Coolant system

Overview
There are several ways of achieving fast sample cooling in a furnace or oven. For most standard furnaces and ovens rapid changes in temperature are not recommended because they can induce stresses in the materials being treated. If rapid cooling is required by the process, Carbolite can design furnaces/ovens to allow it. A few standard products incorporate fast cooling as an option.

Common fast cooling methods

• Moving the sample into a cool area
• Movable insulation panels (manual or automatic)
• Ambient air blown into the furnace or oven chamber
• Ambient air blown around an inert gas retort
• Cycling an inert gas atmosphere through a water-cooled heat exchanger

Moving the sample into a cool area
Samples may be moved from the hot chamber into a cooler zone, for example by removing the sample into ambient air or into a cool metallic retort with a controlled atmosphere. This is a standard feature of the BLF bottom-loading furnace range, which allows the load to be lowered into the cool ambient air with personnel protection via a safety hearth cage. An optional radiation shutter can close the furnace entrance after lowering the hot load to protect the furnace from thermal shock. An optional inverted crucible allows the load to remain under inert gas during the whole process, including fast cooling. This method is also available as an option on some LHT vacuum chamber furnaces: the sample can be lowered into a water-cooled chamber under cool inert gas and blasted with a strong stream of inert gas for extreme fast cooling.

Removable insulation panels
Cooling can be accelerated by opening insulation panels; however opening the door of an oven or chamber furnace from high temperatures is generally not recommended for standard products. Large custom retort furnaces can be designed with panels (e.g. in the roof) that open to speed cooling. The V-L soldering and brazing furnace has a standard feature where the tube furnace surrounding a quartz glass retort can be automatically raised for fast cooling. As an option, the GLO range can be built with heating elements split into two half shells that move apart automatically to cool the retort very quickly; note this reduces the lifetime of the heating elements.

Ambient air blown into the oven or furnace chamber
For larger industrial oven/HT ranges (such as HT and LGP) a fast-cool option can be fitted which blows cooler ambient air directly into the oven chamber. Automatic flaps allow the warmed air to leave the chamber after cooling the sample.

Ambient air blown around an inert gas retort
Where the internal atmosphere must be maintained during cooling, cooling air can be passed around the outside of the retort so the atmosphere inside remains undisturbed. This is a standard feature of the V-L soldering and brazing furnace: when the furnace is raised an ambient-air blower forces cooling on the retort. It is an option for GLO vacuum chamber furnaces above 40 litres capacity, where a fan blows ambient air in the gap between the retort and heating elements and automated flaps exhaust the warmed cooling air. A similar custom option is available on HTMA ovens with modified atmosphere.

Cycling of inert gas atmosphere through a water-cooled heat exchanger
This option is available on HTK vacuum chamber furnaces with capacities greater than 25 litres. The gas is cycled out of the furnace into a heat exchanger and pushed back, after cooling, into the furnace chamber. For this application the furnace must be equipped with a roots pump to cycle the gas through the heat exchanger; after cooling the gas the roots pump pushes the cold gas directly back into the retort with the load so the sample is surrounded by freshly cooled gas. Cycling the inert gas reduces gas consumption and lowers process cost; typically cool-down time of an empty furnace is reduced by about a factor of two using this option. The lifetime of sensitive heating elements and insulation is reduced when using such fast cooling methods.

Examples and typical implementations
Examples of Carbolite custom fast-cooling implementations include: custom debinding furnaces with retort and forced cooling; GLO vacuum chamber furnaces with split heating elements that move apart for forced cooling; V-L soldering and brazing furnaces with cooled quartz glass retorts; bespoke LHTM vacuum chamber furnaces with automated lowering into a water-cooled chamber and inert gas blasters (demonstrated cooling of a load from 1600°C to under 800°C in less than 80 seconds under inert gas atmosphere); ACT air-cooled calibration tube furnaces with twin metallic work tubes to introduce cool air; bespoke vertical GLO vacuum chamber furnaces with air locks for automated movement between hot and cold areas under hydrogen atmosphere.

Caractéristiques / spécifications techniques

• Applicable furnace/oven ranges: BLF bottom-loader, LHT / LHTM vacuum chamber furnaces, V-L soldering & brazing furnaces, GLO vacuum chamber furnaces, HT and LGP industrial ovens, HTMA modified-atmosphere ovens, HTK vacuum chamber furnaces, ACT calibration tube furnaces.
• Capacity thresholds: GLO fast-cool option commonly offered on units > 40 litres; HTK gas-cycle heat exchanger option offered on units > 25 litres.
• Requirements for gas-cycle heat exchanger: roots pump to enable circulation of inert gas through water-cooled heat exchanger.
• Performance example: bespoke LHTM design demonstrated cooling from 1600°C to < 800°C in under 80 seconds under inert gas blasting.
• Benefits: significantly reduced cool-down times, reduced inert gas consumption when cycling gas, ability to maintain internal atmosphere during cooling (when using retort/bypass cooling).
• Trade-offs: increased mechanical/thermal stress on heating elements and insulation reduces component lifetime; some methods (e.g. splitting heating elements) will shorten element life.
• Safety / protection options: safety hearth cage for bottom-loading lowering; optional radiation shutters and inverted crucibles to maintain atmosphere during transfer.