Improving green sand performance through cooling.
When utilizing evaporative cooling, five elements will have a profound effect on final sand temperature. These include:
* kinetic energy of the hot sand;
* retained or added moisture in the sand;
* intimate contact of the sand mixture with moisture and air;
* time factors that affect cooling.
All of the above must interact in order to effectively remove temperature from your molding sand.
Generally speaking, sand will return to the system with a moisture content ranging from 1-2% and at a temperature of up to 350F. Several methods can be used to cool molding and shakeout sand that is being returned to the system for reuse. The most successful incorporate the following steps, which will affect the most efficient cooling:
* storage in a surge hopper;
* controlled water additions;
* mixing sand with water;
* cooling via sand contact with air.
Usually an over-the-belt bar type aerator serves as a lumpbreaker and is mounted on a conveyor belt which returns the sand after magnetic separation. The lumpbreaker pulverizes lumps of green sand from molds that have not had direct contact with molten metal, unpoured molds or pieces of cores.
The surge hopper provides a quantity of in-process sand, which provides a more consistent rate of flow through the cooling process. As a molding line operates, the rate of sand flow varies during start-up, from changeovers or other delays. The surge hopper tends to equalize the rate of sand flow through the rest of the system.
Moisture addition can be accomplished by utilizing a programmable controlled system that meters water additions by sensing the rate of sand flow, and using a belt weigh scale and infrared temperature sensing devices before water additions are made and after the sand has been cooled through the cooler. All data is continuously collected and the controller adjusts the amount of water being sprayed on to the sand.
On a retrofit system, after water is added, usually an over-the-belt aerator is used to assure that the water becomes well mixed with the sand. Over-the-belt cooling aerators utilize blending and cutting wheels with an expansion chamber to achieve air contact with the sand as it is thrown from the cutting wheels. As this system operates over the belt, the retention time is limited, therefore the amount of temperature reduction achieved is less than ideal. These units are usually limited to sand with a discharge temperature of 130-140F.
In the ideal system the most effective (maximum temperature reduction) results are accomplished by using one of several methods. These might include fluidized bed oscillators, rotating drum coolers, fluidized bed vertical coolers or continuous rotating plow coolers.
This important operation must bring the sand into intimate contact with air long enough to effectively remove Btu's from the sand. The cooling effect results when the moisture is removed from the sand, and is commonly called evaporative cooling.
Fluidized bed oscillators are oscillators containing screen or perforated decks and a lower air chamber. Air is blown up through the decks or screens and comes in contact with the sand as it is conveyed through the unit. Use of these units also should incorporate an over-the-belt blending aerator or other means of blending to assure sufficient mixing of water and sand.
A rotary drum cooler has lifting flights that cause the sand to be lifted and cascaded through an air stream passing through the drum. The rotary drum can also serve as a screening unit prior to sand discharges to further remove metallic chips, core butts or other foreign material. Over-the-belt blending or other blending methods are optional in this case as sufficient mixing and cooling is usually achieved in this unit.
A fluidized bed vertical cooler also utilizes air to fluidize the bed of sand to affect cooling. Sufficient mixing occurs as the sand is discharged into the unit and as it flows through the unit, therefore, further blending is not required.
A continuous rotary plow cooler utilizes numerous plows rotating at high speeds to aerate the sand and rapidly mix the cooling water with it. Supplemental blending is usually not needed.
The rotary drum, oscillator coolers and continuous rotary coolers require considerable floor space compared to a vertical cooler or over-the-belt aerators. However, with a vertical cooler the sand has to be elevated or raised to higher elevations than when using the other methods.
In all cases, air contact with the sand, containing the proper amount of moisture and exposure time to air are critical factors. Too much air volume or rate of air flow through the coolers can remove excessive amounts of fines, so proper design of the air flow is critical.
Of course, dust collection is required to contain the dust from the return belt transitions or transfer points as well as the lumpbreaker, aerator, cooler, and water addition to capture resulting steam. In some systems, dry additions are made after cooling rather than being made at the muller.
In summary, consistently cool sand containing a consistent moisture level prior to mulling or sand reconditioning is essential for a green sand system. Sand leaving the cooler should be in the range of 1 00- 1 1 OF, with moisture in a range of 1.0-1.5%. If bond additions are made prior to mulling, the moisture range should be 0.5% higher as it leaves the cooler.
Reducing the variations in the sand condition entering the muller will result in a more consistent and a higher quality molding sand after mulling or mixing. Properly cooled sand will contribute immeasurably to the ability to produce high quality castings in green sand.
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|Author:||Fiser, Richard E.|
|Date:||Jun 1, 1990|
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