Furnace Design

Furnaces have to be designed for particular applications since they have to be compatible with the measuring system and also with problems associated with each technique, for example, convection currents in TG. However, there are some general observations that can be made, first, regarding the thermal capacity of the furnace. If a large furnace is used, then one is going to have a range within the furnace at which a uniform temperature can be recorded. Small furnaces, however, will not have this uniform range of temperature, and the positioning of the sample in smaller furnaces becomes quite important. The second point about the size of the furnace is that large furnaces will take considerable time to reach a particular temperature and also take some time to cool down. The smaller the furnace, the easier it is to cool the temperature of the furnace back to ambient. The sample and reference material in differential measurements must also be subjected to the same temperature change. This generally involves a design feature involving both the unit in which the measurements are made and the furnace itself. One further point about furnace size and shape is that a long narrow furnace will generally give a larger uniform hot zone than a short wide furnace of similar volume. All instrumentation furnaces are electrically powered (although high-frequency inductive and infrared heating furnaces have been put on the market). The resistance wire is generally coiled around an insulating packing. The outside of the furnace is generally well insulated, although this has a bearing on the rate at

TABLE VIII Upper Temperature Limits for Common Furnace Resistance Elements

Furnace

Temperature

winding(s)

limit ( C)

Atmosphere"

Nichrome

1000

A

Chromel A

1100

A

Tantalum

1330

B

Kanthal

1350

A

Platinum

1400

C

Platinum-10% rhodium

1500

C

Platinum-20% rhodium

1500

C

Kanthal super

1700

A

Rhodium

1800

C

Molybdenum

2200

D

Tungsten

2800

D

a A—an oxidizing atmosphere can be used (oxygen or air); B—a nonoxidizing atmosphere can be used (inert or vacuum); C—in these cases oxygen or air can be used at lower temperatures, but at higher temperatures an inert atmosphere is recommended; D—hydrogen should be used.

a A—an oxidizing atmosphere can be used (oxygen or air); B—a nonoxidizing atmosphere can be used (inert or vacuum); C—in these cases oxygen or air can be used at lower temperatures, but at higher temperatures an inert atmosphere is recommended; D—hydrogen should be used.

which the furnace will cool down. Because of the danger of a magnetic field interfering with the measurements of certain physical properties of the sample or system, the furnace should be noninductively wound: that is, it should have two similar windings carrying current in opposite directions so that their magnetic fields cancel, and also, the spacing of the windings should be decreased toward the end of the furnace to compensate heat losses. These points are made because, for special purposes, the operator might well require the construction of furnaces to a specific design. The resistance elements of the furnace control the temperature that can be obtained. Table VIII gives a range of resistance elements which, in theory, allows a temperature of 2800°C to be reached. Furnaces operating above a temperature of 1350°C should do so in an inert or reducing atmosphere. Graphite or silicon carbide resistance bars are used instead of metal bars in furnaces in which temperatures can go over 2000°C, and these are simple to operate, provided the restrictions on atmosphere control are noted.

The furnace should generally be mounted so that it can be moved easily and also put back in exactly the same position each time to minimize any errors due to the geometry of the system being altered by the movement which is necessary to load the sample in and to generally inspect the equipment. Where large furnaces are used, it is essential to have some kind of cooling system to allow the furnace to be programmed rapidly back to ambient temperature.

Relaxation Audio Sounds Relaxation

Relaxation Audio Sounds Relaxation

This is an audio all about guiding you to relaxation. This is a Relaxation Audio Sounds with sounds called Relaxation.

Get My Free MP3 Audio


Post a comment