gilled to provide the optimum surface area for heat transfer and the gases of combustion from the oil or coal fired furnace pass across one side of the Plates, transferring heat to the clean air pulled across the reverse side by the dryer fans. The fans draw the heated air into the dryer proper, either through or across the product which is being continuously conveyed on the dryer Heat exchangers can also be constructed with a series of tubes, instead of gilled Plates; this is very similar to the design of an economizer for a boiler The obvious weakness of this system is that there are considerable thermal losses, in spite of the facility, and indeed practice, of recycling the gases of combustion before discharge to atmosphere. Then there is usually a heavy maintenance factor in cleaning the heat exchanger elements or tubes, to keep the apertures through which the gases flow free from a build up of fly-ash(where solid fuel is used)and carbon and sulphurous deposits from oil. Also, at the cool end of the exchanger, where temperatures are often below the dew point, corrosion can be a problem. Another indirect heat system, which can be used with band, cabinet, tunnel and bin dryers, incorporates the use of steam batteries of gilled tubes through which the drying air passes, either by induction or pressure. Here again there are some thermal losses, and the batteries require constant and careful maintenance to ensure that the air passages are kept free and that they are efficiently trapped on the steam side. Dust is not an uncommon problem in a dehydration factory, and this can be a source of trouble on the air side of this type of steam Combined Systems With air lift and thermal venturi dryers, it is possible to use a combination of the indirect system, with steam batteries, and the direct system of LPG or natural gas-the one system supplementing the other. The use of batteries is often economical when the factory has a surplus of low pressure steam or exhaust steam from a turbine or some other part of the Plant. No heat should go to waste, and every conceivable use should be made of it. Gas heat, as a supplementary source, can then be used to obtain the desired processing temperature The drum dryer is another example of the indirect system. Here, high pressure steam passes through either one or two large rotating steel drums, and the product to be dried is fed by spraying or by feed rollers on to the outer surface of the drums in a thin layer and heat transfer from the inside to he outside surface of each drum evaporates the water content of the product during the course of about 300of the drum s revolution. Owing to thegilled to provide the optimum surface area for heat transfer, and the gases of combustion from the oil or coal fired furnace pass across one side of the plates, transferring heat to the clean air pulled across the xeverse side by the dryer fans. The fans draw the heated air into the dryer proper, either through or across the product which is being continuously conveyed on the dryer belt. Heat exchangers can also be constructed with a series of tubes, instead of gilled plates; this is very similar to the design of an economizer for a boiler plant. The obvious weakness of this system is that there are considerable thermal losses, in spite of the facility, and indeed practice, of recycling the gases of combustion before discharge to atmosphexe. Then there is usually a heavy maintenance factor in cleaning the heat exchanger elements or tubes, to keep the apertures through which the gases flow free from a build up of fly-ash (where solid fuel is used) and carbon and sulphurous deposits from oil. Also, at the cool end of the exchanger, where temperatures are often below the dew point, corrosion can be a pmblem. Another indinxt heat system, which can be used with band, cabinet, tunnel and bin dryers, incorporates the use of steam batteries of gilled tubes through which the drying air passes, either by induction or pressure. Here again there are some thermal losses, and the batteries require constant and careful maintenance to ensure that the air passages are kept free and that they are efficiently trapped on the steam side. Dust is not an uncommon problem in a dehydration factory, and this can be a source of trouble on the air side of this type of steam battery. Combined Systems With air lift and thermal venturi dryers, it is possible to use a combination of the indirect system, with steam batteries, and the direct system of LPG or natural gas - the one system supplementing the other. The use of batteries is often economical when the factory has a surplus of low pressure steam or exhaust steam from a turbine or some other part of the plant. No heat should go to waste, and every conceivable use should be made of it. Gas heat, as a supplementary source, can then be used to obtain the desired processing temperature. The drum dryer is another example of the indirect system. Here, high pressure steam passes through either one or two large rotating steel drums, and the product to be dried is fed by spraying or by feed rollers on to the outer surface of the drums in a thin layer, and heat transfer from the inside to the outside surface of each drum evaporates the water content of the product during the course of about 300" of the drum's revolution. Owing to the 34