Posts Tagged ‘electric motor’

Food Manufacturing Challenges – HACCP Design Principle No. 7

Sunday, November 27th, 2011
     Ever overdraw on your checking account or max out a credit card?  It’s not hard to do if you’re not keeping track of things.  How can we manage household expenses without some sort of record keeping?

      Away from home, in the business sector, record keeping becomes even more important.  In fact, it’s the very thing covered by HACCP Design Principle No. 7. 

     Principle 7:  Establish record keeping procedures. – This HACCP principle requires that all food manufacturing plants maintain records to show they implemented a HACCP plan, are following all principles, and the plan is working effectively.

     Let’s look at an example.  In keeping with the directive of HACCP Design Principle 7, the engineering department of a food manufacturing plant must keep records for each design project.  The design record for a new cookie forming machine would contain things like engineering calculations to determine strength requirements of machine parts and supports, as well as power requirements for the electric motor that drives the machine.  This design record would also contain documentation concerning materials selected to construct the machine, as well as dimensioned mechanical drawings of the machine and its parts.  These dimensioned drawings will show all physical dimensions of the machine and its constituent parts. 

     The record would also contain test results and analysis of the results.  Lastly, the design record must include a risk analysis of potential hazards that could result.  Other activities include identification of CCPs, establishment of critical limits, and other factors in accordance with HACCP Design Principles 1 through 5.  In other words, the record must be complete, bearing witness to an effective adherence to HACCP Design Principles 1 through 5. 

     Principle 7 also encompasses guidelines set in place through Design Principle 6, which calls for the establishment of procedures to govern Principles 1 through 5.  A complete record would contain the procedures themselves, along with any revisions.  It would also contain documentation that the procedures were reviewed and approved by management along the way.

     Finally, of what use would records be if they were incomplete, disorganized, and outdated?  A document control system not only establishes procedures, but assigns responsibilities to personnel within the department for filing design records to make sure that everything is up to snuff.  This system would encompass everything, from the creation of engineering documents, to their timely entry into the record keeping system.

     We have now exhausted our discussion on HACCP Design Principles.  We’ll switch to a new topic next time, examining some basic concepts behind the control of industrial equipment and machinery. 

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Industrial Ventilation – Local Exhaust Ventilation Fans

Sunday, May 22nd, 2011
    When something is said to be the “heart of the operation,” one usually imagines that it is integral to whatever is being discussed, and it is probably centrally located.  The human heart fits this description well.  This amazing organ, centrally located within your chest cavity, moves blood, nutrients, oxygen, and carbon dioxide through your body with amazing efficiency.   During a twenty four hour period it can pump as much as 2,000 gallons of blood through 6,000 miles of arteries, veins, and capillaries.

     At the heart of a local exhaust ventilation system is its fan.  Like the human heart, it is a model of efficiency.  It first creates a vacuum in the intake hood, which is strategically located at a pollution source, pulling in contaminated air and leading it through ductwork.  Sometimes the fan leads the air to a filter or other air cleaning equipment, but eventually the dirty air is exhausted through a stack leading outdoors.

     There are two main types of fan, axial and centrifugal.  You’re probably most familiar with the axial type, because they’re the type commonly used in tabletop, box, and oscillating fans in your home.  These have blades that look like a propeller on an airplane, and they work by drawing air straight through the fan.  As helpful as they are within a personal setting, axial fans are not typically used in local exhaust ventilation systems because the electric motor that drives the blades is in the path of airflow.  This setup can create a problem if the air flowing over the motor contains dust and flammable vapor.  Dust can cause the motor to get dirty and overheat.  Flammable vapor can ignite if the motor wiring fails and creates an electrical arc.

     Because of the technical difficulties presented by an axial type fan, centrifugal fans are what are most often used in industrial settings.  One such fan is shown in Figure 1.

Figure 1 – Centrifugal Fan

     The blades of a centrifugal fan are fully enclosed in air tight housing.  This housing keeps any dust or fumes from leaking out into the building.  The electric motor that drives the fan can be safely located outside of this housing, where it is dust-free and there are no flammable vapors.  If you look inside the housing you will see that the moving part, known as the impeller, resembles a squirrel cage.  See Figure 2.

