What is the Right Oven Size?

Posted on Tuesday, July 11, 2023

By Sheila LaMothe

Determining the optimal cure oven size for your powder coating operation goes far beyond the obvious factor of available floorspace. To better understand considerations for both conveyorized and batch systems, PCT picked the brains of two experts. Alwyn Moody, president of United Finishing Systems LLC, addresses conveyorized systems, and Geoff Raifsnider, senior engineer with Global Finishing Solutions, focuses on batch ovens.

An obvious limitation for oven size is floorspace, but configuration is also important. What should be considered in terms of configuration of a powder line and oven placement?
Al: When designing an oven for a conveyorized system there are several key items to consider. These include part size and how parts will hang on the conveyor, duration the parts need to be in the oven to reach full cure including ramp up time, along with line speed and how it impacts part movement. Product flow, particularly where you want to load and unload the line, is also important.

When it comes to oven placement floorspace is always a critical issue, but there are creative solutions that can help relieve floorspace constraints including an elevated oven or even placing the oven outside of the building.

Geoff: For batch systems, the oven should be located as close to the powder booth as possible to minimize opportunities for disruption or contamination of the coating. For processes where a cart is used, minimize the travel distance and ensure the travel surface is smooth to avoid shaking the cart, which can dislodge powder or drop parts. Also, be sure to consider required clearance to maneuver parts from the powder booth to the oven as effortlessly as possible to avoid compromising the recently coated parts through contamination or contact with items in tight spaces.

What impacts does a multi-pass oven have on sizing?
Al: Specific to conveyorized systems, multi-pass ovens are subject to many of the general design considerations previously mentioned, including part size, how the parts hang, powder cure time, and conveyor line speed. A major factor directly connected to a multi-pass system is ensuring the parts won’t collide with other parts when “passing” each other, or with oven walls, and even support steel. Turning angles, part radius, etc., must be calculated to ensure a smooth path for parts moving through the system.

The types of parts to be powder coated play a big role in oven selection. What are some of the major factors to consider?
Al: Regardless of whether you have a conveyorized or batch operation, the major factors are size, thickness, and material. They drive the oven’s required temperature and dwell time. Thicker gauge metal requires a longer ramp up time and more heat in the oven than lighter, thinner gauge parts. For conveyorized systems, part hanging configurations impact elevation changes and larger, heavy parts typically don’t fare well going up and down.

Geoff: As Al noted, the size, thickness, and material characteristics of the product being cured can impact the time it takes to bring the product up to the necessary temperature which is crucial for powder coating adhesion. In addition, complicated shapes, such as those with pockets or crevices, may be difficult to heat at the same rate as larger, flat surfaces. Airflow is key to driving heat into the product. However, too much air may disrupt the powder coating. Batch ovens that offer variable airflow through the cure cycle can help with first gelling the powder at lower airflows, then increasing the airflow to get to the target substrate temperature.

For ovens that contain burners, how do you determine the best burner size for your oven?
Al: The first step in calculating the burner size is to determine the heat load. To accomplish this, multiple variables are taken into consideration. These include the maximum weight of the products and conveyor or rack weight per hour, thickness and square footage of oven walls, roof, and floor (if there is one), insulated panels, part opening dimensions and type (powered air seal, bottom entry, or none), and maximum operating temperature. Utilizing a formula, the required heat load will be calculated in BTUs per hour. Once this is determined, a safety factor is applied, and a selection of a burner is made to match or exceed this heat load.

Geoff: The burner size is influenced by the amount of fresh air introduced into the oven. Fresh air is needed to dilute products of combustion from the burner, as well as any solvent vapors that are released during the curing process. The amount of work that the burner needs to do to heat up this fresh air accounts for the biggest impact to the burner size. Knowing how much powder you will be processing per batch helps to design the most efficient oven. Less exhaust air means a lower gas bill.

What is considered when determining the recirculation fan and exhaust fan sizing?
Al: Typically, the size of the oven dictates the turnover rate of the recirculation air. Turnover rates should be equal to or greater than three times per minute to move the heat out of the burner box and across the parts. Any lower and the heat is not transferred efficiently to the product.

When it comes to powder coating, the exhaust fan rates can vary based on the product being cured and the appearance/ performance requirements of the finished product. Darker colors with minimal appearance requirements can require fewer turnovers per hour. As colors and requirements become more stringent, turnover rates should accommodate appropriately and can reach up to eight turnovers per minute. If the exhaust fan is being used as the purge fan, then it will be necessary to take this into consideration as well. Currently, the volume of the oven needs to be exhausted at least four times before the burner is allowed to fire. Always follow the most current National Fire Protection Association requirements for minimum exhaust requirements.

