Convection ovens are used for many industrial heating processes performed on a variety of substrates. Regardless of what they are used for, industrial ovens typically are evaluated for functionality and performance based a on few criteria: temperature uniformity, operating efficiency and heat transfer rates. This article will explore these criteria and their importance on performance.

Temperature Uniformity

When convection oven designers and manufacturers talk about oven uniformity, almost always, they are referring to the consistency of the recirculated air temperature profile across the length, width and height in the process chamber of an empty oven. By contrast, when oven end users talk about oven uniformity, they usually are referring to the stability of the part temperature profile for a sample set of product that is processed in the oven. While both these criteria are trending in the same direction, they are certainly not the same thing.

An oven manufacturer has little to no control over how those oven users are going to load the oven. For instance, a batch oven designed for a load of 5,000 lb per batch will perform much better if the product is uniformly racked and spaced throughout the process chamber as opposed to piles of parts randomly thrown into a work basket. Stacking the product allows air to pass over and through the load. The same rationale is true in a belt oven. Purposefully placed and spaced product on the belt better accommodates airflow than product dumped in a pile in the center of the belt.

In any discussion about air temperature vs. part temperature, conventional wisdom favors the oven manufacturer. The most generally accepted pyrometry standard, AMS2750E, covers temperature sensors, instrumentation, system-accuracy tests and temperature uniformity surveys. It specifically calls for temperature uniformity to be measured in an empty oven.[1]

When quantifying temperature uniformity, the convention is to refer to a temperature range that straddles a specific oven setpoint temperature. For example, 300°F ±10°F (150°C ±6°C) might provide fairly decent uniformity for a large oven that is curing a solvent-based liquid coating.

The largest contributing factor to oven temperature uniformity typically is the volume per unit time of air  being recirculated through the oven chamber. Generally, this is quantified in cubic feet per minute. In order to compare the relative performance among ovens of different sizes, the term air change usually is dragged into the conversation.

Oven air change is simply the ratio of the recirculated air volume divided by the interior volume of the oven, or the oven process chamber. Think of it as the number of times per minute that the air volume within the oven passes over the heat source and through the fan. (For more on air changes, see the sidebar, “Typical Air Changes and Target Uniformity for Various Processes.”)

Another significant factor contributing to oven temperature uniformity is the oven balance. This refers to the internal air pressure in the oven vs. the air pressure in the room or building where the oven is physically located. If the oven is severely overpressurized relative to the room, process air will spill or leak out of the oven. If the oven is severely underpressurized relative to the room, cold room air will find its way into the oven. In a batch oven, this usually results in cold spots around the door. In a continuous oven, extreme unbalance can shorten the process length of the oven because the ends cool down as a result of the ambient air infiltration.

Other heat losses — to a concrete floor or through the walls and roof — also affect temperature uniformity. Insufficient or deteriorating insulation in older ovens can cause significant uniformity issues.

The last contributing factor to oven temperature uniformity I will mention deals with getting the air where it needs to be in order to transfer the heat to the product. Poorly designed airflow can short circuit the product. This result in lots of air movement without providing sufficient airflow around the parts.

When convection oven designers and manufacturers talk about oven uniformity, they are almost always referring to consistency of the recirculated air temperature profile across the length, width and height in the process chamber of an empty oven.

Operating Efficiency

Industrial oven operating efficiency is an excellent benchmark in the discussion of operating costs. In its simplest form, operating efficiency is the ratio of gross energy input divided by the energy imparted to the product.

Energy that does not go into heating the product is considered waste and decreases efficiency.

Generally, the biggest detractor from operating efficiency is oven imbalance. If excess heat is allowed to escape up the exhaust stack, the process will be less efficient. In addition, that waste heat contributes to global warming and the unnecessary release of additional greenhouse gases.

Heat that escapes from the oven into the room where the oven is physically located also decreases operating efficiency. Excess heat escaping into the room can be caused by oven imbalance or wall losses. The old rule of thumb was 1” of insulation for every 100°F (56°C) of operating temperature. So, a 300°F (150°C) oven would have 3” of insulation. In this day and age, however, reputable oven manufacturers provide thicker walls to reduce wall losses.

