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/m2

Extraction fans:           34 m3/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

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

Estimated average extraction damper setting: 40%

Actual 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 = 6528 x 0.746 kg = 4870 kgs

The energy required to raise the temperature of this air in the oven from 55oC to 200oC (145oC) 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 100oC to 200oC

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 100oC = 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/m2

Specific heat of carbon steel: 0.12kcal/kgo

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 = 1137.75

Temperature loss: 140 – 105 = 35oC

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:                        200oC

Average temp in heater modules:             350oC

Ave. outer side cover temperature:          55oC

Ave. outer top cover temperature:           55oC

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

Rockwool thermal conductivity (k):          0.066 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:

0.066 x 295m2 x (200 – 55oC) / 0.2 = 14116   W

 

Heat loss from heater modules:

0.066 x 105m2 x (350 – 55oC)/ 0.2 = 10222 W

 

Heat loss from top of oven:

0.066 x 100m x 2.3m x (200 – 55oC) / 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       6m2

Area of oven end hood:                      14m2

Total area                                           34m2

 Heat loss = ∑ σ   (Th4  – Tc4) A

∑ = emissivity

σ = 5.6703 10-8 (W m2 k 4) (Stefan Boltzmann constant)

T = absolute temp. (Kelvin)

A = Area m2

0.5 x 5.6703 x 10-8  x (298 x 108  ) x 34m2 = 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/m3 = 18.9 m3 / hour per burner (average)

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

 Estimated average temperature of flue gases: 200oC

Gas / air weight at 200oC 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 100oC to 200oC

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 100oC = 0.958

Density of air at 2000C = 0.746 kg/m3

1 KJ = 0.000278 KWH

Combustion process: CH4 + 3O2 = Heat + 2H20 + CO2 + O2 (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 flues2              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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