You need to dry components, preheat moulds, heat-treat metal or melt aluminium. You also know the temperature you need to reach. However, that is not enough to select the right industrial furnace.

A furnace may offer the required chamber volume and maximum temperature, yet still be unsuitable for your product, production rate or handling requirements.

If your enquiry only specifies “a 500-litre chamber furnace for 800 °C”, essential information is still missing. A lightweight load arranged on shelves may require a different furnace from several heavy components. Heating in air differs from processing in a protective atmosphere. Simply reaching the target temperature is also different from heating the entire load uniformly within a narrow temperature range.

A well-prepared specification helps prevent you from purchasing equipment that is unnecessarily oversized or unable to achieve the required results under normal operating conditions.

First decide what result you expect from the furnace

The word “heating” can refer to many different processes. Industrial furnaces and ovens are used to:

• remove moisture from the load,
• cure coatings or other materials,
• preheat moulds, castings or semi-finished products before further processing,
• anneal, harden, temper or artificially age materials,
• process metals in a protective atmosphere,
• melt metal or hold molten metal at the required temperature.

Each of these tasks places different demands on the equipment. The correct solution depends on the heat-transfer method, atmosphere circulation, working-chamber design, loading method, ventilation and connection to cooling or subsequent operations.

If an important process factor is overlooked, the problem may only become apparent during production. Components may not heat uniformly, the cycle may take too long, the surface may undergo unwanted changes, or the load may be difficult to insert and remove safely.

Seven questions to answer before choosing a furnace

1. What needs to be different about the product at the end of the cycle?

Specify the material and the name of the process if you know it. It is even more helpful to describe the required result. Do you need to dry or preheat a component, alter its properties through heat treatment, limit surface changes or melt a metal?

For metals, provide the exact material or alloy whenever possible. A general request for “heat treatment of steel” is not enough to determine the temperature, atmosphere or process profile. The process parameters must reflect the requirements of the specific material and product.

2. Do you only need to reach a temperature, or must you follow a complete process profile?

The maximum furnace temperature is a design limit. For production, the actual operating cycle is more important:

• starting and target temperature,
• required heating rate,
• holding time,
• cooling method and rate,
• any repeated process steps.

If you already use a temperature profile, include it with your enquiry. If the profile has not yet been defined, describe the required result and the individual production stages. This information provides a useful starting point for developing a suitable solution.

A furnace selected only according to the target temperature may be capable of reaching that temperature but unable to provide the required heating rate, holding time or subsequent cooling. In practice, this can lead to longer cycle times or inconsistent results between batches.

3. Will the load fit inside the furnace—and will it heat correctly?

Chamber volume in litres does not tell you whether the components can be arranged correctly or how they will heat. The following information is particularly important:

• dimensions of each component,
• number of components in one batch,
• total load weight,
• arrangement or suspension method,
• material and weight of baskets, shelves, fixtures or bogies,
• a photograph or drawing of a typical load.

Do not overlook the weight of the fixtures. They heat up together with the products and affect the cycle. The spacing between components and the ability of the atmosphere to circulate around the load are equally important.

For furnace design, a drawing or photograph of a normally loaded batch is therefore more useful than a chamber volume stated only in litres.

4. What happens to the surface, and what is released from the load during heating?

Air is suitable for many processes. In other cases, the surface must be protected against unwanted changes or processed in a more precisely defined environment. This may require a gastight retort furnace with a supply of the appropriate protective gas. The exact design depends on the temperature, process and other operating conditions.

If the appearance or condition of the surface after processing is important, include this requirement in the specification just as you would the temperature.

For drying and curing processes, you must also specify which substances are released from the load during heating. If flammable vapours may be produced, this must be addressed during equipment design, not after delivery.

5. Must every component in the batch meet the same temperature requirement?

The temperature shown by the controller is the value measured by the control thermocouple. It does not prove that every component—or the core of each component—has reached the same temperature.

The result is influenced by factors such as:

• load size and weight,
• arrangement within the working chamber,
• shielding of individual components,
• atmosphere circulation,
• position of the heating elements,
• holding time after the set temperature has been reached.

Forced convection furnaces and industrial ovens are used where atmosphere circulation supports heat transfer and temperature distribution within the working chamber. Even then, the process cannot be assessed without considering the actual load.

In very wide SV Mk.II chamber ovens, for example, a temperature gradient may develop during heat-up, depending on the dimensions and nature of the load. The temperatures gradually equalise during the holding phase. With powders or freshly applied coatings, it is also necessary to check whether the airflow could disturb or blow material off the components.

6. Can operators load, unload and cool the charge safely?

Large or heavy loads may require a loading bogie, crane, handling fixture or pit-furnace configuration. This is not simply a matter of operator convenience. The handling method affects the usable space, furnace design and arrangement of the entire workplace.

For hardening, the furnace, quenching equipment and transfer between the furnace and quench tank must be considered together.

For melting furnaces, it is important to determine whether molten metal will be removed from a stationary crucible or whether the operation requires a tilting or transport furnace.

Your specification should therefore include information about:

• available handling equipment,
• required loading height,
• space available around the future furnace,
• subsequent cooling or production operations.

7. How much production must the equipment actually handle?

The furnace must meet more than the requirements of a single cycle. It must also provide the required production capacity. Specify:

• number of batches per shift or per day,
• required cycle time,
• expected weight of processed material,
• single-shift, multi-shift or continuous operation,
• available installation space and energy connections,
• requirements for exhaust ventilation, documentation and process recording.

How the specification affects the equipment type

The following table provides initial guidance only. The particular furnace series and configuration must be selected according to the complete load and operating cycle.

Common dead ends when requesting a furnace

Selecting a furnace only by its maximum temperature

The maximum temperature defines a design limit, not the complete operating process. It says nothing about the atmosphere, circulation, temperature uniformity, handling or cooling.

The result may be a more expensive furnace with unused temperature capacity but without a feature that is essential for your production.

Selecting a furnace only by chamber volume

The same chamber volume may work very differently for small components arranged on shelves and for one heavy component placed on a fixture.

The internal dimensions, arrangement and weight of the complete load are what matter. A poorly selected chamber may remain partly unused or force operators to place components too close together.

Assuming that the controller value is the temperature of every product

The controller operates using the value measured by its sensor. The rate at which the components heat through depends on their material, dimensions, weight and arrangement.

If the process result is sensitive to temperature differences, it should be verified by measurement.

Treating a protective atmosphere as an accessory that can simply be added later

A gas inlet alone does not create a defined operating environment. The result depends on the tightness of the system, retort design, gas inlet and outlet arrangement, temperature and any required atmosphere circulation.

If surface protection is only considered after the furnace has been selected, it may not be possible to achieve it through a simple subsequent modification.

Overlooking handling and subsequent operations

A furnace may meet the temperature requirements and still be unsuitable for the operation if the load cannot be inserted safely and within the required time, or if the equipment cannot be connected effectively to quenching, molten-metal handling or the next production step.

If some of this information is not yet available, that does not prevent an initial consultation. The important thing is to describe the actual product, required result and normal operating conditions.

The more specific the information supplied to the manufacturer, the more accurately a standard solution can be compared with a custom configuration.

If you are unsure, test the process with the actual load

For a new production process, it may not be clear in advance how quickly the load will heat through, whether the temperature will be sufficiently uniform or how the material will behave during heating.

What to remember

Maximum temperature and chamber volume are only two parts of the specification. The correct industrial furnace must match the material, load, atmosphere, heating and cooling profile, handling method and required production capacity.

Start with the process and the required result. The furnace model should come afterwards.