By Steven Onsager, P.E.

Years ago, I worked with a guy whose previous job was selling heating elements for Chromalox. 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 typically has a long life. I have often lamented, “If our ovens only lasted ten years, we would sell three times as many”. In and of itself, an Oven does not have many moving parts.

A typical gas-fired convection Oven only has an exhaust fan, a recirculation fan, and maybe a combustion blower for the burner. Larger Oven might have multiple fans. There also might be doors or dampers, sometimes automatic and sometimes manual. There is also an inner shell, an outer shell, insulation, ductwork, maybe some supporting structure and (of course) controls. A conveyorized oven obviously has a conveyor. (More on that subject later).

Some Ovens even have fewer moving parts. An electrically heated Oven has heating elements in lieu of a gas burner, so there is no combustion blower. An infrared or radiant Oven has even fewer fans.

Getting decades of (relatively) trouble-free use out of an Oven is common. All it takes is luck and/or 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, how often you use it, and how critical its functionality is to your production.

For example, I know a company that puts a coating on just a portion of their products to function as a rust preventative. This material will typically air dry overnight, however, if they are trying to rush out an order, they fire up the oven and use it to force-dry their coating in about 20 minutes. This is their only use of the oven.

I also know of companies who use multiple ovens continuously. Every product they manufacture must go through an oven to achieve and maintain a threshold temperature for a specific amount of time. In addition, they must be able to document oven performance during the cycle in which any particular part was processed. Most Ovens will fall somewhere between these two extremes.

CHOOSING THE BEST 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. Identifying a maintenance strategy that specifically fits your requirements starts with understanding how each strategy works. Below are examples of various maintenance strategies for consideration on an Oven:

Using the previous example, if you have an Oven that is only occasionally used, perhaps the most cost effective alternative is Run-to-Fail. However, if every part you produce has to run through a heating cycle, you will need a better plan.

Committing to a maintenance strategy for a particular Oven should be done logically and without emotion, not in the middle of an emergency breakdown with your Plant Manager calling you every 10 minutes asking if the equipment is back online. Review the requirement for your Oven in terms of how critical it is to your process, the cost to maintain it, the cost of downtime, any requirement to collect and/or store process data, and overall safety risk.

I am of the confirmed opinion that any fuel-fired Oven immediately jumps the safety risk to “high”. That is why your oven should at least have a Preventative Maintenance Program. Larger companies tend to use the Condition-Based, Predictive or Prescriptive Maintenance Programs as they have a larger number of “assets” to maintain. There are some 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. 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 and/or wear. Damage can occur inside of a Batch-style Oven due to careless or over-zealous 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 as it may present a fire risk.

Outer Shell and Door Seals The outer shell should be inspected for damage and/or wear. Fork truck 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 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 visually inspected to ensure they are unobstructed and properly labeled.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 fan rotational direction. Also check wire tightness at the motor, disconnect and contactor or VFD. One common issue with Ovens/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 before making an adjustment. You may want to be able to return to that starting point. Another common issue that affects Ovens/Dryers is insufficient make-up air entering the factory. Negative pressure within the building can exacerbate fume containment and even burner performance.

Heat Source – Electric There are several different types of electric heaters used in Ovens/Dryers. The most common is probably the metal sheath tubular heater (aka Cal-Rod). A variant of that is the finned tubular heater. Another type of convection heater is an open coil duct heater. There are also strip heaters and ceramic heaters. If you have an IR or radiant oven, you might see quartz tube, quartz lamp or ceramic heaters. Often there will be several heaters wired into a common three-phase circuit. It is often hard to tell when one heating element in that 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 should also regularly check the tightness of the wire connections at the heating elements and at the contactor or SCR.

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 Maintenance⁴. 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 flame signal.

Heat Source – Other Some Ovens 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 periodically inspected and cleaned. The relief valve should be tested annually as a minimum.

Sensors and Safety Devices We already touched on NFPA 86 requirements for inspecting, testing 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, valve and damper actuators and linkage, etc. 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 initial setting prior to the test

If the safety interlock does not function as designed or required, repair or replace before you use the Oven again.

Conveyor and Material Handling Systems Many Ovens/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, or chains, or rollers, or an overhead monorail or any number of other configurations. The material handling system typically sees the most wear and tear of any major sub-system on the Oven. 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 if often just flashes off and/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 is often the best choice for bearings that have to survive in a high temperature environment. Regular inspection and predictive maintenance are often required to keep ahead of conveyor issues.

PLCs AND REMOTE ACCESS

Trouble-shooting an Oven is easier with more information. In the old days, a flame safety relay would still fault and kick off the burner. However, because all the safety interlocks would 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 Oven 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 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.

A 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. We have customers who allow our Service Team remote access to Ovens/Dryers to assist them in trouble-shooting start-up and/or performance issues. Remote access is extremely helpful when we get a call from a small to mid-size company with little or no maintenance staff and the complaint, “The oven won’t start.” Remote access allows us to identify the potential problem(s) and make recommendations for components to check or replace. Ethernet security is often an issue with companies, so we make use of various VPN devices and applications to connect to an Oven.

SPARE PART AVAILABILITY

Oven downtime is greatly reduced with a minimum quantity of spare parts on hand. The Oven manufacturer can provide you with a list of recommend spare parts. These usually include thermocouples or RTDs, airflow switches, gas pressure switches, flame rod or UV sensor, spark ignitors, ignition transformer, etc.

Calls handled by our Parts Department are evenly split between companies planning for preventative maintenance and companies with an oven that is down and they need a component sent for next day delivery.

For critical Oven applications, we even recommend keeping large components on hand such as fans and burners. It is heartbreaking to tell a customer that a new burner for his non-functional ten-year-old oven is going to take six to eight weeks. Planning ahead and considering the worst-case scenarios keeps the “preventative” in PM.

MAKE A PLAN AND STICK TO IT

The Oven 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 can last for 30 years if properly maintained. If you compare the annual maintenance budget for an Oven versus the quantity of parts produced each year, the cost of a progressive maintenance program is reasonable.

Citations

• 1.Fiix, A Rockwell Automation Company, Short Guide to Preventative Maintenance https://resources.fiixsoftware.com/short-guide-preventive-maintenance
• 2.National Fire Protection Association, Standard for Ovens and Furnaces, 86:2019.
• 3.National Fire Protection Association, Standard for Ovens and Furnaces, 86:2019, Section 7.4.8
• 4.National Fire Protection Association, Standard for Ovens and Furnaces, 86:2019, Section 7.4
• 5.National Fire Protection Association, Standard for Ovens and Furnaces, 86:2019, Section 7.4.2
• 6.National Fire Protection Association, Standard for Ovens and Furnaces, 86:2019, Section 7.4.3