Economic and environmental concerns - coupled with a push for green marketing - are putting pressure on manufacturing engineers to examine their operations for energy efficiency. Paint curing ovens are among the first pieces of process heating equipment to come under scrutiny as they are in common use by many metal product manufacturers. Can the waste heat typically generated and exhausted in these ovens be recovered and reused?
Although many types of ovens are used in paint systems, infrared and convection ovens represent the two most common styles. Of the two, convection ovens are considered less energy efficient, but they are more popular because they are:
- Easier to use.
- Capable of heating three-dimensional products evenly.
- Designed to present little risk of overheating the parts.
Convection ovens typically are gas fired and can have a high exhaust rate - sometimes adding up to 50 percent of total oven heat load. Although paint cure ovens are considered low temperature and generally operate between 200 and 650°F (93 and 343°C), the potentially large amount of heat lost in the exhaust provides an opportunity for waste heat recovery and, therefore, makes this type of oven the focus of this article.
In addition to the exhaust load, the oven design factors in product and conveyor loads as well as wall and opening losses. Exhaust rates are determined by safety codes, insurance requirements and coating process needs. For liquid paints with a solvent constituent, NFPA 86 specifies that the exhaust rate must be sufficient to maintain the oven atmosphere below 25 percent of the lower flammability limit (LFL); therefore, the rate is based upon the amount of paint applied. For non-solvent-based coatings, the oven exhaust rate usually is determined by the number of turnovers required to remove coating off-gases such as electrocoat smoke or to remove combustion products and avoid yellowing the coating. For example, light-colored powder coatings may require 12 to 15 turnovers of the oven volume per hour while dark colors may need only 6 to 8 turnovers per hour.
Generally, the higher the oven temperature, the easier it is to cost-effectively recover waste heat. Because most paint ovens operate at relatively low temperatures, waste heat recovery is marginal. Therefore, it is best evaluated with respect to meeting an acceptable return on investment (ROI) rather than to trying to provide a specific amount of process heat. In other words, any heat recovered from the oven exhaust is beneficial as long as the cost of recovery can earn savings after a reasonable payback period - generally, one to five years.
Typical types of air-to-air heat exchangers that may be used to cost-effectively recover waste heat from paint cure oven exhausts include tubular, duct-in-duct and dimple plate designs.
The first step in the evaluation is to make sure the oven is operating efficiently. Among the things to check are:
- Ensure that the oven exhaust is at a minimum to meet code and process conditions.
- Be sure the burner is clean and adjusted properly with respect to the air-to-fuel ratio.
- Evaluate the coating performance and establish the minimum acceptable cure temperature.
- Thermally scan the oven insulation and repair it as required.
Only when this process is complete can a realistic estimate of available waste heat be established by measuring the actual exhaust airflow.
The next step is to determine where and how the waste heat will be used and the acceptable ROI period. Reuse of the recovered heat within the painting process is considered the best use because it ensures that a heat demand exists at the same time the waste heat supply is available. Other uses may only generate enough demand for the waste heat at certain times of the year. For instance, an air-to-air exchanger can be used to provide heat for comfort heating in the building but, even if a plant is in a northern climate, the payback will only be earned over a short, two- to three-month period each year, significantly extending the ROI. (The remainder of the months, there is no demand for the captured waste heat.) By contrast, if the same exchanger design were used to preheat combustion air in the same oven paint curing system, the payback is available for 12 months, and the heat demand and supply cycles match.
Recovery of the exhaust heat can be accomplished directly through reuse of the exhaust flow or indirectly using a heat exchanger. Direct reuse of waste heat within an occupied area is not permitted because of potential contaminants and generally low oxygen content of the exhaust air. However, if the paint system also uses an oven to dry parts after washing, the higher temperature cure oven exhaust can be directed into the drying oven as a supplemental heat supply. The evaluation of this option should include an analysis of the exhaust contaminants to avoid soiling freshly cleaned parts. Also, the ovens would need to be interlocked with respect to their safety circuits.
