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Hoval STU Wood Pellet Boiler
• Once the door has opened it should be possible to stand
in front of the combustion chamber and inspect the fire,
boiler surfaces etc. Note the ID fan will continue to operate
and should draw any products of combustion or fumes
through the boiler and into the flue and chimney but a
small fire should still be visable within the retort. This will
very gradually subside but the burning fuel will remain hot
for many hours.
• To restart the boiler, the door should be closed and using
the wood burner control on the stoker control panel, set
the Wood burner selector switch from ID FAN ONLY back
to AUTO. It should be possible to restart the boiler in this
fashion even if it has been off for a prolonged period (2-3
hours). If enabled the stoker control will restart by initially
running the ID and FD fans before allowing pellet feed at
the low fire rate. The Stoker is held in low fire for a fixed
period to allow the fire bed to re establish.
Note: A timer is provided in the control to limit the length of
time that the ID Fan is allowed to run (when the wood burner
selector switch is in the "ID FAN ONLY" setting). This will be
preset to two hours.
To Switch Off the Boiler Completely
If the Boiler is to be brought off line and the fire allowed to go out,
it is necessary for the operator to:
• With the stoker enabled and operating, set the
BURNOUT/NORMAL selector switch to the "Burnout"
position. This will run the conveyor auger tube out of
pellets.
• After 15-20 minutes, turn the wood burner ID FAN ONLY/
OFF/AUTO selector switch to the "off" position.
• This will allow the fire to gradually burn itself out.
De-Ashing Procedure
The STU is designed to burn a wood pellet fuel for which the ash
(or non combustible mineral matter) content is carefully control-
led. It is typically less than 1% by weight although this can vary
depending on the pellet source. Accordingly the amount of ash
to be removed from the boiler is relatively small, but depends on
the load placed upon the boiler. The generously sized combustion
chamber and area around the retort means that even during pe-
riods of heavy boiler loading ash removal can be as little as once
per week or possibly longer. Experience will dictate most suitable
de-ash frequency.
The De-Ash Process is relatively straight forward and takes a few
minutes to complete:
• The stoker should be turned to "ID FAN ONLY" via the
ID FAN ONLY/OFF/AUTO selector switch, (to pull any
residual fumes out of the boiler).
• On the 600 - 1000kW boilers the boiler needs to be left to
stand for 30 minutes
• The boiler door can then be opened using the tommy bar
provided and ash/clinker removed with the rake supplied
with the equipment.
• Care needs to be taken not to disturb the fuel within the
retort and only the ash, not unburnt fuel, is taken from
around the retort edge.
• Also clean out the hole for the combustion chamber
pressure switch sensing point which is located on the
LHS of the combustion chamber adjacent to the boiler
door.
• Having cleared all the ash from the boiler, the door needs
to be closed
and the selector switch set back to
the auto position.
• The De-Ashing process does not unduly affect the
performance of the boiler but can be timed to coincide
with a light load period if this is a concern.
• Where fitted, cyclone bins or the trays beneath Ceramic
Filters should be checked and emptied as appropriate. A
suitable vacuum cleaner should be used for this task.
The operator should wear appropriate PPE safety equipment.
Boiler Surface Cleaning
The STU has a bank of horizontal smoke tubes, through which
heat transfer takes place. To ensure optimum boiler efficiency
these tubes should be cleaned of any dust or debris that might
settle. The flue ways or smoke tubes within the boiler should be
inspected weekly and cleaned as required and this will depend
on the boiler load - during a typical heating season brushing the
flues once every 3-4 weeks will suffice. This is a typical figure and
again experience will dictate depending on the boiler operation
and loading.
Again this is a relatively straight forward process:
• The boiler to be cleaned should be turned off to "ID
Fan only" via ID FAN ONLY/OFF/AUTO selector switch
and allowed to cool for at least 1 - 2 hours.
• The operator should wear the appropriate PPE safety
equipment, open the door of the boiler and carefully
remove the spiral retarders from each of the smoke
tubes.
• Each tube should then be brushed - with the tool
provided - and any debris removed. Dust may collect in
the rear smoke box that can be accessed from the rear
of the boiler.
• The spiral retarders should be re-fitted to each tube.
Any dust or clinker should be cleaned from around the
retort - and the tuyeres of the retort checked so they are
clear of any ash/clinker. Also clean out the small hole for
the combustion chamber pressure switch sensing point
which is found on the LHS of the combustion chamber
adjacent to the boiler door.
• Close the boiler door and set the selector
switch back to the AUTO position.
• The operator should check that the pellets are burning
level with the top of the retort before leaving the plant to
run.
Operating Instructions
The operator should again wear appropriate PPE
safety equipment.
Please see Appendix D for step by step operating,
cleaning and fault finding guidance information.
