Metal Finishing Guide Book

2013

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assembled panels are tongue-and-groove design for ease of installation. The outer skins are connected with formed metal channels. These channels form a throughmetal condition, allowing a significant loss of heat at the joint. This panel joint can become too hot. To solve this problem, the channel is slotted, greatly reducing the area available for the migration of heat. This technique can reduce the joint temperature to less than 100°F in a 450°F oven, without losing the structural integrity of the channel. Personnel access must be provided into the enclosure. The door and hardware must seal the opening without the use of a positive latching device for safety reasons. (Any panic hardware with positive latching features must allow the door to be opened from the inside.) A good rule of thumb is to locate access doors so that when someone is working in an oven, once he reaches a wall, an exit is never more than 25 feet away. Windows in oven doors are a good way to make them easy to locate. A great source of oven problems are the enclosure openings. These are required for the product to enter and exit the enclosure. These holes are designed using a minimal clearance for the ware. Bottom entry/exit designs make use of the natural sealing features of hot air and present no real problems. Openings in the sides of ovens require mechanical air seals to contain the environment. To seal an opening, it is best to draw hot air from the oven and force it back into the opening. For this to work, a significant velocity must be developed at the center of the opening. Additionally, the oven must be run on negative relative to the production environment. These two requirements draw factory air into the oven. This pressurization must be relieved by exhausting the enclosure, a considerable source of heat loss. An alternative to traditional construction methods is the oven module, but it is rarely practical due to its configuration. HEATER SYSTEM The second system at work in an oven is the heater unit, which generates the energy necessary for curing the coating and begins the distribution of the energy. The most significant components of the heater are the burner, the supply fan, and the filters. To properly size heater equipment, a detailed heat load must be carefully calculated. Energy losses for the ware load, conveyor load, enclosure, and exhaust must be considered. These losses, expressed in BTUs per hour, are used for selection of the burner and corresponding electrical devices necessary for burner control. The burner, most often a direct-flame device, provides the energy for the cure. The heat-load calculation also provides information for the selection of the oven supply fan. The heat required to maintain a good oven temperature is delivered by heating the supply air to no more than 100°F above the oven operating temperature and distributing this air to the oven proper. The fan volume must be expanded for the elevated temperatures. The supply fan should turn over the oven volume approximately two times every minute. Because the fan is a constant-volume device, the fan motor is sized for cold starts to avoid overloading. These rules will provide an oven temperature profile +10°F throughout the enclosure. Another feature of many heater units is filtration to continuously clean the oven environment. The efficiency of the filters varies with the application, but the most effective are the types used to final filter make-up air, modified for the elevated temperatures. Filters require velocities, which are much lower than in normal heater units. Including these means increasing the size of the heater unit to accommodate this requirement. Oven filters continuously clean the air and, as a result, load very slowly. It is not necessary to prefilter high-efficiency filters. Sometimes, the products of combustion are not compatible with the coating. In these cases, indirectly fired heater units are an option. These use air-to-air heat exchangers and are applied at the cost of the loss of efficiency. In practical applica790

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