Selecting the Heat Exchanger
Heat exchangers are offered in a range of designs, but in general, only a simple, low-cost design will enable an acceptable ROI for paint ovens. Capital costs used to calculate the potential ROI should include the exchanger, all associated ductwork, piping and installation, as well as costs, if required, to upgrade or replace the exhaust fan to overcome the additional static pressure of the exchanger. Filtration, if needed to protect the exchanger, also must be considered. In general, the closer the exhaust location is to the point of reuse, the more cost-effective the recovery will be.
Single-phase heat exchangers for waste heat recovery are either air-to-air or air-to-water designs. Multiphase exchangers also are available, where an intermediary substance such as refrigerant (heat pipes) or media (heat wheels) is heated, but these types typically are more complex, have moving parts or are constructed from expensive materials. Because of their complexity, multiphase solutions often do not provide a reasonable ROI. Many designs are commercially available in each category (air-to-air or air-to-water), but simplicity and cost should drive the selection.
Air-to-air heat exchangers, also referred to as recuperators, transfer heat from the hot exhaust to a cooler airflow as the two airflows travel through opposite sides of a series of chambers separated by metallic or ceramic barriers. The chambers can be a variety of configurations, including tubes, parallel plates or duct-in-duct (figure 1). Air-to-air exchangers can be used to preheat combustion air, introducing the air to the burner box upstream from the burner. A less controllable alternative is to introduce the recovered heat at the conveyor openings through a ducted vestibule.
Typically, the lower cost the exchanger, the less transfer efficient. For example, a duct-in-duct design has the lowest cost but is the least efficient. However, this design is also the simplest and can be installed with the least modification to the existing duct and fan. It also serves a secondary purpose of helping to insulate the hot exhaust.
Air-to-water heat exchangers, or economizers, are widely used to preheat boiler feed water and, in a paint system, can be used to heat washer stages. Air-to-water exchangers, while generally more expensive than air-to-air exchangers, may be more cost-effective overall because piping is less expensive than ductwork.
This type of exchanger uses a series of tubes, usually finned for greater surface area, arranged within a chamber transporting the heated exhaust flow. Waste heat from a 400°F (204°C) oven can fairly easily supplement the 110 to 160°F (43 to 71°C) temperatures used in the chemical stages. To keep scale from building inside the tubes, a dilute acidic solution should be periodically circulated. Using the exchanger to heat a clean rinse water reduces the need for scale removal and helps to heat the part to promote drying, but heating a final deionized water rinse increases its aggressiveness and should be avoided to prevent flash rusting.
The contaminants within the paint oven exhaust flow present several unique issues to the exchanger selection. Access to the exchanger interior and cleanability of the exchanger surfaces are important and may sacrifice some amount of the exchanger design efficiency. Upstream of the exchanger, particulate filtration of the exhaust can be considered but it generally is not necessary: If particulate loading is high enough to foul the exchanger, there will be dirt defects in the paint, and the problem will need to be addressed with improved oven cleaning.
Also, some paint curing agents and constituents such as e-caprolactam can condense out of exhaust air onto cooler surfaces and may degrade the exchanger efficiency over time. Stainless steel is preferred for exchanger surfaces in order to withstand the temperature and reduce corrosion from combustion products, coating contaminants and cleaning materials.
After selecting a heat exchanger design, project costs can be established and coupled with the equipment manufacturer’s estimate of energy savings to establish an ROI. Offsetting costs may be available through rate reduction or rebate programs from the local electric and gas suppliers. Additionally, grants, low interest loans and tax incentives may be available from the government. A helpful state by state list of government programs is listed online. (Visit http://1.usa.gov/dC8gGv or Process-Heating.com for the full link.)
In summary, paint system ovens are on the low end of waste heat recovery feasibility because of the relatively low operating temperature and exhaust flow rate. However, if properly evaluated, a reasonable return on investment may be found by recovering the waste heat employing a simple, low cost, cleanable heat exchanger that reuses the waste heat within the same system in which it is produced.