Heat Treating: Design of Quench Systems for Aluminum Heat Treating, Part II - Agitation
AgitationAgitation and design of agitation systems has been well covered in the literature [10, 11, 12]. Over time, agitation design has been specified as changeovers of tank volumes (gallons or liters per hour), description of surface movement (rolling, still, etc.) or measured flow past the parts (feet/sec or cm/s). The best way to specify the quench flow is a calculated or measured flow past the parts. The maximum flow that should be specified for aluminum batch quenching with water or Polyalkylene Glycol is on the order of 0.8-1.2 ft/sec (24 - 36 cm/s) past the parts. Higher flow will not add to the cooling of the parts unless spray quenching is used. However, this amount of quench fluid might be impossible to move. It will in some cases mean the complete tank volume must be changed over every 1-3 minutes. This is not practical in large tanks. Many tanks are successfully producing heavy gage parts with measured flows in the area of 0.25-0.4 ft/sec (7 - 12 cm/s).
Modern technology and computer simulation has allowed designers and process engineers to design quench systems without expensive trial and error type approaches. The best flow possible around a part is a linear flow with enough turbulence to get into the nooks and crannies of the part to break up the vapor layer and provide the required cooling. Racking methods and flow design must accommodate this. The bottom to top flow is preferred since it will utilize the mechanical agitation from the agitator and the agitation from steam formation, increasing the total flow around the parts. The use of finite element analyses gives the designer a good tool to start with. The tools available are Experimental Fluid Mechanics (EFM) and Computational Fluid Dynamics (CFD).
The use of computer modeling for quench tank and furnace design has been used to verify and predict the mechanical design and process variables [13, 14, 15]. As the capability and sophistication of new computer hardware and software improves, it is very easy to calculate and visualize the fluid flow process. Typically, the whole tank or a section of the tank is modeled (Fig. 11).
Flow modeling is a powerful and versatile tool enabling the designer and process engineer to make decisions necessary to design a good quench system. When the modeling is completed, a tank can be built and will most likely produce good quality parts.
Pumping is versatile and does not take up much space in the tank since sparger pipes, eductors and nozzles can be tucked close to the sidewall or bottom of the tank. Pumping has a low efficiency per gallon (liter) of quench moved compared to other types of agitation devices. The use of an eductor can significantly increase the amount of quench moved inside the tank. The volume goes up by a factor of four, and the velocity goes down with the same factor. However, the overall flow generated will be sufficient to make a good quench. Compared to nozzles, the eductor provides a better distributing of the flow and does not generate point cooling of parts by hitting the part with a very high velocity of fluid at a concentrated spot.
Propeller agitation is divided between open placement and agitation tube placement. In addition, there are marine-type propellers and airfoil-type propellers used for agitation purposes. The following will describe the different steps required to decide which system will work the best. The open-type propellers are most commonly used in side-mounted systems like an integral quench furnace. These propellers are typically marine-type propellers. Marine-type propellers spin slower than airfoil-type propellers. The swirling action of the quench when it leaves the propeller tips generates a good non-linear flow. However, the flow is very uneven and can affect properties in the parts. The horsepower requirements are large compared to airfoil type systems, but it is less than pumping. Table 1 shows a comparison of the required horsepower (energy) between pumping and draft-tubes.
The addition of perforated plates and flow vanes can help direct the quenchants . For example, a 15,000-gallon (56,780 liter) quench tank was agitated by three large side-mounted marine-type propellers. The quench area for the parts was in the top 16" (405 mm) of the tank since parts were quenched one at a time every 20-30 seconds. The flow was very strong but uneven. Several methods were used to solve the problem. Baffling and flow direction vanes did very little to even the flow out. The final fix was to install a perforated plate under the parts. The perforated plate/plenum created a very even and desirable flow. The use of perforated plenums in conjunction with tube or open-type agitators is very successful in generating controlled even flows.
For more information: Scott Mackenzie is Technical Specialist at Houghton International, Inc., Valley Forge, PA 19426. He can be reached at: ph. (610) 666-4000; fax (610) 666-1376; e-mail: firstname.lastname@example.org
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com:Aging, age hardening, agitation, aluminum, aluminum heat treating, quenchant, quenching, agitation, polymer quenching, precipitation hardening, solution heat treating, straightening.