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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.
- Check product spec and baking time to calculate oven output capacity in kgs/hour
- Check actual output of packed, saleable product over 8 hour shif
- 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.
Esa Precision Scale Sartorius Moisture Analyser Konica Minolta Colour
Brecknell Scales Spectrophotometer
Check fuel consumption over the 8 hour shift (gas / oil usage)
Calculate energy in kW/kg of baked biscuits and compare with target consumption
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
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 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)
- 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 + 10oC
- 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
Gas / oil trains
Main gas supply train
Weishaupt gas train for Indirect Radiant oven
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
- 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
Burners: Indirect Fired Ovens
- Check Weishaupt Combustion Manager / diagnostics
- 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
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.
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
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.
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 International 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.
The data logger is slim 25mm, 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. It will be seen that the air temperature shows a (relatively) continuous curve. However, 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.
- 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
The inspection and audit will provide a complete and detail report on all aspects of the oven performance 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
Baker Perkins Ltd: www.bakerperkins.com
Digitron: “Thermaflux”, “Thermal Profile Solutions”, “Data loggers for the direct
measurement of Heat Flux: www.digitron.co.uk
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
The 2nd edition of Biscuit Baking Technology: Processing and Engineering Manual is a guide for designers and operators of the biscuit oven-baking technology.
- Thoroughly explores the engineering of baking, details biscuit baking equipment, oven specifications, installation, operation and maintenance
- Provides details of best industry practice for safety, hygiene and maintenance of ovens
- Contains explanations of heat transfer and all the types of biscuit oven design with clear pictures and drawings
- Gathers all the information on how to select and specify an oven to be purchased for a particular range of biscuits
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About the Author
Iain Davidson graduated from the School of Industrial Design (Engineering) at Royal College of Art in London in 1965 and joined Baker Perkins Ltd. He was Industrial Design Engineer, working in the Technical Department on the design of new biscuit, bakery and candy processing machines until 1975, gaining a thorough technical knowledge of the machines and processes. In 1990 Iain was appointed Regional Manager Asia Pacific for Baker Perkins and re-located to Indonesia and later in 1997 to China. His appointments included Managing Director of Baker Perkins (Hong Kong) Ltd. and Director of Baker Perkins Japan KK. Iain was responsible for a substantial increase in the company’s biscuit business in Asia During this period the company’s Asian biscuit machinery business grew by over 3 times. Iain established a successful manufacturing facility for biscuit ovens in Dalian, China in 1990 for Baker Perkins and subsequently continued a manufacturing capability for Baker Pacific Ltd. In China, India and Indonesia.
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