These catagories are then sub divided and in turn they can be sub-divided. We have listed below a few general topics.

1. Silica Gel is the cheapest, it adsorbs large quantities of moisture and is easily dryed out. It has a pickup rate in the order of 35%, which unfortunately makes it not very tolerant to liquid water. A spoonful of silica gel into a mug of water produces dramatic effects. The beads of gel split ferociously into a powder giving the appearance that the water is boiling (which it is not). This material can be used heatless as well as heat reactivated dryers, the reactivation temperature is comparatively low at 150 deg C.

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2. Alumino-Silicate Gels are available with a variety of properties, which include immunity to liquid water. These are more expensive and therefore some dryer manufacturers use this type of desiccant as a buffer in conjunction with ordinary silica gel. Usually the first 1/3rd of the dryer is filled with this water tolerant desiccant, the remaining 2/3rds with the cheaper standard product. Any liquid water is removed by the Alumino-Silicate Gel and the remaining water vapours are dealt with by the Silica Gel.

3. Activated Alumina is a good all round desiccant which is yet again more expensive. The material is fairly tolerant of liquid water and this makes production and service work much simpler as the whole tower can be filled without the requirement of a buffer zone. It has a pickup rate of 16%, which is much lower than silica gel, this is the main reason why it is so much more stable. Activated alumina can be used in both heatless and heat reactivated dryers, its reactivation temperature is around 200 deg C.

4. Molecular Sieve is widely used when ultra-low pressure dewpoints are required. In fact there is little or no alternative if dewpoints of -70 deg C or lower have been specified. The product is therefore quite expensive. For successful moisture removal a molecular sieve with a 4 Angstrom pore is required, although pores up to 12 Angstrom can be used. The pickup rate is around 18%, which makes the product very stable when used with liquid water. One unfortunate problem is drying a molecular sieve bed which has become accidentally fully saturated. Although the sieve will suffer no long term effects, a fully saturated desiccant bed may take several weeks to dry out. Molecular sieve can be used in both heatless and heat reactivated dryers, its reactivation temperature is around 235 deg C.

These dryers use a small proportion of the processed compressed air to reactivate the wet desiccant bed. Although many manufacturers claim to be more efficient than their competitors, no desiccant dryer manufacturer actually makes any desiccant, it is far too complex and expensive. The result is that they all tend to buy the same products from the same source.

A heatless dryer will need between 15-18% of the dryed air to be used for reactivation purposes.

If you say it quickly 15-18% does not sound much. But it is very expensive. Having gone to the trouble of compressing the air, filtering the air and drying the air, almost 1/5th is blown back through the wet tower on a continual basis to regenerate the desiccant.

The principle of heatless regeneration is simple. When you expand compressed air from 7 barg to atmospheric pressure, the moisture content inside the air remains the same in real terms. For example, if it contained 1 gramme at pressure, it still contains 1 gramme once expanded.

However, the relative humidity of the air will change dramatically. Instead of 1 cu metre of compressed air containing 1 gramme, we now have 7 cu metres of expanded air containing 1 gramme. The air has become much much drier.

By venting 15-18% of the processed compressed air back through the wet tower at atmospheric pressure, the air is significantly much drier than the desiccant bed it is passing over. The molecules of water vapour jump out of the wet desiccant into the dry air, thereby reactivating the bed.

One advantage of heatless dryers is that they can be pneumatically controlled, this allows them to be installed into hazardous areas.

These dry compressed air in much the same way as heatless dryers, the only difference is the way the desiccant bed is reactivated. Heat reactivated dryers rely on a separate heat source to warm the bed up to the appropriate temperature. Moisture is given off in the same way as boiling a kettle. Then a much smaller quantity of air is passed through the wet desiccant bed to carry the water molecules away. This air is called the purge rate, the volume being typically between 3-8% of the processed air.

There are different ways of applying heat to the desiccant bed and different ways of producing the purge. Each manufacturer claims theirs is better and more efficient than anyone else's.

1. Internally Heated. These use heater elements which are buried inside the desiccant bed, usually inside a protective tube which prevents the desiccant abrading the electric heater elements. If you've ever seen a 30kW heater element short to earth you'll know why the protective tube is needed.

2. Externally Heated. These use heated jackets to surround the towers, the jackets are usually either steam or electric.

1. The most common is to use 3-8% of the already dryed compressed air and flush it back through the relevant wet tower.

2. A separate small blower is sited near the towers, the blower produces the equivalent of 3 - 8% of the processed air and this is used as purge. The argument is that you do not waste any dry air, the argument against is that you have yet another item of plant which can go wrong.

3. A separate vacuum pump is sited between the towers and the top of the tower is opened to atmosphere. The vacuum pump sucks atmospheric air over the desiccant, equivalent of 3 - 8% of the processed air. The problem is that the desiccant is screaming hot and vacuum pumps don't usually last very long when running with an inlet temperature of 200 deg C.

1. These dryers use heat which is available from the compressor itself. To successfully work, a desiccant bed must be heated to about 180 deg C to allow regeneration. Re-activation temperatures lower than this will not dry out saturated desiccant. With this type of dryer, the heat is taken from the compressed air immediately after compression and just before it passes through a water cooled intercooler or aftercooler. The air pipework must be plumbed in such a manner that it passes through the desiccant dryer. This means that the dryer must be near or attached to the compressor, or the interconnecting pipework must be well lagged to prevent heat loss. Unfortunately, very efficient air compressors have interstage temperatures much lower than 180 deg C, this means that heat of compression dryers would not work with them. As a rule of thumb, single or two stage compressors will provide enough heat for this type of dryer to work. Three stages and more generally have 'air-off' temperatures lower than 150 deg C and therefore cannot reactivate desiccant.

Although these are not common, it is as well to be aware of a problem so that a subjective risk assessment can be made of the proposed application. The main problem area is when using a heat reactivated desiccant dryer in conjunction with a lubricated compressor. It doesn't matter whether the dryer is internally heated or externally heated, the risk will be the same.

Oil vapour from the lubricated compressor is progressively adsorbed by the desiccant bed until the lower part of the tower is saturated with oil. Depending upon the age and condition of the compressors this may only take months to happen, it may take years.

Bearing in mind the temperatures of desiccant regeneration is around 180 deg C, a fire is waiting to happen. Sooner or later one of the many thousands of heat reactivated dryers will suffer a thermostat fault and the desiccant temperature will climb up to 200-250 deg C. The oil in the desiccant ignites and the whole assembly catches fire. This can be both traumatic and expensive. At best the dryer will be a write-off, although the pressures generated usually open up the adjoining compressed air pipework like a banana skin. At worst you lose a factory.