The industrial sector accounts for about one-third of total U.S. energy consumption. This energy is consumed as electricity that is purchased or self-generated, and as fossil fuels such as natural gas, propane, fuel oils and coal. Understanding these energy sources and their associated uses, equipment, efficiencies, costs, availabilities and waste streams is critical to developing a sustainable energy efficiency program.
Every manufacturing plant has raw materials that come into the receiving dock and finished products that are sent out from the shipping dock. Between the receiving and shipping docks, transformation occurs. Transformation adds value to the materials in a step-by-step process, and energy is required. Evaluating the process transformation steps and energy inputs provides clues about where to look for energy savings.
This article focuses particularly on the transformation process of powder coating. We will look at a 10-step method for performing process energy analysis for four steps in the powder coating line (Figure 1):
- Powder application
Understanding these processes and their associated equipment, technologies and support systems is key to finding energy efficient solutions. This article briefly discusses energy efficiency, energy intensity and transformation, and then presents the approach to conducting a powder coating line process energy analysis. This technique focuses on a process block diagram that shows energy inputs, energy wastes, energy recovery and possible energy improvements.
Energy Efficiency and Intensity
The total energy into a system is Ein, which is the amount that appears on your utility bill. The total energy out of a system is Eout, which represents the useful energy that adds value to the product during the process. The difference between Ein and Eout is the loss. Loss is wasted energy that is not useful to the process and degrades efficiency (Figure 2). For sustainable energy efficiency, energy losses must be identified, documented, tracked, corrected and prevented from recurring. If losses were zero, the system would be 100% efficient; however, this does not occur in the real world.
The energy intensity of a manufacturing process is the amount of energy that is required to produce one logical unit of product (e.g., kWh/ton of metal melted at a foundry, MMBtu/bbl of oil refined at a refinery, MMBtu/lb of polymer produced at a chemical plant). Energy intensity provides an order-of-magnitude estimate of the significance of energy in the production process, and it varies widely across industries.
For the powder coating process, energy intensity could be defined in several ways, including:
- kWh per part
- kWh per pound of powder applied
- MMBtu per square foot of product coated
Transformation in manufacturing is the conversion of a raw material state of a product into a finished state. For the powder coating line, transformation processes include:
- Dirty part → Clean part
- Clean wet part → Clean dry part
- Part with no powder paint applied → Part with powder paint applied
- Part with uncured powder → Part with cured powder
Each step of the transformation process should add value with minimal waste, and every step requires some type and amount of energy to carry out. Certain steps require a large amount of energy, while others require very little. Outlining each step and the required energy inputs is useful for planning and prioritizing energy improvement projects.
The Ten Steps
Step 1. Identify the Raw Materials
Some industrial processes have one main raw material, while others have dozens or even hundreds. Raw materials can come into the process at many places along the transformation journey. To determine the type and amount of energy required in the system, first consider these aspects of the raw materials:
- Type of material, e.g., metal, chemical, mineral, textile, vegetable, finished goods
- Physical state, e.g., solid, liquid, gas, subassembly
- Delivery method, e.g., tanker ship, tanker truck, common carrier, railcar
- Delivery storage, e.g., dry bulk, tank farm, warehouse, sacks, pallets, cardboard boxes
Defining the raw materials and their details is an initial step in creating a process block diagram. We will follow these materials on their journey to their final destination while evaluating the energy used at each point along the way.
For the powder coating process, the raw materials are unpainted parts. These parts come in all shapes and sizes, but at the end of the process, we want to have a properly powder coated part.
Step 2. Identify the Final Products
The final product is the destination of the transformation journey. Manufacturing plants are in the business of making money, so raw materials are brought in, transformed into something useful and then sold for a profit. The manufacturing plant adds value, hopefully very efficiently, to the raw materials and produces a final product of a designated design and quality. Answer these questions to identify the final products:
- Is the final product a completed consumer good that is ready for sale?
- Is the final product an intermediate finished item that will become the raw material at another manufacturing site?
- How is the final product packaged?
- How is the final product shipped?
For the powder coating process, the final product is a properly powder coated part. Typically, the powder coating line is just one step in a larger manufacturing plant. The powder coating can be an intermediate step or a final step in the overall process.
