Book - School of Science and Technology

Special methods for flue gas disposal (Natural gas) 335to bring the CO 2 content of the mixture down to 1%, which involves a fan to handleapproximately 100 m 3 per m 3 of natural gas burnt. Included in the system is a fan failuredevice to shut off the burners in the event of draught failure. The duct sizing with thissystem should be on the basis of a gas velocity of 3±5 m/s.The outlet location requires careful consideration. It must be at least 2 m and preferably3 m above ground level. The outlet must not discharge directly into a courtyard or similarpartially enclosed space. The minimum distance to the nearest building is 3.54 m.Domestic multi-appliance fluesAnother flue system is the SE, as shown in Figure 12.14(a). This is suitable for multistoreybuildings and, as will be seen, air enters at the base and the products of combustion fromthe various appliances discharge into the same duct. Thus the gas combustion space isvirtually sealed off from the occupied space. An alternative, for use where a bottom inletis not possible, is the `U' duct shown in Figure 12.14(b). Fresh air taken in at the roof isconveyed down a shaft running parallel with the rising shaft, which acts as a shared flue asin the SE duct, the products of combustion being exhausted at the roof from a terminaladjacent to the intake, but at a slightly higher level. All gas burning appliances take theircombustion air from, and return the products of combustion to, the rising duct.Balanced fluesThe now conventional balanced flue boiler terminal has adjacent openings for drawing inair for combustion and discharging the products of combustion, configured such thatwind effects are substantially balanced. (Chapter 10, p. 266)Balanced flue domestic boilers are room-sealed, where the combustion system is sealedfrom the room in which the appliance is situated. It is not practical, however, for largerboilers to be room-sealed. In these situations a balanced compartment arrangement canbe adapted. Here, the open-flued appliance is located in a separate non-habitable room,sealed from other rooms. The door into the room should be self-closing. Fresh air into theroom is taken from a place adjacent to, but just below the flue discharge point. Figure12.15, sourced from BS 5440 Part 1, shows typical arrangements. The sizing of this freshair duct must be generous, since it has to convey not only the air for combustion, but alsothat drawn in by the draught diverter where fitted; and the resistance should, in any event,be kept as low as possible. Since limited quantities of ventilation air may be introducedthere may be a risk of overheating in the boiler room.TerminalsCare must be taken to ensure that flues discharge the products of combustion freely to theatmosphere. In buildings with return walls, areas, etc., peculiar atmospheric pressureconditions may occur and it is advisable to extend the flue to a height of at least500 mm above the eaves of the roof. The possibilities of downdraught are then muchreduced.Every flue should be fitted with a terminal of approved design to prevent birds nesting,reduce rain penetration, deflect wind and reduce staining due to down draught.Chimney heights and terminal locations for domestic and small commercial boiler plantare covered under the Building Regulations and B5 5440 Part 1. There are numerous