|08-01-2011, 12:36 PM||#1|
Join Date: Apr 2011
Location: NW suburb of Chicago
Understanding Auto Air Conditioning-Part 1 by Bernie
UNDERSTANDING A/C Part 1
By Bernie 8/1/11
Air conditioning systems operate on the principles that heat flows from hot to cold, a refrigerant’s boiling point temperature increases/decreases with increasing/decreasing pressure, heat is extracted from/released to the environment during a change of state from liquid to gas/gas to liquid.
The basic automotive air conditioning system consists of an evaporator, compressor, condenser, receiver/dryer, metering device and other components such as relief valve and pressure sensor(s).
The evaporator is located inside the passenger compartment. It looks like a radiator with the distinction that it absorbs heat from the surrounding air instead of radiating it.
The compressor is located in the engine compartment and is belt driven by the engine. Its purpose is to compress low pressure, low temperature, superheated refrigerant gas from the evaporator into high pressure, high temperature, highly superheated gas. Superheated gas is gas that has been heated beyond the liquid state boiling point. The compressor is a mechanical device and requires lubrication. A specified quantity and type of oil is therefore added prior to charging the system with refrigerant. An important item to note is that different oils are used with different refrigerants and they are not generally compatible with each other.
The condenser is a radiator mounted in the front of the car for the purpose of condensing high pressure, high temperature, highly superheated refrigerant gas coming from the compressor outlet into lukewarm, high pressure liquid.
The receiver/dryer is a small storage tank in the liquid line between the condenser and the expansion valve. It prevents refrigerant gas from flowing down the liquid line, stores a small amount of liquid refrigerant for use by the evaporator, and incorporates a desiccant bag to keep the system dry.
There are several types of metering devices used in air conditioning. A common approach is to employ a thermostatic expansion valve at the inlet to the evaporator. The valve meters the amount of refrigerant entering the evaporator in order to maintain efficient cooling. The valve also acts as a separator between the high and low pressure sides.
Other components include pressure sensors and a relief valve. These devices monitor system health and ensure personal safety in the event of a catastrophic failure. A sensor on the Saturn SL1 prevents the compressor from turning on unless the system pressure is 40 psig or more. The relief valve is mounted on the compressor housing just below the high pressure outlet.
Refrigerant is the working fluid in an air conditioning system. It is piped through a closed loop where it undergoes repetitive changes of state from liquid to gas, as it absorbs heat, and gas to liquid, as it condenses and releases heat. A significant characteristic of the closed loop is that it consists of a low pressure side and a high pressure side. The high side pressure is significant and great care must be applied when working near or on the system. The refrigerant pressure in a Saturn S Series running at 2000 RPM is roughly 24-30 psig on the low side and 180-215 psig on the high side, at an ambient temperature of 80 ºF. When off and at rest, the system pressures equalize so the high and low side pressures are the same. The pressure in an equalized system is a function of both the refrigerant type and ambient temperature.
Upon turn on, refrigerant is forced to flow through the air conditioning system as a result of compressor action. The compressor compresses low pressure, low temperature, superheated refrigerant gas into high pressure, high temperature, highly superheated gas that is then piped to the condenser. It is important to note that automotive compressors are not designed to handle liquid refrigerant and will suffer damage when liquid refrigerant is introduced.
The condenser, which looks and acts like a radiator, allows the high pressure, high temperature, highly superheated gas coming from the compressor to cool and condense into a lukewarm high pressure liquid.
Liquid refrigerant from the condenser is piped to the receiver/dryer. The receiver dryer, which is filled with a desiccant to reduce moisture, traps refrigerant gas in the line and stores a small amount of liquid refrigerant for use on demand by the thermostatic valve.
The outlet of the receiver dryer connects to a metering device. This can be a thermostatic or other type of valve. A thermostatic valve maintains a prescribed design temperature differential across the evaporator by closely controlling the refrigerant flow. This type of valve is always located at the evaporator inlet.
Liquid refrigerant exits the expansion valve and is released into the evaporator where it absorbs heat from air passing through the fins. The absorbed heat causes the lukewarm, high pressure liquid refrigerant entering the evaporator to vaporize (boil off) resulting in low pressure, low temperature, superheated gas at the exit port.
System And Refrigerant Properties
The saturation (boiling) point temperature of any liquid or refrigerant increases as pressure increases. Special charts referred to as pressure-temperature or saturation charts list a wide range of saturation point pressures and temperatures for a number of common refrigerants.
