As the design of the modular dryers is common for both Heatless and Heat Regenerated types, the following description of the Adsorption phase applies to both types. Photo 5.8.2.1 illustrates the flow paths within the modular dryers as described below.
The open inlet valve allows the wet process air to flow into the dryer and inlet manifold. This directs the air to the desiccant bed currently on-line and carrying out the drying. Due to the close match of the size of the port in the manifold to the desiccant bed cross-section, no desiccant bypass occurs at the entrance of the column. Also, the change in flow direction from horizontally, along the manifold, to vertically, inside the columns, ensures that excessive turbulence or jetting at the bed entrance is eliminated. The wet air flows over the desiccant beads packed within the column. Due to the high length to diameter ratio of the column internals, uniform flow distribution across the column cross-section is easily achieved. The air, and the moisture contained within, comes into intimate contact with the desiccant beads providing ample opportunity for the desiccant to adsorb, and hence remove, the moisture from the air (see Chapter 6).
During manufacture (and subsequent service intervals) the desiccant is evenly filled into the columns using snowstorm filling techniques and a stainless steel perforated screen fitted to the top of each column. These precautions prevent bed fluidisation, attrition and channelling without requiring a quiet zone at the top of the desiccant chamber. This in turn allows the pressure retaining envelope to be fully utilised in its task of containing desiccant without any wasted free space. As the air reaches the top of the column, the moisture has been removed to the specified level. The dry air enters the top manifold and leaves the dryer via the outlet housing check valves. These valves are spring loaded to prevent the air back-flowing from the system to the dryer’s exhaust valve during regeneration.
The design optimisation of the flowrate and column ensures an adequate contact time between the moisture and desiccant. If a customer requires to dry a higher volume of air, the manifold can be extended to include additional columns. The limit to the number of columns per dryer bank is set by the air flow rate through the common inlet valve assembly and the desire to avoid excessive gas velocities along the lower and upper manifolds. Typically, the maximum number of columns per machine is 10 although up to 12 have been used in special circumstances. The flexibility of number of columns per dryer allows its capacity to be matched easily with the requirements of the specific application.
