742 Fermentation and Biochemical Engineering Handbook mounted on the shaft which support inner and outer rows of ribbon flights pitched so as to move the product. The outer ribbon flights usually move the product towards one side of the vessel and the inner ribbons move the product towards the other side. This design would have the discharge port at one end of the dryer. An alternative to this design is to have a center discharge, where the ribbons on one half of the dryer are pitched at 90 to the ribbons on the opposite end The drying here is achieved by means of exposing the product to the surface area of the jacketed vessel. The jacket is a shell of metal(usually carbon steel) welded onto a stainless steel vessel body. This design can include a heated shaft for increased surface area exposure. The heat transfer medium used here is generally steam, hot oil, or hot water. Ports must be provided so as to vent the evaporated vapors being removed from the product 5.5 Paddle dryers Whereas a gaseous medium can be used to transfer heat to the product, in most cases the paddle type is considered to be an indirect dryer. It is similar in design to the ribbon dryer. The differences exist when heated(hollow) paddles are used as opposed to flat blades. (See the previous section on indirect drying )Also, shoe-like paddles or plows can be used which tend to disperse or smear the product against a heated, horizontal, cylindrical wall The advantage of a heated paddle design is that the surface area exposure to the product being dried has been expanded thus increasing the overall heat transfer rate. Most paddle dryers are designed for use under vacuum whicl can supplement the indirect drying process 5.6 Agitated Pan Dryers An agitated open pan dryer is somewhat more complicated mechani cally. This is a short cylinder whose axis and agitator are vertical. The agitator can enter from either the top or the bottom As with the paddle dryers, these are mostly considered to be indirect dryers since heat transfer is from the jacket. If the product is a sticky, pasty material one may wish to use this design. the advantage of the pan dryer is the availability of several heated agitator designs which improve the overal heat transfer rate appreciably over a simple heated jacket; the reason is the same as mentioned in the previous section on paddle dryers. As mentioned earlier, venting of the dryer is necessary to remove the evaporated vapors742 Fermentation and Biochemical Engineering Handbook mounted on the shaft which support inner and outer rows of ribbon flights pitched so as to move the product. The outer ribbon flights usually move the product towards one side of the vessel and the inner ribbons move the product towards the other side. This design would have the discharge port at one end ofthe dryer. An alternative to this design is to have a center discharge, where the ribbons on one half of the dryer are pitched at 90" to the ribbons on the opposite end. The drying here is achieved by means of exposing the product to the surface area of the jacketed vessel. The jacket is a shell of metal (usually carbon steel) welded onto a stainless steel vessel body. This design can include a heated shaft for increased surface area exposure. The heat transfer medium used here is generally steam, hot oil, or hot water. Ports must be provided so as to vent the evaporated vapors being removed fromthe product. 5.5 Paddle Dryers Whereas a gaseous medium can be used to transfer heat to the product, in most cases the paddle type is considered to be an indirect dryer. It is similar in design to the ribbon dryer. The differences exist when heated (hollow) paddles are used as opposed to flat blades. (See the previous section on indirect drying.) Also, shoe-like paddles or plows can be used which tend to disperse or smear the product against a heated, horizontal, cylindrical wall. The advantage of a heated paddle design is that the surface area exposure to the product being dried has been expanded thus increasing the overall heat transfer rate. Most paddle dryers are designed for use under vacuum which can supplement the indirect drying process. 5.6 Agitated Pan Dryers An agitated open pan dryer is somewhat more complicated mechani￾cally. This is a short cylinder whose axis and agitator are vertical. The agitator can enter from either the top or the bottom. As with the paddle dryers, these are mostly considered to be indirect dryers since heat transfer is from the jacket. If the product is a sticky, pasty material one may wish to use this design. The advantage of the pan dryer is the availability of several heated agitator designs which improve the overall heat transfer rate appreciably over a simple heated jacket; the reason is the same as mentioned in the previous section on paddle dryers. As mentioned earlier, venting of the dryer is necessary to remove the evaporated vapors