Posts Tagged ‘heat energy’

Brakes and Braking Systems

Sunday, May 23rd, 2010

     Imagine driving in your car, you’re traveling at a speed of 65 mph and you’re coming up on a curve.  You depress your brake pedal to negotiate the turn, and nothing happens…

     Scenarios just like this one have been in the news quite often lately, brakes which just aren’t operating correctly.  We’ve heard the tales of terror, recounted by those unfortunate individuals who have been placed in this situation, but have we reflected on just why their brakes might have failed?

     Put most simply, a brake is a device whose purpose is to stop a body in motion.  This important task is accomplished by converting the kinetic energy (energy of motion) into heat energy.  This can be accomplished by either of two methods, mechanically or electrically.  In today’s blog we’ll focus on the mechanical aspect.

     A simple mechanical brake is shown in Figure 1 below.  In this arrangement kinetic energy is converted into heat energy when force is applied to a lever, causing a brake shoe to meet up with a rotating wheel.  The brake shoe has a pad attached to its surface that makes direct contact with the wheel, and when the two come together great friction is produced.  It’s this friction that will ultimately stop the object in motion.  Friction turns the kinetic energy into heat energy.

Figure 1 – A Simple Mechanical Brake

     Friction at its simplest is a mechanical resistance to movement.  Whenever two materials in motion come into contact with each other there is always some degree of friction.  The extent to which friction is produced by their meeting is referred to as the “coefficient of friction.” 

     The coefficient of friction varies according to the surface character of the materials coming in contact.  For example, the coefficient of friction for the leather sole of your shoe on smooth ice is very low.  This means you’ll do a lot of slipping when you’re trying to walk, and that’s because ice presents little friction to resist a smoothly soled shoe.  But take this same shoe and apply it to the rough surface of concrete, and you’ll be walking quickly and efficiently.  Coefficients of friction between different materials have been duly measured in laboratories and are tabulated for easy access in engineering reference books.

     Based on our simple example above, one would easily come to the conclusion that a high coefficient of friction is desirable when talking about brake shoes, specifically the one represented in Figure 1 above.  The higher the coefficient of friction, the more the pad wants to grab the wheel, and the less force you will need to apply to the brake shoe to successfully come to a stop.

     That’s mechanical braking in a nutshell.  Next time, we’ll focus on an electrical braking system known as a “dynamic brake.”

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Heat Transfer in Mechanical Engineering, Part III, Radiation

Sunday, February 28th, 2010

     Last week we talked about convective heat transfer and how hot pavement in a parking lot gives up its heat to the environment.  But how does the pavement get hot to begin with?  This week we’ll discuss radiant heat transfer to find out. 

     The sun is a huge nuclear furnace, separated from the earth by 93 million miles of space.  The space between is just a vacuum, almost completely devoid of matter.  Without contiguous solid or liquid matter between the two, heat transfer by conduction or convection can’t occur. The heat that we feel on earth is actually generated when surfaces here absorb electromagnetic energy waves emitted by the sun.  Although these waves have traveled through millions of miles of space, they have not lost their punch.  Our eyes perceive some of them as sunshine, but many others are not visible.  But even if we can’t see them, our bodies often perceive them as heat.

     But radiant heat transfer isn’t a phenomenon exclusive to the sun.  It can also occur when something is on fire.  Intense fires can transmit tremendous amounts of radiant energy across significant distances.  They can even cause combustible materials nearby to burst into flame without any direct contact being necessary.  A line of sight between the source of heat and the receiving object is all that is required, and this is because radiation moves in straight lines, it can’t bend around corners.

     In order to calculate radiant heat transfer, we still must consider the temperature difference between the bodies, as well as the area of heat transfer, just as we did when considering the cases of conductive and convective heat transfer.  But since there is no conduction or convection activity taking place, we need not concern ourselves with thermal conductivity or convection coefficients.  Instead, we have to consider something called the Stefan-Boltzmann constant, a nifty little number that looks like this:   0.000000057 Watts/m2K4.  It was discovered in 1879 by a scientist named Jozef Stefan.  It was later derived by his student, Ludwig Boltzmann, in his work on thermodynamics and quantum mechanics.  Now remember from our discussion last week the unit “K” means Kelvin (°C +273.15).

