Basic Function

• The gas in a chilling system starts at the compressor, where it is both cold and low pressure. The compressor raises its temperature by increasing its density -- as the molecules get closer together, they release their excess energy in the form of heat.
  
  Next, it goes into a condenser, where the gas is moved quickly in order to push heat energy elsewhere. This process also turns the gas into a cool liquid.
  
  This is where the chiller coils come in. The liquid is highly pressurized, then pushed through the chiller coils. This process forces the liquid to turn into a gas, and in order to do so it needs to take heat away from the area around it. The gas then goes back to the compressor, taking with it the heat from the air behind it. The now-cool air is then used to chill the house or appliance in question.
  
  

Coils

• Chiller coils are just one kind of evaporator. They are usually made of copper, and are essentially pipes that the liquid is pressed into. The key point about them is that they have a larger area than the hole that the compressor pushes air through. The extra space decreases the pressure, which is what turns the liquid into a gas.
  
  

Varieties

• Residential coils are usually cased or uncased. Cased coils are inside the air-conditioning system and are, as the name suggests, in a case that holds them. They tend to be in new systems. Uncased coils are attached to an existing furnace. This is done in order to add air-conditioning to a house that previously only had a furnace; it is usually cheaper to add uncased coils to a furnace than it is to replace the entire system.
  
  Coil-in-tank variations also exist. These are coils that are submerged in cold water, which creates a "shield" between the cool coils and the hot compressor. These are not usually necessary in residential applications, but rather tend to be used in higher-end, larger-capacity chillers such as reactor chillers.
  
  

Maintenance

• Chiller coils need to be maintained. If this is not done, they can have a variety of problems, all of which stem from one major source: high head pressure. As corrosion builds up in the chiller coils, the pressure increases. Since chiller coils need low pressure in order to convert the liquid to a gas, this makes the entire system less efficient. So, it is key to check your chiller coils to make sure they are clear of obstructions and generally doing their job in the manner they were designed to do so.