On a popular television show from the 60s, a mythical 23rd century engineer named Montgomery Scott often repeated the maxim “The right tool for the right job.” I say “repeated” because people had been saying something similar long before (and after) Gene Roddenberry created Star Trek. The wisdom in the dictum holds true no matter what century it is applied to, and it is useful when specifying industrial process ovens as well. If you’ve specified an oven before, you may have gone through the thought processes needed to embrace all the engineering details that go into the right oven for the job. If not, or even if it’s been a while, the tips in this article can help if it is your turn to acquire an oven.
Know Your Material and ProcessMaterials define an oven. They also are likely to be that part that you know best. What is the nature of the material you wish to heat? Does the material have a discrete form, or is it an amalgam? Define the form. For example, say “My material is 100 glass bottles that are 7" high by 3" diameter and weigh 10 oz each” or “My material is 150 kg of wet granulation, mostly starch.” For a discrete form, define the individual and the quantity. For an amalgam, define each component in the mix, the quantity of each and the agglomerated bulk density. (In other words, what does it weigh, and how much space does it consume?) Then, identify other characteristics of your material: Is it acidic or caustic? Is it sticky? Is it flammable? Refer to the MSDS for components, reactivity, specific gravity and specific heat.
Think About How the Material Is Held and Handled. The form and nature of the material may dictate how it is held or transported. Consider how you will handle the material before, during and after the oven. Truck-loading ovens have the advantage that the material can be carried on the rolling rack beyond the oven. Tray-drying ovens often use this advantage.
Large and heavy items such as 55-gal drums may be best held on pallets moveable by fork lifts. Small, high throughput items often are transported to, through and from the oven on conveyor systems. Your oven is almost always designed around your material-handling system choices. Think of ways that make handling quick and simple.
Know the Process.Ask -- and answer -- a series of questions about your process.
- How do you wish to change the material with heat?
- What temperatures and other conditions bring about this change?
- What temperatures or conditions are undesirable or have a negative effect on quality?
- Do some parts of the material change while others do not (such as oven drying to remove moisture from the material)?
- How have you conducted this process in the past?
- How should the new process and oven be different?
- How long should it take to warm the material?
- How long should it take to bring about the heated change in the material?
- What steps precede the oven process and follow it?
- Will the oven have any other processes? And if yes, what are they?
Finally, what will happen to the exhausted vapors? Are there other operations external to the oven that could use its exhausted waste heat?
Consider What You Have AlreadyYour goal is to select the best oven style for your material, material-handling equipment and process, so look at what you have first, then consider the two main oven styles: batch and continuous.
Many ovens in service are batch ovens. Batch ovens most often are single chambers designed to match the size of a predefined processing unit. For instance, tray-drying ovens should match the batch size of upstream and downstream processes. If prior or following processes do not define a batch, then the oven should be set up for the most practical content capacity that fits the throughput of other processes to the time it takes for the heated process. The batch oven contains all the process segments (warming, heating and cooling) in one place. There are almost as many configurations of batch ovens as there are applications for heat processing -- form following function.
Continuous ovens are designed to match continuous processes before and after the oven. The material moves through the oven held by a conveyance system. The continuous oven contains all the process segments (warming, heating and cooling) in one sequential line. Given that the conveyance will have a constant speed, the continuous oven design converts the time for each segment into distance. This type of oven often costs more in materials-handling and factory space than batch, but that cost typically is economically justified by high throughput operations.
Process throughput and cost generally resolve the selection between batch and continuous oven styles. The cost considerations need to include the use of precious floor space and increased conveyance maintenance. Think about variations of the two styles that might safely diminish the area the oven process must occupy. One variation would be height. The taller the oven, the less floor space it might use. Taller batch ovens are most often limited by ergonomics and materials handling, but some materials lend themselves to vertical serpentine conveyance to reduce the use of flat area.
There also arises a group of ovens that blend the two styles, called semi-continuous. These include the use of an intermittent conveyance that moves and pauses. An oven design that is stretched in the depth dimension to be more like a tunnel forms the basis of most semi-continuous ovens.