Figure 2 – Centrifugal Fan Impeller

     This impeller is made up of many blades, set up within a wheel configuration.  When an electric motor causes the wheel to rotate, air is made to move off the blades and out of the impeller due to centrifugal force.  This air is sent crashing into the fan housing, shown in Figure 1, which is curved like a spiral to direct the air into an outlet duct which is connected to ductwork that leads to the exhaust stack.  As air leaves the impeller, more air rushes into its center from the inlet duct to occupy the empty space that’s been created.  Hence, as long as the motor keeps spinning the impeller, air will flow through the fan.

     In order for all this to work effectively, the centrifugal fan must be the right size, one that is capable of providing enough suction to capture contaminated air at the hood source, then overcoming the resistance to air flow that is presented by ductwork, filters, and other air cleaning devices. Because air resistance factors such as these impede the fan’s ability to move air through the system, the fan must be of sufficient strength make up for these factors.  To size up the right centrifugal fan for the job, engineers must calculate the resistance to airflow that is expected to be encountered, and to do this they use data supplied by manufacturers of component parts, as well as tabulated data that is readily available in engineering handbooks.  Just as a lawn mower engine won’t provide sufficient energy to power a car, an undersized fan won’t be able to move air through a system which is beyond its capacity limit.

     Next time, we’ll finish our series on local exhaust ventilations systems by looking at the last component in the system:  the exhaust stack.

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Dynamic Brakes

Monday, May 31st, 2010

     Last week we looked at how a mechanical brake stopped a rotating wheel by converting its mechanical energy, namely kinetic energy, into heat energy.  This week, we’ll see how a dynamic brake works.

     Chances are you have directly benefited by a dynamic braking system the last time you rode in an elevator.  But, to understand the basic principle behind an elevator’s dynamic brake system, let’s first take a look at the electric braking system in Figure 1 below. 

Figure 1 – A Simple Electric Braking System

     Here the brake consists of an electric generator wired via an open switch to an electrical component called a resistor.  The weight is attached to a cable that is wound around a pulley on the generator’s shaft.   As the weight freefalls, the cable unwinds on the pulley, causing the pulley to turn the generator’s shaft.

     Unlike last week’s mechanical brake which required a good deal of effort to employ, a dynamic braking system requires very little.  All that needs to be done is to close a switch as shown in Figure 2 below.  When the switch is closed, an electrical circuit is created where the resistor gets connected to the generator.  The resistor does as its name implies: it resists (but doesn’t stop) the electrical current flowing through it from the generator.  As the electrical current fights its way through the resistor to get back to the generator, the resistor gets hot like an electric heater.  This heat is dissipated to the cooler surrounding air.  At the same time, the weight begins to slow down in its descent.  But how is this happening?

     The electric braking system can be thought of as an energy conversion process.  We start out with the kinetic, or motion energy, of the freefalling weight.  This kinetic energy is transmitted to the electrical generator by the cable, which spins the generator’s shaft as the cable unwinds.  Electrical generators are machines that convert kinetic energy into electrical energy.  This energy travels from the electric generator through wires and a closed switch to the resistor.  In the process the resistor converts the electrical energy into heat energy.  So, kinetic energy is drawn from the falling weight through the conversion process and leaves the process in the form of heat.  As the falling weight is drained of kinetic energy, it slows down. 

 Figure 2 – Applying the Electric Brake   

     Okay, now let’s get back to dynamic brakes on elevators.  An elevator is attached by a cable to a hoist that is powered by an electric motor.  When it’s time to stop at the desired floor, the automatic control system disconnects the elevator’s electric motor from its power source and turns the motor into a generator.  The generator is then automatically connected to a resistor like the one shown in the electric brake above.  The kinetic energy of the moving elevator is converted by the generator into electrical energy.  The resistor converts the electrical energy into heat energy which is then dissipated into the surrounding environment.  The elevator slows down in the process because it’s being robbed of kinetic energy.  When the dynamic brake slows the elevator down enough, a mechanical brake is introduced, taking over to bring the elevator to a complete stop.  This two-fold process serves to reduce wear and tear on the mechanical brake’s parts, lengthening the operational lifespan of the system as a whole.

     Next time, we’ll tie everything together and show how mechanical and dynamic brakes work together in a diesel locomotive.

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