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  AC - How it works

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AC - How it works
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In this page we look at the basics of air conditioning in the Scorpio fitted with manual A/C or the more sophisticated Climate Control.

Lets first look some principles of operation

Latent Heat
When a liquid boils (converts to gas) it absorbs heat without raising the temperature of the resulting gas. When the gas condenses (converts back to a liquid), it gives off heat without lowering the temperature of the resulting liquid. This is called Latent Heat.

Relative Humidity
The amount of moisture (water vapour content) that the air can hold is directly related to the air temperature. The more heat there is in the air, the more moisture the air can hold. The lower the moisture content in the air, the more comfortable you feel. Removing the moisture from the air lowers its relative humidity and improves personal comfort.

Effects of Pressure on Boiling or Condensation
As the pressure is increased on a liquid, the temperature at which the liquid boils (converts to gas) also increases. Conversely, when the pressure on a liquid is reduced, its boiling point is also reduced. When in the gas state, an increase in pressure causes an increase in temperature, while a decrease in pressure will decrease the temperature of the gas.

Air conditioning is the process by which air is cooled and dehumidified. The air conditioning in your car, your home and your office all work the same way. Even your refrigerator is, in effect, an air conditioner. While there are many physical principles that relate to air conditioning, this article sticks to the basics. It explains the general concepts of automotive air conditioning, the components used and what you need to know to keep your car's A/C system working properly.
Office A/C Internal Unit Office A/C external Unit

Basically, air conditioning systems operate on the principles of evaporation and condensation.

Here's a simple example of evaporation. Imagine that you're swimming around in your neighbour's backyard pool on a summer day. As soon as you get out, you start to feel cooler. Why? The water on your body starts to evaporate and turns into water vapour. And as it evaporates, it draws heat away from your body, and you get goose bumps. Brrr! Now let's say your neighbour hands you a big glass of ice-cold lemonade. You take a sip and set it down on a table. After a minute or two, you notice that water has collected on the outside of the glass. This is condensation. The air surrounding the glass becomes cooler when it encounters the cold glass, and the water vapour the air is carrying condenses into water.

Both of these examples occur at normal atmospheric pressure. But higher pressures can also change a vapour (or a gas) into a liquid. For example, if you look at a typical butane cigarette lighter, you can see liquid inside it. But as soon as you push down on the button, butane gas comes out. Why? The butane is under high pressure inside the cigarette lighter. This high pressure causes the butane to take liquid form. As soon as the butane is released and it encounters normal atmospheric pressure, it turns back into a gas. Now hold the lighter as you release the gas and feel it get cold - that is because as the liquid turns to gas it requires latent heat and it draws this heat out of you hand and the environment making it feel cold.

The reverse is true too - latent heat is given up when gas is compressed into a liquid.. Hold a bicycle pump in your hands after inflating a tyre an feel how hot it is. Although you can not compress the air enough to become a liquid you can feel the released latent heat being given up into your hand.

OK, those are the basic ideas. But how do they apply to making your car's vents blow cool air? The principles of evaporation and condensation are utilized in your car's A/C system by a series of components that are connected by tubing and hoses. There are six basic components: the compressor, condenser, receiver-drier, thermostatic expansion valve, the evaporator and the life-blood of the A/C system, the refrigerant.

Refrigerant is a liquid capable of vaporizing at a low temperature. In the past, R-12 refrigerant was used in cars. But this chlorofluorocarbon (CFC) is said to be harmful to the earth's ozone layer. Consequently, all vehicles built after 1996 use R-134A, a more environmentally friendly refrigerant. In the case of the Scorpio all the new range had R134a from the start. The refrigerant differs from R12 in that it is about 10% less efficient in cooling and has smaller molecules.

Here's how an air conditioning system and its components work.

Step One: The compressor is the power unit of the A/C system. It is powered by a drive belt connected to the engine's crankshaft. On the Scorpio you can see it on the right hand side of the engine bay and although the drive belt is permanently rotating the front pulley, the pulley is connected to the compressor by a magnetic clutch. So it spins freely with the clutch disengaged and drives the compressor when engaged. When the A/C system is turned on, the compressor 'compresses' the refrigerant vapour heating it up and pumps it under high pressure to the condenser.


Scorpio Condenser Fins increase surface area

Step Two: The condenser is a device used to change the high-pressure refrigerant vapour to a liquid. It is mounted ahead of the engine's radiator, and it looks very similar to a radiator with its parallel tubing and tiny cooling fins. If you look through the grille of a car and see what you think is a radiator, it is most likely the condenser. The hot vapour from the compressor arrives and the fins help the air flowing through the condenser remove heat from the refrigerant, changing it to a much cooler liquid state. When the car is stationary the radiator cooling fans perform the task of moving air through the condenser.

