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Oven efficiency

Energy use

We are concerned not only to bake high-quality biscuits, but also to achieve the lowest cost per kg of baked product. We need therefore to consider the amount of energy required by the oven. As energy costs increase in almost every country, the efficiency of the oven is of growing importance. In certain countries such as India, fuel is very expensive and represents an important element of the total production cost.

The energy used by the oven is predominantly from gas or oil fuel. Electricity is rarely used for baking now, due to its high cost. In a gas/oil fired oven the fuel represents around 95-96% of the total energy usage and electricity (for powering the drive, fans and other electrical systems) about 4-5%.

The energy input to the oven is used primarily to bake the biscuit, to achieve the structure, reduce the moisture content by evaporation and to colour the biscuit. Each type of biscuit requires a certain amount of energy to achieve a good quality result. In the example we will use, a typical rotary moulded product requires 0.2120 kWh (182 kcal) of energy per kg of baked product.

In addition to the energy required to bake a good product, energy is lost in several ways:

  • By extraction of moist air from each oven zone
  • By heat loss through the insulation and outer covers of the oven
  • By the return circuit of the oven band
  • By heat loss from the flues from a heater module or heat exchanger in an Indirect Fired Oven


FIG 2 Energy usage

In order to minimise the heat loss, (wasted energy), we need to pay attention to the extraction system to achieve the final moisture content required, without excessive waste of heat from the burners, insulate the baking chamber adequately and also insulate the return band, particularly where the band temperature is high, for example when baking crackers.

Example                                                                     

The following calculations of the energy balance of an oven are taken from an actual installation. Details of the product and the oven are given below, so that different data for other ovens can be substituted to make calculations of energy use accordingly. In the following energy calculations, we will just consider the energy from the gas or oil fuel.

Density of air at 200oC = 0.7461 kg/m3

Specific heat of air = 1.006 kJ/kgK (at atmospheric pressure)

Specific heat of water vapour: 1.95 kJ/kgK at 200oC

1 kJ = 0.000278 kWh

1 kcal = 0.0011627 kWh


FIG 3  Product: Rotary moulded biscuit

 

Product                                                              

Rotary moulded biscuit

Dimensions:                58 x 37mm

Weight:                        5.1 g

Baking time:                3.8 min                   

 

Indirect Radiant Oven

Baking chamber:         1.25 x 100m

Zones:                         8 (7 burners)

Oven band:                 8.25 kg/m2

Extraction fans:           34 m3/min (maximum)

Oven output:               3,200 kg/hr

 

Data from independent test results

Total energy used by the oven:            0.4043 kWh/kg of baked biscuits

Of this, the energy required to bake

the product to the required quality:       0.2120 kWh/kg of baked biscuits

Waste energy:                                      0.1923 kWh/kg of baked biscuits


Product

The total energy requirement to bake the product is 0.2120 kWh per kg of baked biscuit

This gives a total energy usage (gas) to bake the biscuit of:

3200 x 0.2120 = 678.4 kWh per hour

This is the energy utilised to bake the biscuit, form the structure, remove the moisture and colour the biscuit.

 

Latent heat of evaporation

An important element in energy use is providing the latent heat of evaporation. In order to evaporate the moisture in the dough (14 % by dough weight) to a final moisture content of 3.0% latent heat is required. The latent heat energy required to evaporate the water from the product is 539 cal/g of water.
 

Moisture to be removed to reach final moisture content of 3.0%: 0.135 kg per kg of biscuits

Moisture to be removed: 0.135 x 3200 kg = 432 kg per hour

Latent heat required to evaporate 432 kg of water:

432,000 g x 539 cal = 232848 kcal = 270.73 kWh


Heat loss from extraction system from baking chambers             

Volume of air extracted from each zone

 

34m3/min x 60mins x 8 zones = 16320 m3/hour (maximum)

Estimated average extraction damper setting: 40%

Estimated volume of air extracted from baking chamber = 6528m3/hour

 

