Virtually every manufactured product requires the introduction of heat at some point during the production process. Purchasing the proper piece of thermal processing equipment (Industrial Ovens) is rarely an easy or a casual consideration. Reputable industrial oven manufacturers share those exact sentiments in that they will consider the application, production needs, and concerns of the customer very seriously.
It is also understood that multiple issues become intertwined to create a series of details that must be resolved before trust is gained and oven equipment is purchased. This text serves only as a general oven guide, an overview of heat processing equipment, the appropriate options and features, and the issues that must be considered in order to select the proper industrial oven for a particular application. Of course, for an application specific diagnosis, consultation with a member of your industrial oven manufacturer’s sales team is always recommended.
Industrial ovens are insulated enclosures or tunnels that operate at temperatures from slightly above ambient to 250°F (676°C). Oven loading configurations take two essential forms, batch or continuous, with the more common sources of heat being electricity, gas (natural or propane), steam, hot water, and fuel oil. As for the means by which heated air is introduced into the work area, it is commonly accomplished with forced convection.
The three essential categories of ovens are laboratory, industrial batch and continuous process -- with the distinguishing characteristics being construction, product handling functions, and flexibility.
Laboratory Ovens - Lab ovens have a typical temperature range of slightly above ambient to 650°F (343 °C) and can range in size from 2 to 32 cubic feet. Construction differs from industrial batch ovens in that lab ovens have positive latch doors, pressure release panels, stainless steel interiors, solid-state controllers & contactors, an epoxy/chemical resistant exterior coating, and an extended multi-year warranty. In terms of how a laboratory oven is utilized, its capacity lends itself towards test samples or light duty production of “smaller” parts and products.
Industrial Batch Ovens – Batch type ovens operate at temperatures from slightly above ambient to 1250°F (676 °C) and range in sizes from 3 cubic feet and up. The two general categories of industrial batch ovens are shelf/cabinet ovens and truck/walk-in ovens. Typical features include aluminized steel interior, fully adjustable duct work, scratch resistant enamel paint on the exterior, digital set-point controller, UL listed control panel, and a one-year warranty. Industrial batch ovens are suited for the processing of larger quantities of product in a single batch.
Conveyor Ovens – These oven units tend to be less flexible than batch ovens because they are usually designed for a specific product or production rate. Temperature ranges are the same as the industrial batch, up to 1250°F (676°C), and operate on a continuous or on an indexing basis through one or multiple heat zones. Conveyor oven units tend to be oriented toward automated production in greater quantities of small to medium sized products. The type of conveyance system depends on the product line, the volume of work to be produced, as well as the temperature to be obtained. See also our Whitepaper page on Batch vs Conveyor Ovens.
The type of air flow is an important and essential component to successful industrial oven selection and operation. If the wrong air flow is coupled with the wrong product-loading configuration, the results could be very undesirable. There are six basic types of industrial oven air flow that are commonly utilized:
Horizontal/Vertical air flow is when the air is supplied from the side walls of the industrial oven and is returned to a duct or opening on the ceiling of the oven. It is most applicable in those production situations where the parts are larger in nature and are loaded into the oven on a flatbed cart or skid.
Vertical/Horizontal air flow is when the supply duct is located on the oven ceiling and the return ducts are located on each side wall, and is typically used when a vertical type air flow pattern is needed in a truck or walk-in oven.
Both Vertical/Top Down and Vertical/Bottom Up air flow ovens are suited for those production scenarios where the product is going to be hung or when the parts are smaller in size, which are then placed on perforated shelves. It is important to remember that if shelves are to be used with this oven air flow pattern that they be designed so as to let the air pass from the supply to the return duct with minimal obstruction.
A full Horizontal Air Flow pattern is designed for those products that are loaded into the industrial oven on shelves or if the product is to be hung. This combination of loading configuration and oven air flow allows the recirculated air to pass above and below each shelf, encircling the product with air.
Finally, a Full Horizontal/Vertical oven air flow pattern is best suited for those applications that require extremely tight temperature uniformity in a horizontal/vertical air flow pattern. Or, when two shelf carts (or hanging carts) will be loaded into the unit side-by-side with a space between them, so the air may travel to a top return duct without obstruction.
All industrial ovens have one common element in the realm of air flow: the air needs to pass, with minimal restrictions, from the supply duct to the return duct. If properly adjusted, this will bring about good oven temperature uniformity, a topic of the next section.
While uniform temperatures within the industrial oven work area helps to ensure a uniform product, there are several issues that need to be described and defined:
Temperature Uniformity is defined as the greatest spread of deviation, in °F and/or °C, between the highest and the lowest points within a given work area. For example, it is important to note that ±5°F actually represents a difference of 10°F.
