EQUIPMENT
BUCKETS CARRY WET PRODUCT. Glass fiber-reinforced plastic buckets carry cakes of w e t product through the Thermatool oven between sets of electrode plates. As buckets go through oven, they arc r o t a t e d b y bearings (bottom c e n t e r )
Super-Power Dielectric Dryer Debuts Speedy 200-kw. unit removes moisture from r a y o n , is useful for other heat-sensitive materials .DIELECTRIC
HEATING,
used
exten-
sively in medical treatment (shortwave diathermy) for about 40 years, has since the 1930's been a practical method of uniformly drying many heatsensitive materials. Large-power a p plications today include drying tobacco, baking foundry cores, and curing cork and rubber products. And "superpower" dielectric heating is n o w making its w a y into the textile field for drying rayon "cakes." N e w Rochelle Tool's new, highfrequency (30 megacycles), superpower (200 k w . ) , Thermatool dielectric heater is designed to do this job a thousand times faster than conventional air-dryers. Currently being used by two major viscose producers, the unit dries wet masses of rayon fiber from the viscose pot-spinning operation. These cakes contain about l 1 / 2 pounds of water oer pound of rayon as they go into the dryer. Ovens generally used
hy the industry are sometimes 150 feet long or more, have a 40-hour t o 130hour drying cycle. T h e biggest Thermatool unit, with a 7-minute cycle, measures only 15 ft. long, 7.5 ft. wide, a n d 13.5 ft. high. Since the oven is shipped nearly intact, this means n o major assembly problems and simple installation, the N e w Rochelle (N.Y.) company points out. • Basically a n OscillaÏor. The dryer is a combination higli-frequency oscillator, cake conveyor, and heatrecovery system. Electrically insulated electrode plates line a circular conveyor path in t h e doughnut-shaped oven. The thermally insulated enclosure heats up in the drying process to slightly above 212° F. This prevents condensation of water vapor inside t h e oven. A current-limiting monitor senses incipient surges and automatically interrupts power within a few microseconds to prevent arc discharges d u e t o metal-
lic particles or other troublesome impurities in t h e w e t cake. Glass fiber-reinforced resin buckets, containing w e t cakes of viscose rayon, rotate on special bearings as the metal conveyor travels upon a low-friction Teflon path. Water vapor rises from the cakes a n d i s drawn off from t h e oven. Useful heat can b e recovered during condensation. Adjusting the conveyor speed controls degree of drying of the rayon cakes. Normal time of the drying cycle —from loading w e t cakes to delivering dry cakes—can be as little as seven minutes. A single unskilled operator can dry 5 tons of rayon in a single 8 hour shift, according to Herman C . Morris, N R T president. • Power Efficiency High. This, h e says, is d u e to t h e Thermatool's drying speed and high efficiency—it evaporates two pounds of water p e r kilowatt-hour of electricity use. Most of the energy lost in conversion, a n d transmitted to the product, is eventually absorbed b y the cooling water. A n d , when hot •water is needed, t h e distillate discharged from t h e drying operation provides a fuel credit. In a typical case, Mr. Morris points out, dielectric evaporation for a pound of water costs 6 mills, and from this can be taken a heat credit of 0.5 mill. These figures assume electricity to b e 10 mills p e r k\v.-hr. a n d low-pressure steam 6 5 cents per 10O0 lb. The speed of drying—said to b e three times faster than previous high-frequency methods—prevents thermal a n d oxidative degradation, he explains. " I n process" inventories a r e reduced drastically, too, where drying takes minutes instead of clays. • Dries O t h e r Fibers, Chemicals. The Thermatool heater can be modified to handle blocks of material (cellulose sponge, for example) or continuously moving webs ( felted, woven, or film ), states H u g h Cameron, N e w Rochelle Tool's dielectric engineer. T h e most immediate market is in drying rayon and other fibers—packaged treated yarn, including d y e d and surfacetreated naturals and synthetics in cake, speel, or bobbin. But other heat-sensitive materials needing closely controlled moisture removal can benefit as well from dielectric drying, M r . Cameron points out. An example of purely chemical processing, where he says this technique has proved useful, is drying precipitated tungstic acid, an intermediate in the manufacture of tungsten carbide. Ε 1 NOV.
16,
1959
C&FN
65
