Auto air conditioning unit 3


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Auto air conditioning unit 3

  1. 1. || || || By M A QADEER SIDDIQUI ||
  2. 2. || || || UNIT 3 Air conditioning was an invention that made the South a viable place to live and do business. When they figured out how to add it to your car, the trip there and back got a lot better, too. Your A/C system may seem complicated, and it is, but it's also easy to understand, and has some parts that you can service yourself. How it Works, Basically Any system that lowers temperature operates in similar fashion. First you take a gas, like Freon, and place it in a sealed system. This freon is then pressurized using a compressor. As it's pressurized, it gets hot by absorbing the heat around it. This hot gas is then circulated through a series of tubes that dissipate the heat. Scientifically, the gas removes heat rather than adds cold, but that's a lesson in physics that doesn't really matter to us right now. The gas can lose lots of its heat, in other words it gets really cold, when you reduce the pressure. As it cools it becomes a liquid. This is when you get cold air blowing on your sweaty forehead. To use this system in a car, it needed very little adaptation from its early applications as a refrigeration device. since it was discovered that Freon (R-12) was harmful to the earth's Ozone layer, it's been phased out for automotive use, and replaced with the slightly less efficient, but harmless R-134a refrigerant. This is actually good news because for years it was against the law to service your own air conditioning system without a license. Now that the refrigerant is safer, we can all work on our own A/C systems again! Some cars have not been converted from the old R12 to R134a, but this conversion can be done easily. Components of Automotive Air Conditioning Your air conditioning system is made up of a compressor, a condenser, an evaporator (or drier), refrigeration lines and a couple of sensors here and there.      Compressor: This is the heart of your a/c system. The compressor is what takes the refrigerant (the gas) and pressurizes it so it will cool the air. It's run by an engine belt. The compressor also has an electrically operated clutch that turns the compressor on and off as you demand more cool air. Condenser: The condenser is like a miniature radiator, usually mounted at the front of the car right next to your big radiator. Sometimes the condenser will have its own electric cooling fan, too. The hot, compressed air passes through the condenser and gets lots cooler. As it cools, it becomes a liquid. Evaporator: The evaporator is another little radiator that does just the opposite task as the condenser. As the super-cool liquid is passed through its tubes, air is forced through and gets really cold, right before it hits your face. As it warms up again, the refrigerant starts turning back into a gas. Thermal Expansion Valve: You don't always want to freeze your toes off, so the a/c system has a valve that controls the flow of super-cool refrigerant to the evaporator. This way you can regulate how cold the air blowing on you gets. There are a few types of valves in use these days, but they all do the same thing. Drier or Accumulator: The drier, also known as the receiver-drier, is sort of the safety catch for your system. The compressor is only supposed to compress the gas form of your refrigerant. However, there's always a chance that some liquid could make it back that far. The drier catches this liquid before it can damage your compressor. Since even the tiniest leak or careless installation can introduce water moisture to the system, the drier absorbs this chemically, using what's called a dessicant (similar to that packet of "DO NOT EAT" that comes with electronics). The drier also has a filter that catches any gunk that might be in there. That's pretty much the whole story! Different systems also have sensors here and there to tell it pressure and temperatures, but they are specific to a make and model of vehicle. Pretty coolhuh? If you need to do some work on your car or truck's AC system, be sure to have a repair manual specific to your vehicle. ||
