9 Oven Questions, 9 Oven Answers
This how-to guide will walk you through each consideration and have you on the way to finding the most suitable oven for your needs.
When seeking the best in thermal processing equipment for processing and laboratory applications, there are several factors to consider, including the application itself, the type of oven, the size of the chamber, and temperature requirements. This how-to guide will walk you through each consideration and have you on the way to finding the most suitable oven for your needs.
1. Industrial Heating Process?
The range of applications for which ovens are used is virtually limitless, from the common to the out-of-the-ordinary. It includes:
- Finish baking.
- Laboratory testing.
- Heat treating.
However, no matter the application, most likely there exists a high tolerance, high performance oven that can satisfy the volume and complexity demands it presents. If a standard solution is not quite right, many oven manufacturers offer -- and some even specialize in developing -- custom ovens and process solutions that can speed production and improve product quality.
2. Batch or Continuous?
For applications where the load size or production volumes vary substantially, batch processing is good approach. Batch ovens also are suited for situations that require a high degree of flexibility in terms of process variables such as temperature or dwell (soak) time.
When a large quantity of similar product pieces is processed, continuous operation may be the optimal approach. Continuous ovens help ensure consistent thermal process times for each part in high-volume applications such as manufacturing electronic components or automotive parts. Continuous ovens also may allow several discrete processes to be combined, reducing material handling and increasing throughput.
3. Industrial Oven Chamber Size?
Chamber size depends on the size of the product or parts, the number of products in each batch, and the number of batches required per day to meet production requirements. If the interior space is too small, insufficient space between parts results in poor performance. If it is too large, space, time and energy are wasted.
A few general guidelines can help as a starting point. When using gravity or forced circulating airflow, design the oven to allow 2 to 3" around each part and between the parts and the oven walls. When using forced recirculating airflow, parts still benefit from spacing. They can be loaded more densely vertically, however, because airflow is distributed along the entire sidewall. Parts should still be kept 2 to 3" from the oven walls.
4. Industrial Oven Size?
Because of their convenient small size, laboratory or benchtop ovens are used for process testing and development applications as well as production uses that require small batch loads.
Cabinet. This size is referred to as a reach-in oven because they are ergonomically designed for easy loading and unloading. Cabinet ovens are floor mounted, efficient in terms of footprint, and range in size from 4 to 96 ft3.
Walk-In and Truck-In. Large batch ovens, ranging in size up to thousands of cubic feet, can accommodate a range of product and process needs. They are suitable for loading by fork truck or manually.
5. Temperature Requirements?
When considering an application’s temperature requirements, first note the minimum and maximum operating temperatures required. Other temperature considerations include:
- The required dwell time at temperature.
- The overall cycle time needed.
- The type and amount of product load. The oven design must have sufficient heating capacity to bring the product to the desired temperature within the specified time.
- Whether the heatup rate must be controlled, or if the product can be allowed to reach temperature as quickly as possible.
- Any specific cool-down requirements.
6. Temperature Uniformity Requirements?
Uniformity is critical to consistent heat processing results. Typically, it is expressed as the maximum difference between the highest and lowest temperatures in a chamber at a specified setting. Factors that influence temperature uniformity include:
- Cold air stratification (cold air entering the chamber).
- Controller accuracy and response speed.
- Heat loss through oven walls.
- Workload placement in the oven.
- Ability to direct the air through the chamber.
7. Airflow Type?
Several airflow types can be employed in process ovens. Your application can help determine which method is best.
Gravity-Convected Heat. This is often the simplest and most economical approach. Heated air rises, then returns to the heat source as it cools. This is suitable for powders and other products that may be disturbed by forced air. It can be used when chamber temperature uniformity and time-to-temperature specifications are not critical.
Forced Circulating. This method incorporates a fan to create a vertical or horizontal airflow pattern. It is best for products that air may pass vertically through or around. While forced circulating airflows significantly speed time-to-temperature and heat transfer to parts, it requires proper spacing of the parts to ensure optimal airflow.
Forced Recirculating. Suited for applications involving tray- or shelf-loaded products that require precise temperature uniformity, forced recirculating airflow ovens incorporate a fan that is strategically positioned so air moves across the heater and into a duct on one side, and returns through a duct in the opposite wall. This method creates a true horizontal/vertical airflow that ensures fast, consistent heat transfer and provides precise and consistent process results, even when product is densely loaded.
8. Design and Construction Quality?
A properly constructed oven will improve temperature uniformity and performance, reduce heat loss and energy expenses, and simplify cleaning and service. For example, consider the oven interior. A stainless steel interior provides corrosion resistance and cleans easily. Aluminized and mild steel are less expensive but offer less protection against corrosion, rust and contamination. Shelves should have a sturdy, non-tip design, allow for proper airflow, and simplify product loading and unloading.
The oven itself as well as its control panel should have a UL listing, which indicates that they have undergone extensive tests to verify reliability.
9. Special Processing Needs?
Ovens for special needs do exist. Among the available options are Class A ovens for flammable materials, inert atmosphere ovens and clean process ovens.
Class A Ovens. The National Fire Protection Association (NFPA) requires specially designed “Class A” ovens for processing products involving flammable solvents, volatiles or combustible materials. Class A ovens have:
- Forced exhaust, to keep flammable vapor concentrations well below the lower flammable limit (LFL).
- A purge timer that operates in conjunction with forced exhaust, to purge volatiles before heaters are energized.
- An airflow switch, required to prove exhaust airflow.
- An explosion-relief panel, designed to relieve and vent pressure through an explosion-relief area or plug.
Inert Atmosphere. These ovens provide nitrogen or argon gas, which some processes require to prevent product oxidation at elevated temperatures. Inert gas is injected into the chamber, pressurizing the oven and replacing the oxygen. The oven chamber typically has high-integrity welds and special motor seals to maintain the inert atmosphere and ensure process consistency.
Clean Process. To prevent particulate contamination of sensitive products, clean process ovens have special construction and components such as high efficiency particulate air (HEPA) filters or other special air filtration systems, rounded corners for cleaning, and continuous backwelding to prevent migration of particles into the oven.
By asking and answering some questions about your process before you select an oven, you should be on the way to finding the system with the most suitable chamber size, airflow type, temperature requirements and construction for your application.