When pressure fired boiler doesn’t has an ID Fan (Induced Draft) will may run on positive pressure above several percentage or most of the load variety. In the type of pressure fired boiler, the pressure in furnace ranges when the load is adjusted because of the variance in draft losses together with regard to the load of boiler. Since the amount of firing raises, extra air is delivered by FD Fan (Forced Draft) to raise furnace pressure.
Figure 1 shows the actual design of pressure fired boiler process. An important factor of this type is that the boiler furnace should be flue gastight and also airtight. This is essential to ensure that the quite hot flue gas temperature associated with boiler furnace is unable to leak to environment.
A little leak within these conditions will degrade the material close to it, ultimately doing damage to the boiler furnace walls and also generating a functional risk. Boiler furnace is designed pressure-tight in pressure fired boiler system by using inner casing which is welded or seal somewhere between generating tubes and furnace wall steam. In addition boiler furnace has sealed window to view furnace circumstances and flame.
Figure 1: Pressure Fired Boiler
(Source: Book-Boiler Control System Engineering-G.F Gilman)