Simple Solutions That Work! Issue 11

63 temperature as much as possible, but the water-pipe style heater can accommodate large swings in incoming sand temperature. Heater size. The main disadvantage of a water-pipe design over the electric-resistance units is size. In order to achieve the heating or cooling ranges associated with an efficient modern foundry, retention time must be long enough so that the sand can absorb or release the volume of BTUs needed to achieve set-point. If the application is for a new installation, this larger size can be accommodated easily. For existing installations, many times there simply is not enough elevation or lateral space to locate a water-pipe style unit. A correspondingly sized, water- pipe unit is larger than an electric- resistance unit, not only in elevation but in length. Sometimes it is an advantage to be able to convey sand from supply over to a mixer, but not always (depending on the location and installation.) Height, or headspace, also is needed for the exhaust piping because full- size sand grains can float into the dust-collection piping. Good design calls for a properly sized vertical duct to run vertically for at least 4 ft., so that full-sized sand grains can drop out of the dust- laden air. The sizing of the vertical duct as well as the size of the lateral duct must be carefully engineered to ensure the minimal volume of sand goes to dust collection, and that sand does not accumulate in lateral ducting. If height restrictions do not allow for this vertical duct, the lateral duct size must be calculated to keep the dust in suspension so that it moves easily to the dust collector. A standard drop-out box normally will need to be located close to the heater to ensure sand does not build up in lateral ducts, or go to the dust collector. Fines control. Anytime sand is moved, fines are generated. Because the water-pipe design requires a larger volume and correspondingly longer retention time, the fines generated can be very closely controlled, in comparison to the electric- resistance style. For example, a 500-lb./minute electric-resistance heater has a sand volume of approximately 800 lb., in general. This results in a maximum retention time of less than 2 minutes. A correspondingly sized, water- pipe design will hold as much as 10,000 lb., resulting in a retention time of as much as 20 minutes. Not only will this greatly increase the accuracy potential but any fines that are generated in transport and sand flow can be controlled very well by balancing the fluidization pressure and dust collection. Obviously, for many applications a certain amount of fines is desired, but the water-pipe-style heater has the ability to control this amount very accurately and consistently. It is important that dust collection is set correctly and remains the same through Contact: JACK PALMER jack@palmermfg.com the course of the day. Often, if a water-pipe heater is connected to the plant-wide dust-collection system, there can be huge swings in the volume of dust collected, which will result in less accurate fines control. As always with process equipment there are advantages and disadvantages to all designs. By carefully addressing the above considerations, selecting the correct sand heater-cooler design can be made easier and, hopefully, more quantifiable. This is a longer version of an article originally published in Foundry Management & Technology. Republished with permission from Informa USA. www.foundrymag.co m MAKING YOUR INSTALLATION ASUCCESS