  3. 3. || || || Expansion Valve The expansion valve removes pressure from the liquid refrigerant to allow expansion or change of state from a liquid to a vapor in the evaporator. The high-pressure liquid refrigerant entering the expansion valve is quite warm. This may be verified by feeling the liquid line at its connection to the expansion valve. The liquid refrigerant leaving the expansion valve is quite cold. The orifice within the valve does not remove heat, but only reduces pressure. Heat molecules contained in the liquid refrigerant are thus allowed to spread as the refrigerant moves out of the orifice. Under a greatly reduced pressure the liquid refrigerant is at its coldest as it leaves the expansion valve and enters the evaporator. Pressures at the inlet and outlet of the expansion valve will closely approximate gauge pressures at the inlet and outlet of the compressor in most systems. The similarity of pressures is caused by the closeness of the components to each other. The slight variation in pressure readings of a very few pounds is due to resistance, causing a pressure drop in the lines and coils of the evaporator and condenser. Two types of valves are used on machine air conditioning systems:   Internally-equalized valve - most common Externally-equalized valve special control Internally-Equalized Expansion Valve The refrigerant enters the inlet and screen as a high-pressure liquid. The refrigerant flow is restricted by a metered orifice through which it must pass. As the refrigerant passes through this orifice, it changes from a high-pressure liquid to a low-pressure liquid (or passes from the high side to the low side of the system). Let's review briefly what happens to the refrigerant as we change its pressure. As a high-pressure liquid, the boiling point of the refrigerant has been raised in direct proportion to its pressure. This has concentrated its heat content into a small area, raising the temperature of the refrigerant higher than that of the air passing over the condenser. This heat will then transfer from the warmer refrigerant to the cooler air, which condenses the refrigerant to a liquid. The heat transferred into the air is called latent heat of condensation. Four pounds (1.8 kg) of refrigerant flowing per minute through the orifice will result in 12,000 Btu (12.7 MJ) per hour transferred, which is designated a one ton unit. Six pounds (2.7 kg) of flow per minute will result in 18,000 Btu (19.0 MJ) per hour, or a one and one-half ton unit. Let's look at each valve in detail. The refrigerant flow through the metered orifice is extreamly important, anything restricting the flow will affect the entire system. ||
  4. 4. || || ||  If the area cooled by the evaporator suddenly gets colder, the heat transfer requirements change. If the expansion valve continued to feed the same amount of refrigerant to the evaporator, the fins and coils would get colder until they eventually freeze over with ice and the air flow is stopped.  A thermal bulb has a small line filled with C02 is attached to the evaporator tailpipe. If the temperature on the tail pipe raises, the gas will expand and cause pressure against the diaphram. This expansion will then move the seat away from the orifice, allowing an increased refrigerant flow. As the tail pipe temperature drops, the pressure in the thermal bulb also drops, allowing the valve to restrict flow as required by the evaporator.  The pressure of the refrigerant entering the evaporator is fed back to the underside of the diaphragm through the internal equalizing passage. Expansion of the gas in the thermal bulb must overcome the internal balancing pressure before the valve will open to increase refrigerant flow.  A spring is installed against the valve and adjusted to a predetermined setting at the time of manufacture. This is the superheat spring which prevents slugging of the evaporator with excessive liquid.  Superheat is an increase in temperature of the gaseous refrigerant above the temperature at which the refrigerant vaporizes. The expansion valve is designed so that the temperature of the refrigerant at the evaporator outlet must have 8 to 12°F (4 to 7°C) of superheat before more refrigerant is allowed to enter the evaporator.  The adjusted tension of this spring is the determining factor in the opening and closing of the expansion valve. During opening or closing, the spring tension retards or assists valve operation as required. o Normally, this spring is never adjusted in the field. Tension is adjusted from four to sixteen degrees as required for the unit on which it is to be installed. This original setting is sufficient for the life of the valve, and special equipment is required in most cases to accurately calibrate this adjustment Externally-Equalized Expansion Valve Operation of the externally-equalized valve is the same as the internal type except that evaporator pressure is fed against the underside of the diaphragm from the tail pipe of the evaporator by an equalizer line. This balances the temperature of the tail pipe through the expansion valve thermal bulb against the evaporator pressure taken from the tail pipe. Compressor The purpose of the compressor is to circulate the refrigerant in the system under pressure, this concentrates the heat it contains.   At the compressor, the low pressure gas is changed to high pressure gas. This pressure buildup can only be accomplished by having a restriction in the high pressure side of the system. This is a small valve located in the expansion valve. The compressor has reed valves to control the entrance and exit of refrigerant gas during the pumping operation. These must be firmly seated. ||
