The advantage of this type of system is that the measurement is unaffected by changes in density or viscosity and the accuracy is quite high (.5-1%). The disadvantage is the higher up-front cost.
In loss-in-weight metering (fig. 3), the master flow of product is sensed by monitoring the loss in weight of product flowing from a weigh hopper. In order to accomplish this type of weighing in a continuous flow application, a garner hopper must be used to surge product prior to entry in the weigh hopper. The system cycles drafts of product into a scale hopper and discharges the hopper at a rate that is used as the master flow. This flow is then used to signal a speed control on a positive displacement pump for proportional discharge of a loss in weight liquid scale.
The advantage of this type of system is that the actual weight of the product is being monitored so changes in density are accounted for. Also, calibration of this type of system is simple, since local scale companies can check the system calibration.
The disadvantage of this type of system is that it requires a large amount of head room to accommodate the garner hopper above the loss in weight scale. The up-front cost for this type of system is also greater then either of the systems previously mentioned. However, if the number of ingredients being weighed is greater than three, then the cost can compare well with mass flow technology using coriolis type meters.
Methods of Metering
Delivery of the micro ingredient in liquid form needs to be done in a repeatable fashion. This is accomplished with a positive displacement pump. There are several types of positive displacement pumps available and each has its advantages and disadvantages.
A piston pump uses one or more pistons to draw in and expel liquid. Some of these pumps use two pistons alternately drawing in and expelling fluid. By alternating discharging of the pistons, the amount of pulsation to the liquid delivery is reduced. These types of pumps can have very close tolerances, so the liquid should be filtered so suspended solids do not exceed the tolerance of the pump. A diaphragm pump uses a flexible membrane that moves in and out to move the liquid. This pump is more tolerant of solids, as a mater of fact, a variation of this pump is used to pump sand. Like the piston pump, the diaphragm pump delivers the liquid in a pulsating flow. The peristaltic pump uses a roller or rollers contacting a hose to pump liquid.
The advantage to this type of pump is that none of the pump parts come in contact with the product. The disadvantage is that the hose needs to be changed on a regular basis, since the compressing and decompressing of the hose causes the hose to wear out quickly.
In each of these pumps, the pulsation can be minimized by the addition of a pulsation dampener. These devices are like a pressure accumulator that even out the flow of liquid. In order for the pulsation dampener to work, there must be enough pressure in the system for the accumulator to build up pressure. If the piping run and back pressure for nozzles is not sufficient to build this pressure, then a pressure regulator valve should be added between the pump and the liquid meter. A gear pump and progressive cavity pump use a rotating gear or screw to move the fluid. These pumps deliver liquid with little or no pulsation and can create great pressures. They can also have close tolerances so the proper size filter should be used.
These pumps do not tolerate running dry, so safe guards must be put in place to make sure this doesn't happen. Piston, peristaltic, gear, and progressive cavity pumps can be controlled by using an AC speed control or a DC speed control. In the past, the DC drives were used when more turn down was necessary, but now AC drives have comparable turn down, if a vector type drive is used. The speed control for the diaphragm pump is a controller that controls the speed of an oscillating cylinder. This is accomplished by either actuating a solenoid on an air cylinder, or by energizing a coil directly surrounding the cylinder that moves the cylinder back and forth electro-magnetically. These controls have a timing circuit that determines the number of times the cylinder is energized.