Step 3. Tour the Plant
There are many ways to get from Point A — the raw materials — to Point B — the final product. Touring the manufacturing site with process operators and maintenance personnel as guides is essential to defining the transformation steps and developing the process block diagram. The walk-through should ideally be conducted chronologically, from raw materials to finished products. During the plant tour, take good notes and include:
- Major transformation steps
- Specific process parameters for each step (e.g., temperature, flowrate, pressure, material characteristic)
- Energy inputs into each step (e.g., electricity, natural gas, steam, chilled water, compressed air)
- Equipment used to complete the steps
- Facility equipment used to support the steps (e.g., air compressors, boilers, chillers, cooling towers)
- Waste streams (e.g., combustion stack gases, wastewater, metal shavings, sawdust)
For the powder coating process, your tour will be focused on the powder coating line. During your tour of the line, record process parameters such as wash tank temperatures, drying oven temperatures, curing oven temperatures, etc. Also, note the product flow through the process and record line speed of the powder coating material transport trolley.
Step 4. Develop the Process Block Diagram
You have done your homework and completed a detailed tour of the manufacturing site. Now, you are ready to flesh out the process block diagram. Use your notes, conversations, utility data and possibly some online research to document the transformation steps in the process. The product of your work should look something like Figure 3, which shows the basic process steps for a powder coating line. Figure 3 is just an example, however, and other powder coating lines may have different equipment configurations with different energy sources.
Once the process block diagram is developed, the next steps are to evaluate each process step block to identify the energy inputs, energy wastes, energy recovery possibilities, energy efficiency opportunities and new technology opportunities (Steps 5-9).
Step 5. Identify Energy Inputs
Each step of the process block diagram must be reviewed to identify the primary energy inputs required to perform the transformation. Energy inputs may be direct energy, such as electricity, natural gas, propane and fuel oil, or derived energy, such as compressed air, steam and chilled water. Repeating this analysis for every step helps to produce an overall qualitative energy usage model. From Figure 3, we can observe that for our example powder coating line:
- There are a lot of electric motors consuming electricity for conveyors, fans and pumps.
- There is a significant amount of electricity consumed for the infrared heating in the booster curing oven.
- Natural gas is consumed for heating sources in the boiler, drying oven and final curing oven.
- Steam from the natural gas-fired boiler is used to make hot water for heating the pretreatment line tanks.
- There is some energy input for area support systems such as lighting, HVAC or other items.
Completing an energy input analysis for each block in the diagram creates an overall picture of the process energy consumption. If available, information to help quantify the energy input is valuable, including motor horsepower, actual metered cubic feet of natural gas, electric process sub-metering, etc.
Step 6. Identify Energy Wastes
Energy is wasted to some degree in every step of the manufacturing process. Major wastes should be identified when you are analyzing the process block diagram. Identifying process waste streams is the first step to minimizing them, recovering valuable energy from them and reducing their environmental impact. Figure 3 includes several waste streams for the example powder coating line, including:
- Flue gas from the boiler combustion
- Blowdown from the boiler
- Wastewater from the pretreatment line tanks
- Flue gas from the drying oven combustion
- Waste powder from the powder application booth
- Flue gas from the final curing natural gas convection oven combustion
Step 7. Identify Energy Recovery Possibilities
The energy waste streams should be examined for their potential for energy recovery. Observations of the waste streams in Figure 3 include:
- Using the hot boiler flue gas to preheat the boiler combustion air
- Using the hot boiler blowdown to preheat the incoming boiler makeup water
- Using the hot pretreatment tank wastewater to preheat the incoming city water to the hot water heaters
- Using the hot drying oven flue gas to preheat the drying oven combustion air
- Recovering and recycling waste powder from the powder application booth
- Using the hot final curing oven flue gas to preheat the final curing oven combustion air
These potential energy recovery options should be evaluated for economic feasibility and implementation. Recovering wasted energy can help offset the need for using primary energy and result in good energy cost savings.
Step 8. Identify Energy Efficiency Opportunities
Each block in the process block diagram should be evaluated for energy efficiency opportunities. Depending on the energy input for the process operation, a variety of options may be available to reduce energy consumption. The motors, compressed air and boiler/steam supply in Figure 3 have the potential for energy efficiency improvements.