  5. 5. || || ||  An improperly seated intake reed valve can result in gas leaking back into the low side during the compression stroke, raising the low side pressure and impairing the cooling effect.  A badly seated discharge reed valve can allow condensing or head pressure to drop as it leaks past the valve, lowering the efficiency of the compressor. Two service valves are located near the compressor as an aid in servicing the system.  One services the high side, it is quickly identified by the smaller discharge hose routed to the condenser.  One is used for the low side, the low side comes from the evaporator, and is larger than the discharge hose The compressor is normally belt-driven from the engine crankshaft. Most manufacturers use a magnetic-type clutch which provides a means of stopping the pumping of the compressor when refrigeration is not desired. Compressor Relief Valve Some compressors have a relief valve for regulating pressure. If the system discharge pressure exceeds rated pressure, the valve will open automatically and stay open until the pressure drops. The valve will then close automatically. Compressor Noise Complaints Many noise complaints can be traced to the compressor mount and drive.  If a unit is noisy at one speed and quiet at another, it is not compressor noise.  Many times this kind of noise can be eliminated or greatly reduced by changing the belt adjustment.  Usually tightening mounts, adding idlers, or changing belt adjustment and length are more successful in removing or reducing this type of noise, than replacing the compressor.  Noises from the clutch are difficult to recognize because the clutch is so close to the compressor. A loose bolt holding the clutch to the shaft will make a lot of noise.  The difference, between suction pressure and discharge pressure, also plays an important part on sound level. o A compressor with low suction pressure will be more noisy than one with a higher pressure.  Consider whether the system is properly charged, whether the expansion valve is feeding properly to use the evaporator efficiently, and whether enough air is being fed over the evaporator coil. Condensor ||
  6. 6. || || ||  The purpose of the condenser is to receive the high-pressure gas from the compressor and convert this gas to a liquid.  It does it by heat transfer, or the principle that heat will always move from a warmer to a cooler substance.  Air passing over the condenser coils carries off the heat and the gas condenses.  The condenser often looks like an engine radiator. Condensers used on R-12 and R-134a systems are not interchangeable. Refrigerant-134a has a different molecular structure and requires a large capacity condenser. As the compressor subjects the gas to increased pressure, the heat intensity of the refrigerant is actually concentrated into a smaller area, thus raising the temperature of the refrigerant higher than the ambient temperature of the air passing over the condenser coils. Clogged condenser fins will result in poor condensing action and decreased efficiency. A factor often overlooked is flooding of the condenser coils with refrigerant oil. Flooding results from adding too much oil to the system. Oil flooding is indicated by poor condensing action, causing increased head pressure and high pressure on the low side. This will always cause poor cooling from the evaporator. Too-High Condensor Pressure  Indicated By: Excessive head pressure on high side gauge.  Caused By: Restriction of refrigerant flow in high side of system or lack of air flow over condenser coils. Too-Low Condensor Pressure  Indicated By: Higher than normal pressure on low side gauge.  Caused By: Failed compressor reed valve or piston. Heat exchange in the condenser will be cut down, and the excessive heat will remain in the low side of the system. Evaporator The evaporator works the opposite of the condenser, here refrigerant liquid is converted to gas, absorbing heat from the air in the compartment. ||
