7 DRAFT CONTROL
7.1 Introduction to Draft control
Draft control is used to manage the excess draft from stacks, in both fire-tube and water-tube
applications, so heat transfer from the hot gases to the boiler tubes can be optimised. Both heat
transfer rate and combustion rate depend on the motion of the flue gases; any changes in boiler
pressure can affect the amount of combustion air entering the burner, possibly resulting in unburnt fuel.
An excess of unburnt fuel can lead to unsteady combustion with dangerous consequences. A tall stack
is susceptible to a changing pressure which is caused be stack temperature and wind velocity.
The main benefits of maintain stack pressure through draft control include:
Improves heat transfer
Improves combustion efficiency
Reduces room heat loss
Improves flame stability while reducing chance of pilot light failure
Improves flame retention
Reduce soot accumulation
7.2 Autoflame Fully Integrated Draft Control
The Autoflame draft control stores pressure conditions at the commissioning stage and modulates with
the firing curve to maintain this, irrespective of changing firing rate and stack conditions. Normally
there is a vertical main stack which has a horizontal cross connection from the boiler flue gas outlet;
this is then connected into the main stack. The boiler only works at optimum efficiency when all of the
conditions that effect its operation are held at good commissioned values. Therefore under the new
arrangement, a butterfly valve driven by a positioning motor, is placed in the horizontal back flue
typically two or three metres from the boiler. A differential pressure sensor is then inserted into the flue
that is between the boiler outlet and the butterfly valve. As stack energy alters, the suction or pressure
would vary at this point. It can be seen that by measuring the pressure of the draft at the position of
the damper could be adjusted to bring the pressure or suction back to its commissioned value, the
complete system would then be operating at optimum efficiency again.
Figure 7.2.i Stack with Draft Control
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