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Oven inspection and audit

It is recommended that each oven is given a complete inspection and audit every 12 months. The annual inspection and audit will support the preventative maintenance program and help to ensure excellent oven performance, economy and long life.

1. Oven performance

Output

  • check product specification and baking time to calculate oven output capacity in kg/hour
  • check actual output of packed, saleable product over 8 hour shift
  • check downtime / causes / trends in oven output
  • a log should be maintained of problems causing downtime; any repetitive problems or trends in output can be identified and appropriate action taken  



Product specification / compliance 

Check 10 biscuits from each lane every hour for compliance to the product specification.
Check variation across the width of the oven and variation with time:

  • biscuit weight
  • biscuit size (length, width, diameter)
  • colour (compare to standard samples)
  • moisture content

Identify any quality problems with Quality Control staff, such as “checking”, shelf life, packaging issues.


1. Esa Precision Scale – Brecknell Scales,
2. Sartorius Moisture Analyser
3. Konica Minolta Colour – Spectrophotometer


2. Energy usage 

Check fuel consumption over the 8 hour shift (gas / oil usage).
Calculate energy in kW/kg of baked biscuits and compare with target consumption.


Gass meter: Bell Flow systems


Fuel meter: Great Plains Industries


Energy usage for different biscuit types is given below as a guide. The “energy required to bake the product” is the energy requirement for heat input to convert the dough pieces to baked biscuits, this excludes heat loss from the oven. The “Total energy required” assumes a typical biscuit oven efficiency including heat loss. The final column “Typical range of energy required” is a guide for comparison with actual energy usage. If the energy used exceeds this range, investigation is needed to improve the oven efficiency.

Energy usage guide:

 Biscuit  type  Energy  required to  bake  product  kW/kg  Total energy  required inc.  heat loss from  the oven  Typical range of  energy required  for baking  kW/kg
 Rotary  moulded   0.18   0.36   0.36 - 0.40
 Hard  sweet   0.25   0.50   0.50 - 0.55
 Crackers   0.35   0.70   0.70 - 0.77


3. Oven band

Check the following:

  • band condition (edge damage, cleanliness)
  • band support skids / rollers (all rollers free and rotating, skids and rollers clean)

     
  • band tracking (band wander limit switch settings to detect +/- 10mm band deviation)
  • check alarms and shut down at the set point (over 25mm deviation)
  • band tension: check pneumatic system, air pressures, check actual tension applied compared to calculated requirement for the band type and oven length
  • check oven drum support bearings and slides; check lubrication of slides
  • oven drive: check belts and chains for wear and tension
  • check gearbox and drive transmission lubrication
  • check bearings
  • alarms: check all alarm and safety circuits for band rotation, tracking and tension
  • emergency drive / UPS (shut down power and check UPS system operates to empty the oven of biscuits)
  • band cleaner (condition of brushes)


4. Baking chamber

  • length and zone configuration
  • insulation / temperatures of outer covers
  • check temperatures and heat loss at the oven ends (temperatures   of outer covers should not exceed ambient + 10°C

Example of diagram showing checks of the outer temperatures at the oven delivery end

  • clean out doors/condition inside the baking chamber
  • check for broken biscuits which could cause a fire
  • check for distortion and leaks in the baking chamber structure
  • Extraction system / fans / pressure switches
  • check fan rotation, clean filters, bearings and impellers
  • check pressure switches setting and operation
  • oven end hood and extraction
  • check extraction fan / heat loss around delivery end


5. Gas / oil trains

Check the following:

  • gas train installation / connections / gas filters
  • main gas supply pressure
  • main gas valve operation / pressure gauges / zero governor
  • gas proving system to detect any gas leaks
  • check the gas and air header pipes and flexible pipe connections
     

1. Main gas supply train, 2. Weishaupt gas train

6. Burners: DGF 
  • check burner strips for dirt / blockages
  • check ignition/Flame monitor units
  • adjust electrode gaps
  • check flames: short blue flames (nor long, yellow, lazy flames)
  • adjust gas/air mix as required 


7. Burners: Indirect Fired Ovens

  • check fuel supply / filters
  • check combustion air fan / filter / pressure switches
  • check operation of manual and solenoid valves
  • check gas governor / gas pressure gauges
  • check pressure in burner tube with a manometer, (should be negative pressure/positive pressure may indicate a leakage in the heat exchanger)
  • check burner ignition
  • check flame shape and stability
  • check Weishaupt Combustion Manager / diagnostics


1. Weishaupt burner, 2. Maxon burner

8. Temperature control systems 

A data logger may be used to check the temperature profile of the oven and identify problems of variation across the width of the oven and variation from set temperatures in each zone.