  7. 7. || || || When the liquid refrigerant reaches the evaporator its pressure has been reduced, dissipating its heat content and making it much cooler than the fan air flowing around it. This causes the refrigerant to absorb heat from the warm air and reach its low boiling point rapidly. The refrigerant then vaporizes, absorbing the maximum amount of heat. This heat is then carried by the refrigerant from the evaporator as a low-pressure gas through a hose or line to the low side of the compressor, where the whole refrigeration cycle is repeated. The evaporator removes heat from the area that is to be cooled. The desired temperature of cooling of the area will determine if refrigeration or air conditioning is desired. For example, food preservation generally requires low refrigeration temperatures, ranging from 40°F (4°C) to below 0°F (-18°C). A higher temperature is required for human comfort. A larger area is cooled, which requires that large volumes of air be passed through the evaporator coil for heat exchange. A blower becomes a necessary part of the evaporator in the air conditioning system. The blower fans must not only draw heat-laden air into the evaporator, but must also force this air over the evaporator fins and coils where it surrenders its heat to the refrigerant and then forces the cooled air out of the evaporator into the space being cooled. Fan Speeds Fan speed is essential to the evaporation process in the system. Heat exchange, as we explained under condenser operation, depends upon a temperature differential of the air and the refrigerant. The greater the differential, the greater the amount of heat exchanged between the air and the refrigerant. A high heat load, as is generally encountered when the system is turned on, will allow rapid heat transfer between the air and the cooler refrigerant. A blower fan turned on to its highest speed will deliver the most air across the fins and coils for rapid evaporation. For the coldest air temperature from the evaporator, operate the blower fan at the lowest speed so the heat will be absorbed by the refrigerant from the air Problems of Flooded or Starved Evaporator Coils Changing the state of the refrigerant in the evaporator coils is as important as the air flow over the coils. Liquid refrigerant supplied to the coils by the expansion valve expands to a vapor as it absorbs heat from the air. Some liquid refrigerant must be supplied throughout the total length of the evaporator coils for full capacity. A starved evaporator coil is a condition in which not enough refrigerant has been supplied through the total coil length. Therefore, expansion of the refrigerant has not occurred through the whole coil length, resulting in poor coil operation and too-low heat exchange. A flooded evaporator is the opposite of the starved coil. Too much refrigerant is passed through the evaporator coils, resulting in unexpanded liquid passing onto the suction line and into the compressor. ||
  8. 8. || || || Receiver/driers Receiver/driers (also sometimes called “filter/driers” or “receiver/dehydrators”) look like small metal cans with an inlet and outlet. They are only used in A/C systems that use expansion valves. ||
  9. 9. || || || Receiver/driers are located in the high-pressure section of the system, usually in the plumbing between the condenser outlet and the expansion valve inlet, although some may be connected directly to the condenser. Receiver/driers serve three very important functions: 1. They act as a temporary storage containers for oil and refrigerant when neither are needed for system operation (such as during periods of low cooling demand). This is the “receiver” function of the receiver/drier. 1. Most receiver/driers contain a filter that can trap debris that may be inside the A/C system. 1. Receiver/driers contain a material called desiccant. The desiccant is used to absorb moisture (water) that may have gotten inside the A/C system during manufacture, assembly or service. Moisture can get into the A/C components from humidity in the air. This is the “drier” function of the receiver/drier. Damage can occur if there’s excessive moisture inside an A/C system. It can cause corrosion, as well as possibly degrade the performance of the compressor’s lubricating oil. The receiver/drier should be replaced any time the system is opened for service, and most compressor warranties require it. The desiccant is only capable of absorbing a certain amount of moisture, and when the inside of the system and/or the receiver/drier are exposed to the atmosphere, the desiccant can become very quickly saturated from humidity in the air. If this occurs, the desiccant is no longer effective, and will not provide future protection. Additionally, the filter inside the receiver/drier could be restricted by debris that may have been inside the system. This could diminish refrigerant and oil flow. Accumulators An accumulator is comparable in purpose to a receiver/drier. It serves similar, but slightly different functions. An accumulator is also a metal cylinder, but differs from a receiver/drier in these three ways: 1. An accumulator is considerably larger than a receiver/drier, usually around twice the volume. 1. The accumulator is connected to the evaporator outlet, in the low-pressure section of the system. 1. The accumulator’s primary function is to store liquid refrigerant that is exiting the evaporator, to prevent it from reaching the compressor. If liquid refrigerant were to enter the compressor, it could cause damage, as the compressor is not designed to pump liquid, only vapor. ||
