- Email: [email protected]
Heat cleaning for the coating industry
HEAT CLEANING
FOR THE COATING by Kent F. Whelan Guspro
Inc., Chatham,
INDUSTRY
Ont.
Heat cleaning and thermal stripping is not a modern process. Paint line managers have been burning off coatings from material-handling hardware for many years. The term “burn-off’ was applied years ago when fixtures were burned out back, in a pile. on the go-ound. This uncontrolled burning of fixtures caused lots of environmental problems, including smoke, and was a big concern to local fire departments. The heat-cleaning or thermal-stripping oven was developed to serve II dual purpose. First, it solved the smoke and environmental problems prehent with burning. Second. it provided a controlled process to safely clean material-handling fixtures, racks, trees. and hooks without worry of distortion or damage to the parts being cleaned. During the I%% rapid changes took place in the coating industry and electrostatic coatings. liquid and powder, gained wide acceptance. These modern coatings tended to be highly resistant to chemical attack requiring strong agents for chemical stripping. They also required more frequent cleaning to ensure electrlcal continuity for the electrostatic charge to hold the paint on the component being coated prior to curing. Since the 198Oh regulations governing holvcnt and chemical usage, storage, handling, and disposal have become extremely stringent. These require careful investigation. particularly before expanding liquid stripping operations. Compliance to meet federal and local requirements can be costly. Modern ovens for stripping applications have evolved into sophisticated process cleaning ovens with controls designed for processing a variety of materials found in the coating industry today. They are designed to not only clean material-handling hardware but to also strip reject parts, allowing them to be recycled through the coating proce~\ (see Fig. I ).
THEPROCESS The heat-cleaning process oven involves a chamber, an integral thermal oxidizing afterburner, temperature controls for burners. an air supply, and il water mirt injection system for controlling tlahh temperatures that may occur when processing large loads of combustibles. Modern heat-cleaning ovens operate on the controlled air principal. Combustion and temperature are controlled by restricting air flow to the load. The principal i\ the ~mt’ as uqcd on a backyard smoking oven. The fire is
500
oven chamber’s low oxygen environment, The decomposition rate primarily depends on temperature as long as oxygen remains excluded. As these products thermally degrade, odor and smoke are released and the gaseous products are passed into the thermal oxidizer (afterburner) chamber. These are low oxygen mixturca of volatile vapors and smoke. Thermal oxidation or burning of these volatile vapors and smoke requires an introduction of secondary air into the thermal oxidizer chamber. Raising the temperature with a secondary burner, mixing the ingredients at temperature, and holding them at temperature long enough completes the oxidation process. Carbon monoxide, always present in controlled air heat cleaning, ha\ a retention time requirement of i-O.6 set at 1,400”F for 99% completion. Retention times of about 0.75 to I .O second are desirable and are a standard design feature of modern heat-cleaning ovens. Air flow in the process chamber is very low, velocities across the racks and fixtures are low, and little ash is entrained and carried into the thermal oxidizer. Devolatilization starts for most coatings in the 250 to 450°F range. The oxidizer needs to be preheated to ensure it is up to operating temperature prior to the load devolatilizing. Fast-response burners and controls are standard equipment in ovens designed for today’s process requirements and for compliance with today’s environmental standards.
ENVIRONMENTAL
CONTROL
Current regulations are much more stringent than the regulations in force back in the 1980s. Today, state and local air quality agencies are requiring residence times of 0.75 to I .O second in the oxidizer chamber, not the stack, at temperatures of 1,450 to I ,600”E The U.S. Federal Resource Conservation and Recovery Act (RCRA) and many world air quality agencies require any process that is a potential source of pollution, or one that eliminates a pollutant, to be registered and permitted to specific regulations. RCRA has delegated the authority to control these standards to the individual states. Most states have designed their own standards that are more stringent than the federal standards. As well, many regional or city districts have standards that are more stringent than their state standards. Whether a city. regional, or state agency, a permit system of control is in effect. The permit process typically consists of a Permit to Construct. which is normally nothing more than an application form with technical support material on the oxidizer performance, and a Permit to Operate. Most districts will inspect the installation prior to issuing the Permit to Operate. The regulations do not just require that an oven eliminate odor or visible emissions but are stringent enough to require that a standard far greater than that visible to the naked eye be obtained. Air quality standards vary. With modern and properly designed heat-cleaning equipment, compliance with all of these standards and regulations can be achieved without the purchase of optional controls or accessories. Manufacturers of heat-cleaning ovens should provide test reports and support data for submittal to the local agency for the issuance of permits. Some of the control districts require record keeping of the thermal oxidizer temperature. This will require the purchase of a data acquisition or temperature recorder. These record-keeping devices are available from most heat-cleaning oven manufacturers. The oxidizer’s capacity is an indication of an oven’s ability for processing a specific load in pounds of combustibles per cycle. The larger the oxidizer capacity the shorter the process cycle and the less fuel the process will consume. Large carriers, such as those used in the automotive industry, can have 20 to 30 lb of paint on each carrier. Loads of this nature need ovens equipped with large-capacity oxidizers to reduce cycle times (see Fig. 2). Frequently, a higher oxidizer rating will reduce the size of the oven required. Faster cycles mean more loads can be processed per shift. With the oven’s automatic controls loads can be processed during off hours without operator attendance.