These are factors that impact work area uniformity:
Also, as mentioned under the Air Flow Pattern portion of this industrial oven guide, the loading configuration has to be properly mated with the appropriate air flow. If it is not, then the air may be cut off or reduced to such an extent that good uniformity within the oven unit may be lacking. For example, if a process has product that is set upon flat/solid shelves, the vertical air flow pattern would be the most inappropriate. In this case the air would be blocked and not allowed to follow the desired path to the return duct, and since industrial ovens transfer heat via the movement of air, the oven will not perform as desired. Continuing with the example, if flat solid shelves are coupled with a full-horizontal air flow pattern, the oven will perform properly with the air flowing above and below each shelf.
At this point, let's assume the proper oven air flow has been selected in relationship to the product and loading style at hand. Now it is important to identify those other major oven characteristics that improve the oven’s uniformity and performance.
The second point, which is closely linked to the first, is that the industrial oven fan and motor must be sized properly and rated for the oven’s rated maximum temperature. As the air rises in temperature, it becomes thinner and lighter; consequently, the oven motor and fan are still required to provide an appropriate amount of CFM. This is most easily accomplished with a fan that is rated to withstand the higher temperatures of the oven heat chamber.
The third major component is the interaction of the air, both fresh and recirculating, and the heat source. The return duct and fresh air inlet must be strategically placed so that the recirculated air and the fresh air meet at a given point. This should be on the negative side of the oven fan, prior to its continuing past the chosen source of heat. Upon exiting the supply duct, into the oven work area, some of the air will pass to the return duct and the rest will be ventilated out of the exhaust port, or forcefully exhausted out of the work area by a powered exhaust system. Furthermore, by positioning the fresh air inlet and the ventilation/exhaust opening properly, a slight positive pressure will develop, ensuring that fresh air is introduced into the oven unit at the fresh air inlet and not at possible leak spots (i.e. - door seals).
Note: Higher CFM, motor sizing, fan design, proper fresh air to exhaust ratios and placement all become increasingly critical to the successful balance of an industrial oven, especially at higher operating temperatures. With the proper equipment, design, oven components, and factory tests, consistently good results should be had over years and years of use. If you need help, along with oven maintenance and servicing, Precision Quincy also offers repairs, rebuilding and oven retrofitting.
Control Sensitivity – This is a second major issue, which is the ability of a control instrument to not only measure, but to react to, temperature fluctuations at a given set point. This issue is important for one simple reason: if an industrial oven’s instrumentation is not reading accurately, then it cannot respond accurately.
For example, an oven may have a uniformity reading of +2°F and a set point of 300°F, yet, the control sensitivity may be poor, with temperature "swings" over and below the set-point of 25 degrees. The result: the industrial oven will perform poorly, even with a tight temperature uniformity reading, due to the lack of control sensitivity and being able to hold the oven at a given set-point.
Naturally, one of the most important decisions to make when selecting thermal processing equipment is choosing the proper heat source for your process. The following list represents the six more commonly used sources of heat for industrial ovens:
Electric Heated Ovens - These industrial oven units are the most prevalent, are powered by Incoloy sheathed heating elements, and expressed in terms of KW, which are manufactured to provide long life and quick heat-up times. Also, electric industrial ovens, when in a Class “B” configuration, are not as costly to purchase, are clean/non-polluting, and are more appropriate for those applications where a direct gas fired heat source cannot be applied successfully. An example of just such a process is the aesthetic production of aluminum, which would be “yellowed” or marred by the products of combustion produced by a direct gas fired oven unit. In this case an electric heated oven would be the most appropriate.
Direct Gas Fired Ovens – These industrial ovens are another very common source of heat, expressed in BTUs/Hr., and can be equipped to handle either natural gas or propane. These ovens can be more expensive than electric oven units to purchase due to the fact that they are Class “A” automatically, due to the products of combustion. In that sense, gas industrial ovens offer more flexibility due to the presence of the required NFPA safeties (to be discussed later). Finally, to offset the initial purchase price, the cost of operating a direct gas fired industrial oven is only 25-30% of their electric heated counterparts, which can pay big dividends in the long run.
Indirect Gas-Fired Ovens – These industrial ovens fire into a stainless steel heat exchanger that is then vented to atmosphere, allowing for a clean process by keeping the products of combustion from the work area. As compared to direct gas fired ovens, the initial purchase cost is greater and their efficiencies are rated at approximately 70 percent yet, they are still twice as efficient as an electric heated equivalent and cost less to operate.
