Simple Solutions That Work! Issue 11

Continued on next page Normally, it is best to set up the exhaust line so that dust does not flow back into the silo. These fines and their variability can have a significantly detrimental effect on mold strength. Note, that all the exhaust piping should be vertical, or at least set at a 45° angle to avoid sand build-up in the vent lines. 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. The temperature from the incoming sand may take minutes to go through the thermowell before it sees the thermocouple. By the time the controls react, the cold or hot sand already has exited the heater. On older units with thermowells, the readout will show very accurate temperature when in fact the sand exiting the heater may have temperature swings of 25° to even 50°F — and may not appear on the temperature readout. If the thermocouple junction is directly in the sand, the increase or decrease in power applied to the elements is immediate, and on the reading. Thermocouples wires should be twisted together before putting in a terminal strip, or simply held together with a wire nut 5. Square, not round. The Palmer design has a square body instead of a round body. 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 a longer retention time in the heater. The longer the retention time, the more accurate will be the resulting discharge temperature. is added only to increase the temperature as needed. This results in a much more accurate and repeatable discharge temperature. Also, thermocouple wiring must be correctly “phased.” The operating principle of a dissimilar metal thermocouple is that when the junction of the two metals (thermocouple wires in this case) is heated or cooled, a voltage is produced that can be measured, and correlated to a specific temperature. For this reason, it is important to be sure the correct wire runs from the temperature controller all the way to the terminals on the thermocouples without changing. When going through terminal blocks it is important that these wires do not go from one side of the terminal to the other. The mild steel of the terminal will have an effect 59 PROCESS CONTROL ELEMENTS SCR temperature controls, all together. Old-style heaters typically have control elements arranged in three or four different “banks.” This is a holdover design from a time before SCR temperature controls were readily available and reasonably priced. This older style allows the first bank to get to as high as 1,000°F: If the first bank does not get the discharge sand to the set-point, a second bank comes on, etc. While this design was sufficiently effective for its time, there is a MAKING YOUR INSTALLATION ASUCCESS