You have a sample measuring 200 × 150 × 100 mm and need a furnace that will accommodate it. Can you simply convert its volume into litres and choose a model accordingly?

No. A low, wide chamber and a narrow, deep working space can have the same volume. A sample may therefore fit easily into one ten-litre furnace but not fit into another furnace of the same volume at all.

In addition to the internal dimensions, the weight of the load, loading method and the clearance required around the samples for the process to run correctly are also important.

What does laboratory furnace volume mean?

Volume in litres expresses the approximate geometric size of the working space. In LAC laboratory furnaces, it is also often reflected in the model designation.

For example:

The number before the slash does not always correspond exactly to the actual volume. The LE series, for example, includes the LE 09/11, with an actual chamber volume of 9.4 litres, and the LE 15/11, with a volume of 14.4 litres.

Volume is therefore useful for an initial comparison, particularly between models in the same series. To check whether a specific load will fit inside a furnace, you need its internal dimensions.

Why are internal dimensions more important than litres?

Consider two models with a similar chamber volume:

Both working spaces have a volume of approximately nine litres. The first is narrower and deeper; the second is wider and shallower.

The LE 09/11 may be more suitable for a long sample, while the chamber shape of the LH 09/13 may better accommodate a wider item. The number of litres alone does not reveal this difference.

A similar comparison can be made between muffle furnaces:

The LMH has a horizontal working space and is loaded from the front. The LMV has a cylindrical vertical muffle into which the sample is lowered from above. The difference is therefore not simply two litres, but a completely different shape and handling method.

What must be included in the load dimensions?

The furnace load is more than the sample itself. You will usually place additional items in the furnace, such as:

If the component itself is 150 mm high but stands on a 20 mm base, you need to allow at least 170 mm. Additional clearance may be required for safe loading and suitable process conditions.

There is no single universal value for the required clearance. It depends on:

• The construction and position of the heating elements
• The method of heat transfer
• Atmosphere flow
• Sample shape
• Number of items
• Temperature-distribution requirements
• Handling of the load

Total weight can become the limiting factor before volume

A large but lightweight sample may impose a smaller load on the furnace floor than a small metal component or several heavy crucibles.

In addition to chamber volume, datasheets therefore specify the maximum floor load. This varies between furnace series and sizes.

According to the current datasheets, for example:

This weight must include everything supported by the furnace floor: the sample, containers, bases and any other fixtures.

A larger chamber volume does not automatically mean a proportionally higher load capacity. According to the datasheet, for example, all standard LE models have a maximum floor load of 6 kg even though their chamber volumes range from 5 to 14.4 litres.

Maximum floor load is not the same as area load

The total load capacity indicates how much weight the furnace floor can support. It does not mean that the entire weight can be concentrated at a few small points.

Imagine a heavy sample standing on four narrow feet. Its total weight may be within the maximum floor load, but the load is concentrated over a very small area.

If the load has a small contact area, sharp edges or point supports, mention this in your enquiry. A suitable base or another method of distributing the weight may be required.

Its material and construction must, however, be suitable for both the operating temperature and the chemical environment of the process.

How does load weight affect heating?

Weight affects more than the furnace floor. It also influences the temperature cycle.

A heavy load generally takes longer to heat through. The controller measures temperature with a thermocouple positioned at a particular point in the furnace; it does not automatically measure the temperature inside the sample.

The result depends on:

• The sample material and weight
• Its thickness and shape
• Its initial temperature
• The container used
• The heating rate
• Its position within the working space

If the specified temperature must also be reached inside the load, the process may need to be verified by measurement or a practical test.

The furnace’s rated power alone does not tell you how long your specific load will take to heat through.

How many samples can be processed in one cycle?

This cannot be determined from chamber volume alone.

It depends on:

If you need to process twelve identical crucibles, send their dimensions and proposed arrangement. A simple sketch is usually more useful than saying: “We need a furnace of about ten litres.”

A furnace with sufficient mathematical volume may not have the right footprint for the required number of containers.

The loading method also matters

Working-space dimensions are only part of the issue. You must also be able to place the load inside safely and remove it again after processing.

The standard LT series uses tube bores of 50, 75 or 100 mm and heated lengths of 300, 500 or 750 mm, depending on the model.

The sample’s position within the heated zone and any required tube closure must also be considered.

Internal dimensions are not the external dimensions of the furnace

A small working chamber does not mean that the complete furnace will be equally small.

Insulation, the heating system, casing, controller and electrical components can make the external dimensions considerably larger. The LMH 07/12, for example, has internal dimensions of 170 × 170 × 275 mm, while its external dimensions are 490 × 540 × 720 mm according to the datasheet.

The furnace must not only accommodate the sample. It must also pass through the laboratory door and fit safely on the prepared bench or stand.

How much spare capacity should you allow for the future?

It is sensible to consider whether the number or size of samples may change later. An unnecessarily large furnace should not, however, be selected merely “to be on the safe side”.

A larger model may mean:

• A higher purchase price
• More demanding space requirements
• Higher rated power or a different electrical connection
• A longer or otherwise different real-world cycle
• More difficult handling

Define the expected future requirement specifically. For example:

This makes it possible to assess the actual size required. A general request for “a larger furnace just in case” is less useful for selecting the correct model.

Frequently asked questions

Measure the complete load first

Chamber volume in litres helps you compare model sizes quickly. The suitability of a furnace, however, is ultimately determined by the internal dimensions, chamber shape, total weight and loading method.

Remember to include containers, bases, holders and the clearance required for handling. For heavy samples, check not only the total floor load but also how the weight is distributed across the floor.

What to remember

Chamber volume is useful for an initial comparison, but it does not determine whether a specific load will fit.

Measure the complete load, check its total weight and contact area, and consider the required clearance and loading method before selecting a furnace.