Protective cover held with screws Receiver Dryer

Step Three: Refrigerant moves to the receiver-drier. This is the storage tank for the liquid refrigerant. It also removes moisture from the refrigerant. Moisture in the system can freeze and then act similarly to cholesterol in the human blood stream, causing blockage. The receiver-drier is used on the high side of the Scorpio system and uses a thermal expansion valve. This type of metering valve requires liquid refrigerant. To ensure that the valve gets liquid refrigerant, a receiver is used. The primary function of the receiver-drier is to separate gas and liquid. The secondary purpose is to remove moisture and filter out dirt.

Expansion Valve

Step Four: As the compressor continues to pressurize the system, cool liquid refrigerant under high pressure is circulated from the receiver-drier to the thermostatic expansion valve. The valve relieves pressure from the liquid refrigerant so that it can expand and become refrigerant vapour in the evaporator. Remember that when a liquid becomes a gas it needs latent heat and so it extracts this from its surroundings cooling them down. The expansion valve is the "brain" of the system. By sensing the temperature of the evaporator, or cooling coil, it allows liquid to pass through a very small orifice, which causes the refrigerant to expand to a low-pressure, low-temperature gas. This "cold" refrigerant flows to the evaporator

Evaporator with Filters Filters removed Pollen Filters

Step Five: The evaporator is very similar to the condenser. It consists of tubes and fins and is mounted inside the passenger compartment. As the cold low-pressure refrigerant is released into the evaporator, it vaporizes and absorbs heat from the air in the passenger compartment. As the heat is absorbed, cool air will be available for the occupants of the vehicle. A blower fan inside the passenger compartment helps to distribute the cooler air. The evaporator serves as the heat absorption component. The evaporator provides several functions. Its primary duty is to remove heat from the inside of your vehicle. A secondary benefit is dehumidification. As warmer air travels through the aluminium fins of the cooler evaporator coil, the moisture contained in the air condenses on its surface. Dust and pollen passing through stick to its wet surfaces and drain off to the outside. On humid days you may have seen this as water dripping from the bottom of your vehicle. Rest assured this is perfectly normal.

The ideal temperature of the evaporator is 32 Fahrenheit or 0 Celsius. Refrigerant enters the bottom of the evaporator as a low pressure liquid. The reducing pressure and the warm air passing through the evaporator fins causes the refrigerant to boil (refrigerants have very low boiling points). As the refrigerant begins to boil, it absorbs large amounts of heat from the air entering the passenger compartment and this heat is then carried away with the refrigerant. Several other components work in conjunction with the evaporator. As mentioned above, the ideal temperature for an evaporator coil is 0 centigrade. Temperature and pressure regulating devices must be used to control its temperature. In the Scorpio the de-ice switch has a temperature probe mounted on the evaporator and cycles the compressor off when near to freezing to prevent the evaporator from freezing up. A frozen evaporator coil will not absorb as much heat.

Step Six: The heat-laden, low-pressure refrigerant vapour is then drawn into the compressor to start another refrigeration cycle.

As you can see, the process is pretty simple. Just about every vehicle's A/C system works this way, though certain vehicles might vary by the exact type of components they have.

The best thing about air conditioning is that all you have to do is press a button to make it work. Air conditioning systems are pretty reliable. On a modern and relatively new vehicle, it is rare to have problems. And if there are problems, they are pretty much one of two things: No cool air or insufficient cool air. If you own an older car and its A/C system doesn't seem to be working properly, here are some general troubleshooting tips:


No Cool Air
bulletLoose or broken drive belt
bulletInoperative compressor or slipping compressor clutch
bulletDefective expansion valve
bulletClogged expansion valve, receiver-drier or liquid refrigerant line
bulletBlown fuse
bulletLeaking component: any of the parts listed above or one of the A/C lines, hoses or seals

Insufficient Cool Air

bulletLow refrigerant charge
bulletLoose drive belt
bulletSlipping compressor clutch
bulletClogged condenser
bulletClogged evaporator
bulletSlow leak in system
bulletPartially clogged filter or expansion valve


Most A/C repairs are best left to a specialist repairer or garage. Recharging the refrigerant, in particular, requires special equipment that most people don't own. There are a couple things you can do, however.
First, make sure to have the system checked regularly according to your vehicle's owner's manual - in the Scorpios case that's every 10,000 miles.
Second, if you live in a place with a cold climate, it might not make much sense to run the A/C during the winter months, but many A/C technicians recommend running your A/C system regularly, because it contains a light mineral oil in the refrigerant to keep the compressor properly lubricated. The general rule of thumb is 10 minutes per month. The Scorpio heating, ventilation and air conditioning systems also engage the A/C compressor when the distribution control is in defrost mode.

So those are the basics behind air conditioning. The next time you're riding along in your Scorpio and you hit the A/C button, you can say, Boy, those evaporator tubes sure are cold. It's all thanks to R-134A!

Pete C



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