Routine maintenance is like routine exercise; we know that it is good for us in the long term, but it is easy to find excuses to skip doing it. A convection-style industrial oven or dryer is the type of equipment that can easily achieve a useful life of 20 or 30 years. But regular and ongoing maintenance is critically important in keeping that oven or dryer safe and in good operational order. As a minimum, a Routine Maintenance Program (as opposed to a Reactive or Run-To-Fail Program) is the preferred way to keep that equipment going). And the keys to a good Routine Maintenance Program are inspections and checklists.

Performing inspections of heat process equipment on a regular basis typically gives warning of problems before they result in costly downtime. In addition, appropriate equipment overhaul and replacement of worn-out components ultimately saves the time, money and the anguish associated with emergency shutdown and repair.

Inspection frequency and thoroughness can vary with the function, complexity and criticality of the equipment. An oven or dryer that is used every day on every part produced by a facility needs more tender loving care than one that is only used occasionally on a small portion of the products.

INSULATED ENCLOSURE

The insulated chamber is a part of the oven or dryer that often gets ignored. This is a static structure that doesn’t do anything other than keep the hot air in and the cold air out. But over time and a few thousand heating/cooling cycles, the insulation can start to deteriorate. On an older oven or dryer, you can see sometimes see hot spots on the exterior of the enclosure when scanned with a thermal imaging camera. The other tell-tale warning to look for on the oven exterior is discoloration. Discoloration is a sign that heat, or sometimes fumes, are escaping from the oven. These discoloration marks are often over the product openings (a sign of oven imbalance) or around the access doors (a sign of failed gaskets or seals). Ovens using panel construction (as opposed to shell construction) can sometimes show discoloration marks emerging from the roof trim. This is could be due to insufficient exhaust, oven imbalance or sign of problem with recirculated airflow within the oven.

MOVING PARTS

A second area of focus for inspection and maintenance are the moving parts on an oven or dryer. V-belts wear over time and become loose or break. And there are typically not many moving parts that need inspection:

• Exhaust Fan
• Recirculation Fan(s)
• Combustion Blowers(s), if applicable
• Product Doors, if applicable
• Conveyor System, if applicable

FILTERS

Not all ovens or dryers have filters, but some do. For higher quality painting operations, inlet filters, combustion filters and even recirculation filters have to be properly installed and maintained. Fully loaded filters have higher pressure drops and can directly affect oven operation and product quality.

BURNERS AND HEATERS

Ovens and dryers need a heat source. The two most commonly used heating sources are gas-fired burners and electric heating elements. Electric heaters are typically controlled by contactors, solid state relays or silicon controlled rectifiers. The temperature controller reads a thermocouple of RTD and sends a control signal to the heater controller calling for more or less heat.

Gas burners are more complicated. There is stall a thermocouple, temperature controller and burner controller (gas valve). However, due to the inherently potential danger associated with natural gas or propane, there are also several other associated safety devices:

• High gas pressure switch
• Low gas pressure switch
• Combustion airflow switch
• Flame detection (flame rod or UV scanner)
• Flame safety relay

All these devices need to be in working order to light the burner and maintain the flame.

SAFETY DEVICES

In addition to the gas train safety components described above, there are also other devices that are required by NFPA 86 Standard for Ovens and Furnaces. 2 For Class A ovens, these include devices include, but are not limited to:

• Exhaust fan airflow switches
• Recirculation airflow switches
• High limit (over-temperature) controllers
• Purge timers

NFPA 86 Standard for Ovens and Furnaces provides examples of operational and maintenance checklists for ovens and dryers.3 Inspections are typically separated into specific time periods:

• Daily
• Weekly or Monthly
• Annual

DAILY INSPECTIONS

Every day or every shift, the operators should a quick check of the equipment to ensure that there isn’t anything obviously wrong. Operators should be encouraged to listen for odd sounds or vibrations that develop. If they hear something, they should say

something. The following operational checks should ideally be performed at the start of every shift:

• Verify the set point and process (temperature) reading of control instruments
• Verify positions of manual gas valves, manual dampers, etc.
• Ensure an appropriate purge time is being observed prior to turning on the heating system
• Listen to blowers and fans for unusual noises indicating worn/misaligned bearings, sheaves and/or v-belts
• Verify conveyor speed or oven dwell time

WEEKLY OR MONTHLY INSPECTIONS

Beyond the daily inspections, regular temperature checks are recommended confirm that the oven or dryer is performing as expected. Running a temperature data logger through on the conveyor is a great way to quantify oven performance. Static temperature tests within a batch system are also suggested. In fact, AMS 2750F is a pyrometry specification used extensively through the aerospace industry. It not only tells you how to runs a temperature profile, it also specifies how often you have to change thermocouples based on the operating temperature and required uniformity.4

ANNUAL INSPECTIONS

An annual safety inspection is the mandated by code. NFPA 86 Section 7.4.3 clearly states, “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”. Section 7.4.4 states, “All safety interlocks shall be tested for function at least annually”. And these tests and safety inspections need to be properly documented.

An annual inspection is an opportune time to check non-safety as well as safety related components and devices. Again, NFPA 86 provides a thorough checklist:5 

• Inspect flame rod(s) or UV sensors for condition and cleanliness
• Inspect thermocouples and lead wire for shorts and loose connections
• Check setting and operation of low and high temperature limit devices
• Test visual/audible alarm systems for proper operation
• Check spark igniters and verify proper gap
• Check and test airflow, pressure and/or zero speed switches for proper pressure settings
• Check control valves, dampers, and actuators for free, smooth action
• Test the interlock sequence of all safety equipment. Interlocks should be made to fail manually, verifying that the related equipment operates as specified
• Test the safety shutoff valves for operation and tightness of closure
• Test the main fuel manual valves for operation and tightness of closure
• Visually inspect electrical switches, contacts or controls for signs of arcing or contamination
• Test instruments for proper response to thermocouple failure (up-scale burn-out)
• Clean or replace the air filters
• Inspect burners and pilots for proper operation, air/fuel ratio, plugging, or deterioration
• Check the spark ignition wires and ignition transformers
• Check the operation of modulating control valves
• Examine the interior of the equipment, ductwork, and ventilation systems for cleanliness, damage and flow restrictions
• Lubricate motors, valves, fans, blowers and other components
• Test and recalibrate control instrumentation
• Test flame safety relays. Make a complete burner shutdown and restart to check components for proper operation
• Check electric heating elements for contamination, distortion, cracked or broken refractory element supports, and proper position
• Check motor and electric heating element terminals for tightness

MAKE A LIST

Developing useful check checklists will streamline equipment inspections. (See Table 1 for a sample). Staying current on inspections will allow timely proactive maintenance and component replacement. And regular maintenance will result in years and decades of useful service of your oven or dryer. CLICK HERE FOR SAMPLE LIST

Citations

1. “Selecting a Maintenance Program for Your Oven or Dryer”, Steven Onsager, Process Heating Magazine, 28SEP2021

2. NFPA 86 Standard for Ovens and Furnaces, Chapter 7, 2019

3. NFPA 86 Standard for Ovens and Furnaces Annex C, 2019

4. “AMS 2750F: Latest Revision for Pyrometry Requirements”, Andrew Bassett, Industrial Heating Magazine, 09MAR2021

5. NFPA 86 Standard for Ovens and Furnaces Annex C, 2019

*Cover image courtesy Freepik.com