When it comes to vacuum heat-treating systems, there are a variety of possible methods that can be used to cool the furnace and the workload. These cooling methods are often dictated by the part type, geometry and exact specifications, as well as the end user’s cooling requirements.
What is vacuum cooling?
Considered the slowest method of cooling, it involves heat being removed from the furnace while the vacuum valves remain open. The furnace then cools down without backfill gas or using the recirculating fan. This method also relies on the specific heat loss potential of the hot zone for cooling.
Using this method, if an empty furnace were to be vacuum cooled from 2,400 °F (1,315 °C), it would take approximately 14 hours for it to reach ambient temperatures.
What is partial pressure cooling?
This method is similar vacuum cooling, but uses a small amount of inert gas in the micron range to help negate the insulation characteristics of the hot zone, which provides a slightly faster cooling rate than vacuum cooling alone.
Using this method, if an empty furnace were to be partial pressure cooled at 1,000-2,000 microns from 2,400 °F (1,315 °C), it would take approximately 11 hours for it to reach ambient temperatures.
What is static cooling?
Using this method, the vacuum valves close and the furnace backfills with inert gas to a predetermined pressure, ranging from -5 inches of mercury (inHg) all the way up to 16 bar, depending on the design capabilities of the furnace. The backfill valve will open and close to maintain the desired pressure level in the furnace during the cooling cycle. This mode of cooling does not use the recirculating fan, but the increased presence of inert gas negates the insulation characteristics of the hot zone, which significantly increases the cooling rate through convective heat loss.
Using this method, if an empty furnace were to be static cooled from 2,400 °F (1,315 °C), it would take approximately 6 hours for it to reach ambient temperatures.
What is forced (fan) cooling?
This is the fastest method of cooling. It starts with the vacuum valves closing and the furnace backfilling with inert gas to a predetermined level (negative or positive). Once the furnace reaches -5 inHg (and pressure continues to rise until the specific set point is reached), the recirculating fan comes on to cool the workload and the furnace by moving the hot gas across a water-cooled heat exchanger. Like static cooling, the backfill valve will open and close to maintain the desired pressure level in the furnace during the cooling cycle.
Using this method, if an empty furnace were to be forced cooled from 2,400 °F (1,315 °C), it would take approximately 2 hours for it to reach ambient temperatures.
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