Preventive Maintenance Tips for Your Inline Conveyor Ovens

Part II: The Insulated Box

By Daniel Pierre III, JN Machinery Corp.

Fundamentally, an inline conveyor oven is nothing more than four sections: a heating system, an insulated box, a conveyor mechanism, and a control panel. The heating system was the subject of Part I in the July issue. This time, the focus is on the insulated box, sometimes called the “shell.”

White gasket material surrounding the heat chamber in the lower shell.

Insulation packed around a fan shaft.

Grind out a narrow spot to give the conveyor more room.

The “hot zone” at the entry and exit ends of an oven.

Examples of ceramic-fiber curtains (left) and stainless steel mesh curtains (right).

An oven with casters and independently adjustable legs.

The shell’s main function is to isolate the heat radiating from the heating system and protect the users who must stand near or next to the oven. Its secondary function is to allow the conveyor mechanism some breathing room to expand and contract as it heats up and cools down.

Aging ovens may be beat up with scratches and dents along the shell. As long as a dent doesn’t restrict the motion of a moving part, it shouldn’t detract from the insulated box’s function.

The areas where the heating system and conveyor mechanism interact with the shell are really opportunities for heat to escape from places you don’t want it to. Knowing where these areas are will assist you in evaluating, upgrading and maintaining a functional conveyor oven. Let’s discuss each part of the insulated box.

The Oven Shell: Solid Body vs. Clamshell

From a cost and ease of maintenance point of view, a solid-body oven is by far the least expensive choice. However, it is a choice that’s made at purchase and something you can’t change in the field.

In terms of configuration, a clamshell does give you direct access to the conveyor mechanism and heating system. However, a well-designed solid-body oven will have a belt-reverse switch to free jam-ups, as well as access panels at the top of the oven to get to fan blades and thermocouples. Therefore, direct access to the interior is not really necessary. Also, as mentioned in Part I, using a standoff to exchange heating elements means you don’t have to open up the oven to guide the elements with your hands through the chamber.

If you have a clamshell oven, take care when opening and closing the upper shell, as the hinges and latches are often welded on and they are difficult or impossible to repair or replace out in the field. A bent or broken latch might not hold down the upper shell tight enough. A loose latch can create a gap for heat to escape in the area between the upper and lower shells. To help prevent any potential gaps, good ovens will have a heat-resistant and compressive gasket surrounding the area where the upper shell rests on the lower shell. Be aware that the gasket does degrade over time. This is due to its proximity to the heat chamber, as well as abrasion from hands and tools when tinkering with the interior of the oven. Replacing missing or damaged gaskets will greatly improve the efficiency of the oven.

Insulation

The function of insulation in a conveyor oven is not any different than that of the insulation in your home. In a home, you rarely need to worry about insulation in the walls, as you almost never open up your walls to expose the insulation. However, there is often exposed insulation in an attic. Crawling around an unfinished attic causes wear and tear on the insulation. In the walls of a conveyor oven, the insulation mostly sits there doing its job. A good oven maker will install enough insulation appropriate for the heat output of each oven. The top of the shell (above the heating system) is where the brunt of insulation abuse takes place, so no matter how well the original insulation is installed, it too will deteriorate over time.

The first area that needs periodic checking is where the insulation surrounds the fan shaft. The fan shaft rotates at a high speed and basically smooths out and slightly burns the insulation that rests beside it. The bottom of the fan sits directly in the heat chamber, so heat will travel up the shaft, too. If too much insulation degrades away from the shaft, heat from the chamber will escape up the pathway. Besides creating an inefficient heat system, escaping heat can travel all the way to the fan motor and burn it out. (Indirectly driven fan blades protect the fan motor from such an incident).

There are times when fan blades need replacing (see Part I), and to get to the fan blade, several layers of insulation must first be removed. Care must be taken to properly replace the insulation. It should fit tightly in all places, including around the fan shaft. Do not compress the insulation, or you will reduce its efficiency as well as create more pockets of air that can create new heat-escape pathways.

Four Oven Inspection Points When maintaining or upgrading an old oven, be sure to monitor or look for the following on the shell: 1. Worn out or degraded insulation, especially around the fan shafts. 2. Curtains at the entry and exit ends. 3. The space between the shell and the conveyor mechanism. 4. Gaskets where the upper shell rests on the lower shell, if you have a clamshell oven.

Similar care should be taken when replacing thermocouples. However, the opening in the heat chamber for a thermocouple is quite small to begin with. You basically just need to pull out an old thermocouple carefully and push a new one in the same hole, making sure you don’t poke insulation down into the heat chamber.

