| Last week our kitchen ceiling fan and light combo decided to stop working. We don’t like eating in the dark, so I was compelled to do some immediate troubleshooting. As an engineer with training in the workings of electricity I have a great respect for it. I’m well aware of potential hazards, and I took a necessary precaution before taking things apart and disconnecting wires. I made the long haul down the stairs to the basement, opened the circuit breaker in the electrical panel, and disabled the flow of electricity to the kitchen. My fears of potential electrocution having been eliminated, my only remaining fear was of tumbling off the ladder while servicing the fan.
Just as I took the precaution to disconnect the power supply before performing electrical maintenance in my home, workers in industrial settings must do the same, and a chief player in those scenarios is the motor overload relay discussed last week. It automatically shuts down electric motors when they become overheated. Let’s revisit that example now.
Our diagram in Figure 1 shows electric current flowing through the circuit by way of the red path. Even if this line were shut down, current would continue to flow along the path, because there is no means to disconnect the entire control system from the hot and neutral lines supplying power to it, that is, it is missing disconnect switches. Electric current will continue to pose a threat to workers were they to attempt a repair to the system. Now let’s see how we can eliminate potential hazards on the line.
In Figure 2 there is an obvious absence of the color red, indicating the lack of current within the system. We accomplished this with the addition of disconnect switches capable of isolating the motor control circuitry, thereby cutting off the hot and neutral lines of the electrical power supply and along with it the unencumbered flow of electricity.
These switches are basically the same as those seen in earlier diagrams in our series on industrial controls, the difference here is that the two switches are tied together by an insulated mechanical link. This link causes them to open and close at the same time. The switches are opened and closed manually via a handle. When the disconnect switches are both open electricity can’t flow and nothing can operate. Under these conditions there is no risk of a worker coming along and accidentally starting the conveyor motor.
To add yet another level of safety, disconnect switches are often tagged and locked once de-energized. This prevents workers from mistakenly closing them and starting the conveyor while maintenance is being performed. Brightly colored tags alert everyone that maintenance is taking place and the switches must not be closed. The lock that performs this safety function is actually a padlock. It’s inserted through a hole in the switch handle, making it impossible for anyone to flip the switch. Tags and locks are usually placed on switches by maintenance personnel before repairs begin and are removed when work is completed.
Now let’s see how our example control system looks in ladder diagram format.
Figure 3 shows a ladder diagram that includes disconnect switches, an emergency stop button, and the motor overload relay contacts. The insulated mechanical link between the two switches is represented by a dashed line. Oddly enough, engineering convention has it that the motor overload relay heater is typically not shown in a ladder diagram, therefore it is not represented here.
This wraps up our series on industrial control. Next time we’ll begin a discussion on mechanical clutches and how they’re used to transmit power from gasoline engines to tools like chainsaws and grass trimmers.
Posts Tagged ‘conveyor belt’
| Imagine a doctor not washing his hands in between baby deliveries. Unbelievable but true, this was a widespread practice up until last century when infections, followed by death of newborns, was an all-too common occurrence in hospitals across the United States. It took an observant nurse to put two and two together after watching many physicians go from delivery room to delivery room, mother to mother, without washing their hands. Once hand washing in between deliveries was made mandatory, the incidence of infection and death in newborns plummeted.
Why wasn’t this simple and common sense solution instituted earlier? Was it ignorance, negligence, laziness, or a combination thereof that kept doctors from washing up? Whatever the root cause of this ridiculous oversight, it remains a fact of history. Common sense was finally employed, and babies’ lives saved.
The same common sense is at play in the development of the FDA’s Hazard Analysis Critical Control Point (HACCP) policy, which was developed to ensure the safe production of commercial food products. Like the observant nurse who played watchdog to doctors’ poor hygiene practices and became the catalyst for improved hospital procedures set in place and remaining until today, HACCP policy results in a proactive strategy where hazards are identified, assessed, and then control measures developed to prevent, reduce, and eliminate potential hazards.
In this article, we’ll begin to explore how engineers design food processing equipment and production lines in accordance with the seven HACCP principles. You will note that here, once again, the execution of common sense can solve many problems.
Principle 1: Conduct a hazard analysis. – Those involved in designing food processing equipment and production lines must proactively analyze designs to identify potential food safety hazards. If the hazard analysis reveals contaminants are likely to find their way into food products, then preventive measures are put in place in the form of design revisions.
