Process Oven Know-How
Safety is a primary concern when considering alternative uses for a particular oven design. Improper operation may result in serious consequences such as fires, explosions, bodily injury or even death. Most industrial ovens are specified and designed for unique processes and should never be used for another process or with different operational parameters without a thorough review of all factors related to the installation and application.
The NFPA 86 standard, written by the Arlington, Va.-based National Fire Protection Association, requires that all fuel-fired and/or Class A process ovens are equipped to provide adequate explosion relief (1 ft2/15 ft3 oven volume). In some cases, an oven with explosion-venting latches on the doors will meet this safety requirement. (Depending on the dimensions of the oven, the oven's explosion-relief door latches may be inadequate to meet the NFPA 86 ratio for explosion-relief area to oven volume. On those ovens, an additional explosion-relief panel is provided.)
In the event that the door opening alone does not provide adequate relief area, the balance will be provided by means of an explosion-venting panel typically located in the roof of the unit. It is important to see the manufacturer's design data sheet for details on this specific application.
For processes that involve flammable solvents, Class A ovens are rated for a maximum solvent-handling capacity. For continuous ovens, this is generally stated in gallons per hour for a given solvent at a given operating temperature. For batch ovens, which is the more severe case, it is specified in gallons per batch. Solvent-handling ratings must never be exceeded because an explosion could result.
These ratings also are corrected for both temperature and altitude because the density of air changes with temperature, and the operating pressure changes with altitude. Both of these factors impact the rate of solvent evacuation.
Atmosphere TypesA range of oven atmospheres are used in specialized thermal processing applications. As an example, a process that requires a low oxygen concentration in the oven to prevent oxidation on parts would utilize an inert atmosphere. Inert gas is injected into the chamber, creating positive pressure with the inert gas inside the oven and replacing the oxygen. Specialty gases such as argon or nitrogen often are used to limit oxygen levels to below 50 parts per million (ppm). Atmospheric ovens require specialized construction techniques.
Humidity often is used to control moisture removal rates and to speed curing of certain compounds. Mois-ture is added in two ways:
- Steam is injected directly into the oven.
- Water can be sprayed through atomizers into the oven to maintain a specified humidity level.
The relative humidity inside an oven often is determined by comparing the wet bulb temperature and the dry bulb temperature. Dry bulb temperature is obtained by reading a temperature sensor placed inside the oven. Wet bulb temperature is obtained when a moisture-laden sock is placed over the temperature sensor and the temperature is measured. Relative humidity can be measured by two temperature readings: the higher the wet bulb temperature in relation to the dry bulb, the higher the relative humidity.
Solid-state electronic humidity transducers are used for greater accuracy. These sensors provide accurate humidity readings over a wide range of temperatures and humidity.
Design Considerations for Hazardous AreasSpecial design features are required when the oven and/or controls are located in hazardous areas. Hazardous areas refers to areas where there is an explosion hazard from the possible ignition of dust, vapors or gas in the area external to the oven; it is not limited to only volatiles within the oven chamber. NFPA is the main source of specialized information for the various hazardous classifications and the required design features.
Special design considerations for hazardous areas include: spark-resistant fans, motors rated for hazardous areas, sealed interconnecting wiring and junction boxes, the source of makeup air from outside the hazardous area, and either a remote control enclosure or enclosures rated for the specific type of hazardous area.
Temperature UniformityIn the simplest terms, oven temperature uniformity is the overall temperature variation in the oven workspace. Uniformity generally is stated as +/-oF or +/-oC at a given setpoint temperature. Tight oven uniformities ensure that all parts within the oven are exposed to the same temperature for consistent product quality.
Oven characteristics that affect uniformity (stated in W/in2 for a given wall thickness) include wall losses (including through-metal); oven openings; air distribution and the volume of airflow; control accuracy and construction techniques. Here's what to look for:
- Adequate insulation thickness to minimize wall losses. Insulation thickness varies among different oven designs and depends on the maximum temperature and temperature uniformity required for the application.
- Panel and unitized construction to keep through-metal loss to an minimum.
- Fresh air and exhaust vent openings located strategically to provide a positive pressure differential so cooler ambient air is not introduced into the oven through door seals.
- Fresh air openings located to ensure that recirculated air is mixed thoroughly with the fresh air.
- Air passing through the heating elements mixed adequately with recirculated air before entering the work chamber to prevent air stratification and temperature variance.
The most important factor determining temperature uniformity is providing uniform airflow to all points in the oven.
A simple rule of thumb is that the greater the air volume through an oven, the better the uniformity. It is important to size the fan and motor combination to provide the amount of static pressure drop through the oven to meet the desired uniformity.
Uniformities at low temperatures (typically 150oF [66oC]) are the easiest to obtain due to low wall losses. Minimal amounts of airflow and a simple body construction are all that are required. As temperatures increase, wall losses increase. However, uniformities are harder to achieve. Higher airflow fans and motors, enhanced insulating characteristics and more stringent airflow distribution are required.
Sidebar: 15 Terms You Can UseUse the list below to brush up on your oven-speak.
Back-Welded Seams. A method of joining sheet metal panels by welding seams on the backside.
Conduction. The transfer of heat through a material by passing it from molecule to molecule.
Dry Bulb. Temperature of air as determined by a standard temperature sensor.
Dwell Time. Refers to the amount of time that the product spends in the oven. While the term can be used batch-type ovens, it is used most often with regard to continuous ovens where the product is conveyed into and out of the oven within a specific time interval.
Exhaust Volume. The amount of air leaving an oven system, either passively or forced.
Forced Exhaust. Process air removed from an oven by an exhaust fan.
Load Configuration. The way in which the parts to be processed are situated in the oven.
Lower Explosive Limit (LEL), Lower Flammable Limit (LFL). The point at which process air containing solvents becomes explosive/flammable.
Purge Timer. A settable device that times the replacement of air in an oven with fresh air or an inert gas.
Radiant Heat. A mode of heat transfer in which heat is transferred in a straight line from an emitting surface, without significantly heating the Intervening space.
Setpoint Temperature. An operator-controlled variable. The desired temperature level of the oven is selected as a numeric value on the control instrument.
Soak. The amount of time that a part or component spends at a given processing temperature.
Temperature Stratification. Variation in air temperature across a cross-section of air due to uneven mixing of the air.
Thermocouple. A temperature sensor made of two dissimilar metals welded at the measuring junction. A millivolt signal proportional to the temperature difference between hot and cold junctions is produced.