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.
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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 |
Visit TechTalks Discussion
Heat input required
Can any one tell me how to calculate the Heat energy required to bake one kg of biscuits or one square meter of oven area. Pl consider the moisture loss as 18% when you have 100kgs dough. with at least 2% moisture still there in biscuits.
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As a former product manager and now a consultant for tunnel oven belts, I would be interested to ask for your experience in changing the principal type of belt used in your tunnel oven and for what reason. Did you change from solid steel belt to mesh belt or (multiple spiral) CB5-belt to Z-belt (rolled baking oven belt)? Or was it vice versa?
Oven for water cracker
We are going to buy a new dedicated oven for water cracker, (like Carr's) what are the most suitable configurations of the oven should we looking for? (e.g. heating mode, oven band type, etc.)
Fouling on direct oven flue heat exchanger
I was wondering if anyone can me some insight into fouling issues on a potential combustion air preheater, for use on a direct oven. I am aware that combustion air preheaters have been installed successfully on indirect oven lines, but I am unsure if it is feasible on a direct oven. The flue stream will contain small solid particles and possibly fat alongside the obvious combustion products and water vapor.
Has anyone here successfully installed a heat exchanger on a direct oven flue line? Or heard of such a project?
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References
Armstrong Group, “Specific Heat – Specific Gravity”,
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
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Testo Inc: “Combustion Analysis”: www.testo.com
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