For foundries that prepare chemically bonded sand for molds and cores, optimizing the process means understanding and evaluating the primary features and options for water-based and electric-resistance systems.
Excerpt from the Foundry Management & Technology Magazine February 2019 issue by Jack Palmer
In general, there are two types of sand heaters used to prepare chemically bonded sand for molding and coremaking, and both are designed as fluid bed processes — meaning the sand is moved around the heating/cooling elements with compressed air or blower air. The more popular style is the electric-resistance fluid bed with compressed air fluidization, followed by water-pipe units in which hot or cold water is passed through piping, with fluidized sand moving around them with blower air.
The popularity of the electric-resistance type heater is due primarily to cost (compared with water-pipe designs) and size. The electric-resistance units are much smaller than the water pipe-based designs, though both offer advantages.
The electric-resistance heaters are less expensive and smaller but are not as accurate as the water-pipe units, due to the relative retention time in each. The electric-resistance heater can heat only; it cannot heat and cool like the water-pipe units. In higher-kilowatt sizes, the water-pipe units might be a better long-term choice because the initial capital costs are comparatively higher than the electric-resistance type. Operating costs also are much lower, as the water is normally heated with a natural gas-fired heater and cooled with simple cooling tower water.
Higher-kilowatt (150-500 kW) electric-resistance heaters can have a significant effect on demand charges; they obviously use a high volume of energy. Over time, these demand charges would make the water-pipe design the better choice.
Electric-resistance type units are used in many industries for a variety of purposes and commonly referred to as process heaters. They are used to heat granular materials with fluidization as well as liquids that are pumped through the unit. With normal maintenance and care, the electric-resistance style heater can be a very dependable and consistent piece of the no-bake molding process.
These are a few of the critical fluidized-bed process components that need the operators’ attention:
1. Fluidizing membrane. Depending upon the sand grain size and temperature range, this membrane may be wire cloth or a simple canvas sandwiched between two support members. Both work equally well as long as they are specified for the purpose intended. It is important to keep the membrane in good condition. The most common problem for electric-resistance heaters is that the membrane becomes clogged with moisture, oil, particulate, or rust from the compressed-air piping. The normal solution to this is to install a simple, bowl-type compressed-air filter. While these filters collect most of the moisture and particulate, they are only about 80% efficient.
2. Pressure-chamber pressure gage. The pressure gage in the pressure chamber under the fluidizing membrane usually is a 0-5-psi instrument; if the scale is higher it is impossible to be sure the pressure is set correctly. Normally, the pressure is increased “until the needle bounces,” typically at 2-3 psi, depending on heater size and aggregate type.
3. Exhaust. The exhaust from the fluid bed must be free-flowing at all times. Many times, this exhaust is connected right back to the top of the silo, into a bin vent, or tied into the plant dust-collection system. If there is a restriction in the piping, this will affect the fluidizing pressure, which will affect the accuracy and consistency of the sand-discharge temperature and, in the case of extremely plugged exhaust, sand flow.
4. Thermocouples. Obviously, thermocouples must be in good condition and properly wired because they are the means for controlling the rest of the machine functions. Palmer heaters are designed with a thermocouple junction in direct contact with the sand. It is important that the thermocouple junction is not in a cast iron or steel thermowell: A thermowell is simply a covering over the thermocouple to keep it from getting damaged or prematurely worn. While this works well to protect the thermocouple, it is highly detrimental to maintaining sand-temperature accuracy.
5. Square, not round. While the pipe-style heater is less expensive to manufacture, it is substantially smaller in volume than a unit of the same kW capacity in a square design. This larger amount of sand in the heater results in longer retention time in the heater. The longer the retention time, the more accurate will be the resulting discharge temperature.
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