Another way to think about operating efficiency is to determine the hourly, monthly or annual utility operating cost of the oven. This is particularly useful when making equipment or process changes. With a baseline for comparison, better judgments can be made regarding the effectiveness and contribution of any changes or modifications.

When determining utility operating costs, be sure to include both the energy expended to heat the product and the motors required to move the air. In a typical oven, there are motors on the recirculation and exhaust fans and combustion blowers. On a gas-fired oven, the motor sizes for an exhaust fan and combustion blower typically are small. The current draw (amps) on the recirculation fan is going to be lower when the oven is at temperature than at startup, when the air in the oven is cold.

Among the factors affecting the heat transfer rate are air velocity, relative area-to-mass ratio of the product and airflow characteristics (laminar vs. turbulent).

Heat Transfer Rates

The purpose of an industrial convection oven is to heat parts by exposing them to hot air. The heat transfer rate is quantified using the following equation:

Q  =  m  x  CV  x sT

where m is the mass flow of air over the product, CV is he specific heat of the product and ΔsT is the temperature differential between the air and the product. Other factors affecting the heat transfer rate are air velocity, the relative area-to-mass ratio of the product, and the airflow characteristics (laminar vs. turbulent). By far, though, the primary consideration is the mass of the air impinging on the product.

Purchasing a new industrial oven is an opportune time to consider and incorporate design features that will improve temperature uniformity, operating efficiency and heat transfer.

As air is heated, the density decreases. At 600°F (315°C), air has half the density that it does at 70°F (20°C). Therefore, the same volume of air only has half the mass. That is why as temperature increases, higher airflow provides noticeably better heat transfer.

Better heat transfer through improved airflow also can result in lower operating temperatures. The increase in mass flow reduces the required temperature differential (sT). This has a direct effect on operating efficiency.

Typical Air Changes and Target Uniformity for Various Processes

Over the years, there have been rules-of-thumb as to the relative airflow (as measured in air changes) and temperature uniformity that is desirable for each variable processes. This is not to say that less airflow will not work, and more air is better (except in initial powder curing) and will result in tighter uniformity. The larger the oven, the more challenges there are to maintain good uniformity.

Typical relative airflow and target uniformity for some common industrial processes include those shown in the table.

Suggestions to Improve the Performance for Existing Ovens

The volume of air being recirculated in a convection oven has a direct effect on temperature uniformity, heat transfer rate and operating efficiency. Higher airflow (as measured in cubic feet per minute) improves temperature uniformity and heat transfer; however, higher airflow also lowers the efficiency because it requires more horsepower to move that air. As explained earlier in this article, fan airflow (as measured in cubic feet per minute) is directly proportional to uniformity and heat transfer. Also fan airflow is directly proportional to motor speed (rotations per minute) over the linear portion of the fan curve.[2]

By increasing airflow by 25 percent, uniformity and heat transfer (theoretically) will improve by 25 percent. Based on the Fan Laws, however, the motor requirement will increase by 95 percent.[3] That increase in operating cost is probably worth the improved performance in many cases, however.

Cost-effective modifications for existing ovens include:

• Properly balancing the oven.
• Increasing the recirculated air volume.
• Racking or stacking parts to allow for adequate airflow.
• Getting the air where it needs to be via ductwork modifications.
• Instituting a preventive maintenance (PM) program.

Perform Oven Balancing. As noted, excessive exhaust rates are a waste of heat and money. At the same time, insufficient exhaust creates cold spots within the oven process chamber. Try to get as close to neutral as is practical.

Increase Recirculated Air Volume.

Generally, recirculation fan motors are sized to allow for cold-start requirements. Depending on the existing motor size, replacing the across-the-line motor starter with a variable-frequency drive (VFD) may allow the existing motor to be fully utilized. Use the VFD to start and run the recirculation fan motor at a lower speed while the oven is coming up to temperature. Once at temperature, increase the motor speed (thereby increasing airflow) to improve uniformity and performance.

