a) Volume flow
The initial calculation steps for heat regenerated adsorption dryers are identical to those for adsorption dryers with heatless regeneration and are listed solely for the sake of completeness.
Formula 7.3.1.1
b) Operating volume flow
The volume of air per unit time is generally stated at atmospheric pressure and a temperature of 20°C. For design calculations of a drying installation, determination of the operational state V
o is required.
Formula 7.3.1.2
c) Operating volume per cycle
Adsorption dryers with heat regeneration make use of dynamic capacity (Diagram 7.3.1.2) up to the maximum possible value. This leads to the achievement of a loading cycle t l of several hours. With an adsorption period of 6 - 8 hours, the optimum can be obtained.
Formula 7.3.1.3
d) Moisture load per cycle
The size of the adsorber is determined by the humidity load hc per cycle. Moisture content h at inlet temperature T
i can be obtained from curve a) of Diagram 7.3.1.1. This moisture content refers to air volume V
oc at operating pressure p
o and inlet temperature T
i (f/1000 = kg/m
3/g/m
3).
In order to avoid the desiccant bed being subjected to elevated moisture loads, the operating temperature should not exceed T
i = 40° C when drying compressed air. If the operating pressure is low, the moisture loading will also be high.
Formula 7.3.1.5
e) Theoretical temperature rise
When moisture is adsorbed by the desiccant, adsorption heat of up to 2.2°C/gH
2O/m
3 is released. This causes the desiccant and the compressed air passing through it to be heated up. Heating causes the relative humidity of the compressed air to be reduced.
This is based on a specific heat for air of 0.31 kcal/m
3°C. The moisture content h
o for air T
i can be obtained from Diagram 7.3.1.1, curve b, and then inserted into the equation. The moisture content of the humid air h
o , to be inserted into Formula 7.3.1.5, is referred to the dry air volume and the operating pressure.
Formula 7.3.1.6
f) Theoretical outlet temperature
The theoretical outlet temperature, is derived from the addition of inlet temperature plus temperature rise, and should not exceed 60° C. From this temperature value onward, the capacity of the drying medium is reduced as the transition to regeneration temperatures is flexible.
Formula 7.3.1.7
g) Secondary relative humidity
Particularly in the adsorption zone, the rise in temperature causes a reduction in the relative humidity of the air to be dried. However, calculations are intentionally based on the assumption that the entire heat of adsorption leads exclusively to the heating of the air. Losses are not taken into account.
Moisture content ho refers to inlet temperature T
i, and moisture content h
o1 refers to outlet temperature T
o ex Diagram 7.3.1.1, curve b).
Formula 7.3.1.8
h) Load factor
Load factor K
l is the proportion of the adsorbed quantity of water in kg, per kg of drying material utilised, referred to the operating conditions. The load factor (breakthrough capacity) results from the secondary relative humidity Srh and the assumed dwell time. According to Diagram 7.3.1.2, the mean value K
l = 8 - 20 %.
Diagram 7.3.1.2 applies to the achievement of a stable dewpoint of P
dp -40°C at the end of the drying period, measured at 1 bar absolute.
During the greater part of the drying period, the dewpoints are significantly better. If a one layer filling of water proof silica gel is used exclusively, the capacity read from the diagram should be multiplied by factor 0.7.
Diagram 7.3.1.2
The influence of load factor K
l on the size of the dryer is frequently underestimated in practice, as this value depends on the secondary relative humidity can become extremely small at low operating pressure and high inlet temperature
Formula 7.3.1.8
i) Quantity of desiccant
An adequate quantity of desiccant mdr, a reserve margin for the particular application, is important. The desiccant quantity is determined from the moisture load per cycle and the load factor.
j) Adsorber volume
In order to protect the adsorption material from liquid water, a protective layer of 20 - 25% of the drying medium is formed at the inlet side of the drying bed, using waterproof material. The adsorption dryer with heat regeneration used for normal compressed air drying is filled with a drying medium of silica gel :
20 - 25% waterproof material at the inlet
75 - 80% non waterproof material at the outlet
The total packed density of this combination of waterproof and non-waterproof material is to be inserted into the formula, using the values from Table 6.1.1.
Formula 7.3.1.9
k) Flow velocity
The permitted flow velocity for the gas/air relative to be the operating pressure can be obtained from Diagram 7.3.1.3 and should not be exceeded by more than 20%. If the velocity in the adsorber is exceeded, the vessel diameter must be increased.
Diagram 7.3.1.3
l) Adsorber surface
With the operating volume flow V
o from Formula 7.3.1.2 and the flow velocity we from Diagram 7.3.1.3, the adsorber cross sectional surface area A
dr is determined, using Formula 7.3.1.10 while taking into account the units which apply, i.e. m
3/h or m/s.
Formula 7.3.1.10
m) Filling height
Adsorbers are dimensioned in such a way that the filling height amounts to at least 500 - 600 mm. At the same time, the relation ship of diameter to filling height, as well as the air inlet, must be arranged in such a way that even flow through the drying material is ensured.
Formula 7.3.1.11
n) Dwell time
Given normal applications in the air drying field, the dwell time should be of the order of about 5 seconds. As can be seen from Diagram 7.3.1.2, the capacity of the desiccant diminishes if the dwell time is shortened. The dwell time also influences the degree of compressed air drying. The values from Diagram 7.3.1.3 serve as a guide.
Formula 7.3.1.12
o) Pressure loss
At the specified flow velocity w
e and operating pressure p
o , the pressure loss l1,2 of the air when flowing through a layer of silica gel, which has been compacted by vibration, can be determined in line with Diagram 7.3.1.4 per metre of filling height F
hm.
The pressure losses specified in the diagram apply if the desiccant bed is packed as tight as possible without destroying the beds of desiccant. In practice, these values are reached only after a lengthy time of operation.
When determining the pressure loss in order to specify the pressure capacity of the regenerating blower, there should be a sufficiently large safety margin.
However, what is read from Diagram 7.3.1.4 is solely the pressure loss caused in the desiccant bed per metre of filling height.
Diagram 7.3.1.4
In order to determine the total pressure loss in the adsorption dryer, all individual pressure losses arising from the dryer components, such as inlet valve, piping at inlet and outlet, outlet valve and the fittings within the adsorber vessel have to be added up.
Formula 7.3.1.13
Calculating the total pressure loss using Formula 7.3.1.13 is relatively difficult. For this reason, this is generally obtained in practice with relative accuracy by using nomograms and tables.