For example, suppose a food processing machine is designed and hazard analysis reveals that food can accumulate in areas where cleaning is difficult or impossible. This accumulation will rot with time, and the bacteria-laden glop can fall onto uncontaminated food passing through production lines.
As another example, a piece of metal tooling may have been designed with the intent to form food products into a certain shape, but hazard analysis reveals that the tooling is too fragile and cannot withstand the repeated forces imposed on it by the mass production process. There is a strong likelihood that small metal parts can break off and enter the food on the line.
Next time we’ll move on to HACCP Principle 2 and see how design engineers control problems identified during the hazard analysis performed pursuant to Principle 1.
| My wife and I have an agreement concerning the kitchen. She cooks, I clean. Plates and utensils are easy enough to deal with, especially when you have a dishwasher. Pots and pans are a little more challenging. But what I hate the most are the food processors, mixers, blenders, slicers and dicers. They’re designed to make food preparation easier and less time consuming, but they sure don’t make the clean up any easier! Quite frankly, I suspect the time involved to clean them exceeds the time saved in food preparation.
Food processors on a larger scale are also used to manufacture many food products in manufacturing facilities, and being larger and more complicated overall, they’re even more difficult to clean. For example, I once designed a production line incorporating a dough mixer for one of the largest wholesale bakery product suppliers in the United States. A small elevator was required to lift vast amounts of ingredients into a mixing bowl the size of a compact car. Its mixing arms were so heavy, two people were required to lift them into position. It was also my task to ensure that the equipment as designed was capable of being thoroughly cleaned in a timely and cost effective manner.
Food processing machinery must be designed so that all areas coming into contact with ingredients can be readily accessed for cleaning. And since most of the equipment you are dealing with in this setting is far too cumbersome to be portable, the majority of the cleaning must be cleaned in place, known in the industry as CIP. To facilitate CIP, commercial machinery is designed with hatches and special covers that allow workers to get inside with their cleaning equipment. Small, portable parts of the machine, such as pipes, cutting blades, forming mechanisms, and extrusion dies, are often made to be removable so that they can be carried over to an industrial sized sink for cleaning out of place, or COP. These potable machine components are typically removable for COP without the use of any tools and are fitted with flip latches, spring clips, and thumb screws to facilitate the process.
Everything in a food manufacturing facility, from production machinery to conveyor belts, is typically cleaned with hot, pressurized water. The water is ejected from the nozzle end of a hose hooked up to a specially designed valve that mixes steam and cold water. The result is scalding hot pressurized water that easily dislodges food residues. Bacteria doesn’t stand a chance against this barrage. The water, which is maintained at about 180°F, quickly sterilizes everything it makes contact with. It also provides a chemical-free clean that won’t leave behind residues. Once dislodged, debris is flushed out through strategically placed openings in the machine which then empty into nearby floor drains.
As a consequence of the frequent cleanings commercial food preparation machinery requires, their parts must be able to withstand frequent exposure to high pressure water streams. Parts are typically constructed of ultra high molecular weight (UHMW) food-grade plastics and metal alloys such as stainless steels, capable of withstanding the corrosive effects of water. And since water and electricity make a dangerous combination, gaskets and seals on the equipment must be tight enough to protect against water making its way into motors and other electrical parts.
Next time we’ll look at how design engineers of food manufacturing equipment use a systematic approach to minimize the possibility of food safety hazards, such as product contamination.
| Ever wonder why the burger you get at your favorite fast food chain never looks like the one on TV? The bun isn’t fluffy, the beef patty doesn’t make it to the edges, and the lettuce is anything but crisp. Well, it’s because a professional known as a Food Stylist, working together with a professional marketing firm and production crew, has painstakingly created the beautiful, bright and balanced burger used to lure you in. The process can take days or even weeks to create and has nothing to do with reality. The burger you’re really going to get will look more like a gorilla sat on it.
Many of the same issues must be dealt with when mass producing food. Chances are human hands will never even touch the product, like they did when creating the prototype in the test kitchen. In the world of food manufacturing, the “look” part can be extremely challenging. How do you get machines and production lines to create visually appealing food that entices prospective buyers to make an investment in it? How do you get it to taste good, or at least acceptable to the palate?
