Oven Operation: Direct Gas Fired Oven

June 28th, 2021

by Iain Davidson of Baker Pacific Ltd

1.Starting the DGF oven

Preparation

Baking Programme

Allow 30 minutes from start up before baking for the required baking temperatures to be reached – high baking temperatures may take longer.

Burner Pattern

Normally all burners will be alight and the automatic temperature control system will adjust the heat input to maintain the set baking temperatures by modulating the burner flames. If a very low heat input is required for a product, some burners may not be required and the oven may be operated without these burners. Burners can be closed by an on/off switch on the Ignition Control Unit at each burner. The burners switched off should be evenly distributed throughout the zone.

Before starting the oven

1. Check that the stripping knife at the oven end is disengaged
2. Check that the oven feed and delivery ends are clear of obstructions
3. Ensure that all inspection and access doors are closed
4. Before starting the oven, ensure that the air supply is turned on for the band tension and band tracking systems
5. Open the main supply manual gas valve
6. Switch on the main control panel isolator
7. Switch on the zone panel isolators

Zone Control Panels

At each zone panel:

1. Start the extraction fan
2. Start the burner air fans (for top and bottom burners)
3. Set the temperature controllers to the required temperatures

Zone panel

Main control panel

Calculation of oven zone lengths

Main Control Panel

At the main control panel:

1. Switch the panel isolator on; check panel live lamp is on
2. Set the baking time
3. Start the oven band.
4. Check band running lamp is on and check band tension and tracking warnings
5. Check that burner ignition is switched to ‘Off’
6. Check that “Gas Pressure OK” lamp is on
7. Check that the "Purge On" lamp is on - this indicates that the 6 minute purge timer has started to run to ensure that the fans clear the oven of any unburned gases
8. At the end of the purge period the "Purge Complete" lamp will be on

2. Lighting the burners – direct gas fired zone

The following conditions must be met before the oven burners will light:
1. The band is tensioned
2. The main manual gas valve is open
3. Each panel isolator is "On"
4. The extraction fans on each zone are running. This is monitored by pressure switches
5. The oven drive panel AC & DC isolators are on
6. The oven drive is started
7. The gas pressure detected by the gas pressure switch is within the set limits.

If these conditions are met the oven can be purged. The purge will last for 6 minutes. On completion of the purge the main gas valves will open.

1. When the oven purge is complete, turn burner switch to "On". Those burners that have been set to ‘"On" at the ignition control unit will light. This occurs, in sequence in each zone in banks of four burners.
2. If any additional burners are required, select at the ignition control unit
3. If any burner fails to light, switch the ignition control off, then on again.

Flynn Ignition unit

3. Heating Up / start of production

1. Set the extraction dampers to 20% open during the warm up period
2. Approximately ten minutes before baking commences, set the extraction dampers to their baking settings
3. Check baking time is correct
4. Start band cleaner unit
5. When baking temperatures are correct, start production

4. Shutting down the DGF oven

1. Switch to "Burner Off" at the main control panel
2. Set the extraction dampers in each zone to fully open
3. Keep inspection doors closed.

When Temperature Drops to 100°C

1. Stop burner and extraction fans
2. Leave zone panel isolators on
3. Press the oven drive “Stop” button
4. Turn off the main manual gas valve
5. Turn off panel isolators

In the event of power failure

1. Empty the oven by using the hand winding-out mechanism or the DC motor drive. The DC drive will start automatically on power failure.
2. Turn the ignition switches to "Off"
3. Turn off the gas supply at the main manual valve.

In an emergency

1. Press the "Emergency Stop" button on the main drive panel or at the oven ends
2. When any danger to personnel is removed, re-set the Emergency Stop and press the "Main Drive Start" button.

5.Oven safety monitoring and alarms

Oven band

The oven band will be monitored for excess movement using limit switches at each end of the oven. Early warning and trip positions are provided. After band wander trip, there is an override facility for re-start of the band.

