a) Drying medium
The energy requirement for the quantity of drying material (silica gel), or other desiccant, is calculated with the specific heat value cdr from Table 6.1.1, using Formula :
Formula 7.3.4.1
b) Moisture load
The adsorption heat QH depends on the load factor Kl from Diagram 7.3.1.1. Adsorption heat QH from the diagram is multiplied by the moisture load hc from Formula 7.3.1.4.
Diagram 7.3.4.1
Depending on the load variations at the adsorber inlet, the heat of adsorption for a given moisture content may differ considerably from the design assumptions. Adsorption dryers are, in principle, designed for the theoretical maximum moisture load. However, under practical operating conditions, they are very frequently charged with a lower moisture load. Utilisation of the reserve resulting from this takes place as a function of loading.
Formula 7.3.4.2
c) Vessel
In order to obtain the percentage of the energy taken up by the vessel, a rough estimate of the vessel weight has to be made in the first instance. For this, specified data is necessary, based on loading receiver technical regulations : wall thickness s =xmm
thickness of bottom s1 =ymm
length Sh1 =zmm
fitting mz =in%
spec.grav. Stdb =7.85kg/dm3
Figure 7.3.4.1
The vessel weight mv is determined. For the energy requirement Qst of the adsorber, a supplement mz has to be added to the vessel weight mz, as fittings within the vessel increase the weight and not only the adsorber but also further components such as connecting piping and valves are partly heated up during desorption. Whereas the energy requirements arising from the desiccant and the moisture loading are determined with a high degree of certainty, the energy requirement of the total mass of steel mst including components must be within a range of tolerances. The determination of the mass of steel to be heated depends on a significant variety of factors, e.g. differing surrounding conditions and different locations. The specific heat value for standard steel cst = 0.11 kcal/kg°C has been taken into account in Formula 7.3.4.3. A correction for the cst value is required depending on the material selection.
Formula 7.3.4.3
d) Energy requirement
The energy for desiccant Qdr, moisture loading QH2O and vessel Qst are combined to obtain the sum of the energy requirement QS.
Formula 7.3.4.4
e) Additional energy requirement, radiation
The radiation energy requirement Qra of an adsorber depends on its’ location. The following may serve as a general indicator: The higher the logarithmic temperature difference for desorption, the higher the heat loss through radiation. In the absence of all required parameters, it is hardly possible to predetermine radiation in practice. For this reason, and depending on the type of application and the location, qra is assumed to be 4 - 8 % of the total energy requirement QS.
f) Grand total energy requirement
Putting forward a grand total of energy requirements for heat regenerated adsorbers can only be theoretical. A general tolerance of 2 - 5% is to be expected. As the parameters, to some extent, mutually affect each other, an appropriate and low as possible safety factor, based on experience and visual observation, is required.
Formula 7.3.4.6
g) Energy requirement, cooling
The energy requirement for cooling is considerably lower than that for desorption. For one thing, only the drying materials and the adsorber are cooled, for another, we have the favourable temperature gradient of cooling. Under normal conditions, the relationship between desorption and cooling is about 4:1.
Formula 7.3.4.7
