Give a brief outline of the sequence of operation of a pressure jet oil-fired burner.
Answer : by Gordon Goodfellow
1. Boiler required to fire.
The burner control box determines the operation by selecting different processes by means of camshaft-operated micro-switches.
After powering up the forced draught fan starts.
This has two main functions; firstly to purge the boiler and secondly to create the movement of air
through the boiler and flue to ensure efficient burning.
Attached to the end of the fan shaft there is an oil pump, this pump starts to build up pressure approximately
5 to 10 seconds after starting up.
2. After initial purging as described above the burner control box then starts the electrodes sparking
to ensure ignition of the oil / air mixture.
3. After purging and electrode arcing, a solenoid valve is opened to allow oil to the jet, here atomisation
occurs and the electrodes ignite the mix of fuel and air.
If the electrodes fail to start combustion, a photocell comes into operation.
The photocell consists of a light sensitive resistor whose signal is monitored by the control box.
Should the boiler fail to ignite after a pre-determined period then the control box shuts off the oil supply and ‘locks out’.
In most boilers ‘lock-out’ is indicated by a red or orange neon light and a button is pressed to re-set the burner control system.
The sequence of operation described above is important as the boiler is initially prepared for firing by establishing the forced draught, then time is given for the electrodes to arc, then the oil pressure is built up prior to ignition an lastly time is allowed to ensure ignition before safety measures are operated.
Answer : by David Johnston
On switching on the electricity supply to electric motor turns the fan and the oil pump and runs for several seconds to purge the boiler of any unwanted gases. After this period a solenoid valve opens on the high pressure side of the pump to allow oil to be fed to the nozzle, which is so designed to atomise the oil to the smallest droplets possible where it is mixed with the incoming forced air from the fan. At this point a spark is provided across the electrodes and the fuel/ air mixture ignites.
A photo electric cell than ‘sees’ the flame and shuts down the transformer that was providing the spark.
Ignition continues while the fuel/air mixture is supplied to the combustion chamber.
The solenoid valve and photo electric cell and transformer are all controlled by an electronic control box. A diffuser plate is fitted before the nozzle to form a proper shaped air pattern and the type of nozzle fitted is also important.
The nozzle or jet is usually
The boiler will continue to run until the set temperature on the boiler thermostat is reached when the burner will stop. On reaching the lower limit of the thermostat the burner will start again and provide heating to the water as before.
The sequence of operation is important i.e. to purge the boiler of unwanted gases, the solenoid valve stops oil leaking from the pump to the nozzle when shut down. If the photo electric cell does not ‘see’ a flame it stops the sequence because if it didn’t the boiler combustion chamber would be saturated in oil and maybe ignite from the electrode spark.
The boiler thermostat is set by the occupant for comfort usually at flow water temperatures around 60oC to 80oC and if it fails there is a high limit stat (factory set at about 90oC ). The high limit stat has to be manually reset, therefore alerting someone to control stat failure. In the event of both failing a pressure safety valve can be fitted to the water side of the boiler.
A fire valve and fusible link are also incorporated into the oil feed line to a boiler.