Any number of apartments may be built, and each apartment may be arranged to handle any number of cars, generally about three or four, or they may be so constructed that the material is piled directly upon the floor of the drying room. [Illustration: Fig. 36. Exterior View of Six Apartment Dry Kilns, each 10 Feet wide by 52 Feet long, End-wise Piling. They are entirely of fire-proof construction and equipped with double doors (Hussey asbestos outside and canvas inside), and are also equipped with humidity and air control dampers, which may be operated from the outside without opening the kiln doors, which is a very good feature.] When cars are used, it is well to have a transfer car at each end of the kilns, and stub tracks for holding cars of dry material, and for the loading of the unseasoned stock, as in this manner the kilns may be kept in full operation at all times. In this type of dry kiln the material receives the same treatment and process that it would in a Progressive kiln. The advantages of Apartment kilns is manifest where certain conditions require the drying of numerous kinds as well as thicknesses of material at one and the same time. This method permits of several short drying rooms or apartments so that it is not necessary to mix hardwoods and softwoods, or thick and thin material in the same kiln room. In these small kilns the circulation is under perfect control, so that the efficiency is equal to that of the more extensive plants, and will readily appeal to manufacturers whose output calls for the prompt and constant seasoning of a large variety of small stock, rather than a large volume of material of uniform size and grade. Apartment kilns are recommended for industries where conditions require numerous kinds and thicknesses of material to be dried, such as: Furniture factories; Piano factories; Interior woodwork mills; Planing mills; etc. The "Pocket" Dry Kiln "Pocket" dry kilns (see Fig. 37) are generally built in batteries of several pockets. They have the tracks level and the lumber goes in and out at the same end. Each drying room is entirely filled at one time, the material is dried and then removed and the kiln again recharged. The length of "Pocket" kilns ranges generally from 14 feet to about 32 feet. The interior equipment for this type of dry kiln is arranged very similiar to that used in the Apartment kiln. The heating or radiating coils and steam spray jets extend the whole length of the drying room, and are arranged for the use of either live or exhaust steam, as desired. Inasmuch as Pocket kilns have doors at one end only, this feature eliminates a certain amount of door exposure, which conduces towards economy in operation.