Improving the Product Quality and Cost-Effectiveness of Industrial Furnaces with Inert Gas Atmospheres by Optimizing Atmosphere Replacement

The importance of anne­al­ing pro­ces­ses under inert or reac­ti­ve gas has been gro­wing for years. In par­ti­cu­lar, the qua­li­ty requi­re­ments for pro­cess gas atmospheres—especially for high­ly flamma­ble or com­bus­ti­ble pro­cess gases—are gene­ral­ly very high due to the need to ensu­re per­so­nal and plant safety.

Pur­ging the free fur­nace volu­me with non-flamma­ble gases to ensu­re a safe atmo­sphe­re chan­ge can be per­for­med under all pro­cess con­di­ti­ons. The free fur­nace volu­me is pur­ged with pro­cess gas until the con­cen­tra­ti­on of the pro­cess-rele­vant gas com­pon­ents meets the tar­get values for safe­ty and quality.

The rese­arch objec­ti­ve is to gain a deeper under­stan­ding of atmo­sphe­re chan­ge in inter­mit­tent-ope­ra­ti­on inert gas fur­naces. The aim is to ana­ly­ze various gas-chan­ge stra­te­gies in terms of their effi­ci­en­cy. This will make it pos­si­ble to iden­ti­fy influen­cing para­me­ters and incor­po­ra­te them into new con­cepts for atmo­sphe­re change.

The tar­get is to redu­ce the requi­red spe­ci­fic gas volu­me and the pur­ge time (ther­eby incre­asing pro­duc­ti­vi­ty). The influen­cing fac­tors under inves­ti­ga­ti­on are:

• Lay­out and con­fi­gu­ra­ti­on of the gas inlets and outlets,
• pur­ge gas flow rate,
• pro­cess gas flow rate (flow rate cir­cu­la­ted by the fan).

To this end, phy­si­cal and nume­ri­cal simu­la­ti­ons were per­for­med on cold models, and inves­ti­ga­ti­ons were con­duc­ted on an indus­tri­al inert-gas cham­ber furnace.

The results of the labo­ra­to­ry tests wit­hout a fan (forced con­vec­tion) show that

• With the same spa­ti­al arran­ge­ment, the inlet and out­let dia­me­ters have a very minor effect on the gas exchange.
• Alt­hough spa­ti­al varia­ti­ons in the inlets and out­lets have a signi­fi­cant influence on the flow struc­tures that form, they have only a minor effect on the over­all gas exch­an­ge due to the high level of tur­bu­lent mixing, so that the cour­se of the atmo­sphe­ric exch­an­ge has near­ly con­ver­ged in all vari­ants by the time θ = 5.

Labo­ra­to­ry tests using a fan have shown that

• When the gas recir­cu­la­tor is run­ning, free con­vec­tion cau­sed by tem­pe­ra­tu­re or con­cen­tra­ti­on gra­di­ents plays a negli­gi­ble role in gas exchange.
• The arran­ge­ment of the pur­ge gas inlets is signi­fi­cant only at the start of the gas exch­an­ge; as the pro­cess con­ti­nues, all tes­ted vari­ants converge.
• The recir­cu­la­ted flow rate (gas recir­cu­la­tor) and the pur­ge gas flow rate also have an effect only at the start of the gas exchange.

Ope­ra­tio­nal tests on the inert-gas cham­ber fur­nace show that

• Under ope­ra­ting con­di­ti­ons (hot test), the atmo­sphe­ric chan­ge rea­ches the O₂ safe­ty limit of 1% within 5 times the safe­ty pur­ge volu­me (θ = 5); in the cold test, this is not relia­bly achieved.
• A hig­her fur­nace tem­pe­ra­tu­re signi­fi­cant­ly acce­le­ra­tes the atmo­sphe­ric change.

For safe­ty reasons, it should be noted that

• Par­ti­cu­lar­ly for fur­naces that have been idle for exten­ded peri­ods, it is important to note that the fur­nace design and insu­la­ti­on can delay the initi­al atmo­sphe­re chan­ge due to poor gas flow (dead zones).
• Moni­to­ring the 5‑fold safe­ty pur­ge gas volu­me (DIN-EN 746–3) is not suf­fi­ci­ent as a safe­ty cri­ter­ion for atmo­sphe­ric chan­ge. Addi­tio­nal moni­to­ring of safe­ty-rele­vant gas com­pon­ents is neces­sa­ry at cri­ti­cal points.