Adjust Product Racking. The recirculated air needs to flow against and around the parts to transfer heat. Avoid blocks and stacks of parts that you cannot see through.

Consider Ductwork Modifications. Ductwork modification can improve oven performance. For instance, some ovens are designed for one type of product and then used for parts that were never contemplated. Oven ductwork with nozzles and slots tends to have a limited throw, with the discharge velocity being maintained for only four times the effective nozzle diameters, then dropping off significantly.[4] Providing adequate airflow to the middle of a large oven can be problematic with that type of ductwork.

Perform Preventive Maintenance. A good PM program should be required for any industrial oven. The PM should include regular inspections of safety components such as airflow switches and dual-blocking valves. NFPA 86 mandates that the end user perform and document testing of safety devices at least annually.[5] Frequent temperature profiling with a datalogger also is useful to stay ahead of problems before they develop.

A few limitations and pitfalls should be avoided when modifying existing industrial ovens. They include:

• Excessive airflow in a powder-cure oven.
• Increasing recirculation fan speed.
• Decreasing exhaust airflow.

Manage Excessive Airflow. If freshly applied powder is subjected to high air velocities, the powder can be dislodged from the part, resulting in lower coating thickness.

Control Recirculation Fan Speed. Know the rated safe speed of the fan at the intended operating temperature. Do not exceed the safe speed! If the fan allows and requires a larger motor to operate at maximum safe speed, the size of the wire, conduit and main circuit breaker needs to be verified by a qualified electrician or engineer.

Manage Exhaust Airflow. Know the minimum exhaust rate that is allowed while still meeting the requirements of NFPA 86, the standard for ovens and furnaces. Typically, the minimum rate is based on the amount of solvents or powder being processed in the oven.[6]

Design Improvements in New Ovens

Purchasing a new industrial oven is an opportune time to consider and incorporate design features that will improve temperature uniformity, operating efficiency and heat transfer. Consider features such as oven construction, recirculation system design and oven controls to meet these challenges.

Construction. Look for an oven with an outer shell that is comprised of solid, welded 0.1875” plate. This will help minimize wall losses and eliminate the NFPA 86 requirements for explosion relief. Thicker insulation also reduces wall losses, and oven efficiency is improved. Also helpful are designs where there is no need for additional platforms for maintenance access on the oven roof. If roof access is needed, simply add railings and a ladder.

Recirculation System Design. Turbine fan blades blow directly on the product in either a downward vertical or horizontal orientation. The lack of supply ductwork results in lower required system static pressure and longer throw of the air. This increases heat transfer rates while also considerably reducing motor horsepower requirements for the fans, thereby saving on electrical utility costs. The improved uniformity and heat transfer often allow operating temperatures to be reduced, resulting in lower fuel costs.

Enhanced Controls Package. Variable-frequency drives on the recirculation and exhaust fans allow optimal flow rate tuning, maximizing heat transfer to the product and increasing operating efficiency.

In conclusion, many manufacturers produce products that must traverse through an industrial oven during manufacturing. Some products require multiple industrial heating processes. Improved oven performance and functionality can decrease operating costs, processing time and reject rates. Allow the air to do its work. And remember, it is mostly about mass flow, not temperature.     

Citations:

1. AMS2750E Pyrometry, https://www.sae.org/standards/content/ams2750.

2. Fan Laws and System Curves, Engineering Letter 2, New York Blower Co., https://www.nyb.com/pdf/Catalog/Letters/EL-02.pdf.

3. Ibid.

4. Fan Engineering, Buffalo Forge Company, 1961, page 152.

5. NFPA 86:2019, Ovens and Furnaces, Chapter 11.6 “Safety Ventilation for Class A Ovens.”

6. NFPA 86:2019, Ovens and Furnaces, Chapter 7.4 “Inspection, Testing and Maintenance.”