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.

Oven performance


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

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

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

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


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)

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 + 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

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

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.

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 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.

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


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

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

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

Visit store.elsevier.com to purchase your copy today! Use discount code “STC215″ at checkout and save up to 30%!

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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