Adsorption dryers with heat regeneration in closed loop versions are based on physical processes operating when air is being compressed. The compressed gas is heated, thus utilising the energy fed externally to the compressor in the form of work by converting it into heat for the benefit of the dryer system. The quantity of energy required for compressing the air is low, if the temperature is kept constant while the pressure is being increased. However, in order to achieve this, compression must be cooled to such an extent that the added energy is immediately conducted away. This would mean achievement of the isothermal state and would represent the ideal compression process. If, however, the change of state of the gas to be compressed in the compression chamber takes place adiabatically, then heat is neither fed into nor withdrawn from the compression process. The entire added energy is utilised for increasing the inner energy. As it is hardly possible in practice to withdraw at once the entire heat generated or to create a heat insulated compression chamber, compression in reality usually takes place between these two changes of state, i.e. polytropically. The following is valid for establishing the external energy used for polytropic compression:
Formula 5.7.1
Herewith, analogously, the establishment of the final temperature of compression:
Formula 5.7.2
In the case of one-stage compression up to about p = 3 bar, temperatures of up to tult = 135°C are reached. With two-stage compression, however, the compressor outlet temperature would amount to 140 - 180°C. This heat energy is utilised in a targeted manner when adsorption dryers of the closed circuit type are used, as these use the compressor heat for purposes of regeneration. In conjunction with heat of compression dryer systems, solely oil free compressors are permissible. This decisive aspect must always be observed as a point of principle! Oil free compressing systems are needed, inter alia, in the food and luxury consumables industry, in paper factories and printing works, in chemical or also pharmaceutical enterprises.
Diagram 5.7.1
The argument in favour of this dryer system states that no energy for desorption has to be provided, as this is available free of cost from the compressor system in the form of the heat generated by compression and available for desorption. The adsorption dryer of the closed loop type always depends on the compressor. Dryer and compressor form one combined unit. Differing parameters are in an interactive relationship. The pressure dewpoint, normally a parameter independent of the compressor is, with this adsorption dryer system, dependent on the compression temperature of the compressor.