Maximum deviation +/- 10mm

Deviation over +/- 10mm sounds alarm

Deviation over 25mm stops the oven band and shuts down burners

The band tension is monitored and tension must be present before starting the oven band.

The band will stop under certain conditions e.g. power failure, excessive band deviation. The band movement is monitored by photocell and if the band stops all burners will be automatically extinguished.

When the band stops automatically, for example due to a tracking problem, the fault must be rapidly found by an engineer. The band can be re-started by using an override push button and the band run slowly to empty the oven of biscuits, until the fault is corrected and the oven can be run normally under automatic control.

Euchner multi-position limit switch to monitor oven band movement

Ignition

The burner ignition / shut down sequence are controlled by a dedicated control unit. Flame failure, gas pressure, air pressure, fan pressure are monitored by the controller.

Purge system

Before lighting the burners on a DGF oven, the purge system will ensure that any unburnt gas present in the baking chamber is exhausted. The extraction fans will operate with the dampers open to purge the baking chamber. The purge cycle is 6 minutes in duration and this is controlled by a pre-set timer. The duration of the purge cycle and the volume of air expelled from the baking chamber are designed to comply with FM Global insurance standards.

Excess Temperature

The zone temperature is monitored by a separate temperature controller. If the automatic temperature control system fails and the temperature in the baking chamber continues to increase over a safe level, the oven will automatically shut down. An excess temperature is pre-set on the controller and if the temperature exceeds the pre-set, the gas supply will be shut off and all burners extinguished.

Power Failure

If the main power supply fails, the gas supply will be automatically shut off and all burners will be shut down. The DC motor or factory generator will start automatically to empty the oven of biscuits to avoid a fire.

E – STOPS
Emergency stop buttons, clearly identified should be positioned at each end of the oven and on the main control panels. If an emergency stop button is actuated, the drive, gas supply and burners will shut down.

6. Control equipment for the DGF oven

Baker Pacific Direct Gas Fired Oven with Eratec MFB burners

Gas train: Manual Gas Shut-off Valve

The gas manual shut-off valve is situated on top of the oven. It is opened before start up and it is good practice to shut it off at the end of every production run.

Gas System

The gas supply to the oven can be isolated by the manual gas valve. A high/low gas pressure switch is fitted to stop the oven being operated with unsafe gas pressures. An electrical solenoid valve is fitted; this allows the gas supply to the oven to be automatically switched on at the end of the oven purge periods and automatically switched off in the event of unsafe conditions.

The gas leakage system checks that both of the main safety shutoff valves are operating correctly and able to completely stop the gas supply.

High/Low Pressure Switch

The gas high/low pressure switch is situated in the main gas supply to the oven. Its function is to shut off the burners if the gas pressure supply falls below 12 mbar or exceeds 20 mbar.

Main Gas Solenoid Valves

These valves are situated in the gas train to the oven. Their function is to open on completion of the oven purge and shut off immediately a fault is detected.

Gas leakage

This system checks that both of the main safety shutoff valves are operating correctly and there are no leakages in the gas train.

Zero Gas Governor
The zero gas governor is situated in the gas train. Its function is to provide an even, low (3mbar) gas pressure to the burners in each zone.

Temperature Control for DGF Zones

The Direct Gas Fired oven has four thermocouples in each zone, two above the band and two below. The thermocouples are connected to temperature controllers on the zone control panel. The controller will average the top two thermocouple temperatures and the bottom two temperatures. The DGF oven has a separate controller and separate system for controlling the top burners and a separate system for the bottom burners.

Omron controllers

The thermocouples provide temperature data to automatic controllers. The required baking temperature is set at the controller by push button and the controller will display the set temperature and the actual temperature.

Over temperature safety

In addition to the baking temperature controllers a simple controller is used to detect an over temperature condition which could be unsafe. A thermocouple in the zone flue will detect an excessive temperature (as set on the controller) and the safety system will automatically shut down the burners by closing the gas solenoid valves. An alarm will sound.

Manual Control of Top and Bottom Burners

In special circumstances, the engineer may operate the burners by manual control. Manual control may be selected on the temperature controller and the "Up" and "Down" push-buttons used to achieve the required air pressure to the burners.

