ON THE SETTING OF STEAM BOILERS. 



141 



to its constitution tlirough the furnaco. This gas has to be 

 heated up to the temperature of the furnace by the com- 

 bustible gases ; and the difference between the temperature 

 of the escaping gases after passing the flues and the tem- 

 perature at which the air enters the furnace, shows the loss 

 of heat from this cause. 



Assuming that air enters at G0° and escapes at G(XJ°, the 

 loss is 540°, which multiplied by 18, the weight required 

 for 1 lb. of coal, and by -23, the specific heat of air = 2,235 

 thermal units which have to be deducted from 14,500 = 

 12,265 units available for making steam. Two things thus 

 are seen to be important— (1) that the temperature of the 

 escaping gases should be as low as possible, and, (2) that 

 while a quick draught is essential to good combustion, there 

 should bo the means of regulating it, so that the tempera- 

 ture of the furnace should not be lowered by too great an 

 excess of air. Both of these conditions are obtained in the 

 setting before us. 



1. The temperature of the escaping gases is found in all 

 cases whore the setting is properly done to be very little 

 more than the temperature of the water — namely, from 800° 

 to 400°, of which there is abundant and reliable evidence. 



2. The damper, which is one of the features of this set- 

 ting, allows of very accurate regulation of the draught. 

 Opening back as it does in the direction of the flow of the 

 gasos, instead of vertically as ordinary dampers do, it 

 checks the rush of air into the furnace, without offering 

 unnecessary resistance to tlie gasos ; and an intelligent fire- 

 man, having the handle close to his fire door, may regulate 

 the draught to the over-varying conditions of the furnaco. 

 Some men are found to be careful enough to partially close 

 the damper when the furnace door is open for firing, a 

 practice which, by preventing a rush of cold air into the 