     Now, ideal radiant heat transfer problems involve calculations that need only consider the Stefan-Boltzmann constant.  By “ideal,” I mean that there is perfect emission of radiation by one object and perfect absorption of that radiation by another.  But reality is not typically so kind, and radiant heat transfer problems typically involve calculations that involve more than just the Stefan-Boltzmann constant.  They involve additional calculations of terms like emissivity factors and geometric factors.  What’s that?  Read on.

     Emissivity factors relate to how well objects actually emit and absorb radiation compared to an ideal case.  For example, a shiny object doesn’t absorb radiant energy as well as a dull, black object.  Geometric factors are included in radiant heat transfer calculations to account for the shapes and relative orientation of the objects emitting and receiving radiation.  For example, do you ever notice how the sun is hotter at noon than it is at sunset?  Well, that’s because an object with a surface that’s parallel to the surface emitting radiation will receive more radiation than one that isn’t.

     Just to give you a basic idea of how radiant heat transfer calculations work, let’s consider an ideal situation.  Suppose you own a store building with a flat roof.  The store is right on the equator and it’s the vernal equinox.  The roofing material is dull black, measures 20 meters by 10 meters, and it absorbs radiant energy like a sponge.  But today is a dark, cloudy day, and the temperature of the roof is a cool 25°C.  Now, at some point in your life I’m sure you’ve seen a documentary where scientists declared that the surface temperature of the sun is a blistering 5,400°C.  Keeping this in mind, if the sun were to suddenly pop out of the clouds directly overhead at high noon, what would be the amount of radiant heat it would transfer to the roof?

     Well, according to Jozef Stefan, the radiant heat transfer rate can be calculated to be:

     Heat Flow = (The Stefan-Boltzmann Constant) x

                               (The Area of the Roof) x

                                   ((The Sun’s Temperature)4 – (The Initial Roof Temperature)4)

Now the terms we’ll need to plug into the heat flow calculation above are found as follows:  The sun’s temperature is:  5,400°C + 273.15 = 5,673.15K.  The temperature of the roof is:  25°C + 273.15 = 298.15K.  The area of the roof is:  20 meters x 10 meters = 200 meters2.   So, the heat transfer rate is:

Heat Flow =  (0.000000057 Watts/m2K4) x (200 m2) x ((5,673.15K) 4 – (298.15K) 4)

                      = 11,808,605,250 Watts

This would be the maximum rate of heat transfer that the roof could absorb at the instant the sun popped out of the clouds.

     From our example we can conclude that even in less than ideal conditions, radiant energy from the sun has the potential to generate tremendous amounts of heat on the surface of the earth.  Much of this heat drives our weather as a result of convective heat transfer that takes place between the earth’s surface and our atmosphere.

     That wraps things up for our discussion of heat transfer in mechanical engineering.  Next week we’ll talk about the importance of vibration analysis in design.  Remember what happened to Jody Foster’s character in the movie Contact when the space/time device she was in began to shake violently?  That’s the kind of thing vibration analysis seeks to correct!    

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Heat Transfer in Mechanical Engineering, Part II, Convection

Sunday, February 21st, 2010

     Last week we talked about heat transfer by conduction, with an example that showed how heat flowed from hot to cold through a solid block of metal.  This week we’ll focus on another method of heat transfer known as convection.

     Convection occurs when heat is transferred between a surface and a moving fluid.  As with conduction, heat always wants to flow from a higher temperature to a lower temperature.  There are two types of convective heat transfer, natural and forced.

     An example of natural convection would be hot air rising off a blacktop highway on a sunny summer day.  The cool air near the ground picks up heat from the sun-baked blacktop.  As the air heats, its density decreases and it gets lighter.  The warmer, lighter air rises off the highway and more cool air rolls in to take its place.  This creates a continuous natural airflow that removes heat from the blacktop.  You can actually see this airflow as ripples in the air just above the highway.  It produces a mirage effect and almost appears to be water on the highway, particularly on very hot days.

