Both factors have an enormous impact on producing consistent, quality parts.
When shopping for an oven, countless items need to be considered when trying to select the best vendor for your project. Obviously, the initial investment is a major factor, but purchasing the cheapest oven may actually cost your company more money in the long run. Besides price, factors to be considered include the vendor’s ability to meet your requirements and specifications to ensure the equipment will work for the application. It is also important to pay close attention to airflow design and temperature uniformity, both of which can have an enormous impact on obtaining quality results on a consistent basis. Although proper airflow design and temperature uniformity are closely related, they should be evaluated separately to avoid issues once the equipment arrives at the job site and you begin running your product.
Remember, you can buy an oven with excellent empty oven chamber temperature uniformity and still not obtain acceptable results. If the airflow is not designed for the way you load your parts inside the oven, you will not achieve the projected uniformity. Only by buying an oven with acceptable temperature uniformity and the proper airflow design will you be on your way to maximizing your production rates and improving your product quality.
Understanding what is necessary to achieve optimal temperature uniformity in your oven and how to match the airflow to the way the products are loaded into the oven will help you get the results you desire.
Ensuring Oven Temperature Uniformity
Oven temperature uniformity is defined as the overall temperature variation from a given setpoint. Acceptable uniformity tolerances are stated as a deviation (±10°F, for example), which means the temperature of all points that are measured within the work chamber need to be within a 20°F band around a specific setpoint.
Temperature tolerances within the work chamber seem to become more stringent each year. In many applications, better temperature uniformity will provide more consistent heating, which allows the manufacturer to provide better quality parts to its customers.
Typically, uniformity is measured through a web of thermocouples located inside the work chamber (figure 1). The most common arrangement is a nine-point survey, where one thermocouple is located near each corner of the work chamber, and another is located in the center. After the oven is heated to the required temperature and allowed to come to equilibrium, the points are recorded, most often with a chart recorder. If there are points outside the acceptable tolerance, adjustments are made to bring the points into spec.Many different features and designs influence uniformity:
Accuracy of the temperature controller.
Accuracy of the thermocouple.
Location of exhaust outlets and fresh air inlets.
Although each one of these impacts the ability of the equipment to achieve a given temperature tolerance, this article focuses on airflow volumes, distribution of the air through the work chamber, and shell losses. These combined factors keep the heat uniform within the work chamber, ensuring each area within the oven receives the same air temperature regardless of its location.
If you are responsible for purchasing an oven, please check with others within your organization to determine what temperature uniformity will be required. Although reputable oven manufacturers will include this question as a standard requirement for a firm proposal, researching your requirements ahead of time will help establish a firm requirement for all potential vendors to adhere when they are preparing their bids.
Select the Appropriate Airflow Volume
Typically, airflow volume is stated as either cubic feet per minute (cfm) or as the number of air changes, which is calculated as cfm divided by the internal volume of the work chamber.
It stands to reason that when heat processing equipment has a higher amount of cfm, the air will flow through the chamber in a faster manner. This is called airflow velocity. Aside from some paint finishing applications, in most cases, higher airflow is better. However, a larger recirculation system costs more, so the value the larger blower adds to your process should be compared to the increase in price.
Most oven manufacturers base the airflow volume on several factors specific to the process:
The process or application being heated.
Temperature uniformity requirements.
Required heating rates.
The configuration of the work chamber.
The load within the chamber, in both size and configuration.
These factors help define how the air is distributed into the work area, and how it is collected from the chamber on its way back to the heating plenum.
Air Distribution Affects Temperature Uniformity
An oven with a greater number of air changes will not always have better temperature uniformity. Unless the air is distributed evenly throughout the chamber and returned to the heating plenum in a controlled manner, uniformity will suffer. Airflow means heat. If you have areas within your chamber that have little or no airflow, those areas will not reach the same temperature as areas with adequate airflow. It sounds obvious, but this often is overlooked.
A common airflow pattern used throughout the finishing industry is combination, or uni-flow, airflow, where air is introduced into the work chamber along each lower side of the work chamber near the bottom. This provides a combination of horizontal and vertical-upward airflow through the chamber.
As shown in figure 2, both schematics show combination airflow, but the oven on the right has a return sheet. It distributes the return area over the entire ceiling of the chamber and helps ensure the air flows in a more vertical direction after it exits the supply plenum. Even a partial return plenum is better than none. Equipment without a return sheet will be prone to cool areas as shown in the schematic on the left.
Another common batch oven design is where the heating plenum is located on the rear of the oven (figure 3). Again, the addition of a return sheet helps ensure even airflow through the entire work chamber. The oven on the right has a return sheet that distributes the return area over the entire ceiling of the chamber. Equipment without a return sheet will be prone to cool areas.
There are certain applications where an oven without a return sheet will work, but use caution when considering ovens without a means to control the return of the air from the chamber back into the heating plenum.
Account for Shell Losses
As with a house, areas not well insulated (i.e., windows, doors, etc.) allow a greater transfer of energy than well-insulated areas. The same is true of industrial heating equipment. Those areas which are not well insulated or that have large amounts of through metal will transfer heat energy out of the equipment. This not only can create problem areas regarding uniformity, but it also is less energy efficient, which means it will cost more to operate the equipment.
Tongue-and-groove panels with punched rails are one common way to reduce the through-metal area and thereby, the overall shell losses.
The quality and quantity (thickness) of the insulation must be considered. A general rule of thumb to follow is for every 100°F (55°C), an oven should incorporate 1" of 6-lb density insulation. So, for instance, an oven rated for a maximum continuous temperature of 300°F (149°C) will typically have 3" of insulation. Some companies use a higher quality, more dense insulation to reduce the insulation thickness.
What Type of Airflow Is Best for Your Application?
Many applications handle a variety of products, which can range from small parts (measured in inches) to much larger parts (measured in feet). Combination (uni-flow) airflow will provide the necessary flexibility to handle this range of parts. If the majority of parts are cured on a multi-tiered rack, where air cannot easily rise vertically upward through the tiers, then an oven with horizontal airflow will be a better choice.
Ovens with horizontal airflow will typically have a supply duct located on one entire side of the work chamber, and a matching return duct located on the opposite side. This will help ensure the airflow is evenly distributed throughout the entire work chamber, from front to back, and top to bottom.
Airflow volume, air distribution and quality insulation are a few of the key factors that affect temperature uniformity in work chambers. These items should be at the top of your list when addressing temperature uniformity with vendors. Although temperature uniformity and proper airflow design are closely related, it is important for buyers to evaluate them separately.