  10. 10. || || || Accumulators are only used on systems that contain orifice tubes. It is a characteristic of orifice tube systems to have large amounts of liquid refrigerant leaving the evaporator. In other words, unlike in expansion valve systems, where all or most of the refrigerant turned into a vapor while passing through the evaporator, in orifice tube systems, the refrigerant leaves the evaporator still as a liquid. The accumulator is the component in which the refrigerant gets the opportunity to warm up and change from a liquid to a vapor before being drawn back into the compressor Like receiver/driers, accumulators also serve as a temporary storage containers for oil when the oil is not needed by the system. Lastly, accumulators also contain the system desiccant and a small filter, so compared to receiver/driers, the same “rules of replacement” apply. Things that can go wrong with receiver/driers and accumulators Receiver/driers and accumulators rarely fail themselves, but as mentioned previously, need to be replaced whenever the system is opened for any other type of service. When a failure does occur with a receiver/drier or accumulator, it is usually due either to clogging from debris inside the A/C system (like from a failing or failed compressor), or that the bag containing the desiccant has broken open, allowing desiccant material to circulate throughout the system with refrigerant and lubricant. Sometimes, the desiccant material will disintegrate into small sand-like particles. This can cause possible clogging in other system components. Auto Cabin Air Filter Did you know know that many cars have cabin air filters built into the air conditioning system. Automotive aftermarket industry surveys have demonstrated that a majority of vehicle owners are unaware of this feature and whether their car can filter cabin air. Even fewer can find the cabin filter after they are sure their vehicle has one. These filters have been added to vehicles in the last 20 years without much fanfare. Marketed on European vehicles since the mid 1980's, cabin air filters began to appear on stateside cars and trucks around 1995. High projected U.S. growth failed to materialize, but now the cabin air filter is common on American and Asian automobiles. Cabin filters, as with air filters generally, do not have standardized terminology. ||
  11. 11. || || || They may be called; pollen filters, dust filters, car air conditioning filters, ACC filters, breather filters, micro filters, micron air filters, interior air filters, and interior ventilation filters. Many cars are running with dirty, clogged cabin filters. AC performance will be degraded and inside air filtration will suffer if the filter is not serviced. Leaves and dirt will clog the filter media, and road grime will turn it pure black in about 12,000 miles. This is a filter in the incoming air duct of the car's heating ventilating and AC system. The cabin air filter cleans the air coming into the auto's passenger cabin, collecting pollen and dust. Since automotive servicing is an area of lower consumer sophistication than the air purifier market in general, rip-offs and exaggerated claims are everywhere. The question for consumers desiring cleaner air in their cars is fourfold; Is my auto equipped with a cabin air filtration device, or the hardware to add one? What features and products are desirable in a replacement filter? If so, how difficult and costly is it to change or add? Can I do it myself, or is this a job for professionals? Replacement Air Filter Features Like the air conditioning filters commonly found in home furnace ducts, a big function of the automotive interior air filters is protecting heater and AC components. The reduction of visible dust, which builds up on dashboards, confuses us into believing car cab air is cleaner. Like the home environmental quality scenario, appearances can be deceiving when things just look clean. ||
  12. 12. || || || Two types of cabin air filtration media are offered in the after market: single and multiple stage filters. A simple single stage particulate filter, maybe just paper, is common in original equipment installations. Single Stage filter These one stage filters trap airborne dust, the largest soot, and allergens. Quality cabin air filters typically use an electrostatic charged fiber mat to capture airborne particles. While better than plain paper, these are not HEPA air filters. What particle size do they allow through? Evaluation of car cab air filters begins at about the 10 micron size, with particle filtration efficiency falling rapidly in particle sizes smaller than 3 microns. Few vendors mention particle efficiency and those that do may say: "The Cabin Air Filter can remove airborne particles that are larger than 3 microns." Premium products capture particles as