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Air treatment

The only way to guarantee compressed air water vapour levels that meet the standard and having dry air reaching the dental delivery unit is by employing some sort of air drying system.

It is worthwhile to note that when air is compressed its temperature elevates, but on leaving the compressor motor it begins to cool to ambient temperature. Its ability to hold water vapour decreases two fold. Firstly, compressed air can hold less vapour than the uncompressed air, and the amount of condensate, which occurs, is dependant on the initial degree of saturation (% relative humidity) and the final pressure. Secondly, as this compressed air moves along the delivery system it gradually cools to that of the ambient surroundings, which varies from that outside to that inside which is often air-conditioned, so more and more condensate will appear all the way to the control blocks of the dental delivery unit.

So unless this vapour is physically removed by a dryer to a level where it will not condense at any point along the whole system, some condensate will inevitably be present in untreated air systems. 

In tropical zones, the relative humidity experienced for a large proportion of the year is high, hence the amount of condensate removal needed is proportionally high. The most economic means of removing large quantities of moisture in this situation is by employing a refrigerant dryer. Their service needs are minimal and running cost minor. They are best located so that the dried air can be stored in the receiver vessel so it also remains dry. Desiccant air dryers reach a much lower PDP but cope poorly with extended periods of high humidity at high temperatures and in these types of situations are far more expensive to service and are more susceptible to breakdown. Repair costs can be as high as the cost of an industrial grade compressor.

Hydrocarbons are easily removed from dry air by means of an appropriate filter cartridge.

Some dedicated dental compressors now come fitted with Membrane driers. These air drying devices are used to depress the pressure dewpoint by selectively passing water vapour out of the compressed air stream as it passes through the inside hollows of a bundle of narrow, thinly-coated, membrane fibres. A small amount of "sweep air" must then be purged around the outside of the bundle of hollow fibres carrying the compressed air. Purge or "sweep" air is thus expanded (to atmospheric pressure) and subsequently vented to the atmosphere. The "dryer" purge air which is carried or swept along the outside of the hollow bundle of fibres creates a vapour pressure gradient between the "wet" compressed air on the inside the fibres and the "dryer" sweep air on the outside the bundle. This results in delivery of cleaner, dryer air before the compressed air is sent to the reciver. This style of dryer is well suited to tropical conditions but does require a certain volume of air (around 10%) to be used in the “sweep”. Therefore the compressor needs to be of a slightly higher capacity than used on other drying systems.