The “taste” part of food manufacturing can be even more challenging. For example, in the test kitchen of a pastry product manufacturer, a recipe will be developed using home pantry products like flour, butter, and eggs. Ever made bread or a pie crust? The stickiness factor is enough to drive many insane. Even nimble human fingers have a hard time dealing with it. Now enter food processing machinery and conveyor belts into the scenario. This brings the possibility of stickiness to a whole new level. Huge messes that gum up the machinery are common, and production line shutdowns are the result.
When faced with these challenges, plant engineers have to work closely with chefs in the R&D kitchen to come up with some sort of compromise in the recipe or final form of the food product. The goal is to cost effectively produce food products acceptable to consumers for purchase, and it’s often an iterative process involving many successive changes to recipes and equipment designs, coupled with a lot of testing and retesting, before success is finally met. If testing ultimately proves that the product appeals to consumers’ tastes and flows nicely through production lines, then there’s a good chance it will be a commercial success. In any case, cost is the dictating factor as to whether the food product will successfully make it onto the shelves of your supermarket. A margin of profit must be made.
But this success is only part of the design process. Before full commercial production can commence, processing equipment and production lines must be designed so that they:
Next time we’ll explore how cleanliness requirements factor into food manufacturing equipment design.
| If you’ve ever read a book to a small child on the subject of food or digestion, you’ve probably come across the analogy that our stomachs are like a furnace and our digestive system much like an engine. We explain to the youngster that what we eat is important, because our body needs the right fuel in order to operate properly. If little Susie or Danny insisted on eating only candy day after day, their bodies would become weak and sick.
In much the same way a coal power plant is like a living organism, eating fuel in order to function. But instead of meats and vegetables, it eats coal, and the coal handling department of a power plant acts as a dinner table. It’s where the food is placed and prepared before it enters the diner’s mouth.
The coal our power plants consume comes from one of two places, underground mines or strip mines. It all depends on the particular geology of the area from which the coal is harvested. According to the US Energy Information Administration, underground mines are more common in the eastern United States, while strip mines are more common in the western states. The coal from underground mines is excavated by means of shafts and tunnels which are dug deep beneath the earth’s surface in order to provide access to the buried coal deposits. In strip mines the deposits are just below the surface, so the topsoil is merely stripped away with heavy earthmoving machinery, like bulldozers, to reveal the coal. In both types of mining activity excavating machines and conveyors are required to remove the coal from the mine so it can be loaded for shipment to its ultimate destination.
Once harvested, coal is shipped to power plants primarily by train, river barge, or ship. Its journey can cover thousands of miles. It culminates in delivery to a power plant, where it is unloaded by means of a huge piece of machinery called a rotary dumper. This machine is capable of grabbing onto 100 ton railcars and turning them upside down. The coal spills into a large collection hopper positioned next to the railroad track.
If the coal has found its way to a plant located near a waterway, that means of transport was most likely have been made by flat barge or ship. In this case a large crane with a clamshell bucket is used for unloading. The crane drops its bucket into a pile of coal located within the ship’s hold, takes out a large bite, then hoists and dumps its contents into a large collection hopper next to the crane.
To get an idea of how coal flows within the coal handling system of a power plant, let’s refer to the flow chart in Figure 1.
Figure 1 – Schematic Diagram of the Coal Handling System
Collection hoppers and have slanted bottoms which allow coal to easily spill out onto a conveyor belt. Within the plant coal is transported by means of conveyors into what’s known as a “breaker building.” This building lives up to its name because it contains a very large machine whose job it is to break the chunks of raw coal that have been harvested from mines into smaller chunks which the boiler can work with.
Once broken down, the coal will go to one of two places, either directly into silos or coal bunkers in the power plant building for short term storage, or into an outside storage pile, usually a prominent feature of a power plant due to its formidable size. The coal pile can be several stories tall and much larger than a football field. It acts as a reserve supply should the regular delivery of coal be interrupted by labor strike, natural disaster, or equipment failure. When necessary, the coal is removed from the pile and sent into the plant to fill the coal silos. Coal from the silos is used to feed the power plant boilers.
Next week we’ll continue to follow coal’s journey, on its way to arguably one of the most important pieces of equipment in a power plant, the boiler.