Years ago, I worked with a guy whose previous job was selling heating elements. When a customer would ask, “How long will these heaters last?” he would reply, “Forever — if you never turn them on.” This was his glib way of indicating that equipment and component life is dependent upon usage. The more you use something, the sooner you will use it up. This is even true of industrial ovens and dryers.
A well-made oven or dryer typically has a long life. I have often lamented, “If our ovens only lasted 10 years, we would sell three times as many.” In and of itself, an industrial oven or dryer does not have many moving parts. A typical gas-fired convection oven/dryer has only an exhaust fan, a recirculation fan and, perhaps, a combustion blower for the burner. Larger oven/dryers might have multiple fans. There also might be doors or dampers (sometimes automatic and sometimes manual). An inner shell, an outer shell, insulation, ductwork, perhaps some supporting structure and controls help complete the package, and a conveyorized oven has a conveyor, obviously.
Some oven/dryers have even fewer moving parts. An electrically heated oven/dryer has heating elements in lieu of a gas burner, so there is no combustion blower. An infrared or radiant oven/dryer has even fewer fans.
Getting decades of (relatively) trouble-free use out of an oven/dryer is common. All it takes is luck and a little tender loving care. Tender loving care translates to maintenance. The amount of maintenance required is a function of how you use your oven/dryer, how often you use it and how critical its functionality is to production.
For example, one company may put a coating on just a portion of its products to function as a rust preventive. Typically, they allow this coating to air-dry overnight. If they are trying to rush out an order, however, they may fire up the oven and use it to force-dry the coating in about 20 min. This is their only use of the oven.
By contrast, another company uses multiple ovens continuously. Every product manufactured must pass through an oven to achieve and maintain a threshold temperature for a specific amount of time. In addition, the company must be able to document the oven performance during the cycle in which any particular part was processed.
The oven maintenance required at the latter company will be different than that at the former. Most oven and dryer applications fall somewhere between these two extremes.
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Choosing Your Maintenance Strategy
Preventive maintenance (PM) is a phrase familiar to everyone. One classic example is changing your car’s engine oil every 5,000 miles. This minimizes wear and tear to the engine components.
Preventive maintenance is often encouraged, but it is not always the best maintenance solution for an industrial oven or dryer. Identifying a maintenance strategy that specifically fits your requirements starts with understanding how each strategy works. Table 1 shows examples of various maintenance strategies for consideration on an oven/dryer.
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Using the previous examples, if you have an oven or dryer that is only used occasionally, perhaps the most cost-effective alternative is run-to-fail. However, if every part you produce must run through a heating cycle, you will need a better plan.
Committing to a maintenance strategy for a particular oven or dryer should be done logically and without emotion — not in the middle of an emergency breakdown with your plant manager calling you every 10 min asking if the equipment is back online. Review the requirement for your oven or dryer in terms of:
- How critical it is to the process.
- The cost to maintain it.
- The cost of downtime.
- Any requirement to collect and store process data.
- Overall safety risk.
I am of the confirmed opinion that any fuel-fired oven/dryer immediately jumps the safety risk to “high.” That is why your oven should at least have a preventive maintenance program.
Larger companies tend to use condition-based, predictive or prescriptive maintenance programs because they have a larger number of assets to maintain. There are good software packages worth considering for these types of plans.
Areas of Attention
A good PM program should focus on several different areas of an oven or dryer. Some of the inspection, testing and maintenance requirements are detailed in the National Fire Protection Association Standard for Ovens and Furnaces (NFPA 86).2
Inner Shell and Ductwork. The interior of the oven should be cleaned and regularly inspected for damage or wear. Damage can occur inside of a batch-style oven or dryer due to careless or overzealous loading of parts, pallets or baskets. Distorted ductwork can inhibit airflow. Loose interior trim can get caught while loading or unloading and cause worse damage. Exposed insulation can break loose and end up on freshly coated parts. Broken parts, coating residue and debris should be removed from the oven. They may present a fire risk.
Outer Shell and Door Seals. The outer shell should be inspected for damage or wear. Forktruck tines have been known to pierce oven panels. Discolored paint around an access door is usually a sign of a worn door seal. The exterior oven or dryer surface is going to be hot around the door, but it will be hotter when the door seal is leaking.
A non-contacting infrared pyrometer or thermal imaging camera is a handy tool to have when inspecting the outer shell. Explosion-relief mechanisms should be inspected visually to ensure they are unobstructed and labeled properly.3
Fans and Blowers. All fans and blowers should be inspected periodically. Most fans have V-belts and sheaves. The belts should be checked and properly tensioned. Replace any worn belts before they break. If any electrical work has been done on the oven, it is a good idea to verify the fan rotational direction. Also, check wire tightness at the motor, disconnect and contactor or variable-frequency drive (VFD).
One common issue with ovens and dryers used for coatings is controlling fumes or smoke from escaping, particularly with large end openings. Too little exhaust can result in fumes escaping. Too much exhaust may shorten the effective heated length and dwell time. Before changing any exhaust damper settings, make sure to mark the initial damper position. You may want to be able to return to that starting point.
Another common issue that affects ovens and dryers is insufficient makeup air entering the factory. Negative pressure within the building can exacerbate fume containment and even burner performance.
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Heat Source — Electric. Several different types of electric heaters are used in ovens and dryers. The most common is the metal-sheathed tubular heater (aka, a Cal-Rod heater). A variant of this design is the finned tubular heater. Another type of convection heater used in ovens and dryers is an open-coil duct heater. There are also strip heaters and ceramic heaters. If you have an infrared or radiant oven, you might see quartz tubes, quartz lamps or ceramic heaters.
