herrmann HL 60 GLV.2-S Technical Information, Assembly And Operating Instructions Download Page 26

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2.1 Mixing device

A blue flame burner mixing device is used to burn the mixture. The fuel spray
that escapes from the nozzle evaporates before the actual reaction of com-
bustion by a mixture of hot flue gases. The low temperature level within the
evaporation zone, and the water content of the returned combustion gases,
prevent the formation of soot. The intensity of the backflow is indicated by
the rate of recirculation that measures the proportion of the recirculated flow
of flue gas of the entire gas mass flow. Low rates of recirculation favour the
formation of soot. The solid state of soot particles confers a yellow colour
to the flame. An increase of the combustion gas recirculation reduces the
formation of soot and leads finally to a completely soot-free flame that emits
a hardly visible bluish radiation to the human eye.
In order to achieve an intensive recirculation of flue gas over the entire
performance range in combination with a high stability of the flame the
combustion air is supplied in a swirled jet. The image below shows
schematically the mode of operation of the mixing device. The combus-
tion air enters through a nozzle into the flame tube.
Due to the rapid cross-sectional enlargement of the air jet a vacuum
accrues at the edge of the air nozzle through which the hot flame gases
are transported from the inside of the flame conduit into the evaporating
zone. Besides this, already cooled-off combustion gases reach through
the openings of the flame tube from the combustion chamber into the
evaporation zone. Additionally, a back streaming is formed due to the
twisted streaming of the combustion air in the rotation centre of the flame.
Besides the avoidance of soot formation the intensive return of flue gases
to the flame’s root also achieves a reduction of nitrogen oxide emissions
(NOx). Two mechanisms essentially help this occur. On the one hand the
oxygen partial pressure of the mixture is reduced. Therefore the local con-
centration of dissociated oxygen molecules that react with the nitrogen
of the combustion air to NOx is reduced. On the other hand the flame
temperature is reduced through the recirculating flow of inert flue gases
with a higher specific heat capacity (CO

and H²O).

Swirl generator

Ignition electrode

Air nozzle

External recircu-
lation zone

Internal recircu-
lation zone

Flame

Flame tube

Injection nozzle

Oil preheater

Air

Mixing device

2.2 Combustion air blower

The combustion air is supported through a patented hybrid blower that
distinguishes itself by its extreme compressive rigidity. This ensures a pul-
sation-free and delay-free start of the burner, particularly at high combus-
tion chamber counter-pressure. The immense efficiency of the fan creates
a clear reduction of electrical energy over conventional fan solutions. With
room dependent or air dependent operating methods it is possible to re-
place the protection cover at the fan inlet by a suction air silencer that is
deliverable as an accessory. In the case of room air independent operation
there are inlet adapters with fitting diameter of Ø 50 mm or Ø 80 mm avai-
lable. In addition a turnable suction socket is offered with a fitting diameter
of Ø 50 mm that can be combined with a upstream mounted silencer.

2.3 Fuel pump and nozzle closing system

A gear pump is used as the fuel pump. The figure shows a diagram of the
hydraulic system of a single stage oil pump. The gear wheel set of the

pump delivers the fuel through a cartridge filter from the storage tank of
the oil supply system to the oil nozzle. The required injection pressure is
adjusted on the pressure control valve. A solenoid valve is fitted to control
the process of injection. This solenoid valve is closed in current-less con-
dition. In this switching state the entire fuel stream flows back through the
pressure control valve into the storage tank. For the fuel injection the so-
lenoid valve will be provided with current and consequently opened. The-
reupon the fuel reaches the nozzle due to the adjusted pressure at the
pressure control valve.

In order to reduce the start-up
and switch-off emissions the bur-
ner has been equipped in series
with a nozzle closing system by
Danfoss (LE system).
For this a spring-loaded dia-
phragm valve is installed into the
oil pre-heater. This will open at an
oil pressure of approximately 5
bars, and closes due to the spring
resistance at approximately 3
bars. To speed up the closing pro-
cess of the valve when turning off

the burner, or to prevent a pressurisation in the nozzle line by external ef-
fects (e.g. oil pre-heating at start of burner, radiation from the combustion
chamber after the burner’s switch-off), the LE oil pump is provided with a
bypass channel between the pressure and suction side. A spring-loaded
overflow valve with an opening pressure of 2 bars has been integrated in-
side the bypass channel. Due to the volumetric expansion caused by the
temperature the pressure increases in the oil pre-heater. As soon as the
pressure increases to 2 bars the overflow valve in the bypass channel
opens. However, the diaphragm valve in the pre-heater remains closed
due to the higher opening pressure and consequently prevents an esca-
ping of the fuel. Upon expiration of the heating up phase the burner motor
will start up, whereby the adjusted pressure on the pressure controller in-
side the pump builds up. At the end of the pre-ventilation period the so-
lenoid valve opens. The rising injection pressure inside the oil pre-heater
opens the diaphragm valve and the injection process starts in a controlled
manner, with the opening pressure pre-adjusted by the diaphragm valve.
Since the decrease of pressure on the diaphragm valve is negligible the
prevailing pressure on the oil nozzle corresponds with the pressure that
has been measured on the pump. In order to keep the partial stream that
is flowing off through the bypass as low as possible during the operation

of the burner a faceplate has
been additionally integrated in-
side the bypass channel. As soon
as the burner turns off the sole-
noid valve will close and the in-
jection pressure relieves itself
through the nozzle. As soon as
the pressure falls below 3 bars
the diaphragm valve inside the oil
pre-heater will close. This will en-
sure a controlled end of the in-
jection process without any furt-
her dripping. The LE oil pump
may also be used as a standard
oil pump. By turning the adjust-
ment screw in accordance with
the illustration you may activate
or de-activate the LE system.

2.4 Flame monitoring

Three optional systems are available as flame monitoring systems, namely
two optical flame detectors as well as an ignition unit with integrated ioni-
sation flame monitoring. These systems will be represented subsequently.

Optical flame detector, KLC 2002

Real flames release luminous radiation with an unsteady changing fre-
quency. This kind of “flickering“ of the flame is used for the especially de-
veloped optical flame detector (BST Solutions KLC 2002) for blue flame
burners to recognise flames. The evaluation of the optical signal, as well
as the conversation into an evaluated signal, occurs through a micro pro-
cessor based control that is integrated into the flame detector. This flame
detector differs from other optical flame monitoring devices in that it eva-
luates only the flickering of the flame. The constant luminous radiation of
the glowing recirculation tube or other components inside the combustion
chamber is completely cut out. A radiation with constant frequency, also
leads to no flame detection. It is not necessary to adjust the sensitivity.
Only an LED in the flame detectors housing indicates the current operating
state of the flame sensor. Following modes can be distinguished:

LED is OFF:

Flame monitor is carrying no current

LED is flashing:

KLC is active, no flame has been detected

LED is permanently ON:

KLC is active, flame has been detected

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