Whether starting up a new oven for the first time or trying to determine what your real operating conditions are, it is important to understand oven performance. Oven users need to compare the current operating parameters with the oven’s design capabilities based on the original oven design. By comparing that information to the daily operating condition, oven users can better understand the oven’s daily performance.
Unfortunately, over time, the personnel responsible for specifying and purchasing the original equipment move on, and the original design information may no longer be available. At the same time, aging of the components and changes to the original design that deviate from the optimal running conditions of the machine can occur. The changing conditions affect both mechanical and electrical operation.
What Is a Balanced Oven?
To discuss oven balancing, we must first define what a balanced oven is. A balanced oven will meet three basic criteria:
This article concentrates primarily on the mechanical operating equipment and the operating conditions of the heating and air-handling systems for proper balancing of the system. The inspections and checks described can be applied to continuous ovens, walk-in ovens and low temperature furnaces.
Oven air-system balancing is a multi-step process that involves many variables. The first step is to gather data on the oven and to determine if there is a problem. The second step is to restore the oven to factory condition in order to establish a known baseline from which the oven can be adjusted. The third step is to make small, incremental changes to those variables to achieve a balanced oven.
Oven balancing is essential. An out-of-balance oven will consume an excessive amount of energy. It can create potentially unsafe operating conditions. In cases where the oven is equipped with an oxidizer, an out-of-balance oven with increased exhaust flow also will cause increased oxidizer fuel usage. Additionally, an out-of-balance oven will affect the plant HVAC, potentially disrupting the work environment and adding operating costs. Oven balance will have a direct influence on the quality, function and, many times, appearance of the product.
Measuring Current Oven Operating Data
The first step to balancing an oven is gathering information about the oven and the process. Data needed to balance an oven include:
- Oven (zone) temperatures.
- Desired part temperatures.
- Cycle heating times.
- For continuous ovens, zone lengths.
- Part temperature desired ramp rate (time).
- Part time required at temperature.
- For moving systems, line speed.
- Product physical size and loading configuration.
- Work chamber size.
- Burner size and specifications.
- Recirculation fan specifications.
- Exhaust fan specifications.
- Current heat profile curves.
- Positions of all internal and external dampers for recirculation airflow, exhaust airflow and fresh airflow.
- Fan variable-frequency drive settings (if equipped).
- Minimum allowable safe exhaust fan volume.
- Exhaust fan speed (rpm).
- Recirculation fan speed (rpm).
- All recirculation and exhaust fan motor amperages.
Design charts such as general assembly drawings and electrical schematics as well as nameplate data of the various devices are valuable sources of information. Information also must be gathered regarding the problems — or perceived problems — with oven operation as well as the length of time they have existed. Finally, maintenance records are useful in establishing a cause-and-effect scenario regarding the air-balancing problems.
What to Do Before Beginning to Balance Airflow in Your Industrial Oven
Before an oven can be balanced, it must be restored to factory conditions to provide a known reference and a stable base that can be adjusted. The following is a checklist of items that must be in good working condition before an oven can be balanced:
- The burner should have a strong flame signal with the combustion air volume adjusted to the specific burner output.
- Clean combustion air inlet filters.
- Work chamber access door gaskets must seal properly.
- For flow-through ovens, end openings must be in place and adjusted to their minimum clear position.
- Plant atmosphere pressure must be neutral.
- Exhaust stacks and end caps must be clean and properly installed.
- The oven should be cleaned of any debris, dirt or condensate buildup in the ductwork and dampers that will negatively affect the performance of the oven.
If there is a problem in any of the areas listed, the oven may appear out of balance due to an issue of component malfunction.
Industrial Oven Balancing Principles and Adjustments
Once the oven is in good working order with the burner adjusted properly and all obvious leaks or problems corrected, any remaining problems can be fixed by adjusting various parameters of the oven (balancing). The adjustments will vary depending upon the problem encountered with the oven.
Please note, the minimum exhaust rate — based upon the volume of volatiles given off by the product — must be established per NFPA 86 in order to operate the oven in a safe manner. Engineering calculations must be completed to establish this exhaust volume based upon any VOCs produced while heating as well as the total combustion air input to the oven.
Balancing procedures will be defined by the nature and condition of the problem. Some typical problems include high gas usage and imbalanced airflow into the oven.
Condition 1: High Gas Usage in Your Industrial Oven
First, it must be established that the oven is indeed using more gas than normal. Assuming that both temperatures and production rates have not changed, this can be confirmed by comparing gas meter readings over time. If there is no totalizing gas meter, a meter for the entire oven or for each zone can be installed and the readings recorded over time.
