A sequel to 'Men, women and air conditioners',
March 2003 issue of SA I&C, (Apologies to George Lucas)
Us technical types, having to share offices with the non-technical, are frequently frustrated by the lack of logic and understanding when it comes to the things that happen to that most essential piece of all office equipment, our beloved airconditioner. The next time they want to argue with you about why the air conditioner is 'leaking', or icing up - do not stress, just hand them this article!
The basics of airconditioner operation
A compressor compresses a gas, which makes it get quite hot. This very hot, compressed gas then passes through a heat exchanger, where it is cooled (like the water in a car's radiator). With so much heat 'squeezed' out of it, the gas condenses and becomes a liquid - so this heat exchanger is called a condensor.
This liquid emerges from the condensor and squeezes through an expansion valve (skinny hole) into a region of pipe that is at a lower pressure. This sudden drop in pressure causes the liquid to begin evaporating, so it becomes very cold and absorbs heat. This evaporating liquid enters a second heat exchanger (called the evaporator) - which is exposed to the air in the room that is being cooled. So the heat from the air in the room is effectively used to boil the liquefied refrigerant back into a gas. The evaporator (which also looks like a car radiator) is colder than the air in the room, so it absorbs heat from the room air, making the room air cooler. The evaporated gas emerges from the evaporator and is delivered by a pipe, via a liquid trap, back to the compressor, where the cycle is repeated as long as the compressor is running.
Help, the airconditioner is leaking on my PC!
The evaporator often runs at a temperature lower than the dewpoint of the air in the room. What this means, is that water from the air in the room will condense on the outside of the evaporator. (Remember that wet stuff on the outside of your icy beer mug?) This is quite normal, and the water is allowed to run off, drip onto a tray (inside the air conditioner), and through a pipe, out of the building. Should the drip tray become corroded, or the drain hole becomes blocked, the water could end up dripping through gaps in the airconditioner's casing. If the evaporator becomes severely iced up, the ice could block the drain hole, or create channels to allow water to drip over the sides of the drip tray, with similar results.
Where does all that ice come from?
It all starts with the pressure of the gas in the airconditioner's cooling circuit. The temperature at which the liquefied gas boils is determined by the pressure within the evaporator. (The higher the gas pressure, the higher the boiling point.) So if the airconditioner has a gas leak, the boiling point of the liquid in the evaporator will begin to drop. This may not seem to be too serious, but should the refrigerant's boiling point drop below 0ºC, any water that condenses on the evaporator will freeze solid instead of trickling down and falling into the drip tray. So the evaporator accumulates ice. This ice acts to insulate the evaporator from the air that is supposed to boil the refrigerant, so more and more refrigerant returns from the evaporator in liquid form...look out.
You can break your airconditioner's heart
Ultimately the excess, unboiled liquid refrigerant can overwhelm the trap and find its way back into the compressor. The compressor (the heart of the machine), which is designed to work with a gas, and not a liquid, will then in all likelihood sustain fatal injuries in its gallant efforts to compress a liquid. Best take action before things get this far.
De-icing action
Some newer airconditioners automatically de-ice themselves. If yours does not, you have to make sure that the evaporator does not ice up. If you see ice on the evaporator (white stuff on the thing that looks like a radiator, inside the airconditioner), stop the compressor by selecting a 'fan only' option, turn fan speed to maximum and ensure that a good flow of air is getting through the frozen evaporator. This will warm up the evaporator and allow the ice to melt and run off. Next ensure that there is nothing causing an obstruction to the airflow, and that the fan is functioning properly. Get the airconditioner system checked for correct charge pressure.
Normally, icing problems are the first sign of loss of refrigerant gas pressure. The problem becomes particularly noticeable when the air is humid and the weather outside quite cool, and the airconditioner's fan speed is set low. This is because the ability of the room air to impart heat to the evaporator would be considerably less than it would be on a hot day, with the fan speed on maximum. So on a cool, humid day, the evaporator gets too cold, and icing begins. The problem gets worse as charge pressure is lost from a leaky system.
What to do:
* If you like your aircon, do not allow the compressor to run when the evaporator is iced up.
* Get an airconditioner technician to check the gas in your airconditioner.
* Run the airconditioner with fan speed at maximum, watch for ice - and keep those windows closed!
* If all hope of winning an argument is gone, turn off the airconditioner, open the windows and fan yourself with the nearest magazine - the latest copy of SA I&C - of course.
Stay cool.
John Gibbs, Associate Editor, SA Instrumentation & Control
031 764 0593, john@technews.co.za, www.technews.co.za
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