# Oven efficiency

**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 heater module or heat exchanger

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.

In the following energy calculations, we will just consider the energy from the gas or oil fuel.

**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.

**Product *** *

*Rotary moulded biscuit*

*Dimensions: 58 x 37mm*

*Weight: 5.1 gms*

*Baking time: 3.8 mins*

* *

*Oven*

*Baking chamber: 1.25 x 100m*

*Zones: 8 (7 burners)*

*Oven band: 8.25 kgs/m ^{2}*

*Extraction fans: 34 m ^{3}/min (maximum)*

*Oven output: 3,200 kgs/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/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/gm 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 3200kgs = 432 kgs per hour

Latent heat required to evaporate 432 kgs of water:

432,000 gm x 539 cal = 232848 kcal = 270.75 KWh per hour

**Heat loss from extraction system from baking chambers **

**Volume of air extracted from each zone**

34m^{3}/min x 60mins x 8 zones = 16320 m^{3}/hour (maximum)

Estimated average extraction damper setting: 40%

Actual volume of air extracted from baking chamber = 6528m^{3}/hour

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

Weight of the air extracted = 6528 x 0.746 kg = 4870 kgs

The energy required to raise the temperature of this air in the oven from 55^{o}C to 200^{o}C (145^{o}C) is:

145 x 1.009 KJ/kg x 4870 kg = 712505 KJ = **198 KWh perhour **

Energy required to raise the temperature of the water vapour from 100^{o}C to 200^{o}C

1.996 x 432 x 100 = 86227 kJ = **23.97 KWh perhour**

**Heat loss from extraction system per hour = 222 KWH**

*Specific heat of water vapour: 1.996 kJ/kgK*

*Density of water vapour at 100 ^{o}C = 0.958*

*1 KJ = 0.000278 KWH*

**Heat loss from return band**

*Oven band drum centres: 111m*

*Band width 1.25m*

*Band weight: 8.20kgs/m ^{2}*

*Specific heat of carbon steel: 0.12kcal/kg ^{o}C *

*Band temperature at delivery end: 140 ^{o}C*

*Return band temperature at feed end: 105 ^{o}C*

*(estimated temperatures)*

Weight of band (on return circuit): 111 x 1.25 x 8.20 = 1137.75

Temperature loss: 140 – 105 = 35^{o}C

Heat loss: 0.12 x 1137.75 x 35 = 4778.55 kcal (5.56 KWh) per revolution of the band

Bake time: 3.8 mins

**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*

*Height of covers 1.23m*

*Average bake temperature: 200 ^{o}C*

*Average temp in heater modules: 350 ^{o}C*

*Ave. outer side cover temperature: 55 ^{o}C*

*Ave. outer top cover temperature: 55 ^{o}C*

*Rockwool insulation thickness (s): 200mm sides and 250mm top*

*Rockwool thermal conductivity (k): 0.066 W/m. ^{o}C *

**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 = 400m^{2}

This includes 7 heater modules and baking chamber sides

Area of heater modules on burner side: 13m^{2} x 7 = 91m^{2}

Area of heater modules on non burner side: 2m^{2} x 7 = 14m^{2}

Total area of heater modules = 105m^{2}

Total area of oven sides (less heater modules) = 295m^{2}

Heat loss from sides of baking chamber sections:

**0.066 x 295m ^{2} x (200 – 55^{o}C) / 0.2 = 14116 W**

Heat loss from heater modules:

**0.066 x 105m ^{2} x (350 – 55^{o}C)/ 0.2 = 10222 W**

Heat loss from top of oven:

**0.066 x 100m x 2.3m x (200 – 55 ^{o}C) / 0.25 = 8804 W**

**Total heat loss through the insulation of oven sides and top: 33.1 KW**

**Heat loss from radiation from oven delivery end:**

Area of oven delivery end covers 6m^{2}

Area of oven end hood: 14m^{2}

Total area 34m^{2}

* **Heat loss = ∑ **σ (T _{h}^{4 } – T_{c}^{4}) A*

*∑ = emissivity*

*σ **= 5.6703 10 ^{-8} (W m^{2} k ^{4}) (Stefan Boltzmann constant)*

*T = absolute temp. (Kelvin)*

*A = Area m ^{2}*

0.5 x 5.6703 x 10^{-8} x (298 x 10^{8} ) x 34m^{2 }= 28725 W

### **Heat loss from radiation at the delivery end: 28.7 KW**

**Heat loss from burner flues**

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

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

Gas consumption: 185/9.8 kWh/m^{3} = 18.9 m^{3} / hour per burner (average)

Gas/air volume required per burner: 18.9m^{3} gas + 301m^{3} air = 319.9m^{3}

^{ }Estimated average temperature of flue gases: 200^{o}C

Gas / air weight at 200^{o}C per burner = 319.9 x 0.746 kg/m3 = 239 kgs/hour/burner

Estimated energy in flue gases: 239kgs x 200 x 1.026 KJ = 49043 KJ

= **13.6 kWh /hour/burner**

Estimated water vapour content of flue gases: 119.5 kgs /burner/hour

Energy required to raise the temperature of the water vapour from 100^{o}C to 200^{o}C

1.996 x 119.5 x 100 = 23852 KJ = **6.6 kWh /hour/burner**

### **Total heat energy in flue gases: 7 x (13.6 + 6.6) kWh = 141.4 kWh per hour**

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

*Specific heat of water vapour: 1.996 kJ/kgK*

*Specific heat of air: 1.026 kJ/kgK*

*Density of water vapour at 100 ^{o}C = 0.958*

*Density of air at 200 ^{0}C = 0.746 kg/m^{3}*

*1 KJ = 0.000278 KWH*

*Combustion process: CH _{4} + 3O_{2} = Heat + 2H_{2}0 + CO_{2} + O_{2 }(see Appendix 3)*

*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 50.6 %

Heat loss from extraction 222.0 16.6 %

Heat loss from burner flues^{2} 141.4 10.5 %

Heat loss from return band: 87.8 6.5 %

Est. heat loss of air from oven end: 75.0 5.6 %

Est. loss from thro’ metal, fans etc. 75.0 5.6 %

Heat loss through insulation: 33.1 2.5 %

Heat loss from radiation: 28.7 2.1 %

**Total 1341.4 kWh 100.0%**

*Note 1: estimated accuracy in the assumptions and base data is +/- 5%*

* **Note 2: the heat loss can be considerably larger than given depending on the design of the heat exchanger and flue. Please see Appendix 3, “Combustion Analysis”.*

** **

**The overall oven efficiency is 50%****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)

** **

** ***Indirect Radiant Oven with Heat Recovery System*

**Comparison of Oven Efficiencies**

### **Comparison of oven efficiency for different oven types**

Product |
Oven type |
Oven size |
KWH/kg of biscuits |

Snack cracker | DGF/conv | 1.2m x 90m | 0.477 |

Rotary moulded | DGF/conv | 1.5m x 100m | 0.441 |

Rotary moulded | DGF/conv | 1.2m x 60m | 0.430 |

Rotary moulded | DGF/cyclo | 1.2m x 60m | 0.492 |

Rotary moulded | IR (cyclo) + HRS | 1.2m x 100m | 0.404 |

Rotary moulded | IR (cyclo) | 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, Unive 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

“kW and kWh explained” www.energylens.com

“Thermal Conductivity” www.quats.com

“Thermal properties for water” www.engineeringtoolbax.com

“Thermal properties for air” www.engineeringtoolbax.com

“Food and Foodstuff” www.engineeringtoolbox.com

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