502
THE
BENEFITS
Caustic chemicals and their costly disposal can heat-cleaning ovens. This reduces the liability associated transport liability during and after disposal. An added cleanliness and labor savings that are achieved with the
504
be eliminated with the use with chemical usage as well benefit is the actual degree use of a heat-cleaning oven.
Fig. 2. Basket
loading
of muteriul-handling
hardware
for hutch rlearting.
Heat-cleaning ovens thermally degrade virtually all combustible materials from coatingline hardware such as hooks, fixtures, trees. racks, and baskets. Many users reclaim reject-coated parts, including aluminum components, without difficulty. There is virtually no end to the type of parts that are being cleaned in heat-cleaning ovens. Metals that melt under 900°F cannot be heat cleaned. These include zinc and its alloys. Aluminum and alloys, melting at about 1,20O”F, can be heat cleaned. Manufacturers’ instructions include precautions and procedures. Heat cleaning rarely affects the temper of heat-treated ferrous parts but can affect that of some aluminum alloys. Trial testing is encouraged prior to the purchase of an oven. Spring steel fixtures and magnetic components usually can be safely cleaned in a properly controlled oven. Some magnetic materials may deteriorate after repetitive cycles. Some coatings, such as “black oxide” and white color pigment coatings with titanium oxide, may require an aggressive postoven cleaning rinse. Typically, the simple shaking of the ash off of the component or the rinsing with fresh water will allow the part to return to the process line for continued use (see Fig. 3). Virtually all straight hydrocarbon-based coatings can be heat cleaned. Chlorinated compounds (PVC) will generate hydrochloric acid. Processing should be verified by local authorities to insure that the chloride discharge from the stack does not exceed local regulations. Fluorinated compounds (Teflon) and other noncombustible ingredients should be investigated with your oven supplier before processing. In virtually all instances the ash residue that is remaining on the parts after heat cleaning is a nonhazardous substance. Some exotic compounds and unique coatings can be exceptions. Ash samples should be periodically taken and sent out to a testing laboratory for compliance with disposal regulations. This is a prudent procedure and eliminates a lot of stress should a local agency ask about your disposal methods. IMPROVE
QUALITY
AND
REDUCE
COSTS
Clean hooks and clean fixtures minimize overspray and enhance the coating quality. thereby reducing rejects and conserving coating materials. This is particularly visible in
505
Fig.
3. Ovens
with
ash
wash
chamber
electrostatic powder coating applications and electrocoating lines where you can physically view the coating being applied. A properly cleaned fixture will provide the electrical conductivity required for proper coating. A dirty hook or improperly cleaned fixture wastes coating materials and increases the probability of an improperly coated component. There are wide differences in various equipment manufacturers’ capacities and their oven construction. Very low capacity can lead to long cycles and low production rates. Long cycles will lead to high fuel costs per cycle and may require larger equipment than is necessary to meet production requirements. Lightweight construction will deteriorate and not provide the long service life expected from such a capital investment. With thousands of installations throughout North America and the world the heatcleaning oven and process has proven to be an effective and cost-saving investment. It is not uncommon for the purchaser of a heat-cleaning oven to report within months of installation that the savings in labor, materials, and waste disposal costs have provided a rapid return on their investment.
The Technology
of Anodizing
Aluminum,
Third Edition
$210.00 NEWEDITION This valu&le book has been co~npletely reedited .md considers significant new developments in anodizing rechnology. The expanded volume will satisfy the dnodizer who requires more detailed technology. After an introduction, the reader is presented with practical applicxion of the new technology, and the nxure of the industry with cdpirdl investment appraisal, budgeting, and cost control. An excellent summxy of anodizing technologies in clear language encomp.&ng the advice of experienced technicians. hy A. W. Bruce
410 pages
Send Orders to: METAL FINISHING, 660 White Plains Rd., Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570
506
FOR THE COATING by Kent F. Whelan Guspro
Inc., Chatham,
INDUSTRY
Ont.
