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Choosing an industrial oven does not have to be difficult if you know what questions to ask about the equipment and your process. Oven manufacturers understand that their businesses are not just about selling ovens -- their job is to help match the correct oven design with a customer's application. Paying careful attention to your product and process is only part of the job. These frequently asked questions can help ease the process of choosing an industrial oven by providing a list of questions to help focus the search.
Q: What are my Temperature Requirements?
Not all airflow patterns are created equal. Different airflow patterns are needed for different part geometry and processes. Choices include vertical airflow, full horizontal airflow, and horizontal/vertical (combined).
- Vertical Airflow - is best suited for processes where parts are hung from racks or hooks, with the air supplied from the top down or bottom up.
- Full Horizontal Airflow - is used when product is loaded onto shelves or a shelf cart for processing. With the supply on one side and the return duct on the other, the product becomes encircled with air.
- Horizontal/Vertical Airflow - is suitable for applications with large parts, and where there is a need for air circulation to be supplied from both sides and then returned to the top of the oven.
Q: How Essential Is Temperature Uniformity?
Oven temperature uniformity is important to ensure uniform heating of the product. This is different from control sensitivity.
- Uniformity – is the greatest deviation, in degrees, between the lowest and highest temperature points within the work area. For example, it is important to note that ±5°F represents an actual difference of 10°F (5.5°C).
- Control Sensitivity – relates to the ability of a control instrument to measure and react to temperature fluctuations at a given set point.
Many factors influence uniformity:
- Controller calibration
- Sensor calibration
- Sensor placement within the work area
- Oven operating temperature (higher temperature/greater variables)
- Air circulation (the greater the air circulation, the better the uniformity)
- Placement of the product within the work area
- Product geometry
- Airflow pattern
- Heat loss through walls
- Metal-to-metal conduction
It is important to take all of these variables into consideration and to utilize a 10-point thermocouple test to ensure that the specified uniformity is obtained for the application. Although this 10-point test is not always included with the oven purchase, it is recommended and usually available for an additional cost. Some applications such as paint or resin curing require a temperature uniformity of ±10°F (±5.5°C). Many drying or preheating processes only require a uniformity of ±20°F (±11°C). Some technical processes need a uniformity of ±5°F (±2.7°C). Knowing your temperature uniformity requirements will help with oven selection.
Q: How Can the Proper Oven Chamber Size Be Determined?
Some factors to consider when determining the proper oven chamber size are:
- The maximum dimensions of your products
- The required spacing between multiple parts, and between the parts and any ductwork (Usually this dimension is 3” to 6")
- The quantity of units to be processed in a single batch, and the space they will require after considering the required clearances
- The method of material handling, being sure you have enough room to move the product in and out of the oven safely
After these factors have been determined, you will have arrived at an approximate work area dimension. If the work area is too small, there may be an inadequate amount of space between parts, which will result in less than optimal oven performance due to poor airflow. Similarly, when the work area is too large, there is an excess of space to heat and circulate air through, which wastes energy, space and, most importantly, time.
Q: What Role Do Volatiles Play?
The National Fire Protection Association (NFPA) has stipulated multiple classes of ovens that determine what can be processed within them. Class A can be used with volatiles while Class B cannot be used with volatiles. Therefore, the heating method determines the controls and instrumentation that must be implemented on them.
Class A electrically heated ovens
Class A gas-fired ovens
- Process volatiles
- Required controls include airflow switches, manual reset, excess temperature control, backup contractors, powered exhaust, and a purge timer
Class B electrically heated:
- Process volatiles
- Required controls include airflow switches, manual reset, excess temperature controls, powered exhaust (sized per NFPA requirements), designed relief area, high/low gas pressure switches, purge timer, flame-sensing unit and controlled spark ignition
- Does not process volatiles
- Required controls include airflow switch, manual reset, excess temperature control, and backup contractors
It is important to note that the rating of Class A is determined by examining the volatile gallons per hour at a given operating temperature. Volatile ratings are never to be exceeded. Physical injury or death may occur if the volatile ratings are not strictly followed. To properly size the rate of exhaust, you must know the amount and type of volatiles you will be processing. If you have any concerns as to whether your process utilizes volatiles, please consult your oven manufacturer.
Q: Should the Unit Be Gas, Electric or Steam?
If you have equal access to gas, electricity or steam as potential power sources, knowing which to use may not be obvious.
Electrically heated ovens:
- Depending on your energy rates, gas-fired units may be more cost effective to operate than electric heated ovens. However, direct gas-fired units cannot be used in some processes because the products of combustion will affect the product
- An indirect gas-fired oven can be used as an alternative. Keep in mind, though, that the initial cost of this option is much higher, and it often is not available in small ovens or high temperature units
- Electrically heated ovens are clean, non-polluting, and can be used in applications where direct gas-fired units are not suitable
- Large loads or costly electrical power may make this option a poor choice
- Steam-heated units provide an efficient means of power when operating in lower temperature ranges.
- Steam heated ovens are often advantageous when a facility already has a boiler in place and has extra steam available to use
Q: How Do the Materials of Construction Impact Productivity?
Together with the correct materials, proper oven construction will help prevent heat loss, increase efficiency and facilitate cleaning and service. Two materials typically are used for oven interiors: aluminized steel and stainless steel.
- Aluminized steel resists corrosion from simple moisture, heat and other sources.
- Stainless steel is recommended when the work area will be exposed to corrosive materials or must be cleaned with caustic solutions.
For exterior surfaces, three materials are used:
- An oven manufacturer's standard finish – a cold-rolled steel surface is primed and painted to provide durability and service.
- Aluminized steel – which resists oxidation that is caused by moisture, is offered as an option on most units.
- Stainless steel – resists oxidization and withstands corrosion from chemical exposure and is an option on most units.
Related to the materials of construction is the decision to insulate the oven's floor. In walk-in designs, insulated oven floors provide numerous benefits at operating temperature.
- Offer increased operating efficiency
- Lower operating costs
- Improved temperature uniformity within the work area as compared to units without floors
- Not made to withstand the stress placed on it by heat. Even units that operate below 250°F (121°C) will cause damage. Over time, this will result in the floor cracking or "powdering," which will result in product contamination and ruin the floor.
In conclusion, matching a customer's needs with the proper piece of equipment is just one of the many important considerations involved in choosing the proper oven for your company's application. Work with a reputable oven supplier to find a system that addresses all of your plant's specific considerations effectively.