.com/ac1.htm)