Energy Efficiency Comparison

Reference ID: MET-2A13 | Process Engineering Reference Sheets Calculation Guide

Introduction & Context

The energy efficiency comparison between extrusion cooking and oven baking is a critical analysis in food process engineering, particularly for the production of Ready-to-Eat (RTE) cereals. This calculation evaluates the total primary energy consumption required to transform raw dough into a finished, dried product. By establishing a gate-to-gate system boundary, engineers can quantify the energy intensity of mechanical energy delivery (extrusion) versus thermal energy delivery (baking). This analysis is essential for optimizing operational costs, reducing carbon footprints, and selecting the most efficient processing technology for high-volume cereal production lines.

Methodology & Formulas

The energy assessment is divided into two distinct processing routes. The extrusion route focuses on Specific Mechanical Energy (SME) and steam usage, while the baking route focuses on sensible heat, latent heat of evaporation, and oven thermal efficiency.

Extrusion Route

The energy consumption for the extrusion process is calculated as follows:

\[ E_{\text{steam}} = \frac{\dot{m}_{\text{steam}} \cdot h_{fg}}{3600} \] \[ E_{\text{dryer,ext}} = \frac{\dot{m}_{\text{water,ext}} \cdot h_{fg}}{\eta_{\text{dryer,ext}} \cdot 3600} \] \[ E_{\text{total,ext}} = E_{\text{steam}} + (SME \cdot \lambda_{\text{primary}}) + E_{\text{dryer,ext}} \]

Baking Route

The energy consumption for the baking process accounts for the theoretical heat required for phase change and temperature rise, adjusted for oven efficiency. A post-oven dryer removes residual moisture with its own thermal efficiency:

\[ E_{\text{theoretical,oven}} = \frac{(C_{p,\text{dough}} \cdot \Delta T) + (h_{fg} \cdot \dot{m}_{\text{water,oven}})}{3600} \] \[ E_{\text{fuel,oven}} = \frac{E_{\text{theoretical,oven}}}{\eta_{\text{oven}}} \] \[ E_{\text{total,bake}} = E_{\text{fuel,oven}} + \frac{\dot{m}_{\text{water,post}} \cdot h_{fg}}{\eta_{\text{dryer,post}} \cdot 3600} + (E_{\text{fan}} \cdot \lambda_{\text{primary}}) \]

Operational Regimes and Constraints

Parameter	Constraint/Threshold	Description
SME Range	\( 0.08 \leq SME \leq 0.18 \)	Valid range for optimized corn-based extrusion cooking (kWh/kg).
Oven Efficiency	\( \eta_{\text{oven}} \leq 0.35 \)	Typical upper bound for direct gas-fired band ovens without heat recovery.
Dryer Efficiency	\( 0.40 \leq \eta_{\text{dryer}} \leq 0.65 \)	Practical thermal efficiency range for continuous convective dryers.
Latent Heat	\( h_{fg} = 2256 \)	Constant for water evaporation at \( 100^{\circ}\text{C} \) in kJ/kg.
Primary Energy Factor	\( \lambda_{\text{primary}} = 3.0 \)	Conversion factor for electrical energy to primary fuel equivalent.

To establish an accurate baseline for energy efficiency comparisons between processing routes, follow these steps:

Collect historical utility data (electricity, natural gas, steam) for at least 12 months to account for seasonal variations in ambient conditions and production schedules.
Normalize the data against production throughput using the specific energy consumption metric (kWh per kg of finished product) for each utility stream.
Identify and isolate non-process loads, such as administrative building HVAC or lighting, to ensure the baseline reflects only the process unit performance.
For extrusion lines, separately meter the extruder drive, preconditioner steam, and dryer fuel to attribute energy correctly. For baking lines, meter oven fuel, combustion air fans, and post-dryer fuel independently.

The oven thermal efficiency has an outsized impact because it appears in the denominator of the fuel consumption calculation (\(E_{\text{fuel}} = E_{\text{theoretical}} / \eta_{\text{oven}}\)). Key points:

Direct gas-fired band ovens typically operate at only 15–35% thermal efficiency, meaning 65–85% of fuel energy is lost to exhaust gases, radiation, and convection from the oven shell.
Halving the efficiency (e.g., from 0.40 to 0.20) doubles the practical fuel consumption for the same theoretical heat load, making the baking route extremely sensitive to oven design and maintenance.
In contrast, the extrusion route delivers mechanical energy directly into the product via viscous dissipation, achieving effective energy transfer that bypasses large thermal losses. This fundamental difference in energy delivery mechanism is why extrusion often outperforms baking in primary energy comparisons.

Optimizing SME in twin-screw extrusion involves balancing mechanical energy input with product transformation requirements:

Screw configuration: Increasing the number of kneading blocks and reverse screw elements raises shear intensity and SME, while more forward-conveying elements reduce it. Match the screw profile to the desired SME within the valid range (0.08–0.18 kWh/kg for corn-based products).
Moisture content: Higher feed moisture reduces melt viscosity and lowers SME. Adjust preconditioner water and steam addition to fine-tune the rheology without exceeding final product moisture targets.
Screw speed: SME scales approximately with the square of screw speed in the fully filled zones. Use this relationship to dial in target SME while monitoring product quality attributes such as expansion ratio and water solubility index.
Barrel temperature profile: Higher barrel temperatures reduce viscosity and SME. Optimize the profile to balance energy efficiency with adequate starch gelatinization and texturization.