Steam Heated Ovens – These industrial ovens present a viable alternative when the appropriate circumstances present themselves. The first component is when a facility already has steam in sufficient quantities, measured in PSI, to obtain the desired temperature. Furthermore, the application must be in the lower temperature ranges of approximately 300°F or below, or the process cannot accept a “red hot” source of heat (heater elements or a burner); or when quick time-to-temperature is not of critical importance. In this scenario, steam would not only be very cost efficient, but would also be a very clean means of processing product.
Hot Water Heated Ovens - This source of heat is similar to the steam heated unit mentioned above except that the temperature range tends to be more constricted, approximately 160°F and below. Essentially the hot water will travel through “radiator coils” in sufficient quantities, it will emit heat and the recirculating air will be warmed and carried into the work area. In terms of efficiency, cleanliness and compatibility with certain processes, hot water heated industrial ovens provide a very viable alternative in lower temperature ranges when a quick heat up is not critical.
Oil Fired Ovens - Fuel oil is typically used when natural gas is unavailable and electricity is too costly to operate however, as a source of heat, it has several drawbacks. First, the initial purchase price tends to be higher than the gas fired or electric heated industrial ovens. The byproduct of burning fuel oil is soot, which will eventually recirculate through the oven work area if the oven is not cleaned or maintained on a regimented basis. Also, using an oil fired oven is a dirty process from the standpoint of what is exhausted into the atmosphere and the environment. An increasingly popular alternative to fuel oil is propane, which costs less to purchase, is more cost efficient to use, and the maintenance costs are lower.
To further protect your operation, see our Whitepaper page on Industrial Oven Care And Maintenance.
All industrial oven manufacturers are required to follow the guidelines as specified in the NFPA 86 Code Book. Essentially there are two oven classifications and definitions that need to be considered and defined:
Class B Industrial Ovens - A Class “B” oven unit is that which is not intended or manufactured for the processing of solvents or volatile materials. A possible process that would be Class “B” would be the annealing of plastics, which produce no off-gassing or the stress relieving clean metal parts.
Class A Industrial Ovens - The first grouping is Class “A”, which includes those oven units that are equipped to handle and process solvents, volatile materials or combustible materials. Specifically, this rating is determined by calculating the volatile type, gallons per hour, and operating temperature. An example of a Class “A” application would be the curing of a solvent based paint or the curing of a rubber by-product.
Another area that needs to be examined is the way in which your industrial oven manufacturer complies and meets with the appropriate NFPA 86 Codes.
Typically, all Class “B” ovens are either electric, steam or hot water heated - not gas or oil fired. Essentially, a Class “B” unit will include the following safeties:
Conversely, all gas (natural or propane) and oil-fired units are Class “A” as standard and include the following safety features:
In terms of electric heated Class “A” ovens, they are equipped with the following safety equipment:
Note: volatile ratings are never to be exceeded. Physical injury or death may arise if the volatile ratings are not strictly followed.
The five specifications that insurance companies and government agencies may want businesses to meet are the following:
NFPA 70 – As a counterpart to NFPA 86, in its National Electrical Code book NFPA 70 addresses the required safeties, standards and rules that industrial oven manufacturers must obtain or surpass as they relate to electrical components and installations in the workplace.
Underwriters Laboratories (UL) - This is a group founded to test for and analyze the results of product manufactured and sold in the United States. As it pertains to the industrial oven industry, UL is primarily interested in the use and integration of electrical components and controls.
Factory Mutual (FM) - This is an association of mutual insurance companies that are dedicated to loss prevention. Many of its efforts are made through investigations, analyzing, and determining those means by which fire, and other losses, can be greatly reduced.
Industrial Risk Insurance (IRI) - Formerly the Factory Insurance Association (FIA), IRI comprises member stock insurance companies concerned with all phases of fire protection and other perils. It constantly reviews and studies methods of making the industrial workplace safer.
Occupational Safety and Health Administration (OSHA) - This federal agency concerns itself with a variety of different issues; examples of this are motor guards, exterior skin surface temperatures, insulating materials, and general industrial oven construction.
A quality industrial oven manufacturer will not only have some of the above specifications (that is: OSHA & NFPA 70) as a standard feature, but also the capacity to completely build to the additional specifications mentioned.
An additional industrial oven category that needs to be considered is that of explosion-proof. An explosion proof industrial oven is manufactured with specialty rated electrical components, gas train, control panel, as well as with other rated equipment. Other features include spark resistant fans & motors, explosion-proof wiring/conduit and control equipment.
