GUIDE TO HEAT PROCESSING EQUIPMENT

This guide discusses general issues related to selecting an industrial oven for heat processing. The appropriateness of the guide is evident when you consider that almost all products manufactured today require the application of heat at some point during the manufacturing process.
The complexity of issues related to selecting the optimum equipment for any particular process indicates the need for a universal guide as a starting point.

Process Description

Heat processing applications vary widely from industry to industry. Curing, drying, heat treating, sterilizing and bonding represent just a few of the many requirements for heat processing.

Basic Oven Considerations

Ovens generally are classified as heating equipment operating from ambient to 1000°F
(538° C), Ovens may be designed for intermittent loadings (a batch at a time), or for a continuous flow of work using some form of conveyor. The source of heat is normally derived from steam, hot water, electricity or from combustion of fuel (gas, oil, etc.). Heat can be transferred to the work primarily by natural gravity, forced convection, conduction or by radiant heat sources. Ovens may be designed to contain special atmospheres such as argon or nitrogen, or incorporate special materials of construction to lend themselves to the specific application.

The quantity of material to be processed; the uniformity, size and shape of the products; whether the products lend themselves to logical grouping for batch processing or are better suited to continuous in-line processing; the temperature tolerance that is permissible: these are all issues that must be considered when selecting an oven.

Batch Ovens or Furnaces

Batch-type ovens represent the largest category of ovens used to manufacture products. Batch-type ovens can be classified as cabinet-style or truck-loaded type. The size can range from small bench top units to large industrial installations with thousands of cubic feet.
Bench-mounted and cabinet-type ovens are most often used for laboratory applications such as sterilizing, curing, drying and other general laboratory activities. They range in capacity from 2 to 24 cubic feet. Typical temperature ranges are from 100°F to 650°F (380°C to 343°C). Usually, these ovens are used in lighter duty applications than are industrial production ovens.

Ranging in size from 3 cubic feet interior volume and up, production-type cabinet ovens are used extensively for curing, baking, drying, finishing and annealing processes and an almost limitless number of other applications.

Box-type furnaces are utilized for higher temperature processes (heat treating or annealing). Often a vertical lift door is employed to keep the hot face of the door away from the operator, and for floor space savings.

Continuous Ovens/Furnaces and Material Handling

Continuous ovens include the entire spectrum of oven equipment operated on a continuous or indexing basis. These ovens have construction characteristics that generally are similar to batch ovens. The distinguishing characteristics of continuous ovens include such items as the means of conveyance, air distribution techniques and product loading methods.

Some of the common types of continuous conveyor ovens classified by their method of conveyance are:

The type of conveyor selected for a continuous oven or furnace depends almost entirely on the type of products to be processed and their configuration at the time of processing. Monorail conveyors are adaptable to a wide variety of workpieces, because many individual workpieces can be positioned on fixtures or racks suspended from the monorail.

Heat-Up/Soak/Cooldown Times

Heat-up, Soak, and Cooldown times are critical elements to consider for selecting the correct equipment. There are three primary design considerations:

1. What is the required heating capacity to bring the product to the desired temperature within the specified cycle time?
2. Can the product absorb heat at a rate sufficient to reach temperature within the specified time frame?
3. Must the heat-up rate be at a controlled rate, or is it sufficient to allow the product to reach temperature as quickly as possible, given the oven's heating capacity?

Temperature Uniformity

Oven temperature uniformity has different definitions depending on the type of oven and application in question. A basic definition is the overall temperature variation in the oven workspace. Uniformity is generally stated as +/- °F or +/-°C at a given setpoint temperature. The obvious advantage of tight oven uniformities is that all parts within the oven will be subject to the same temperature. therefore insuring consistent product quality.

Oven characteristics that affect uniformity are: wall losses, including through-metal; air distribution and the volume of airflow; control accuracy; oven openings; construction techniques. In order to minimize wall losses, insulation thickness should vary depending on the maximum temperature and uniformity required. Through-metal loss should be kept to an absolute minimum by special panel and unitized construction.

Be sure that oven openings for fresh air and exhaust are strategically located. The location helps to provide a positive pressure differential in relation to the outside of the oven so cool ambient air introduced into the oven through door seals is minimized. The fresh air opening should also be located so that the fresh air can mix thoroughly with the recirculated air.

Similarly, the oven airstream should be designed so air passing through the heating elements is adequately mixed before entering the work chamber. if fresh air is insufficiently mixed with recirculated air, air layers at different temperatures, called air stratification, will affect oven uniformity. (Air duct design, placement and geometry also contribute to uniformity).

