The essential characteristics of the different types of adsorption dryer have now been described. The question is asked as to which adsorption dryer system is the best. Numerous publications have been issued on this subject, furnishing very interesting results. It is understandable that every manufacturer of adsorption dryers tends to allocate a premier position to his own system. It is the aim of a system comparison, not to evaluate a system as such but the peculiarities of a particular method of drying. Using fundamental parameters, it is possible to arrive at an objective evaluation. It is assumed that the principal calculation schemes of adsorption dryers with heatless or heat regeneration are known. Such calculations form the basis of a logical comparison between systems The mathematically based relationships, form the basis for an objective study of the systems which are available. Thus, a system comparison with reproducible dryer data for the varying performance ranges can be developed and utilised for practical purposes. Every system comparison is based on specific dryer data. This data is supplemented by the effective operating costs using different types of energy, e.g. steam, hot water, hot oil or electricity, which are available to the user. Estimated compressed air cost data, are out of place here. System comparisons based on assumed compressed air costs are not logical, for:
- It is the purpose of the system comparison to find out the cost of compressed air. The aim of the investigation cannot be used as its starting point.
- The installation is being supplemented by a new item so that only units which have already been installed can be taken into account.
- The unit present in the system is being exchanged for a new system with totally different performance data.
- Because of a demand for additional purified compressed air, the units already installed in the system are being extended and supplemented.
Other operating costs arising from the application case in question, such as power, water and steam costs, are input and are based on reliable experience. Adsorption dryers with heatless regeneration are regenerated using a fraction (purge) of already dried compressed air, whereas adsorption dryers with heat regeneration have the energy for this supplied mainly from an external source. A considerable error could be included into a system comparison, unless the real and differing energy costs have been specified correctly. Only by the use of true compressed air parameters can an objective evaluation of various systems be achieved. This is based on allocating a uniform energy cost to the energy requirements for all types of regeneration. An unbiased comparison between all types of regeneration methods then becomes possible. For example, the specific energy requirement of oil injected screw compressors amounts to about 6 kW/m3 /min (see Table 9.8.1). Given an output flow volume of 1000 m3/h, adsorption dryers with heatless regeneration working at 7 bar require a quantity of regeneration air of 143 m3/h. The compressor power requirement is then:
- The output flow of 6 kW/m3/min x 1000 m3/h x 1h/60 min = 100.0 kW.
- The regeneration air quantity 100 kW x 143 m3/h/1000 m3/h = 14.3 kW.
Pressure losses must also be taken into account if there is to be an objective system comparison. The proportion of pressure losses is obtained from the difference between p1 and p2 and is expressed in %. As a result, there is less energy available at the outlet of the adsorption dryer and therefore fed into the compressed air system. The difference in pressure is also taken into account when evaluating performance, all energies and losses are allocated to a comparable unit. On this basis, a comparison of adsorption dryer data from the most varied designs can be objectively carried out without bias. In addition, various factors have to be considered, all of which influence the selection of an adsorption dryer, objectively and subjectively, with reference to:
a) Investment
- cost effective investment
- pressure dewpoint required
- specific energy requirement
- low-cost energy source
- economical service life
- effective operating load
1pressure/volume
b) Operation
- ecological evaluation
- low waste material disposal
- appropriate noise protection
- good access
- reliable servicing
c) Installation
- uniform spares storage
- dust loaded ambient air
- explosion prone location
- computer assisted monitoring
- location subject to frost
- off-shore operation
To mention only the most important decision characteristics. Every system manufacturer and operator of adsorption drying installations is called upon to list all aspects relevant to the particular application, thus facilitating a purchase decision. It would be irresponsible to arrive at a universally valid statement advocating or rejecting a particular system without an adequately based evaluation. The wide range of applications, from the small receiver mounted compressor to the sophisticated compressed air plant, have to be considered in a different manner.