An example of a work situation that requires an explosion proof oven is when the oven is near an open paint spraying application that is within “...20 feet (6.10 m) horizontally and 10 feet (3.05 m) vertically...” Naturally, these rules change from year to year and edition to edition, therefore, it is critical that you either refer to the NFPA 70 Code book, or contact the factory and let them analyze your situation and determine if the industrial oven must carry an explosion-proof rating.
Industrial batch ovens have multiple means by which to load product, the most popular of these are:
Batch Oven Shelves – These are available in a couple of configurations; typically, batch oven shelves rest on rails that are attached to the side of the industrial oven through the use of keyhole slots. As a variation on this theme, roller shelves can be an important feature when a load has to be removed from the industrial batch oven with minimal effort or when product has to be picked up with an overhead hoist or crane.
Batch Oven Carts – These are used for truck-in or walk-in style industrial ovens where the carts typically come in three different configurations -- shelf, hanging, or flatbed. Either style of batch oven cart can be rated for different processes and varying weights. Each style of cart tends to be more compatible with a particular oven air flow, as mentioned earlier in the air flow pattern section of this industrial oven guide.
One of the more critical considerations, Oven Cycle Dynamics refers to the following four sub-issues:
Several factors must be considered and calculated to determine the necessary amount of heating capacity for a specific material during a given time frame:
Let’s assume we are curing hard rubber, which has a mean specific heat of 0.33, in 1,000 pound loads at 350°F (177°C) for one hour. Following are the formulas to determine the proper capacity for this example:
In order to raise the temperature of the load to 350°F, it is important to apply the following equation:
Product Wt. x (Operating Temperature °F - Ambient Air °F) x Mean Specific Heat ÷ Cycle Time = Btu/Hr.
That is: 1,000 pounds x (350°F - 70°F) x 0.33 Btu ÷ 1 hour = 92,400 Btu/hr.
Let's assume we need 100 cfm of powered exhaust in order to comply with NFPA 86 Codes and remove the appropriate amount of fumes from the cycle, the following equation would be applied:
Exhaust system cfm x (Operating Temperature °F - Ambient Air °F) x Correction Factor = Btu/hr.
That is – 100 cfm x (350°F - 70°F) x 1.08 Btu/Hr. = 30,240 Btu/Hr.
Let’s assume we are using a standard walk-in oven (72” x 72” x 72”) with a four inch insulated wall and four pound density Rockwool insulation, the total wall losses for one hour would be as follows:
Total Sq. Ft. of Oven Exterior x Btu/Hr. Loss Per Sq. Ft. of Oven Exterior = Total Btu/Hr. Wall Losses Per Hour
That is – 277 Square Feet x 70 Btu/Hr. per square foot = 19,390 Btu/Hr.
By adding the above totals, we're able to determine the minimum amount of Btu/Hr. needed in this application:
If a person wants to think in terms of kilowatts, simply divide the total number of BTU/Hr. (i.e., 142,030) by 3,412 and arrive at the minimum number of kW necessary. In our rubber curing example, the minimum number of kW needed would be 42.
Once the appropriate amount of Btu/hr. or kW has been determined, it's important to assess how the set-point (350°F) should be reached.
A desired temperature can be obtained in two ways:
We know that the recirculating air within an industrial oven will reach temperature faster than the product load. Therefore, soaks can be viewed as a technique that allows the product temperature to catch-up to the recirculating air temperature.
For example, the oven air reaches 350° F and the product is only at 280°F. At this point, a thermocouple attached to the product or placed within the work area senses the discrepancy in temperature and stops the oven from rising in temperature until the controller detects that the product has obtained 350° F as well. At that point, the timer begins to count until a predesignated time has been reached, thus providing for an assured soak. As mentioned above, programmers allow for ramps & soaks, which allow for the coordinated and controlled increase in product temperature as it relates to the recirculated air temperature.
Cool down is categorized as the removal of recirculating and residual heat from the industrial oven work area. Normally, this is accomplished by the exhausting of the hot air and the introduction of ambient air. If this rate in decline does not have to be controlled, then the opening of the adjustable dampers on the ventilation/exhaust system and the fresh air inlet ports will suffice. However, if the cool-down has to be controlled then there are two basic options:
When calculating industrial oven capacity, heat-up time, soak parameters or cool-down segments, remember that much depends on the product configuration and loading pattern as it relates to the selected air flow type and the way in which the air interacts with the product.
