Adsorption dryers are used to dry compressed air or gases. Although many conditions influence the design of an adsorption dryer, dryer size has to be specified. The size of a dryer, depending on the particular application, is determined by critical factors such as gaseous medium, volume flow, operating pressure, inlet temperature and pressure dewpoint. Apart from the nature of the gaseous medium, the size of an adsorption dryer is primarily determined by the maximum moisture loading of the gas to be dried. The maximum moisture loading is determined at the highest throughput performance, the highest inlet temperature and the lowest operating gauge pressure. Alternatively, the minimum moisture loading of the gas to be dried is fixed by the lowest throughput performance, the lowest inlet temperature and the highest operating gauge pressure. Between these limit values of the loading situation (Fig. 7.1.1), minimum and maximum moisture loading, the loading factor varies with the pressure dewpoint.
Figure 7.1.1
Minimum moisture loading has little influence on the size of the adsorption dryer but does, however, determine the limiting value of the maximum possible adsorption time. The necessary calculation schemes are based on logic and outline important criteria when designing an adsorption dryer. The following demonstrates the calculation scheme for heatless and heat regenerated adsorption dryers. Adsorption dryers with heatless regeneration require the calculation of dryer size on the basis of the optimum quantity of desiccant, supplemented by the specification of the regeneration volume flow.
Calculations for adsorption dryers with heat regeneration are considerably more complex, because the load factor has to be established first and then the regeneration energy as a function of the overall heat allowance The calculation examples of heat regenerated adsorption dryers are based on normal data and is meant solely to stimulate understanding of the complicated interrelationships. It is, therefore, presented in a generally valid manner, on the basis of external blower regeneration. Values indicated take into account the recommendations of desiccant manufacturers. All limiting values are input parameters and, in all likelihood, judged differently by various manufacturers of adsorption dryers. To ignore physical laws or process technological limitations involves risks which will not be dealt with in detail here. Layout design is divided into three steps : Determination of the moisture loading per cycle Dimensioning the adsorber vessel Establishing the regeneration energy The examples can be used as a framework for estimating or checking calculations and modified for specific applications.