An oven with an unknown or questionable history should be checked for signs of (a previous) fire. Severe burn marks on insulation in areas close to the heat chamber are a clue that the oven had a fire at one time. Fresh insulation in an old, well-used oven is also suspect; requiring further investigation. Ovens that had a fire can be perfectly restored and continue to have long lives, but sometimes only external damages are fixed. If the insulation shows signs of a fire, check the top of the heat chamber (inside the oven) for black residue. If it is clean, chances are high that the oven has been properly restored.

Keep the Shell and Conveyor Mechanism Apart

Both solid-body and clamshell ovens need to have a small clearance in the “hole” where the conveyor mechanism will lay. It is a bit of an art to know exactly how much clearance is enough. If there is no clearance, when the conveyor mechanism heats, it will expand, push against the body of the oven and perhaps buckle the shell. If there is too much clearance, the conveyor mechanism can expand, but the extra space just lets heat out.

In no case should a conveyor mechanism be permanently attached to the shell. Even an accidental hit from the side by a forklift could press the shell in too close to the conveyor mechanism. Don’t let the fact that the belt still moves make you think all is well with the conveyor. If necessary, grind out the edges to gain back the breathing room for heat expansion.

Curtains at the Oven Entry and Exit

No matter how well an oven is insulated, heat will escape from its entry and exit ends. The ends are the “hot zone” where the oven bodies will be the warmest. The hot zone is created mostly by heat escaping out the ends of the oven and up the outside of the shell. So, as the insulation inside the shell does its best to contain the heat, a completely open entry and exit will offset a lot of the insulation’s function.

Curtains over the entry and exit area are absolutely essential. The better the curtains are the “cooler” the hot zone will be.

Curtains need to form a solid enough surface to hold back and reflect the heat back into the chamber, yet they have to be light and flexible enough to let parts through them. Two materials most often used for curtains are reflective ceramic woven-fiber material, and stainless steel mesh. Sometimes you also see solid stainless plates.

You can lose up to 40 percent of the heat in the chamber if the curtains are missing or completely worn away. The loss of heat makes the elements work harder, increasing the energy costs and reducing the life of the heating elements. In addition, the lack of curtains turns the heat chamber into a heat “tunnel,” which can create an uneven distribution of heat. Next to burned-out elements, missing curtains often turn up as the culprit when parts no longer get heat-treated properly.

The cost to replace curtains is well worth the increase in oven efficiency. As an example, simply replacing a ceramic-fiber curtain with a stainless steel curtain on a JN-15 oven resulted in immediate reduction of energy loss. In 40 hours of production, the curtains paid for themselves, and thereafter the customer enjoyed cost savings. This example is not intended to suggest stainless curtains are always better than ceramic-fiber curtains. Light springs may not have enough mass to push their way through a heavier curtain. Please use the curtain best suited for your application. Any of the major oven makers can assist you in selecting the appropriate curtain.

Supporting the Shell

Although not physically part of the insulated shell, most conveyor ovens have legs attached to the shell. There are not many maintenance issues for legs, with the exception of casters. Not all conveyor ovens have casters, but for those that do, it is best to replace or fix a caster if the wheels get jammed. Usually the problem is a partial spring or scrap piece of metal getting embedded in the wheel material. Without freely rotating wheels, there is a chance you could push over an oven when trying to move it around. Other than this, here is a short list of safety tips to keep in mind:

• Ovens with adjustable legs should only be moved when the legs are in the lowest position. An oven with fully extended legs can become top heavy, creating the potential of tipping over if not handled carefully.

• Ovens with independently adjustable legs allow you to finely adjust the entry and exit height. Often, springmakers first determine the perfect belt height to accommodate springs going into the oven, only to find the exit is a half inch too low to fit their container underneath. You can adjust the exit to be slightly higher than the entry.

• To a lesser extent, independently adjustable legs were designed this way to facilitate leveling if the floor area under the oven could not be cleared completely or leveled.

• The difference between the entry height and exit height should be no more than one inch. Excessive height differential lets the heat chamber act like a chimney, with the majority of the heat flowing up and out, rather than down onto the parts to be heat-treated.

• For that matter, it is unwise to directly prop up one end with boards or other material. Not only would this alter the heat-treatment efficiency, but also the oven could roll off the boards and injure someone.

In conclusion, as far as the insulated box is concerned, it is the least complicated of the four sections of a conveyor oven, and perhaps the most durable.

The next installment in this series of articles will discuss the conveyor mechanisms, including belt styles and options for upgrading or replacing an old conveyor system that is just plain worn out.