small as one micron, but at lower efficiencies, like 30%. Even the best cannot remove dangerous sub-micron particles. In the scheme of pollution and its impact on the human immune system, 1 micron is boulder-sized. Many single stage filter media are limp, without stiff edges to hold the filter firmly in the duct or plenum chamber. This allows airflow to bypass. Sometimes bypassing is intentionally engineered to protect downstream AC components from overload. Multi-stage filter A multiple stage car air filter, generally using some activated carbon, reduces toxic gases and odors. Vendors claim tests have shown carbon monoxide levels are lowered with the multi-stage filter models, but I think this is due to restricted intake rather than the actual ability to remove carbon monoxide. Multiple stages on premium filters can have progressive size layers like prefilters in room purifiers. The carbon layer comes near the end. In general, the more stages the better. Better models have reinforced framing and sealed edges. A premium 5 stage charcoal filter may cost $20 to $35, about double the original equipment replacement. This is a very small price for filtered air in the cabin. ||
  13. 13. || || || Filter Replacement Rip-Off Environmental conditions will determine the useful life of a cabin air filter. Urban pollution and/or rural dust call for more frequent changes. Leaf and twig debris will build up in many installations. Filter replacement is needed when reduced air flow lowers air conditioner, defroster, and heater performance, but filters should be changed long before this is noticeable. Odors from the ducts may be a signal. Owners' manuals usually recommend replacement every 15,000 miles or so. Here is a serious issue: installing a cabin filter can be a 20 second job anyone can do as easily as opening and closing a kitchen drawer. Or it can require 30 minutes of a skilled mechanic's time. Car manufacturers "hide" the cabin air filter in various locations, making them difficult for green mechanics to service. While many quick-lube operators were slow to adopt to the new cab air filter products, industry journals now report that cabin air filters are their highest profit add-on service. It's a new version of the old service station low oil scam. Workers are trained to recognize the car models with the easiest to change cabin media, and to offer an inferior new filter only to drivers of those car models. The charge is less than a dealer service department, but is exorbitant for the easy job performed. It is important to get the correct filter for the application, sometimes these operators will substitute loose fitting air filter media. If you are not the mechanically inclined type, I recommend insisting on premium multi-stage filter media and observing the process to learn how time consuming it really is. Do-it-yourselfers and shade tree mechanics will have no trouble with 99% of these installations. A few may require removal of the passenger side windshield wiper assembly. A cabin air filter mostly protects against outside air infiltration, but cab air quality is determined by emissions inside the cabin as well. Running the AC on recirculate will help, but is not a complete solution. Many particulates do not enter through the AC system. I recommend frequent replacement of original equipment cabin filter media, using 5 stage premium aftermarket products. But don't stop there, consider a car air purifier or running a room purifier in the parked car periodically. ||
  14. 14. || || || The open and closed positions of a thermostat. HSW Thermostat The thermostat's main job is to allow the engine to heat up quickly, and then to keep the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. At low temperatures, the outlet to the radiator is completely blocked -- all of the coolant is recirculated back through the engine. Once the temperature of the coolant rises to between 180 and 195 F (82 - 91 C), the thermostat starts to open, allowing fluid to flow through the radiator. By the time the coolant reaches 200 to 218 F (93 - 103 C), the thermostat is open all the way. If you ever have the chance to test one, a thermostat is an amazing thing to watch because what it does seems impossible. You can put one in a pot of boiling water on the stove. As it heats up, its valve opens about an inch, apparently by magic! If you'd like to try this yourself, go to a car parts store and buy one for a couple of bucks. The secret of the thermostat lies in the small cylinder located on the engine-side of the device. This cylinder is filled with a wax that begins to melt at around 180 F (different thermostats open at different temperatures, but 180 F is a common one). A rod connected to the valve presses into this wax. When the wax melts, it expands significantly, pushing the rod out of the cylinder and opening the valve. ||