A final thought on style: A customer once told me that “An adequate number of batch ovens is equal to a continuous oven, based upon the practicality of redundancy and lower maintenance.” While his assertion certainly is not true for every process, it proves once again that one size does not fit all.
Inside Your OvenYou know your material size and the quantity you wish to process. Next up is the configuration of the load in three dimensions to determine the oven workspace size. To continue the example from earlier, you know that your material is 100 glass bottles that are 7" high by 3" diameter. This load might be configured to five tiers of 20 that are spaced five bottles wide and four bottles deep. Of course, in addition to the space for the material, you must allow space for the airflow around the pieces, and space for any materials-handling system. So, your configuration might resemble a workspace 27" wide by 22" deep by 48" high. Much less blocks or accelerates the airflow; much more slows the air velocity at the price of local temperature conformity.
Remember that compromises on proper sizing can end up being costly in the long run. Likewise, load arrangements different from the intended (designed) configuration will have different results. The maxim -- the right tool for the right job -- still applies.
In cases where the oven’s duty is to process different materials at different times, size for the largest, and consider means of altering the airflow in the workspace to minimize the compromise.
Think About the Air in Motion. In addition to configuring your materials to allow airflow space, you should consider how the airflow may be affected by your material. Heated air moves over and above a tray in a drying oven, giving its heat to the tray and cooling. (It also collects the vapors of liquids evaporated.) The cooled air is quickly supplanted by new, dry, heated air, which also cools. Cooled air is recirculated; that is, reheated and reused, saving its residual energy. In this example, the trays are configured like the fins of a radiator: Surfaces whence heat energy is moved (to or from).
The aerodynamics of your material will affect how and where the heat is delivered. For instance, if your material is a cylinder, airflow directed at it from the side (toward the curved surface) will “slip stream,” which means it will divide to pass around it and re-mix as the streams merge after it. This is likely to deliver more heat to one side than the other. In such a case, it may be better to flow the air in parallel to the axis of the cylinder (from the top or bottom) to minimize turbulence and maximize heat delivery -- as in the case of many drum-warming ovens.
Air added or subtracted from the recirculating pattern is considered ventilation. Ventilation costs energy. Heat is discarded with the exhaust, and the fresh air must be warmed to the process setpoint temperature. In a glass annealing oven, for instance, ventilation may be only as much volume as the heated air has expanded, with no intake. In a tray-drying oven, ventilation must be adequate to remove the moisture. In a Class A oven, ventilation must be adequate to prevent the concentration of volatile materials from rising to dangerous levels.
Ventilation also affords you the opportunity to adjust the quality of the process air. Intake air may be filtered, dehumidified or preheated. The exhaust stream may be filtered or its energy recuperated by heating intake air without mixing.
Know Your FacilityPlan for the space the oven will occupy and the areas where loading, unloading and maintenance will take place. Can the oven ship assembled, or should it be assembled on site? Check your doorways. Expect some heat entropy, so do not put it next to something you pay to cool. Be sure to select an appropriate energy source for heat and consider the altitude and climate of your area. Adjust for barometer and humidity accordingly.
Plan Your Control and Safety Strategy. Learn more about oven requirements from the National Fire Codes and from articles in trade journals and web sites about oven controls and safety. Minimally, you will need a temperature control instrument with thermal probe and final controls for the heating system. You need an excess temperature limit instrument for a gas or electric oven. Recirculating and ventilating fans will require motor controls. Class A ovens must have additional safety devices and interlocks.
Take time to think about what the oven can “tell” you about itself, the process and your material by the effective use of controls and instrumentation. Any technology that lets you “see” into the oven process can add to the quality of your work.
Finally, select oven manufacturers who will partner with you in your quest for the right oven for the job. Give them all the information above. Have them make suggestions. Ask questions. Working collaboratively, and with a thorough understanding of what you need, will help deliver an oven that delivers pleasing results.