Ribbon type burners

The burner consists of a tube with a slot along part of its length into which are placed a number of formed stainless steel strips. This provides numerous small ports through which the gas/air mixture passes for combustion. This strip design ensures:

1. Continuous flame of uniform height
2. Good flame retention properties on high flame
3. Prevention of light back on low flame

Multi-lane burner control

A proportion of the burners are of the multi-zone distributor type, (3 lane). This control enables flame variations along the burner strip i.e. variation of heat output across the oven. The burner barrel is divided by baffles into three sections – oven control side, centre and non-control side. Each section of the burner strip is supplied with gas/air mixture by a tube from the burner head.

Air System

The combustion air is provided by separate fans for the top and bottom burners. The fans have pressure switches to monitor operation.

By means of a zero pressure regulator, the gas header provides gas at zero pressure at the burner. An air blower feeding the air header provides air under pressure at the burner to draw in the gas and provide a gas/air mixture for the combustion.

Burner flame supervision

The Flynn Ignition Control Unit provides for automatic ignition and safe supervision of the gas burner.

The DGF burners have a single electrode for ignition and flame detection. These functions are controlled at each burner by an Ignition Control Unit. Ignition is achieved from a spark directed across the burner strip at the electrode from a high tension transformer in the flame ignition unit.

The start up sequence is initiated by a switch on the unit or by a switch on the zone control panel. The control starts its sequence by checking that the flame probe (electrode) is signalling no flame. The high voltage spark generator is then energised together with the solenoid valve. The spark and valve will remain energised until the gas is lit or the ignition safety period is completed.

When the gas is lit the flame is detected by the electrode causing the spark igniter to be de-energised. The solenoid valve will remain open until the electrical supply to the unit is terminated or the flame goes out.

If the burner fails to ignite or the flame subsequently fails, the gas valve will be closed and the control unit will go to lockout. Re-set of the lockout followed by a new start up sequence can only be achieved by interrupting the electrical supply to the unit for a minimum of three seconds.

Individual Burner Safety Circuit

In order to light the burners, the following conditions must be met:

1. The switch on the flame supervision unit is "On"
2. The "Ignition" selector switch is turned to "On"
3. The burners will now light up in banks. As an additional safety precaution, there is a 10 second delay between each bank of burners lighting.

Burner adjustment: Air/Gas Ratio Adjustment to the burners

To weaken the mixture, (i.e. less gas), turn the regulating screw clockwise. Turn the screw counter-clockwise to make the mixture richer.

Start with a rich mixture, which is indicated by a green or red coloured flame, and slowly weaken until the base of the flame is blue. The flame should sit on the strip with a bright blue cone.

Burner heat output is controlled by variation of air pressure and since the gas is maintained at zero pressure, the gas/air ratio will remain constant and ensure a completely combustible mixture at all air pressures.

Flynn burner installation

Extraction fans

The extraction fans operate at fixed speeds and the extraction air flow is adjusted by dampers. The extraction fan operation is monitored by pressure switches. The pressure switches are set by the commissioning engineer.

Oven band operation, tension and tracking

Oven band drive

The oven band drive speed is monitored by a Synatel unit located on the oven drum shaft. A band running warning lamp is located on the main control panel.

Synatel speed monitoring unit at the oven drum

Oven band tension and tracking

Warning lamps are located on the main panel to warn of band tension fault, tracking at feed and delivery ends of the oven and trips for band stop. In addition, control panels are located at the oven ends to show warnings for tracking and tension faults.

Quality Control during baking

Every 30 – 60 minutes, the following checks for the product may be made:

1. Take a sample of biscuits (usually 5 or 10 biscuits)
2. Weigh the biscuits and record the weight
3. Measure and record the biscuit length or diameter
4. Measure and record the biscuit width (if applicable)
5. Measure and record the biscuit thickness
6. Check and record the moisture content (in the laboratory)
7. Check the colour of the tops and bases. Check for dirty bases.