The chemistries used in industrial cleaning processes are predominantly either dry (granular or powder) or liquid. Dry chemistries cover a wide range of density and flow characteristics. In fact, two batches of the “same” dry chemistry may have considerably different properties. Each liquid product has its own specific gravity and viscosity characteristics but may also vary in other ways including concentration. As a first step in metering chemistry, then, we must first depend on the chemical manufacturer to supply a consistent product help assure metering accuracy.
Dry Chemistry –
Overall, it seems more difficult to automatically meter dry chemistry than it is to meter liquid chemistry. This, of course, leads to a preference for liquid chemistry if there is a choice but that luxury is not always possible. One of the main problems with dry chemistries is that many are not “free-flowing.” In short, if you attempt to draw a dry chemistry from the bottom of a container relying on gravity, the supply may not be consistent as particles of the ingredient tend to stick to one another (think of something similar to flour). Another problem with “dry” chemistry is that many are hydroscopic (think of salt or or sugar with a small amount of water added). Once wet, the product won’t flow. Other “dry” chemistries are “wet” by design with some liquid ingredients mixed in (think brown sugar).
Assuming that it is possible to move the dry chemistry to the metering device reliably, there are automated devices that will deliver by weight and/or volume. The final complication, of course, is that once the chemistry is metered and delivered, it must be mixed until it dissolves. This involves the use of stirring devices as the turbulation of pumping is many times not adequate.
Liquid chemistry is nearly always measured by volume. Liquid chemistries present less challenge than dry chemistries when it comes to automated metering. Unlike dry chemistries, liquids can be pumped and most liquid chemistry metering devices rely on a pump of some sort to get the job done. Similarly, once metered, the liquid chemistry can be delivered to the cleaning tank by pumping and mixed in using turbulation.
In general, liquid metering is achieved using “positive displacement” pumps. A positive displacement pump is one that assures that a specific volume of liquid is delivered in response to a specific input. There are many different types of positive displacement pumps.
- Parastaltic – A parastaltic pump uses a flexible tube compressed by a series of rollers driven against it. As the pinched area progresses along the tube, the liquid within is moved along with it. A pump of this type requires a speed control to assure that the volume of liquid moved is proportionate to the time that the pump is operated.
- Gear – A gear pump uses two rotating gears which engage one another with pockets designed to trap and move liquid in small quantities as the gears turn against one another. Again, a speed control is required to assure that delivery is proportionate to time.
- Piston – A piston pump uses a cylinder with a piston to move liquid. Valves assure that the liquid enters and exits the cylinder in a single direction as the piston moves within the cylinder. This is the same way the cylinders in an automotive engine work! Each time the piston is moved from one end of the cylinder to the other, a measured volume of liquid exits.
There are, of course, other schemes used to meter liquids but most fall into one of the above groups. One distinct benefit of positive displacement pumps is that they are, in general, self-priming. This means that they can draw liquid up (as from a barrel) as opposed to relying on gravity to deliver liquid to the inlet.
The above will serve as an introduction to further discussions of how various chemistry metering devices are applied to cleaning processes be it for initial filling of a cleaning tank or providing makeup in response to a chemical concentration monitoring device.
– FJF –