Atmospheric Tubular Ported
The gas jet in atmospheric burners injects primary air into the burner with the
air supply at room or atmospheric pressure. It derives its momentum from the gas
pressure in the manifold. A negative pressure is produced in the venturi throat
by the air-gas mixture flow through it. Air at atmospheric pressure flows through
the primary air openings into the venturi throat. After passing through the venturi
throat, the air and gas mix in the mixer tube, enter the burner head and flow
from the burner ports. The air-gas mixture in the burner leaves through the ports
to burn on those ports. Not all of the air needed for combustion is supplied as
primary air. The burner flames draw in secondary air from around the burner to
complete burning of the gas.
Infrared radiant burners rely on radiation, rather than conduction or convection,
to transfer heat from the flame to the object to be heated. The most common infrared
burner is the surface combustion burner. It uses a porous refractory material
(e.g., ceramic, stainless steel) with small ports or a metal screen. The air/gas
mixture flows through the ports or screen and burns close to the surface at a
very high temperature, typically 1400° to 1650°F. Higher surface temperatures
are possible with a power burner. The hot, glowing burner surface becomes the
source of radiation.
Burner designs vary. In radiant tube burners–found,
for example, in infrared tube fryers–the radiating surface is placed between
the flame and the oil to be heated. Broilers, on the other hand, often use metal
screens heated directly by flames.
Unlike conventional atmospheric burners, most
infrared atmospheric burners require 100% primary air unless designed in such
a way that secondary air can reach the burner. More or less air causes the burner
surface to cool and cease to glow or give off radiant heat.
Advantages of atmospheric infrared burners are
that the heating is very fast, requires a very short warm-up period, and can be
designed to provide even heating of the object to be heated. Infrared heat is
not affected by air between the burner and object to be heated and warms only
the target object, not the surrounding air. The greater heat transfer rates allow
infrared-fired equipment to be more efficient and designed more compactly. On
the downside, low input rates to accommodate the higher primary air input mean
infrared burners are larger than comparable non-radiant burners, limiting the
applications suited to infrared.
Power Burners/Forced Draft
In power burners, pressure of the gas or air, or both, is boosted by mechanical
means. In the forced draft burner, the air-gas mixture is pushed through the system.
Combustion air is supplied under pressure by a fan. In forced draft systems, the
mixing rate of air and gas is fairly slow. Gas and air can be admitted separately
and mix in the combustion chamber. Alternately, in a nozzle mixing burner the
gas and air mix at the point of combustion. Since air and gas mix slowly, combustion
takes place over a large volume. High temperature in the combustion chamber usually
can be achieved only by preheating the combustion air.
An induced draft burner uses an exhausting device such as a fan or air injector
to draw combustion products from the system simultaneously pulling combustion
air in. The blower is placed at the flue outlet. This type of burner is often
used when resistance to flow of flue gases is high, such as in water-immersed
heating tubes or tubular heat exchangers. The fan at the flue outlet of an induced
draft power burner must be in operation before gas is admitted to the burner.
Air and gas supplies must be interlocked. A pressure or vacuum switch activated
by the forced draft fan can be used for this purpose.