Space conservation is a significant issue. Selecting the proper heat chamber location (walk-in ovens), door type (walk-in and cabinet ovens), or both can prove critical.
a) The first style, bi-parting horizontal swing, is the most common, is the least costly to purchase and is widely found throughout industry. However, when allowing for the physical size of the industrial oven, a person must take into consideration the swing of each door. This can represent a significant amount of space that must be kept free or empty in order for the door to travel unobstructed.
b) The alternate door style: vertical lift, all but eliminates this type of scenario. With the door traveling straight up, there is no “swing” to contend with, which means the area to the front/side of the oven may be used for other functions. This also applies to cabinet style ovens; that is, door swing is eliminated and the area can be used more advantageously.
As stipulated by NFPA 86, section 2-1.5, industrial ovens that operate at temperatures over 300°F do require an insulated floor. At this point, it is important to discuss the multiple floor options available as well as cart guide tracks:
Multi-Inch Insulated Oven Floor - These floors come in different intervals and are rated for certain temperature ranges: 2.5 inches (rated at slightly above ambient to 750°F/399°C); 3 inches (rated at 850°F/454°C to 1,000°F/538°C); 6 inches (rated at 1250°F/676°C). Insulated oven floors represent the utmost in flexibility due to the fact that if the oven has to be moved, the insulated floor automatically moves with it.
Plate Floor Over Insulating Cement - This type of floor is a viable option when a multi-inch insulated floor and cart guide tracks are not an option (often due to varying cart track centers). Plate floors are available on all units, even those rated at 1250°F/676°C. The benefit of the plate is the industrial oven chamber becomes an enclosed area, which is not as susceptible to concrete dusting or powdering. A possible drawback to this type of technique is if the oven has to be moved since insulating concrete must be poured again at the new oven location.
No Floor And Insulating Cement Only - This is acceptable when a process isn't sensitive to the possible effects of powdering. It also proves to be advantageous when there is little or no chance of the oven being moved. If the industrial oven is moved, high temperature concrete has to be poured again.
In terms of loading product, cart guide tracks, a flat channel or v-groove configuration is most common and available on all walk-in styles, except those operating at 1250°F. Tracks aren't available because of the extremely high operating temperatures and the corresponding need to protect a normal plant floor. Such a high temperature unit will have its batch loaded via a forklift and a high temperature stainless steel pallet. In the eventuality that guide tracks are not desirable, a ramp and an insulated oven floor reinforced to the appropriate weight are viable alternatives.
Numerous elements need to be considered in industrial oven construction, more than could possibly be listed here. The following topics represent the more significant elements:
Industrial Oven Exterior - An oven exterior should be constructed of heavy gauge cold rolled steel and primed and painted with chip/scratch resistant enamel paint. Under non-corrosive circumstances, this will provide for many years of corrosion resistance. In the eventuality that an operating environment has corrosive fumes, a stainless steel exterior is recommended.
Industrial Oven Interior - An oven interior ought to be of aluminized steel, not cold rolled steel with aluminized paint. Simply put, aluminized steel is manufactured to resist the corrosive effects of moisture and heat, as well as other materials. In addition, aluminized steel doesn't require the maintenance, and doesn't flake, peel or contaminate a given process, like aluminized paint may. Naturally, if a process contains acids or corrosive materials, a stainless steel interior should be selected.
Oven Ductwork - The ductwork of an industrial oven, in terms of materials, follows the same guidelines as the oven interior prescriptions made previously. The style of quality ductwork can be described as louvered or as slotted. Both are fully adjustable (without removal of the ductwork) and allow for the fine-tuning of air flow and temperature uniformities within the oven work area. In addition, an impinging style of oven ductwork may be necessary. Available in all of the air flow patterns mentioned previously, impinging air is applied to those applications where a part has grooves or crevices that must receive a very high rate of air flow and wouldn't be properly served with a more ‘typical” style of ductwork.
Oven Insulation & Wall-Panel Construction - These are the two key elements that allow an industrial oven, which is running at 1250°F, to remain cool to the touch on the oven’s exterior. Quality oven insulation should be the proper density, oversized and compressed into the panel to avoid sagging. Some companies use a heavier pound of oven insulation and make the oven wall thinner; this produces a thermal processing unit that has poorer structural rigidity or integrity compared with the thicker oven wall. The physical strength of an industrial oven is found in the wall panel construction and the overall frame. Finally, the wall panel should allow for minimal transfer of heat to the outer skin.
Oven Control Panel - The control panel is of significant concern because it's the primary component with which the operator will interact. Consequently, a high quality control panel must possess the following characteristics:
We hope this industrial oven guide has served as a primer to provide some insight and useful information about industrial process ovens.
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