Often in industrial ovens and dryers, several heaters will be wired into a common three-phase circuit. It can be difficult to determine when one heating element in an array has failed. The easiest way to check is to know what the amp draw of a fully functional array of heaters should be. Then, periodically put an ammeter on the circuit and check each leg of the three-phase power. You also should check the tightness of the wire connections at the heating elements and at the contactor or SCR regularly.
Heat Source — Fuel Fired Burner. Most ovens with burners are fueled by natural gas. However, some are fueled by propane, fuel oil or even landfill gas. Every burner has a valve train with approved safety devices. Minimum maintenance requirements for gas train components are detailed in NFPA 86: 2019, Section 7.4 Inspection, Testing and Maintenance.4 These include annual testing of all safety interlocks and safety shut-off valves. In addition, periodically inspect and test the spark ignition and flame sensors for proper operation and strength of the flame signal.
Heat Source — Other. Some ovens or dryers are heated by steam, hot water or heat transfer oil. The common denominator of these systems is the heating coil and pressure-relief system. The coils should be inspected and cleaned periodically. The relief valve should be tested annually, as a minimum.
Sensors and Safety Devices. We already touched on NFPA 86 requirements for inspecting, testing and maintaining safety devices. In addition to the gas-train components already mentioned, safety interlocks may include airflow or air pressure switches, rotary speed switches, high and low gas pressure switches, and high limit controllers. Ancillary components include thermocouples or RTDs, sensing tubes for flow or pressure switches, and valve and damper actuators and linkage. Testing of safety components is of paramount importance. An easy way to test a pressure switch or high limit controller is to:
- Note and record its initial setting.
- Change the setting to force a fault.
- Confirm that this fault shuts off the heat source and other interlocks as designed.
- Change the setting back to the initial setting prior to the test.
If the safety interlock does not function as designed or required, repair or replace it before you use the oven or dryer again.
Conveyor and Material-Handling Systems. Many ovens and dryers include provisions to move product into and out of the heat zone. These can be as simple as a cart in a batch oven. Material handling might mean a conveyor belt, chains, rollers, an overhead monorail or any number of other configurations. The material-handling system typically sees the most wear and tear of any major subsystem on the oven or dryer. This is especially true of belt conveyor slide beds.
Keeping the conveyor running smoothly can be a challenge, particularly in a high temperature environment. Lubrication helps, but it often just flashes off or contaminates the product being processed. Bearings can be greased, but even “high temperature grease” has a limit. Experience has shown that sealed-for-life bearings with a dry lubricant often are the best choice for bearings that have to survive in a high temperature environment. Regular inspection and predictive maintenance are required to keep ahead of conveyor issues.
PLCs and Remote Access. Troubleshooting an oven or dryer is easier with more information. In the old days, a flame-safety relay would fault and kick off the burner. However, because all of the safety interlocks were wired in one string, you could not immediately tell if there was an issue with the exhaust fan airflow, recirculation airflow, combustion air pressure, high gas pressure or low gas pressure. You had to climb up on the unit to look at a switch indicator, or you had to get out a VOM and start probing live circuits.
One benefit of having a programmable logic controller (PLC) as part of the control scheme is the abundant information available for component and sensor failure. This is particularly true with a multi-zone oven or dryer where you have several recirculation fans and burners. Typically, the fault screen on the HMI identifies the problem.
Another benefit of the PLC is the ability to save fault history and identify periodic maintenance issues. Collecting and retaining this information is the first step to a predictive maintenance program.
The third benefit of a PLC in the control panel is the ability to provide remote access to the manufacturer or a third-party service organization. Some end-users allow a service team to remotely access their ovens or dryers to assist in troubleshooting startup or performance issues. Remote access allows the oven manufacturer to identify the potential problem(s) and make recommendations for components to check or replace. Ethernet security is often an issue with companies, so various VPN devices and applications can be used to connect to an oven or dryer.
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Spare Part Availability
Industrial oven or dryer downtime is greatly reduced when a minimum number of spare parts are kept on hand. The oven or dryer manufacturer can provide a list of recommended spare parts. Typically, it includes thermocouples or RTDs, airflow switches, gas pressure switches, the flame rod or UV sensor, spark ignitors or ignition transformers.
For critical oven or dryer applications, I recommend keeping large components such as fans and burners on hand. It is heartbreaking to tell an end-user that a new burner for his non-functional 10-year-old oven is going to take six to eight weeks. Planning ahead and considering the worst-case scenarios keeps the “preventive” in preventive maintenance.
In conclusion, the oven or dryer manufacturer has the responsibility to “provide instructions for inspection, testing and maintenance.”5 But, “it shall be the responsibility of the user to establish, schedule and enforce the frequency and extent of the inspection, testing and maintenance program, as well as the corrective action to be taken.”6
A good maintenance plan starts with pride of ownership. An oven or dryer can last for 30 years if properly maintained. If you compare the annual maintenance budget for an oven/dryer vs. the number of parts produced each year, the cost of a progressive maintenance program is reasonable.
- Fiix, A Rockwell Automation Co., Short Guide to Preventative Maintenance, https://resources.fiixsoftware.com/short-guide-preventive-maintenance.
- National Fire Protection Association, Standard for Ovens and Furnaces, 86:2019.
- Ibid, Section 7.4.8.
- Ibid, Section 7.4.
- Ibid, Section 7.4.2.
- 6. Ibid, Section 7.4.3.