The first step to balancing an oven is gathering information on the oven and the process. Data that may be needed to balance an oven is as follows:
1) Oven zone temperatures
2) Desired can temperatures (peak metal temperature and hold time)
3) Zone dwell times
4) Zone lengths
5) Can temperature desired ramp-up rate (time)
6) Can temperature dwell times
7) Belt speed
8) Can height
9) Duct height (chamber height)
10) Burner specifications
11) Recirculation fan specifications
12) Exhaust fan specifications
13) Current Datapaq curves
14) Note position of all internal and external dampers for both the recirculation and exhaust dampers
15) Fan VFD settings, if equipped
16) Minimum allowable safe exhaust fan volume
17) Recirculation fan rpm
18) Exhaust fan rpm
19) Record all recirculation and exhaust fan motor amperages
Oven drawings such as general assemblies and electrical schematics as well as nameplate data of the various devices are valuable sources of information.
Information must also be gathered regarding what the problem(s) or perceived problem(s) are and how long they have existed. Along with this information, maintenance records are very useful in establishing a cause and effect scenario regarding the problem(s)
Before an oven can be balanced it must be restored to factory condition to provide 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:
1) Burner adjustment. The burner should have a strong flame signal with the combustion air volume adjusted to the specific burner installation.
2) Clean combustion air inlet filters
3) Access door gaskets must seal properly
4) All supply nozzles and gaskets are in place and sealing properly
5) All perforated plates are in place and seated properly
6) End openings must be closed off as much as possible. Make sure that all close offs are in place and adjusted to there minimum position.
7) All can tipping issues must be resolved
8) Plant atmosphere must be neutral
9) Exhaust stacks and end caps must be clean and properly installed
10) If the oven has a considerable amount of condensate build up inside, it is highly recommended to clean the oven. Condensate builds up in the duct work, nozzles, perforated plates, and dampers will negatively affect the performance of the oven.
If there is a problem in any of the above areas the oven will act out of balance, when it is really an issue of component malfunction.
Balancing Principles and Adjustments
Once the oven is in good working order with the burner adjusted properly and all obvious leaks or problems correct, 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 production rate (cans per minute), wet/dry can weight, and total combustion air input to the oven. Balancing procedures will be defined by the nature and condition of the problem. Some typical problems are discussed below:
Condition 1 – High gas usage
- First, it must be established that the oven is indeed using more gas than normal. This can be confirmed by comparing gas meter readings over time assuming that both zone temperatures and production rates have not changed. If there is no totalizing gas mete,r a meter for the entire oven or for each zone can be installed and the readings recorded over time.
- Oven exhaust volume higher than required will use a very large amount of energy. The proper exhaust volume can be calculated from formulas and guidelines in NFPA 86 based upon the type and amount of volatiles and water given off from each can multiplied by the maximum number of cans per hour.
Condition 2 – Both the entrance and exit openings are positive (air exiting the oven to plant atmosphere)
- This condition can be caused by too much fresh air coming in through one or more fresh air inlets or too low of an exhaust rate. It is best to establish and set the exhaust at the minimum safe exhaust rate based upon volatiles given off by the product, and then close off the fresh air inlet(s) until both end openings are neutral.
- A very negative plant atmosphere will also cause this condition to occur and proper oven balancing is not possible.
Condition 3 – Both the entrance and exit openings are negative (air entering the oven from plant atmosphere)
- This condition can be caused by not enough fresh air coming in through one or more fresh air inlets or too high of an exhaust rate. Again, it is best to establish and set the exhaust at the minimum safe exhaust rate based upon volatiles given off by the product, and then open up the fresh air inlet(s) until both end openings are neutral.
- A very positive plant atmosphere will also cause this condition to occur and proper oven balancing is not possible.
Condition 4 – One end opening is positive and one end opening is negative – This condition is known as a directional oven. This can occur because one end opening is negative. The negative end draws cool ambient air into the oven where it is heated and expands and is then 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 zone and/or too little fresh air is coming in through that zones fresh air inlet. If this is the case the exhaust dampers in that zone can be closed down and/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 may also be set up with too little exhaust and too much fresh air as well. The fresh air inlet dampers may need to be closed and/or the exhaust rate increased to correct the problem.
- An oven can also 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 or have blocked off part of the return air flow.
- Another scenario is that the nozzle bank at the positive end is not sealed either because it was installed incorrectly or because a gasket is missing, torn, or leaking. This would cause that end to be positive and the other end could become negative. The solution is to correct the cause of the leaking supply air.
Condition 5 – Poor zone temperature zone segregation.
- Analysis of a Datpaq 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 will normally 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 and/or the exhaust rate 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.
The above conditions are separate, unique problems. Many times a combination of the above conditions exist in the field. These combination problems are harder to diagnose and cure, but the same approach to problem-solving should be used.
Oven Balancing Recommendations
It is advisable to set up a schedule to regularly perform temperature audits (Datapaq) to ensure that the oven is in balance. HeatTek also strongly recommends regularly scheduled safety audits per NFPA 86, to ensure that all safety devices are working properly.
The firing rates of the burners, as well as inspection of the oven in general, is also recommended on a regular basis to keep the oven running in an efficient manner that will produce an acceptable, quality product. Also, a gas usage totalizing meter installed on each oven with its usage recorded daily can act as a monitor to the ovens state of balance as an out of balance oven will use more energy.
Instrumentation/Tools Required for Oven Balancing
- Slack tube manometer
- inclined manometer
- Smoke Sticks
- Hand Held Tachometer
Problems Outside of Balancing
Various problems besides balancing can occur in can making ovens. The following section will categorize problems to specific ovens in the can making process and listing the specific problem, cause, and solution.