a) Desorption temperature
Desorption takes place, using heated blower air. As a source of heat, electrical energy, steam, hot water or even heated oil could be used. The available energy must be sufficient to heat the adsorber to the appropriate desorption temperature TRE.
Diagram 7.3.3.1
Desorption temperature and pressure dewpoint Pdp vary indirectly in accordance with Diagram 7.3.3.1, curve E). In order to be certain of a pressure dewpoint Pdp -40°C, a desorption temperature of TRE = 180°C is necessary.
b) Switch off temperature
In the course of desorption, the temperature level within the adsorber is continuously moved along the latter’s length towards its outlet. A temperature change at the adsorber outlet signals completion of temperature migration and is used to trigger the switching off of the heating phase. Switch off temperature TRO can, correspond to the pressure dewpoint, use curve A) of Diagram 7.3.3.1 as a guideline value. Changing the switch off temperature exerts considerable influence on the pressure dewpoint.
c) Temperature difference
Given a high level of moisture loading on the desiccant (QH2O/QS > 0.7), it suffices to determine the regeneration air quantity via the desorption temperature. In practice, a situation is rarely met, and is not described in further detail in this section. Given low loading of the desiccant (QH2O/QS > 0.7), a significant portion of the heat is used (see Section 7.3.4) for heating the drying medium and the equipment. The quantity of regeneration air required is calculated, using the logarithmic temperature difference. The method for determining the temperature difference is explained later.
d) Logarithmic temperature difference, desorption
The logarithmic temperature difference, desorption, is obtained from the desorption temperature TRE , the switch off temperature TRO and the temperature of the desiccant bed TBC.
Formula 7.3.3.1
Figure 7.3.3.1
e) Logarithmic temperature difference, cooling
The logarithmic temperature difference during the cooling stage is obtained from the desorption temperature TRE, the inlet temperature Ti and outlet temperature TCO of the cooling gas. The inlet temperature of cooling gas equals the ambient temperature plus the temperature rise caused by the blower (about 5 - 15°C on top of the ambient temperature). The outlet temperature TCO of the cooling gas should not exceed 70 - 80°C, in order that the humidity within the blower air on the dry side of the adsorber bed is kept as low as possible. The lower the outlet temperature obtained, the greater the probability of an elevated pressure dewpoint peak remaining constant (see Section 7.3.7) over a long period of time.
Formula 7.3.3.2
Figure 7.3.3.2
Figs. 7.3.3.1 and 7.3.3.2 show the previously mentioned temperature zones for the desorption and cooling phase for determining the logarithmic temperature difference. An accurate and, realistic determination of the logarithmic temperature difference is essential for the overall heat allowance. The desiccant medium and the adsorber are cooled to a low operating temperature during the cooling phase. The cooling phase (see Section 5.5.3) is terminated after a specified time period. It is important to limit cooling in order to avoid a harmful pressure dewpoint peak when cycling from regeneration to adsorption. As the ambient air used for regeneration has a certain moisture component, it is when cooling with moist ambient air, the upper layer of the desiccant material will always be slightly preloaded. This preloading causes a pressure dewpoint peak so that, during adsorption, the dried compressed air contacts this humid zone, and re-entrains some of this moisture again and carries it into the compressed air piping. Using Diagram 7.3.3.2 and assuming an inlet temperature Ti = 30°C and an operating pressure po = 7 bar, the logarithmic temperature difference can be roughly determined for different pressure dewpoints. For different values, Formulae 7.3.3.1 and 7.3.3.2 are used to obtain the logarithmical temperature difference.
