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
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
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