Oven exhaust volumes larger than required will use significantly more energy as a result of excess fresh air being drawn into the oven or furnace. The proper exhaust volume can be calculated from formulas and guidelines in NFPA 86 based upon the type and amount of volatiles given off per batch or per hour of product.
Condition 2: Air Exits the Industrial Oven to the Plant Atmosphere
This condition can occur with continuous ovens. In this condition, both the entrance and exit openings are positive (air exits the oven to plant atmosphere).
This condition can be caused by too much fresh air or too low of an exhaust rate. It is best to establish and set the exhaust at the minimum safe exhaust rate and then close off the fresh-air inlet(s) until both end openings are neutral.
A negative plant atmosphere also will cause this condition to occur. In those cases, proper oven balancing is not possible.
Quite often, balancing issues occur in spring and fall when factory pressures may change due to the building doors being closed or opened.
Condition 3: Air Enters the Industrial Oven from the Plant Atmosphere
This condition also can occur with continuous ovens. In this condition, both the entrance and exit openings are negative (air enters the oven from plant atmosphere).
This condition can be caused by a lack of makeup air. Again, it is best to establish and set the exhaust at the minimum safe exhaust rate based upon VOCs given off and products of combustion (gas burning), and then open up the fresh-air inlets until both end openings are neutral.
Although this condition is not very common, a positive plant atmosphere also will cause this condition to occur and proper oven balancing is not possible.
Condition 4: Excessive Heat Losses in a Directional Oven
This condition also can occur with continuous ovens. With a directional oven, one end opening is positive, and one end opening is negative. It occurs because the negative end draws cool ambient air into the oven, where it is heated and expands. The heated air then is pushed out the other end opening.
The cause of one end being negative can be that too much exhaust is being drawn from that particular end. Alternately, too little fresh air is coming in through that zone’s fresh-air inlet. If either of these conditions exists, the exhaust dampers in the affected zone can be closed down or the fresh-air inlet can be opened.
The positive end will be positive from the relatively cool air coming in the other end and expanding, but it also may be set up with too little exhaust and too much fresh air as well. The fresh-air inlet dampers may need to be closed or the exhaust rate increased to correct the problem.
In addition, an oven can be directional if the return air to the heater box is not centered in that particular zone. If this is a problem from day one of operation, it may be a design issue that the return was not centered. If this problem suddenly arose, an internal baffle may have moved or fallen and blocked off part of the return airflow.
In some cases, the problem can be caused by drafting, in factory, from an open door located in line with end of the oven.
Condition 5: Poor Zone Temperature Segregation in Your Industrial Oven
In this situation, an analysis of the heat profile curve will show temperature migration from one zone to another.
In principle, the higher temperature zone will try to migrate to a lower temperature zone; therefore, you normally will see an elevated temperature near the end of a lower temperature that is adjacent to a higher temperature zone. To minimize this effect, the fresh-air inlet blast gate can be opened on the lower temperature zone, allowing in more ambient air. Alternately, the exhaust rate can be increased on the higher temperature zone by opening up the internal or external exhaust dampers.
After these adjustments are made, both end openings must be checked that they are neutral.
Condition 6: Air Imbalances in the Work Zone of a Batch Industrial Oven
To balance the work zone in a walk-in or batch-loaded oven, the work chamber balancing is done primarily by adjusting fresh-air intake dampers and the exhaust outlet damper. Balancing temperatures inside the oven can be accomplished by opening and closing air-supply ducts supplying air to “cold spots” inside the work chamber. (A heat survey needs to be performed to see the original condition and then again after changes were made.)
In conclusion, while the conditions described may be separate, unique problems, in some cases, a combination of conditions can exist in the field. These combination problems are harder to diagnose and cure, but the same approach to problem solving should be used. To be effective, make only one adjustment at a time until the results of each change are confirmed as being beneficial to the operating performance.
In addition, it is advisable to set up a schedule to regularly perform temperature audits (temperature heat profiles) to ensure that the oven is in balance. In order to ensure a safe operating system, it is strongly recommended that regularly scheduled safety audits, per NFPA 86, are made to ensure that all safety devices are working properly.
Checking the firing rates of the burners as well as inspection of the oven are recommended on a regular basis to keep the oven running in an efficient manner. Doing this will produce an acceptable, quality product. Finally, a gas usage totalizing meter installed on each oven with its usage recorded daily can act as a monitor to the oven’s state of balance. An out-of-balance oven will use more energy.