178 HOT-WATER BOILERS AND PIPES. 



what is here stated ; for, although many causes tend to modify 

 the effect, the above calculation will be found a good average 

 proportion, under ordinary circumstances. The effect very much 

 depends upon the quality of fuel, the force of draught, the con- 

 struction of the furnace, &c., which, from what has been already 

 said on these matters, will show that they will, in a great 

 measure, influence the intensity of the heat received by the 

 boiler. It is always safest, however, to work with a larger sur- 

 face of boiler, at a moderate heat, than to keep the boiler Avork- 

 ing at the maximum of its power. 



There is another cause, however, that will tend to modify the 

 proportions which may be adopted. The data from which the 

 calculation of the boiler surface is made assumes the difference 

 to be 140 between the temperature of the pipe and the air with 

 which it may be surrounded ; the pipe, in this calculation, being 

 200, and the air 60. But if this difference of temperature be 

 reduced, either by the air in the house being higher, or by the 

 apparatus being worked below its maximum temperature, then, 

 in either case, a given surface of boiler will suffice for a greater 

 length of pipe. For, if the difference of temperature between 

 the water and the air be only 120, instead of 140, the same 

 surface of boiler will supply the requisite degree of heat to one 

 sixth more pipe ; and if the difference be only 100, it will sup- 

 ply one third more pipe than the quantity stated in the table. 



It will, therefore, frequently occur, in practice, that the quan- 

 tity of pipe, in proportion to a given surface of boiler, may be 

 considerably increased beyond the amount which is given in the 

 preceding table ; because, in forcing-houses, the temperature of 

 the air may sometimes be above the number of degrees here 

 given, and frequently the temperature of the water may be below 

 100, the pipe not being required to be worked at its full heat; 

 and, therefore, in both these cases, a larger proportion of pipe 

 may be worked by a given sized boiler. 



In order to estimate the quantity of surface which is acted 

 upon by the fire, an allowance must be made for the flues which 

 circulate round the exterior of the boiler, (and all boilers should 

 be so erected as to admit of the action of the heat round their 

 sides.) Thus, suppose an arch boiler (Fig. 35) to be 30 inches 



