The heated regeneration of modular dryers utilises a combination of technologies, namely modular PTC (Positive Temperature Co-efficient) heater elements, in-bed location and self-regulation. The use of rod type in-bed heaters has been described (see 5.4.1 and 5.4.3). However, with a modular dryer design, several additional benefits can be obtained. The regeneration heat is supplied to the desiccant bed from two heaters located in different positions along the column length. The heat energy is therefore transported to the desiccant over very short distances with very low heat losses. The heaters are located toward the middle and lower part of the bed where the desiccant is wettest and needs the most heat energy for regeneration. The top of the bed, where heating is not present, is regenerated by the purge air alone which is at its dryest at this point. Whereas internal rod heaters are orientated axially in the desiccant bed, the heaters of a modular dryer are positioned perpendicularly to the purge flow direction. This ensures that all the purge air flows through the heaters, closely contacting the hot surfaces, and receives an even amount of energy which it then transfers to the desiccant. As mentioned in the previous section, the high length to diameter ratio of the columns ensures even flow of the purge air through the desiccant.
Photo 5.8.7.1
The modular dryer heaters are made up of Positive Temperature Coefficient (PTC) heater discs. These are a sintering of conductive compounds that achieve a certain temperature (their Curie temperature) after which, any increase in temperature causes an increase in electrical resistance. This reduces the current flowing through that element, causing it to cool. In this way, the heaters become self-regulating and will not reach higher temperatures than the Curie temperature, even if the purge flow is shut off completely. This is obviously a good safety feature when, during its service life, the compressed air dryer could be subjected to oil vapour and perhaps oil aerosol carryover if the correct pre-filtration has not been fitted. In modular dryers fitted with PTC heaters, the purge air is used to carry the heat from the heaters to the desiccant to be regenerated. This means that the quantity of purge air required, when compared to a heatless dryer, can be reduced. The purge flow direction is counter-current as per heatless dryer design. A combination of the low moisture content of the purge air, the reduced pressure and the elevated temperature drive the moisture from the desiccant, thereby ensuring efficient bed regeneration. The exhaust silencer arrangement is the same as that fitted to Heatless modular dryers, an example of component/assembly re-use available to designers of modular dryers.