Heat cleaning and thermal stripping is not a modern process. Paint line managers have been burning off coatings from material-handling hardware for many years. The term “burn-off’ was applied years ago when fixtures were burned out back, in a pile. on the go-ound. This uncontrolled burning of fixtures caused lots of environmental problems, including smoke, and was a big concern to local fire departments. The heat-cleaning or thermal-stripping oven was developed to serve II dual purpose. First, it solved the smoke and environmental problems prehent with burning. Second. it provided a controlled process to safely clean material-handling fixtures, racks, trees. and hooks without worry of distortion or damage to the parts being cleaned. During the I%% rapid changes took place in the coating industry and electrostatic coatings. liquid and powder, gained wide acceptance. These modern coatings tended to be highly resistant to chemical attack requiring strong agents for chemical stripping. They also required more frequent cleaning to ensure electrlcal continuity for the electrostatic charge to hold the paint on the component being coated prior to curing. Since the 198Oh regulations governing holvcnt and chemical usage, storage, handling, and disposal have become extremely stringent. These require careful investigation. particularly before expanding liquid stripping operations. Compliance to meet federal and local requirements can be costly. Modern ovens for stripping applications have evolved into sophisticated process cleaning ovens with controls designed for processing a variety of materials found in the coating industry today. They are designed to not only clean material-handling hardware but to also strip reject parts, allowing them to be recycled through the coating proce~\ (see Fig. I ).
THEPROCESS The heat-cleaning process oven involves a chamber, an integral thermal oxidizing afterburner, temperature controls for burners. an air supply, and il water mirt injection system for controlling tlahh temperatures that may occur when processing large loads of combustibles. Modern heat-cleaning ovens operate on the controlled air principal. Combustion and temperature are controlled by restricting air flow to the load. The principal i\ the ~mt’ as uqcd on a backyard smoking oven. The fire is
500
oven chamber’s low oxygen environment, The decomposition rate primarily depends on temperature as long as oxygen remains excluded. As these products thermally degrade, odor and smoke are released and the gaseous products are passed into the thermal oxidizer (afterburner) chamber. These are low oxygen mixturca of volatile vapors and smoke. Thermal oxidation or burning of these volatile vapors and smoke requires an introduction of secondary air into the thermal oxidizer chamber. Raising the temperature with a secondary burner, mixing the ingredients at temperature, and holding them at temperature long enough completes the oxidation process. Carbon monoxide, always present in controlled air heat cleaning, ha\ a retention time requirement of i-O.6 set at 1,400”F for 99% completion. Retention times of about 0.75 to I .O second are desirable and are a standard design feature of modern heat-cleaning ovens. Air flow in the process chamber is very low, velocities across the racks and fixtures are low, and little ash is entrained and carried into the thermal oxidizer. Devolatilization starts for most coatings in the 250 to 450°F range. The oxidizer needs to be preheated to ensure it is up to operating temperature prior to the load devolatilizing. Fast-response burners and controls are standard equipment in ovens designed for today’s process requirements and for compliance with today’s environmental standards.
ENVIRONMENTAL
CONTROL
Current regulations are much more stringent than the regulations in force back in the 1980s. Today, state and local air quality agencies are requiring residence times of 0.75 to I .O second in the oxidizer chamber, not the stack, at temperatures of 1,450 to I ,600”E The U.S. Federal Resource Conservation and Recovery Act (RCRA) and many world air quality agencies require any process that is a potential source of pollution, or one that eliminates a pollutant, to be registered and permitted to specific regulations. RCRA has delegated the authority to control these standards to the individual states. Most states have designed their own standards that are more stringent than the federal standards. As well, many regional or city districts have standards that are more stringent than their state standards. Whether a city. regional, or state agency, a permit system of control is in effect. The permit process typically consists of a Permit to Construct. which is normally nothing more than an application form with technical support material on the oxidizer performance, and a Permit to Operate. Most districts will inspect the installation prior to issuing the Permit to Operate. The regulations do not just require that an oven eliminate odor or visible emissions but are stringent enough to require that a standard far greater than that visible to the naked eye be obtained. Air quality standards vary. With modern and properly designed heat-cleaning equipment, compliance with all of these standards and regulations can be achieved without the purchase of optional controls or accessories. Manufacturers of heat-cleaning ovens should provide test reports and support data for submittal to the local agency for the issuance of permits. Some of the control districts require record keeping of the thermal oxidizer temperature. This will require the purchase of a data acquisition or temperature recorder. These record-keeping devices are available from most heat-cleaning oven manufacturers. The oxidizer’s capacity is an indication of an oven’s ability for processing a specific load in pounds of combustibles per cycle. The larger the oxidizer capacity the shorter the process cycle and the less fuel the process will consume. Large carriers, such as those used in the automotive industry, can have 20 to 30 lb of paint on each carrier. Loads of this nature need ovens equipped with large-capacity oxidizers to reduce cycle times (see Fig. 2). Frequently, a higher oxidizer rating will reduce the size of the oven required. Faster cycles mean more loads can be processed per shift. With the oven’s automatic controls loads can be processed during off hours without operator attendance.
