Successful aluminum heat treatment is dependent on understanding a number of important factors principal of which are proper quenchant selection, agitation and quench tank design. Part two of this two-part article explains the fundamentals of quenchant agitation. Part one of this article can be found on page 53 of the June 2006 issue.

Fig. 10. Mechanical model of an impeller for flow visualization


Agitation

Agitation 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).

Fig. 11. Results of CFD (Computational Fluid Dynamics Modeling) of a quench tank
The use of Experimental Fluid Mechanics, or flow visualization, can provide direction for how to design an agitation system. The simulation of loads and flows will be able to solve almost any questions related to the quench tank. The design and building of the models, however, are time consuming and expensive. Secondly, scaling up the model to the finished tank size and product might not be a straightforward process (Fig. 10).

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.

Figure 12. Typical locations of agitation tubes in rectangular tanks
Flow is generated utilizing several methods. Pumping and the use of different types of propeller agitation provide the most common method. Part or basket movement is used in rare occasions. It is important to realize that quench agitation is different than mixing of chemicals. Heat-treat facilities are specifically looking for the linear flow with some turbulence past the part that gives the best and most efficient cooling of the part in a predictable manner across the whole section of the product rack or part each time a quench is performed.

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 draft-tube is widely used in the larger open-tank systems. The draft-tube consists of a propeller (airfoil or marine type) placed inside a tube. The placement of the propeller inside the tube increases the efficiency of the prop in addition to giving the designer the ability to direct the quench flow in a more controlled and predictable manner. Figure 12 shows typical placements of agitation tubes in square tanks. The draft-tube design has been covered in detail [16]. The distance from the water to the edge of the flared-tube entrance must be big enough to prevent air from being pulled down into the tube, creating bubbles in the quench. The bubbles can create an insulating layer on the parts and must be avoided in the quench tank. Foaming can also result. Several methods are available to prevent the vortex from being started. One way is to place a flat plate several inches under the surface and force the water to enter the agitator in a more horizontal manner. This will create a slight restriction in the inlet, but normally this will not reduce the volume significantly. The other method is to place the propeller and the flared cone deep enough to prevent the inlet vortex from forming. Flow modeling and field measurements [16] proved that additional flow of up to 20% could be generated in the tank. Placing parts baskets in the maximum natural flow area and using the proper method for generating the flow will ensure the best quenching possible.

The addition of perforated plates and flow vanes can help direct the quenchants [10]. 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: smackenzie@houghtonintl.com


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