The air extracted has been heated from ambient temperature over the oven (55oC) to an average baking temperature (200oC). This requires an energy input as follows:

Weight of the air extracted per hour = 6528 x 0.746 kg = 4870 kg

The energy required to raise the temperature of this air in the oven from 55oC to 200oC (145oC) is:

145 x 1.006 kJ/kg x 4870 kg = 710387 kJ = 197 kWh per hour

 

Energy required to raise the temperature of the water vapour from 100oC to 200oC

1.95 x 432 x 100 = 84240 kJ = 23.4 kWh per hour

 

Total heat loss from extraction system per hour = 220 kWh
 



Heat loss from return band

Oven band drum centres:  111m

Band width 1.25m

Band weight: 8.20kgs/m2

Specific heat of carbon steel: 0.12kcal/kgoC  

Band temperature at delivery end: 140oC

Return band temperature at feed end: 105oC

(estimated temperatures)

 

Weight of band (on return circuit): 111 x 1.25 x 8.20 = 1138

Temperature loss: 140 – 105 = 35oC

 

Heat loss: 0.12 x 1138 x 35 = 4780 kcal (5.56 kWh) per revolution of the band

Bake time: 3.8 min

Heat loss per hour: 5.56 kWh x 60/3.8 = 87.8 kWh
 



Heat loss from the insulation and outer covers of the oven:                                

Oven baking chamber  1.25m x 100m
Width over covers  2.3m
Overall height of covers   2.0m
Average bake temperature  200oC
Average temp in heater modules  350oC
Ave. outer side cover temperature  55oC
Ave. outer top cover temperature  55oC
Mineral wool insulation thickness (s)  200mm sides and 250mm top
Mineral wool thermal conductivity (k)  0.04 W/m.oC

 

Heat loss from sides and top of the oven through the insulation

Heat loss = k A dT / s

 

Total area of oven sides: 100m x 2m x 2 = 400m2

This includes 7 heater modules and baking chamber sides

            Area of heater modules on burner side: 13m2 x 7 = 91m2

            Area of heater modules on non burner side: 2m2 x 7 = 14m2

 

Total area of heater modules = 105m2

Total area of oven sides (less heater modules) = 295m2

 

Heat loss from sides of baking chamber sections:

0.04 x 295m2 x (200 – 55oC) / 0.2 = 8555 W

 

Heat loss from heater modules:

0.04 x 105m2 x (350 – 55oC)/ 0.2 = 6195 W

 

Heat loss from top of oven:

0.04 x 100m x 2.3m x (200 – 55oC) / 0.25 = 5336 W

 

Total heat loss through the insulation of oven sides and top: 

20.1 kWh

 


 

Heat loss from oven delivery end

 

Area of oven delivery end covers and hood:  20m2

Estimated heat loss from radiation at the delivery end: 17.0 kWh

 

Estimated heat loss from air escaping from oven end at approx. 180oC:

Estimated volume of air in the baking chamber: 1.7 x 0.8 x 100 = 136 m3

Estimated volume of air escaping per min = 136 / 4.0 = 34 m3

Weight of air 34 x 0.746 = 25 kg

25 x 60 x 1.006 x (180-50) = 196170 kJ = 54 kWh

 

Estimated heat loss from oven delivery end: 71 kWh
 



Heat loss from burner flues

 

Total energy used: 0.4043 kWh x 3200 kg = 1294 kWh/hour

7 burners: average energy used per hour per burner = 1294/7 = 185 kWh

Gas consumption: 185/9.8 kWh/m3 = 18.9 m3 / hour per burner (average)

Gas/air volume required per burner: 18.9m3 gas + 301m3 air = 319.9m3

 

Assume that the volume of combustion air drawn in by the burners is exhausted through the burner flues.

