Water in air lines displaces lubricants from critical wearing surfaces in air tools, valves and cylinders, decreasing the life of their components and increasing downtime. Water also creates rust in metal piping which can be carried downstream to tooling.
Another impurity commonly found in air lines is oxidized compressor oil, caused by the concentration of oxygen and heat during compression. Oxidized oil causes sticky brown varnish deposits on metal surfaces in valves and cylinders, has very poor lubricating qualities, and invariably mixes with the condensed water in the lines to form a green, frothy slime.
Air line filters will only collect moisture that has condensed out of the compressed air. They will not reduce the humidity level at all. Also, filters must be drained and the elements cleaned or replaced on a regular basis or they will restrict the flow of air and cause downstream equipment to malfunction. The proper answer is to eliminate the water and oil carryover at, or near, the compressor.
In our previous column (Machinery & Equipment MRO, Feb. 2002, p.28), we discussed why water is found in compressed air lines and the effect that temperature has in condensing the humidity into a liquid form. In a nutshell, for every 20°F drop in temperature, the ability of the air to hold on to moisture, in the form of humidity, is cut in half.
Aftercoolers, either water- or air-cooled, with a separator attached, should always be used immediately downstream from the compressor to provide an economical way of initially cooling the air to within 15°F of the cooling medium temperature. This is called the approach temperature and can be anywhere from 5°F to 20°F, depending on the size of the aftercooler and the temperature of the cooling water.
Cooling the air causes water to condense into droplets which can be collected by the separator. It should be noted that the compressed air is still at 100 per cent humidity after passing through this process. If the temperature is lowered, even one degree, more water will condense in the air lines.
The air-cooled aftercooler is more costly to purchase than a water-cooled type, but as it does not need a water supply or a drain, it is less expensive to run.
The exiting compressed air temperature from the aircooled aftercooler is about 15 degrees higher than the ambient air, therefore further cooling downstream can cause as much as an additional 40 per cent of the remaining water in the air to condense out. Obviously, an air-cooled aftercooler by itself will not totally solve the problem. It will however provide an economical method of preconditioning the air before it enters a refrigerant-style compressed air dryer.
The water-cooled aftercooler has the advantage of using a cooling medium which, during Canadian summers, is usually at 55°F to 60°F, a lot cooler than the normal, in-plant, air temperature. This results in an exiting compressed air temperature very close to the in-plant ambient temperature, so that the chance of more water condensing in the air lines is reduced considerably.
The water used in water-cooled compressors can be used initially to cool an aftercooler, making its operation very economical. The direction of water flow through the aftercooler should always be opposite to the direction of the air flow for maximum efficiency.
Oil vapour generated by the compressor behaves in much the same fashion as humidity. Reducing the temperature of the compressed air causes the oil vapour to condense into oil droplets that can be collected by a special coalescing filter and drained away. However, coalescing filters are only designed to remove oil from compressed air lines, not water or dirt particles.
As the air flow enters the inside of the element and passes through the filter, the oil "wets out" against the coalescing surfaces of the element and is carried down the outside surface of the element to gather in the bottom of the filter bowl.
A coalescing filter should always be protected by a 5- to 20-micron pre-filter to remove any water and solid particles, thus protecting the relatively expensive coalescing element. The two filters should be positioned immediately following the aftercooler and separator combination. In addition to an automatic drain, both the separator and the coalescing filter should have a manual drain, mounted in parallel with the auto drain, in case the auto-drain becomes plugged and fails.
At this point, if you think you can buy and install a suitable aftercooler, separator and coalescing filter for your compressed air systems and have your problems solved, you are wrong. Keep your wallet open, because you're only halfway there.
Air is moved through piping by creating a pressure difference from one point to another. This pressure difference or pressure drop creates a cooling or refrigeration process which lowers the temperature of the compressed air below ambient temperature, thus condensing out more water.
This situation is, of course, aggravated at the "work piece" where the air is allowed to expand during the process of doing work. The only answer is to add a compressed air dryer to the foregoing system to reduce the compressed air dew point, or the temperature at which water will condense, to well below the ambient temperature.
In the next column, we will discuss several types of dryers and how to use them. The use of a dryer in a compressed air system does not eliminate the need for the aftercoolers, separators and filters discussed in this article. In fact, the economical operation of any dryer depends on the efficient use of these items.
Ted Grove is the corporate training manager of Wainbee Ltd., Mississauga, Ont., He can be reached at 905-568-1700 or firstname.lastname@example.org. In upcoming columns in this series, Ted's Tech Talk examine various ways of solving problems in compressed air systems.