NO. I FUEL ECONOMY — MITMAN I3 



around the boiler heating surfaces and so on out of the stack, but 

 the resistances met with in its flow prevent complete equalization, 

 so that a difference in pressure, or differential pressure, as it is called, 

 always exists and in amounts proportional to the resistances. Any- 

 untoward condition, however, taking place within the furnace, such 

 as a hole in the fuel bed, alters the resistance to the flow of air which, 

 in turn, causes the differential pressure to change. In other words, 

 the pressure differential affords a measure of the air feed and an 

 indication of furnace conditions as well. Such is the draft gage, 

 for it is a pressure recording instrument made especially to indicate 

 the pressure difference between the outside and inside of a furnace. 

 When furnace conditions are right and the percentage of carbon 

 dioxide is such as to give the maximum steam output, a certain 

 amount of resistance exists in the furnace, represented by a certain 

 pressure difference indicated on the draft gage. As long as this con- 

 dition exists, there is an assurance of the existence of a uniform 

 pressure, but if some change takes place within the furnace which 

 alters the resistances, permitting an increase or decrease in air supply, 

 it is immediately indicated by a change in pressure. In many instances 

 the condition can be corrected in time to prevent any appreciable 

 change in steam output and before its effect is indicated in the quality 

 of the flue gases. In other words, the draft gage intelligently used is 

 the mainstay of an established furnace practice. 



Flue Gas Temperature 



Experience in chemical control has demonstrated that success is 

 attained in the degree to which each and every operation involved is 

 under observation. Thus, in the case in hand, the objective is the 

 most economic production of heat for the steam output required, and 

 while there is a surety that with the proper co-ordination of flue 

 gas analyses and steam output maintained by a draft gage the objec- 

 tive is being attained, still a further check sitch as the temperature 

 of the gases as they leave the boiler heating surfaces, will, in a 

 measure, constitute more or less proof, inasmuch as the generally 

 accepted permissible temperature of the gases as they go up the stack 

 is around 500 degrees Fahrenheit. Obviously, therefore, the use of 

 a thermometer or some other temperature recording device will be 

 beneficial. It is conceivable, too, that even though proper combustion 

 is taking place and sufficient heat is produced, a condition may arise 

 whereby the heat so produced is not being absorbed by the boiler 

 heating surfaces or that the latter are deprived of sufificient oppor- 