Worked Example: Energy Efficiency Comparison – Extrusion Cooking vs. Oven Baking

A food processing plant produces ready-to-eat (RTE) corn flakes at a rate of 1,000 kg/h of final product (3% moisture wet basis). The engineering team must compare the primary energy consumption of two candidate routes: (1) twin-screw extrusion cooking with a preconditioner and linked dryer, versus (2) traditional sheeting, direct gas-fired band oven baking, and a post-oven dryer. The functional unit is 1 kg of dried product exiting the system boundary.

Knowns (Input Parameters)

Latent heat of evaporation, \(h_{fg} = 2256.0\) kJ/kg
Primary energy conversion factor (electrical to fuel), \(\lambda_{\text{primary}} = 3.0\)
Specific heat of dough, \(C_{p,\text{dough}} = 1.8\) kJ/(kg·K)
Temperature rise in oven, \(\Delta T = 120.0\) K
Steam addition in preconditioner, \(m_{\text{steam}} = 0.03\) kg/kg product
Extruder specific mechanical energy, \(SME_{\text{extruder}} = 0.12\) kWh/kg
Water evaporated in extrusion-route dryer, \(\Delta m_{\text{evap,extruder}} = 0.3\) kg/kg product
Dryer thermal efficiency (extrusion route), \(\eta_{\text{dryer,extruder}} = 0.50\)
Water evaporated in the oven, \(\Delta m_{\text{evap,oven}} = 0.35\) kg/kg product
Oven thermal efficiency, \(\eta_{\text{oven}} = 0.20\)
Water evaporated in post-oven dryer, \(\Delta m_{\text{evap,post}} = 0.10\) kg/kg product
Post-oven dryer thermal efficiency, \(\eta_{\text{dryer,post}} = 0.50\)
Fan electrical consumption in oven, \(E_{\text{fan,oven}} = 0.02\) kWh/kg

Step-by-Step Calculation

Extrusion Route – Steam Energy
The preconditioner steam energy per kilogram of product is:
\[ E_{\text{steam,kJ}} = m_{\text{steam}} \cdot h_{fg} = 0.03 \times 2256.0 = 67.68 \text{ kJ/kg} \] Convert to kWh/kg:
\[ E_{\text{steam,kWh}} = \frac{E_{\text{steam,kJ}}}{3600} = \frac{67.68}{3600} = 0.0188 \text{ kWh/kg} \]
Extrusion Route – Dryer Energy
The thermal energy required by the dryer (gas-fired) is:
\[ E_{\text{dryer,extruder}} = \frac{\Delta m_{\text{evap,extruder}} \cdot h_{fg}}{\eta_{\text{dryer,extruder}} \cdot 3600} = \frac{0.3 \times 2256.0}{0.50 \times 3600} = \frac{676.8}{1800} = 0.376 \text{ kWh/kg} \]
Extrusion Route – Total Primary Energy
Sum the contributions, converting electrical to primary energy equivalent:
\[ E_{\text{total,extrusion}} = E_{\text{steam,kWh}} + (SME_{\text{extruder}} \cdot \lambda_{\text{primary}}) + E_{\text{dryer,extruder}} \] \[ E_{\text{total,extrusion}} = 0.0188 + (0.12 \times 3.0) + 0.376 = 0.0188 + 0.360 + 0.376 = 0.7548 \approx 0.755 \text{ kWh/kg} \]
Baking Route – Theoretical Oven Energy
The minimum energy needed to heat the dough and evaporate water is:
\[ E_{\text{theoretical,oven}} = \frac{(C_{p,\text{dough}} \cdot \Delta T) + (h_{fg} \cdot \Delta m_{\text{evap,oven}})}{3600} \] \[ E_{\text{theoretical,oven}} = \frac{(1.8 \times 120.0) + (2256.0 \times 0.35)}{3600} = \frac{216.0 + 789.6}{3600} = \frac{1005.6}{3600} = 0.2793 \text{ kWh/kg} \]
Baking Route – Practical Oven Fuel Input
With a direct gas-fired oven efficiency of 20%:
\[ E_{\text{practical,oven}} = \frac{E_{\text{theoretical,oven}}}{\eta_{\text{oven}}} = \frac{0.2793}{0.20} = 1.3967 \approx 1.397 \text{ kWh/kg} \]
Baking Route – Post-Oven Dryer Energy
The additional drying step, accounting for dryer thermal efficiency, requires:
\[ E_{\text{post,dryer}} = \frac{\Delta m_{\text{evap,post}} \cdot h_{fg}}{\eta_{\text{dryer,post}} \cdot 3600} = \frac{0.10 \times 2256.0}{0.50 \times 3600} = \frac{225.6}{1800} = 0.1253 \text{ kWh/kg} \]
Baking Route – Total Primary Energy
Combine oven fuel, post-dryer thermal, and fan electrical (converted to primary):
\[ E_{\text{total,baking}} = E_{\text{practical,oven}} + E_{\text{post,dryer}} + (E_{\text{fan,oven}} \cdot \lambda_{\text{primary}}) \] \[ E_{\text{total,baking}} = 1.397 + 0.1253 + (0.02 \times 3.0) = 1.397 + 0.1253 + 0.060 = 1.582 \text{ kWh/kg} \]

Final Answer
The extrusion route consumes 0.755 kWh/kg primary energy per kilogram of product. The baking route consumes 1.582 kWh/kg primary energy. Extrusion is approximately 52% more primary energy efficient in this comparison, driven largely by the low thermal efficiency (20%) of the direct gas-fired oven, the high latent heat load in the baking step, and the additional post-oven drying energy required in the baking route.

"Un projet n'est jamais trop grand s'il est bien conçu." — André Citroën

"La difficulté attire l'homme de caractère, car c'est en l'étreignant qu'il se réalise." — Charles de Gaulle