     In natural convection we are concerned with heat transfer between a surface and a fluid moving over that surface.  To calculate the heat transfer, we need only consider the area of the surface and the temperature difference between the surface and the fluid.  There is no material thickness to consider like we saw in last week’s conductive heat transfer example, where heat was moving through a solid object.  So rather than working with a conductive heat transfer coefficient, we must work with a convective heat transfer coefficient, and our engineering reference guide will guide us to the correct convective coefficient to be used to calculate heat flow.

     With this said, we can calculate the natural convective heat flow to be: 

         Heat Flow = (The Convective Heat Transfer Coefficient) x

                                 (The Area That The Heat Is Flowing Through) x

                                     (The Difference In Temperature)

      Now let’s go back to the blacktop to see how this all works.  Suppose you have a parking lot and you want to know how much heat it is pumping into the atmosphere on a hot, sunny summer day with no wind.  We must first determine the area of the lot, and we measure it out to be 100 meters by 100 meters.  That’s 100 X 100, or 10,000 square meters of blacktop we’re talking about.  We now measure the surface temperature of the blacktop and find it to be 65°C, but the air temperature near the ground away from any source of blacktop is a cool 30°C.  Okay, so what is the heat transfer?

     First, you consult your friendly engineering reference manual.  It tells you that the convective heat transfer coefficient is 15 Watts/meter2K for still air over a horizontal surface.  The “K” represents temperature measured in degrees “Kelvin,” and this is calculated by simply adding 273.15 to any temperature measured in degrees Celsius, or °C.

     So, to get all of our units to match up in order to perform the calculation, the blacktop would be at a temperature of 65°C + 273.15 = 338.15K.  The cool air would be at a temperature of 30°C + 273.15 = 303.15K.  Our equation becomes:

Heat Flow = (15 W/m2K) x (10,000 m2) x (338.15K – 303.15K)

= 5,250,000 Watts 

     We have calculated that the air in the atmosphere picks up heat energy from the parking lot pavement at a rate of over 5 million watts.  This explains why it always seems to be warmer in cities compared to the surrounding countryside.  The presence of dark asphalt pavement and dark roofing materials absorb heat from the sun like any dark surface will, and this heat buildup then dissipates into the surrounding atmosphere through the process of natural convection.

     The other type of convection, the forced type, is just as its name implies.  It requires a powered device to move the fluid, that is to say, it does not rely on a natural source of energy like the sun.  An example of forced convection can be found in a hair dryer, which uses a small blower to move air over an electric heating element.  Another example of forced convection can be found in the water pump of your car.  This pump circulates water through the engine, absorbing heat as it goes, and then gives that heat up to the air which is flowing over the radiator fins.  This keeps the engine from overheating.

     Calculating heat transfer rates in forced convection problems can get extremely complicated, involving higher level mathematics and concepts of advanced fluid dynamics.  Some problems are so complex they can only be solved with the aid of specially written computer programs, so an example problem would be beyond the scope of the basic discussions in this series of articles.

     Next week we’ll analyze how the sun, which is separated from Earth by 93 million miles of the vacuum that we call Outer Space, is able to heat our blacktop pavement up from so great a distance.  It does this by the process of radiant heat transfer.

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Heat Transfer in Mechanical Engineering, Part I, Conduction

Sunday, February 14th, 2010

     Last week we finished up our series on fluid mechanics with a look at fluid dynamics, which considers fluids that move.  This week we’ll talk about heat transfer, which is the study of how heat moves through vacuums, gases, liquids, and solid objects.

     Understanding heat transfer is important when designing insulating materials, because they’re responsible for conserving energy by keeping heat contained inside things, things like pipes, boilers, and steam turbines.  Done in reverse, the concepts of heat transfer can also be used to determine how to dissipate excess heat, like in automobile engines and electrical equipment, to keep them from overheating.

