Depending on the size and type of application, preassembled and tested packaged process heat recovery systems can simplify and speed installation while reducing investment costs compared to a custom system. This unit draws heated air from the exhaust duct or stack for energy recovery and draws in fresh outdoor air, which is controlled within the setpoint range of 70 to 125°F (21 to 52°C) for supplemental heating or cooling for the building.
Many products go through a dryer, an oven or drying process, and fluctuating natural gas prices make it difficult for companies to price products competitively and manage profit margins. There is also increased pressure to reduce greenhouse gases and the products of combustion typical of the drying process.
Fortunately, it is possible to drive down dryer energy costs and boost heat recovery return on investment (ROI). For example, dryer energy costs for printing or coating may be only 1 percent of total expense, but they could easily add up to $1 million a year. Reducing usage by 25 percent -often an achievable goal - saves $250,000. Moreover, in many cases, savings could be realized with little or no capital outlay.
Make the Most of What You Have
Major heat recovery projects are not always cost effective for all processes. Luckily, several far less extensive steps can yield significant savings. You should benchmark energy usage up front so you can measure progress.
Step one is to audit your equipment and make sure routine preventive maintenance procedures are followed. Loose or worn belts; dirty filters, nozzles or plenums; and leaking gaskets, seals or burners constantly rob energy.
Next comes setup and operation of the dryer itself. Creating a “cookbook for success” may be as simple as developing a checklist of optimum parameters, or “recipe,” for each product. Recipe “ingredients” include substrates, coatings and line speed, any or all of which can vary from product to product or shift to shift, even for the same product. A change in substrate thickness may require less tension. A new coating may require less heat or less drying time. The list goes on.
Minimizing dryer exhaust safely is the task of this closed-loop system, which monitors the lower flammable limit (LFL) and opens a series of dampers or AC drives to control the exhaust flow.
Many companies “make do” with used or relocated equipment that is working okay, yet wasting energy. Many of these systems can be optimized to run more efficiently. For instance, in a flotation dryer, the air-bar gaps, quantity, widths and center-to-center settings can be adjusted for efficiency, or the air bars can be replaced with different style air bars.
Remember, the idea is to apply only as much heat or air as necessary to achieve the desired speed and product quality. This is especially important if exhaust air is treated by an oxidizer or pollution control device. Process supply air, makeup air and exhaust air flows, temperatures and critical parameters can be “balanced” for each dryer or process. In addition, the flows often can be optimized for each product, plus factors such as ambient conditions, humidity, temperatures or building pressures.
An OEM or qualified service provider can help model your system so you can document parameters along with baseline energy usage. You can use the data to create a roadmap to optimum energy use.
Dryer Controls and Configuration
Controls to measure the humidity are used to monitor the relative humidity within the dryer in order to control the exhaust rate.
Many vintage dryers have obsolete controls or instruments. Replacing limiting devices for better control could help them run more efficiently, faster - perhaps at 800 ft/min instead of tripping out at 400 ft/min - and more reliably. System upgrades may require little money or downtime.
- Air-to-gas ratio control keeps your burner at stoichiometric combustion throughout the operating ranges, optimizing the amount of combustion air supplied to the burner.
- Controls to measure the lower flammable limit (LFL) or humidity are used to monitor the solvent concentration or relative humidity within the dryer in order to control the exhaust rate. They will maintain only the exhaust necessary to safely provide the desired product quality and production speed.
- Outdated or problematic PLCs, discrete controllers and other devices can play havoc with your process. Fortunately, OEMs can provide replacement kits that include all parts, programming or other instructions needed for installation.
Improper configuration, as noted earlier, robs energy every day. This also can create web-handling problems, reduce product quality and limit production speeds. Here again, a qualified OEM can help model your processes and recommend alternatives.
Dryer Heat Recovery Criteria for Success
The air-to-air heat recovery system recovers energy from the exhaust airstream and passes it through a heat exchanger, where ambient air is pulled in and cost-effectively converted to heated makeup air.
Before investing in dryer energy recovery, you need to determine a cost-effective, reliable means of collecting and distributing waste energy, along with suitable uses for the energy recovered.
While every exhaust stack has a different temperature and flow, to be used efficiently, your waste stream will need to be:
- Clean. Particulates, condensates or dirt will accumulate and plug up equipment. In some cases, particulate can be filtered.
- Reasonably Dry. The proper level can vary, but 25 percent humidity, for instance, may create unmanageable condensation.
- Kept at a Temperature Above 250°F (121°C). Unless the volume is substantial, any airstream below 250°F probably will not “pay its way” for heat recovery. Also, a minimum temperature within the heat recovery system must be maintained to prevent dewpoint condensation.
Systems that generally provide the most payback deliver heat or energy back to the process for year-round savings. Examples include:
- Combustion air heating.
- Process makeup air.
- Building makeup air.
- Direct building makeup air and space heating.
- Building hot water systems (plant boiler loop).
- Low temperature steam applications.
When considering uses for recovered heat, keep in mind that the need for area or space heating can be limited by the local climate.
Also, the recovery method, devices and capital expenditure depend on the application and distance between the source and user. Temperatures and flows also may impact technology used. Optional methods include:
- Air-to-Air Systems. These systems pass exhaust air through a heat exchanger and return fresh air into the process or building.
- Air-to-Water Systems. These are used to preheat water for boilers or other uses.
- Air-to-Oil Systems. These systems use high temperature thermal oils for applications above 300°F (149°C) where glycol or water may not work.
- Direct Hot Water Systems. Though primarily a pulping industry application, these systems run air through a water mist.
- Hybrid and Custom Systems. These systems can be any combination of the above.
Many OEMs can customize a system for you. Don’t overlook off-the shelf solutions though - standard packaged system could be used for many applications. A local sheet metal company could install the ductwork and you’d be running in a few days.
Finally, when projecting payback, don’t overlook incentives in the ROI mix. For any heat recovery system, consider:
- Equipment costs (with installation and commissioning).
- Current and potential future energy costs and savings.
- Maintenance and related upkeep for the expected lifetime of equipment.
Improved operating conditions and productivity should be factored as well. Acceptable ROI varies among applications, but projects with a two-year payback typically merit serious consideration.