Rotating Dryers That Require Syphon Drainage

There are two general types of rotating dryers that require syphon drainage. In the first, the rotating cylinder is filled with steam. The product is partially wrapped around the circumference of the cylinder or drum as it passes. Such devices are found extensively in the paper, textile, plastics and food industries, where they serve as dry cans, drum dryers, laundry ironers and paper machine dryers. Rotating dryers of this type can vary in size from 6 inches (15 cm) to 14 feet (4 m) in diameter. Pressures vary from sub-atmospheric to 200 psig (14 bar) or more and rotational speeds range from 1 to 5,000 rpm. In these machines the condensate falls to the bottom of the drum itself, where it is lifted by a rotating syphon.

The second type of rotating drum dryer places the product inside a drum containing steam-filled tubes (the drum may be steam-jacketed as well). The product is heated by direct contact with the tubes. Examples of rotating dryers with the product inside the drum are grain dryers, rotary cookers and bean dryers that are found in meat packing and food processing plants. These dryers commonly rotate very slowly and pressures range from 0 - 150 psig (0-10 bar). Normally, the condensate accumulates at the bottom of a condensate collection chamber and is lifted out by a stationary syphon.

The construction of syphon-drained rotary dryers of either type results in a syphon pipe that is jacketed by steam. This causes the condensate temperature in the syphon pipe to rise as the pressure falls. Therefore, some of the condensate will flash back into steam during the lifting process.