An air conditioning system is said to be equalized when the high and low side pressures are the same. The pressure observed when an air conditioning system is off and equalized is called static pressure. Static pressure is temperature dependent, the greater the ambient temperature the greater the pressure. The saturation pressure corresponding to the ambient temperature of an equalized system, as taken from a saturation chart, should closely match the measured static pressure. If not, then the system is definitely low on refrigerant or contaminated with air.
The refrigerant in a working air conditioning system changes states repeatedly from gas to liquid and liquid to gas as it circulates. The process of changing states involves large transfers of energy that happen without change in temperature. Under normal atmospheric conditions the temperature of liquid refrigerant, or any other substance for that matter, increases as heat is added and continues to increase until the liquid boils and turns into gas. The point at which liquid boils or gas condenses is known as the saturation point. During the liquid to gas change of state, heat energy continues to be absorbed by the refrigerant even though its temperature remains constant. Once the change of state process is fully complete, the refrigerant gas temperature once again continues to rise with applied heat, resulting in gas that is superheated. The process is completely reversible. The superheated gas temperature decreases during cooling until a change of state occurs. During the change of state, even though heat is being released, the temperature of the refrigerant remains constant until the change of state from gas to liquid is complete. Once completed, the liquid’s temperature will again decrease as it cools. Subcooling refers to liquids whose temperature is below the saturation point.
The magnitudes of superheat at the evaporator and subcooling at the condenser reflect system performance. The amount of superheat or subcooling is obtained by measuring the pressure and temperature of the corresponding outlet, converting the observed pressure to saturation temperature using a saturation chart, and then calculating the difference between the two temperatures. The difference is the amount of superheat or subcooling above/below the saturation point. For example, say that the refrigerant is R-134a and that the indicated pressure is 26.1 psig with a measured temperature of 55 ºF, or 213.5 psig with a measured temperature of 120 ºF. According to the saturation chart, 26.1 psig corresponds to a saturation temperature of 30 ºF, a difference of 15 ºF (55-30) of superheat. Again looking at the saturation chart, 213.5 psig corresponds to a saturation temperature of 135 ºF, yielding 15 ºF (135-120) of subcooling.
Automotive air conditioning systems incorporate two service valves, one on the low side and one on the high side. These are usually located near the compressor. The pressure drop from the compressor to the inlet of the thermostatic valve, or from the outlet of the evaporator back to the inlet of the compressor is negligible. This means that superheat and subcooling can be calculated for nearly every point within the system where temperature can be measured. Two points of particular interest are the outlets of the evaporator and condenser, which should fall within given parameters. In the end, knowing the manufacturer’s specifications for the system is crucial in evaluating system performance.
The thermostatic expansion valve, designed to maintain a preset temperature difference between the inlet and outlet of the evaporator (superheat), exerts no control over the evaporator’s actual temperature. That means that the evaporator temperature can float up or down, as long as it stays above 32 ºF to prevent condensation from freezing. An evaporator outlet temperature of 32 ºF corresponds to a refrigerant saturation pressure, according to the saturation chart, of less than 27.8 psig. It has to be less than 27.8 psig or heat could not flow from the evaporator to the refrigerant. SaturnFan member JerryHughes tells us in one of his posts, that at 2000 RPM and 70 ºF ambient, his Saturn system pressures are 26-30 psig on the low side, and 115-170 psig on the high side, with outlet air temperatures of 40-48 ºF. He goes on to say that at 80 ºF, the system pressures are then 24-30 psig on the low side, and 180-215 ºF on the high side, with outlet air temperatures of 42-49 ºF. Low side pressures of 24-30 psig correspond to saturation temperatures of roughly 27.5-34.5 ºF, indicating a minimum superheat of 4.5ºF (32-27.5).
An evaporator that is starved of refrigerant results in high superheat. Conversely an evaporator that is flooded with liquid refrigerant exhibit low superheat. In either case this points to a thermostatic valve malfunction though it is possible for moisture to freeze and or contaminants to clog the valve and prevent proper flooding of the evaporator. High superheat can also mean that the system is low on refrigerant. If so, bubbles would be seen through the sight glass if installed, and both low side and high side pressures would be low.
An abnormally cold evaporator results from poor air circulation. The thermostatic valve, in response to a drop in evaporator temperature, restricts the flow of refrigerant to maintain proper superheat. This action, in turn, has the effect of lowering the low side pressure.
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|10-08-2012, 11:09 PM||#2|
Join Date: Oct 2012
Re: Understanding Auto Air Conditioning-Part 1 by Bernie
hmmmm the thread is now for like very old well in fact before reading your post i really didn't knew more of the principles and the components kind of stuff and now i do ........ i do really appreciate you for such kind of info.......
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