Measuring baking profiles 

In order to monitor actual baking temperatures, a thermal data logger is used. These units are slim and will pass through the oven carried on the oven band.

ThermaPro Data Logger 40
ThermaPro Data Logger from Digitron

Data loggers are available with multi channels, usually 6-8 for biscuit oven applications. Each channel has a thermocouple sensor which is attached to a bar spanning the width of the oven band. In this way the baking temperature is monitored and recorded at multiple positions across the width of the oven.

Data loggers such as the TCK2000, which is made in China, monitor and record baking temperatures. A TCK system comprises the following: the data logger or profiler unit with 3 up to 9 channels within an insulated box, thermocouples with high temperature connections, all accessories, and software to run on a PC. This unit is a convenient way to monitor oven performance, detect problem areas in the oven which may cause poor structure, colour, and moisture level.
TCK 9 channel data logger to monitor and record baking temperatures
TCK 9 channel data logger to monitor and record baking temperatures

Our main baking control in most ovens (direct gas fired, indirect radiant and convection ovens) is by temperature. We set zone temperatures to match the required baking profile and control the heat input from the burners to maintain the set baking temperatures.

Monitoring the baking temperatures through the oven and across the width of the oven band is therefore a valuable guide to the performance of the oven and a good starting point for trouble shooting problems such as uneven colour and moisture content.
 
Heat flux 
 
However, heat transfer depends not only on temperature, but is also affected by other conditions such as air movement. In an oven the heat energy will be transferred to the dough piece by radiation, conduction (from the oven band) and convection. In principle, heat flux sensors measure radiant, conducted and convective heat transfer. This gives a more complete understanding of the rate of heat transfer to the dough pieces than measuring temperature alone.

Heat flux is the rate of heat energy transferred to a given surface and it is measured in watts per square metre (W/m²). (1 W/m² = 0.86 kcal/hr/m²). This is expressed as:

Q” = Q/A

Where:

Q” = heat flux (W/m²)

Q = heat transfer rate (W/h)

A = area (m²)

Measurement of heat flux 

Data loggers, such as the ThermaFlux from Digitron record the rate of heat transfer, taking into account temperature, air velocity and conduction. They utilise the direct method of heat flux sensing (surface mounted heat flux sensors).

When heat passes through the heat flux sensor, a temperature gradient is developed across it. The difference in temperature is detected by a thermopile in the heat flux sensor. The sensor head is connected to a data logger with a high memory capacity and can measure up to 4 samples/second.

ThermaPro data logger for measurement of heat flux
ThermaPro data logger for measurement of heat flux

The data logger is slim, 25 mm, lightweight and portable with battery power. It has a dual sensor head enabling top and bottom heat flux measurements to be made simultaneously with recording air temperature.
 
The data logger will produce a graph showing both air (baking) temperature and heat flux at top and bottom of the sensor. Normally it can be seen that the air temperature shows a (relatively) continuous curve. However, in a convection oven, the heat flux traces are more complex. They show very low dips at the end of each zone and a spiky trace reflecting the convection air jets impinging on the sensor through each oven zone.
 
9. Electrical Panels  
 
  • check all temperature controllers are operational, including over-temperature controllers
  • check variation in set temperatures and actual temperatures on the controllers
  • check PID settings and adjust as required
  • check thermocouple positions and wiring
  • check cleanliness and vacuum
  • check all wiring secure
  • operate safety systems and alarms
  • check all E-stops function correctly


10. Reporting 

The inspection and audit will provide a complete and detail report on all aspects of the ovenperformance as outlined above. The aim is to:

1.  To secure the best product quality
2. To achieve the optimum oven efficiency
3. To reduce downtime and achieve the highest possible production
4. To support development of preventive maintenance and a program of equipment and operational improvements

 


References

Baker Perkins Ltd:   www.bakerperkins.com
Digitron:  “Thermaflux”, “Thermal Profile Solutions”, “Data loggers for the direct measurement of Heat Flux:   www.digitron.co.uk
Eratec:  www.era-tec.fr
Flynn Burner Corp. USA:   www.flynnburner.com
Maxon Corp:  www.maxoncorp.com
Haas Meincke:  www.haas.com
Weishaupt Corp:  www.weishaupt-corp.com
Wikipedia: “Heat Flux”, http://en.wikipedia.org

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