﻿KILN DRYING HANDBOOK. 7 



PIPE COILS AND RADIATORS. 



The form, construction, and arrangement of the kiln radiators is 

 of importance. Those constructed of pipe coils are in most common 

 use. Pipe coils are made of ordinary merchant pipe, extra heavy 

 pipe of various kinds, and wrought-iron pipe, the last being 1 par- 

 ticularly suitable for severe drying schedules. Among the advan- 

 tages of pipe-coil radiators are low first cost, ease of manufacture 

 and installation, ready adaptability to a great range of shapes and 

 sizes, and ease of repair by the shop mechanic or millwright. There 

 are several essentials which a good pipe coil must possess : First, it 

 must be of such size and shape and so located that it can properly 

 heat the air in the kiln; second, it must be mechanically strong and 

 durable and provided with means for permitting the expansion and 

 contraction of the individual pipes in the coil; third, it must pro- 

 vide for the ready escape of air and water of condensation from the 

 entire system ; fourth, it must provide for adjustment in the amount 

 of heating surface to be used by cutting certain pipes in or out. 

 As it is difficult to combine all these essentials in the highest degree 

 in any one type of coil, different ones have been found best adapted 

 for various special conditions. 



A large portion of all pipe coils used for dry-kiln heating are 

 located in the kiln proper, between or under the rails. These fall 

 into two general classes, known as header and return-bend coils. In 

 the former, a number of pipes spring from the same supply pipe or 

 header and return to a similar drip pipe or header, usually but not 

 alwaj^s, located at the other end of the kiln. In the return-bend 

 type, however, the pipes of each group are connected end to end by 

 means of return bends or double-elbow fittings; steam enters at the 

 front of the first or top pipe, and condensation is removed from 

 the end of the last or bottom pipe. Figure 2 illustrates various types 

 of header and return-bend coils. 



PLAIN HEADER COIL. 



The action of the two types of coils is quite different, especially when 

 operated with a thermostat. When steam is turned on in a plain header 

 coil with a header at each end of the kiln that end of the kiln nearest the 

 supply header will heat up first ; the other end will not heat until the 

 front end has become hot and all the air has been exhausted from the 

 coil. This uneven heating takes place each time the thermostat 

 opens. If the heating surface is unduly large, as it may be when low 

 temperatures are used, the thermostat will operate often, and there 

 will be a marked difference in temperature between the two ends of 

 the kiln. Another characteristic of the header system is that the 

 large heating surface of the headers themselves causes an uneven 

 distribution of heat by causing a " hot spot " at each header. 



RETURN-BEND COIL. 



In the return-bend type the top pipes in each group become hot 

 first, since the steam must pass through them before reaching the 

 lower ones. Each pipe runs the full length of the kiln, and heating 

 will be practically uniform from end to end. The return-bend type 

 also has disadvantages, among: which are the first cost and the amount 



