Heat Exchanger

One way to transfer heat from one fluid to another is through a device called a heat exchanger.

When you put your pot on the stove element, the heat from the element is exchanged to the pot, and then from the pot to the liquid in it.

The refrigerator in your home has a heat exchanger too. It is usually located on the outside back wall of the cabinet. The heat that is being pumped out of your fridge travels through the intricate web of lines on the back of the fridge cabinet, and with the abundance of surface area available to the cooler kitchen air, heat from within your fridge is exchanged with the cooler air surrounding the lines.

That is why it is a good idea to look behind the fridge periodically, and if the back is covered with dust, vacuum it off. Your fridge will work less hard, and that will cut down a bit on your energy bill.

The radiator in your car is a heat exchanger too. The hot coolant from your engine is pumped through the lines in the radiator, and as air passes through and around the fins of the rad, heat is exchanged, the liquid within the lines of the rad is cooled, and is ready to go back to your engine to pick up more heat. And of course, the coolant heat exchanges within the engine block to cool the block down.

Compressor Heat Exchangers

As you can see, heat exchangers are everywhere and there are many different kinds. So how does this affect compressed air and air compressors?

In our case, the fluid we are primarily concerned with is the compressed air exiting our compressor receiver and heading for the plant air mains and air tools, air valves, air motors, air pumps, etc.

The other fluid is the water surrounding the air line in some types of heat exchangers.

Water Jacket

The heat exchanger referred to here is a water jacket type.

The hot compressed air from the receiver is still carrying lots of water in vapour form.

Here is why there is water in your compressed air and the receiver.

If the compressed air temperature is not lowered down past the dew point before the air gets to the mains and the drop lines, then the compressed air will continue to de-water in the lines, vapor condensing into water, and creating free water in the lines to flow down to your air tools, air valves, air motors etc. This is not good!

Here is where see what problems can occur with water in your compressed air lines.

In the water jacket heat exchanger, the two fluids (compressed air and cool water) will be separated by the metal pipe of the air line.

Cold water will flow around the air pipe, and heat exchange will take place.

The heat exchanger must have a surface area large enough - or the dwell time of the two fluids in the exchanger is long enough - for sufficient heat to be transferred from the hot compressed air to the cold water.

Sizing Heat Exchangers

Things you'll need to know to help determine the size of your heat exchanger are:

• the temperature of the compressed air as it enters the heat exchanger
• the maximum flow of compressed air to the plant - you will want the exchanger to be sized to be able to heat exchange sufficiently at the plants maximum compressed air demand
• what temperature the compressed air should be as it leaves the heat exchanger
• how the heated water will be cooled

With this information, your heat exchanger vendor should be able to help you determine the size you need.

If you wish more details on the science of heat exchanging, look up the following topics:

• "The first law of thermodynamics"
• "Fourier's law of conduction"
• "Newton's law of cooling"