Types of airflow

When selecting an airflow pattern, the most important consideration is the load configuration. The main goal is to minimize obstructions to the airflow for more uniform heat distribution and to maximize the product surface area with which airflow will come into contact.

Horizontal airflow, supply on one side and return on the other, is most often used for a product that is tray or shelf loaded. The load is such that air can pass above and below.

Vertical airflow, supply at top or bottom and return opposite, is for products that allow air to pass vertically through or around. Products may be shelf loaded, suspended from stationary hooks or monorails, or placed on conveyor belts.

Uniflow airflow, supply along both lower sides and return at the top, is a combination of vertical up and horizontal airflow and is best suited for larger products with an uneven shape. Products are usually truck loaded with this type airflow.

Atmosphere Type

Thermal process ovens utilize different atmosphere medium in order to satisfy certain test criteria. For example, inert atmosphere ovens are generally utilized when a test requires a low oxygen concentration in the oven due to oxidation of the test parts at elevated temperatures. The inert gas is injected into a sealed chamber, pressurizing the oven and replacing the oxygen. With proper construction, special atmospheric ovens such as argon or nitrogen injected ovens can be built to control the oxygen level below 500 parts per million (PPM). Construction techniques like high integrity welds, special fabrication methods and special motor shaft seals are examples of such construction.

Humidity may be added to an oven to achieve controlled moisture removal rates and to speed curing of certain compounds. The addition of moisture into the oven to maintain humidity is accomplished in two ways. Either water is boiled in a steam generator and directly injected as steam into the oven or water is sprayed through atomizers into the oven to maintain a specified humidity level. Ovens built to maintain high humidity levels must be constructed with a stainless steel interior and have continuously backwelded seams to prevent rusting and migration of moisture into the insulation. (Backwelded seams refer to a technique whereby inside seams are welded on the insulation side of the wall.)

Inert Atmosphere Oven

Process Control and Monitoring

One of the most misunderstood concepts in heat processing involves instrumentation accuracy for temperature control and monitoring. Because microprocessor control offers extreme accuracy, typically one tenth of one percent, it is assumed by many that is accuracy they will achieve. However, like any other precision device, quality of output depends on quality of input.

Since these instruments are capable of converting only the information they receive, the most critical factors in temperature measurement are the sensor itself and where it is located in the oven. Sensor accuracy is a function of the specific sensor selected and can be compensated for by calibrating the instrument-sensor combination,
The most important consideration is the positioning of the sensor. The sensor can sense the temperature only at its location. Therefore, if the sensor is not strategically located, or if the oven temperature is not uniform at the sensor location, the temperature displayed on a highly accurate instrument will give an inaccurate indication of part temperature.

Oven Construction

Generally, a well constructed oven will be of a type suitable for the temperature range of the oven and the environment in which it will operate. Look for these standards: a mild steel exterior finished in a scratch resistant paint, sufficient insulation to minimize heat loss, easily readable controls, and a door system with sufficient thermal expansion and structural integrity to avoid warping.

Construction may be particularly important when a corrosive material is to be processed through the oven, or when possible contamination of the work load can occur. You will need a stainless steel interior whenever high degrees of cleanliness, cleanability and resistance to corrosion are required. This type of interior includes stainless steel material throughout the air stream portion of the oven and in the heat chamber itself.

Aluminized steel interiors provide a thin layer of aluminum fused to a steel surface. This surface resists corrosion from moisture and other sources by forming a thin oxide coating on the aluminum to protect the underlying steel.
Mild steel interiors with corrosion resistant aluminum/silicone paint are acceptable for drying above 212°F (100°C) and for general non-corrosive heating/curing operations.

Several process specific factors need to be taken into account in regard to oven construction, Maximum temperature of the oven and size of the work chamber will affect the construction methods used. Number and type of expansion joints used will be affected by these as well. As indicated in the discussion under uniformity, fan size will depend on process needs and your oven may require special door seals and breaker strips to get the uniformity desired. Surface temperature specifications may require special construction techniques and load support design will depend on the type and weight of the load.

Also, inert atmosphere ovens require special construction techniques for expansion because interiors are continuously welded.

Door seal design is determined by maximum temperature, size of the opening and door style. Lower temperature ovens often use two point silicone seals. Higher temperature ovens use a combination of fiber glass or ceramic and silicone. in addition, the seals can be water cooled if necessary.

Be sure to specify vertical lift doors (with pneumatic locking cylinders) for larger oven openings. They are more convenient to use and easier to seal.