Motors. Motors consume a significant amount of process energy. The biggest cost over the life of a motor, by far, is the electricity to turn it — typically accounting for 96% of a motor’s total life cycle costs (Figure 4). Maximizing the overall efficiency of the plant’s motor population has energy-saving benefits. NEMA Premium efficient motors should be used for all new motors.
Variable frequency drives (VFDs). Where variable loads and good feedback parameters exist, VFDs help save energy. Conduct a detailed analysis of VFD/motor combinations and implement where operationally and economically feasible. Possible areas to consider VFDs on the powder coating line include ventilation and exhaust fans, pretreatment line pumps, and drying and curing oven air circulation fans.
Compressed air. Compressed air is a very expensive and inefficient energy source. A 1/8-inch diameter leak on a 100-psig compressed air system costs approximately $1,000 per year for the electricity to compress the air for just that leak. Multiply this by 100 leaks across a large process system, and a plant can spend up to $100,000 per year on wasted electricity for compressed air alone. Compressed air energy efficiency recommendations could include:
- Establish and maintain a compressed air leak survey and repair program, which is inexpensive and has immediate payback.
- Where feasible, replace air-driven mixers and diaphragm pumps with electric-driven mixers and pumps.
- Use zero-loss condensate drains on the compressed air system throughout the plant.
Boilers, steam and combustion. The process steps in Figure 3 use a boiler to make steam that is used for process heating. Assuming a delivered cost for natural gas of $6 per dekatherm (1 dth = 1 million Btu), exposed uninsulated steam piping can cost $200 per foot per year in lost heat. Extrapolating this over a large processing plant, 100 equivalent linear feet of uninsulated steam piping would cost $20,000 per year in lost heat. There are also opportunities for energy efficiency improvements on these systems, including:
- Ensure combustion equipment and steam piping are properly insulated.
- Monitor the oxygen content of the flue gas to ensure the most efficient combustion and reduce nitrogen oxide and sulfur oxide releases, which will also reduce the amount of natural gas consumed, thereby lowering the amount of carbon released.
- Conduct a proper steam trap survey using a thermographic camera or ultrasonic leak detector, and perform maintenance to save energy on steam systems.
Step 9. Identify New Technology Opportunities
Implementing new or existing process technologies can provide energy savings in addition to those identified in Step 8. A goal is to reduce energy intensity, and a different technology may lower the energy required to transform one logical unit of product. Look for opportunities to improve process equipment with new technology.
Possible energy-saving technology improvements from Figure 3 could include:
- Update spray nozzles in the pretreatment line wash and rinse tanks to optimize spray patterns, which could result in pump motor energy savings.
- Upgrade pretreatment chemicals to low temperature chemicals that do not require tank heating and gain the savings of heating the tanks.
- Convert the natural gas convection drying oven to infrared drying.
- Upgrade the hybrid infrared/natural gas convection curing system to a full infrared curing system.
Process quality and part specifications must always be considered when upgrading to a new technology. Process parameters must still be met. In addition, the energy and financials would need to be evaluated to see if the idea is feasible. Once a careful analysis is completed, this type of new technology implementation can frequently provide energy efficiency and energy intensity savings without sacrificing any required process parameters.
Step 10. Implement Solutions
After you develop the process block diagram and perform the process energy analysis, the next and most important step is to implement some of the energy-saving solutions you have identified. Savings will not be realized until the results are actually applied.
Your analysis will produce a detailed set of opportunities for energy improvements. Compile the results in a table or spreadsheet so they can be evaluated, prioritized, budgeted and tracked for implementation. Then, repeat the approach periodically for continual improvement.
Pursuing energy improvements often produces benefits in other areas as well. These non-energy benefits may include greater plant productivity, higher product quality, fewer process bottlenecks, better worker safety, more available floor space, lower emissions and lower waste stream volumes.
Taking a practical, process-oriented approach to your powder coating line can result in many potential energy-saving ideas. The process block diagram is key in this analysis. Once you understand how energy is consumed, it is much easier to find ways to conserve it.