Estimated average temperature of flue gases: 200oC

Gas / air weight at 200oC per burner = 319.9 x 0.746 kg/m3 = 239 kg/hour/burner

Estimated energy required to heat the combustion air: 239kgs x 200 x 1.0 kJ/kgoC = 47800 kJ

= 13.3 kWh /hour/burner

 

Total heat lost in burner flues: 7 x 13.3 kWh = 93.1 kWh per hour

Of this 50% can be used in the Heat Recovery System

 

Combustion process: CH4 + 3O2 = Heat + 2H20 + CO2 + O2

Note 1: For complete combustion 10% excess air is required (this amount can vary considerably depending on the burner and heat exchanger design)

Note 2: air contains 20.9% oxygen
 



From calculations above, the energy consumption of the oven per hour:

 

For product:                                                    678.4 kWh      56 %

Heat loss from extraction                                220.0               18 %

Heat loss from burner flues2                           93.1                 7 %

Heat loss from return band:                            87.8                 7 %

Est. heat loss of air from oven end:                71.0                 6 %

Est. loss from thro’ metal, fans etc.                30.0                 3%

Heat loss through insulation:                          20.1                 2 %

Heat loss from radiation at oven end              17.0                 1 %

 

Total                                                               1217 kWh       100.0%

 

Note 1: estimated accuracy in the assumptions and base data is +/- 10%

Note 2: the heat loss can be considerably larger than given depending on the design of the heat exchanger and flue.


 


FIG 4 Heat energy diagram

 

  • The overall oven efficiency is approximately 56%

 

  •  Of the heat loss through the burner flues, up to 50% can be recovered and used for baking in a Heat Recovery System

 

  • In this Baker Pacific oven installation (picture below), the Heat Recovery System reduced the energy requirement by 15% (as calculated by an independent test)

     

     FIG 5 Baker Pacific Indirect Radiant Oven with Heat Recovery System

 


 

Comparison of oven efficiency for different oven types

(based on actual installations)

Product Oven type Oven size kWh/kg of biscuits
 Snack cracker  DFG/convection  1.2m x 90m  0.477
 Rotary moulded  DFG/convection  1.5m x 100m  0.441
 Rotary moulded  DFG/convection  1.2m x 60m  0.430
 Rotary moulded  DGF/cyclotherm  1.2m x 60m  0.492
 Rotary moulded  Indirect Radiant + HRS  1.2m x 100m  0.404
 Rotary moulded  Indirect Radiant  1.2m x 100m  0.475

References

Armstrong Group, “Specific Heat – Specific Gravity”,

www.armstrong-intl.com

J.S. Alakali and others: “Specific Heat Capacity of Palm Oil”; Dept. of Biosource Engineering, McGill Univ. Canada, Dept of Food Science, University of Agriculture, Makurdi, Nigeria

Y.S. Kim and others: “ Physical, Chemical and Thermal Characterisation of Wheat Flour Products”; Dept. of Bio. and Agriculture Eng. Kansas State Univ., USA

Sugar Engineers, “Specific Heat Capacity”, www.sugartech.co.za

Testo Inc:  “Combustion Analysis”: www.testo.com

Bizee Software Ltd: “kW and kWh explained” www.energylens.com

The Engineering ToolBox: “Thermal properties for water”, “Thermal properties for air”, “Food and Foodstuff”’, “Specific Heat” 

 www.engineeringtoolbox.com

Hyper Physics: “Specific Heat” www.hyperphysics.phy-astr.gsu.edu

Lantau Group (2014): Gas pricing in Southeast Asia”: www.lantaugroup.com

BBC (2014): “Indonesia raises fuel prices by more than 30%”: www.bbc.co.uk

Global Business Guide (2014): “Indonesia’s subsidies on electric powered down”: www.gbgindonesia.com

The Jakarta Globe (2015): “Indonesia to cut subsidised fuel prices”: www.thejakartaglobe.beritasatu.com

CEIC (2014): “Keeping up with Indonesia’s rising energy needs”: www.ceicdata.com

Wikipedia. Latent Heat. http://www.en.wikipedia.org/wiki/Latent_heat 2010

The Boiling Point of Water and Solutions. http://www.chestofbooks.com/food/science/Experimental-Cookery/The-Boiling-Point-of-Water.....

 

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