Geoff: The exhaust fan is sized to provide the removal of the solvents from the coating as well as by-products of combustion from the burner. Solvent vapors from powder coatings can be significantly less than from liquid coatings. This typically means the exhaust rate from a powder oven is less than a similar-sized liquid coating oven. This is especially true for batch ovens in which there is no need for air seals at the work openings because they have product doors.

Energy consumption is a major consideration in the curing process. Please share design attributes that impact energy usage, for example wall thickness, opening size, entrance and exit air seals or vestibules, etc.
Many factors can impact energy usage in a cure oven. Oven panel thickness is a major contributor. A systems house will typically use 6-inch-thick oven panels for temperatures ranging from 300 to 500 degrees Fahrenheit. Great care is also taken in the assembly of the oven to maximize energy efficiency. Non-silicone high temperature cement is used for all connecting joints, inner and outer. All inner voids are filled tight with four-to-six-pound mineral wool insulation.

In addition, powered or non-powered air seals and/or doors can be utilized to help contain the energy in an oven. If possible, a non-powered air seal at the bottom entry/exit of an elevated oven is preferred. An extended vestibule at the entrance and exit can also help contain heat loss. Regardless of the technique used, the oven opening must be minimized but still must have enough room to accommodate any sway or mis-hung part. Typically, the goal is to keep the opening within a few inches around the part window.

Geoff: Similar considerations apply to batch ovens. The wall thickness and insulation type impact the amount of heat that escapes the oven, which has a direct impact on fuel consumption. Oven openings should be kept as small as possible to reduce the amount of heat loss. Air seals at work openings can help keep the heat in the oven by creating an air curtain that counteracts the rising heated air that is trying to escape the oven. However, air seals use a directed high- velocity air stream that may blow some of the powder off the product.

When should an oven include filters and what type is recommended?
Al: Oven filter decisions are customer specific. Required finish quality and capital constraints (filtration provisions are more costly) can drive decisions. Any addition of filtration in the oven recirculation stream requires additional space in the burner box, facility, or oven itself. Filters will require additional fan capacity to overcome the additional load added to the airstream. A systems house can recommend the ideal filter based on customer requirements and specifications.

Geoff: Filters are not typically used in the recirculation air stream unless there is a specific need to address contamination. If needed, filters rated for the oven operating temperature must be used. Filtering both the fresh air and combustion air supply may be needed if the workspace around the oven contains dust or other materials that could have a negative impact on the coated parts as they cure in the oven.

In your opinion, which is easier to maintain, and why: a larger or smaller oven? What can be done to make the oven easier to clean?
Al: This is a bit of a trick question. Typically, accessibility is the key regardless of size, and oven design and requirements typically dictate accessibility. For example, a large oven with an in-floor chain conveyor with side clean wall discharge enables floor and wall scrubbing equipment to be utilized for cleaning. When a system is designed to accommodate the use of cleaning equipment rather than sending someone in with a rag to wipe things down, cleaning is easier.

Custom ovens can be designed to customer requirements or specifications, and to reduce dirt accumulation. Typically, system ovens do not have any flat surfaces other than the oven floor itself and floor ductwork. If floor ductwork is required or specified, the duct can be designed to avoid gaps between the ductwork (interior frame design) and the floor. Access doors can be provided for ductwork and hard to access areas.

Geoff: While larger ovens may have more room to work within for cleaning activities, smooth surfaces can make cleaning easier regardless of size. If accumulations are possible, ensuring the oven has a means to access those areas can make cleaning more effective.

Avoiding buildup of materials in the oven can be achieved with proper maintenance of the burner and exhaust system. Proper burner setup can minimize contaminants from combustion and appropriate exhaust airflow can minimize condensation of the vapors released during the curing process. This condensation can build up both within the oven and outside the oven near openings and doors.

Can you future-proof your oven decision? What future considerations should you think about? Can the oven be expanded in the future and if so, what would that entail?
Al: Futureproofing can absolutely be a part of the initial oven design and a systems house will include future expansion options upon request. Physical space is almost always a constraint when planning for the future. Multiple design scenarios can be developed, and each scenario can be overlaid onto the original design to aid in decision making. In terms of specific considerations for future expansion, floorspace, additional burner box(es), utility load requirements, product routing, and oven balancing all come into play.

Geoff: Ovens can be expanded, but since the burner and fans are a function of the size of the oven, those items may also need to be changed.

Since product configurations and coatings change over time, requiring more or less airflow or higher or lower temperatures, a good way to provide maximum flexibility is to choose controls that allow easy adjustment in the operating temperature and airflow. This can be achieved by using variable frequency drives (VFDs) for airflow control and programmable recipes for both temperature and airflow.

Determining oven size for applications today and tomorrow is an undertaking. Sourcing input from supplier experts such as systems houses and oven manufacturers can facilitate the process and help ensure the right oven size for your operation.

Sheila LaMothe is editor of Powder Coated Tough.