Design Features of Scraped surface heat exchanger
SSHEs are employed in the continuous processing of virtually any pumpable fluid or slurry system. Choice of design depends on the processing needs for a particular product. For example, pasteurization, slush freezing, cooling, and crystallization will have different design requirements. The selection of design must account for the specific operational needs. Design parameters for some commercially available SSHE systems are presented in Table 1. Details of design are mostly proprietary and beyond the scope of this review. A general description is provided here to give readers a broad perspective on the design issues involved.
Design of SSHEs is based on the simple concept of continuous heat transfer of the moving product with periodic removal of fluid from the barrel wall in contact with the heating or cooling medium. Most commercial designs have a rotating shaft in the center with the product being pumped through the annular gap between the shaft and the outer cylindrical heat transfer tube (Figure 1a). As the product is pumped under pressure through the annulus, the central shaft with the scraper blades is rotated. Due to the centrifugal force fields, the product is continually forced to the tube surface. Simultaneously, blades on the rotating shaft continuously scrape the surface to enhance heat transfer rates and agitate the product. The main design features include the number of scraper blades, angle between the blades and the axis, the annular gap for the product flow, gap for the heat transfer media flow, length of the tubes, and number of the tubes. The blades may also have holes to control the product flow. Although the shafts are typically in the center, certain eccentric designs, where the shafts are off-center, are also available. Waukesha Cherry-Burrell sells SSHEs that have oval tubes (Figure 1b) and eccentric shafts (Figure 1c). In the eccentric design, the shaft arrangement increases product mixing and reduces the mechanical heat load. The channeling of products is reduced in oval tubes.
Increased throughputs may be obtained by equipment with several tubes that are bundled in parallel. Some commercially available SSHEs can be adjusted to flow rate needs by changing the number of tubes (Table 1). Gerstenberg and Agger’s Perfector 250 has an option of construction with varying number (from 1 to 6) of tubes. Waukesha Cherry-Burrell’s Votator II SSHEs have the option of vertical or horizontal mount, which can change flow properties due to the force of gravity. Alfa Laval sells SSHEs under the brand name Contherm and Convap .
A new SSHE design, with reciprocating scraper blades, is now available (for example, from HRS Spiratube SL, Murcia, Spain). Individual tubes are bundled in a shell with heat or cooling medium. Within each tube is a reciprocating scraper, moving forward and backward along the axial length of the tube, that periodically removes fluid from the heating or cooling surface. The manufacturer claims that this heat exchanger is more efficient than the regular SSHE and can be used for all the same applications.
Because of the variety of commercially available SSHEs to suit specific processing needs, the key is to identify the product specific processing needs and procure the available SSHE that is most suitable. For example, heating of highly viscous fluids requires equipment suitable to withstand higher processing pressures than for evaporative crystallization of low viscosity fluids.