502
THE
BENEFITS
Caustic chemicals and their costly disposal can heat-cleaning ovens. This reduces the liability associated transport liability during and after disposal. An added cleanliness and labor savings that are achieved with the
504
be eliminated with the use with chemical usage as well benefit is the actual degree use of a heat-cleaning oven.
Fig. 2. Basket
loading
of muteriul-handling
hardware
for hutch rlearting.
Heat-cleaning ovens thermally degrade virtually all combustible materials from coatingline hardware such as hooks, fixtures, trees. racks, and baskets. Many users reclaim reject-coated parts, including aluminum components, without difficulty. There is virtually no end to the type of parts that are being cleaned in heat-cleaning ovens. Metals that melt under 900°F cannot be heat cleaned. These include zinc and its alloys. Aluminum and alloys, melting at about 1,20O”F, can be heat cleaned. Manufacturers’ instructions include precautions and procedures. Heat cleaning rarely affects the temper of heat-treated ferrous parts but can affect that of some aluminum alloys. Trial testing is encouraged prior to the purchase of an oven. Spring steel fixtures and magnetic components usually can be safely cleaned in a properly controlled oven. Some magnetic materials may deteriorate after repetitive cycles. Some coatings, such as “black oxide” and white color pigment coatings with titanium oxide, may require an aggressive postoven cleaning rinse. Typically, the simple shaking of the ash off of the component or the rinsing with fresh water will allow the part to return to the process line for continued use (see Fig. 3). Virtually all straight hydrocarbon-based coatings can be heat cleaned. Chlorinated compounds (PVC) will generate hydrochloric acid. Processing should be verified by local authorities to insure that the chloride discharge from the stack does not exceed local regulations. Fluorinated compounds (Teflon) and other noncombustible ingredients should be investigated with your oven supplier before processing. In virtually all instances the ash residue that is remaining on the parts after heat cleaning is a nonhazardous substance. Some exotic compounds and unique coatings can be exceptions. Ash samples should be periodically taken and sent out to a testing laboratory for compliance with disposal regulations. This is a prudent procedure and eliminates a lot of stress should a local agency ask about your disposal methods. IMPROVE
QUALITY
AND
REDUCE
COSTS
Clean hooks and clean fixtures minimize overspray and enhance the coating quality. thereby reducing rejects and conserving coating materials. This is particularly visible in
505
Fig.
3. Ovens
with
ash
wash
chamber
electrostatic powder coating applications and electrocoating lines where you can physically view the coating being applied. A properly cleaned fixture will provide the electrical conductivity required for proper coating. A dirty hook or improperly cleaned fixture wastes coating materials and increases the probability of an improperly coated component. There are wide differences in various equipment manufacturers’ capacities and their oven construction. Very low capacity can lead to long cycles and low production rates. Long cycles will lead to high fuel costs per cycle and may require larger equipment than is necessary to meet production requirements. Lightweight construction will deteriorate and not provide the long service life expected from such a capital investment. With thousands of installations throughout North America and the world the heatcleaning oven and process has proven to be an effective and cost-saving investment. It is not uncommon for the purchaser of a heat-cleaning oven to report within months of installation that the savings in labor, materials, and waste disposal costs have provided a rapid return on their investment.
The Technology
of Anodizing
Aluminum,
Third Edition
$210.00 NEWEDITION This valu&le book has been co~npletely reedited .md considers significant new developments in anodizing rechnology. The expanded volume will satisfy the dnodizer who requires more detailed technology. After an introduction, the reader is presented with practical applicxion of the new technology, and the nxure of the industry with cdpirdl investment appraisal, budgeting, and cost control. An excellent summxy of anodizing technologies in clear language encomp.&ng the advice of experienced technicians. hy A. W. Bruce
410 pages
Send Orders to: METAL FINISHING, 660 White Plains Rd., Tarrytown, NY 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570
506