     In the most basic of terms, heat transfer takes place because heat always wants to travel from a place of higher temperature to a place of lower temperature.  Heat will continue to flow in this direction until temperatures reach equilibrium, and this is true whether we’re considering heat moving through gases, liquids, or solids.  For example, if you pull the plug on your refrigerator, the heat from the air in your kitchen will begin to flow through the walls of the refrigerator where it will get absorbed by the cold food and ice cubes.  Eventually the heat will stop flowing when the temperature of the stuff inside of the refrigerator equals the temperature of the air in the kitchen.

     Now, there are different means by which heat can be transferred.  These include conduction, convection, and radiation.  Heat transfer analysis can get complicated, especially if it involves a combination of these means.  For now, let’s focus on conduction.

     As its name implies, heat transfer by conduction occurs when heat is conducted through a material.  Let’s consider the simple conductive heat transfer problem shown in Figure 1. 

Figure 1 – Heat Flow Through A 3-Centimeter Thick Copper Plate

 

    Figure 1 shows a 3 centimeter (cm) copper plate.  One side of the plate has a temperature of 400 degrees Centigrade (°C), the other side that of 100°C.  For this example let’s say that the plate is 30cm high and 20cm wide.  So, how do you calculate the heat flow?

    Now remember, heat always flows from hot to cold, so we know that it’s going to flow from the 400°C side to the 100°C side of the plate.  But let’s get into a little more detail.  Conductive heat flow depends on this temperature difference and some other things, like the ability of the material itself to conduct heat.  As with other materials and processes commonly used, scientists have performed lab experiments to measure heat conducting ability for all sorts of materials and recorded them in resource materials under the heading, “thermal conductivity,” so this element of information is readily available to us if we only seek out the reference manuals.  But in addition to this thermal conductivity factor, heat flow also depends on the area of the material that it is flowing through, as well as the material’s thickness.  So with this said, we can calculate the heat flow in Figure 1 to be:

        Heat Flow  =  (The Thermal Conductivity of Copper) x

                                   (The Area That The Heat Is Flowing Through) x

                                   (The Difference In Temperature) / (The Plate Thickness)

     From reference manuals, we know that the thermal conductivity of copper is 370 Watts/meter °C.  The second part of the equation asks us to calculate the area through which our heat will be flowing, and we calculate this to be 20cm x 30cm, or 600cm2.  Now in order for the units to match those of the thermal conductivity constant, we have to convert the area and thickness from centimeters (cm) to units of meters (m).  Therefore, 600cm2 becomes 0.06m2, and 3cm becomes 0.03m.  So our completed equation becomes:

Heat Flow = (370 W/m °C) x (0.06m2) x (400°C – 100°C) / (0.03m)

= 222,000 Watts

     This means that 222,000 Watts of heat will flow from the 400°C side to the 100°C side.   “Watts” don’t just apply to light bulbs, as we can see in this example.  Here, the number of Watts represent how much heat energy is flowing in a given amount of time.  This heat energy flow will stop when the temperature on each side of the copper plate becomes the same.  

     That wraps things up for our discussion of heat transfer via conduction.  Next week we’ll consider heat transfer by convection, and the next time someone talks about their convection oven, you’ll know what they’re talking about.

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Thermodynamics in Mechanical Engineering, Part V, Psychrometry

Sunday, January 3rd, 2010

     Last week we looked at the arithmetic behind chemical reactions in an area of thermodynamics known as stoichiometry.  This week we’ll learn about psychrometry and the value of a summer breeze.  Well, more specifically, psychrometry involves the analysis of gas and vapor mixtures, like air and water. 

     You may not have ever heard of psychrometry or psychrometrics, but your body is familiar with it.  In fact, it adheres to its principles every time it sweats.  See, sweating keeps you cool, and that’s because when liquids like water evaporate, they absorb heat in the process.  When sweat, which is mostly water, evaporates from your skin, it takes some of your body heat with it, dissipating it into the atmosphere.  That’s why a roomful of sweaty bodies is so uncomfortable to be in.

     Now let’s say you’re outside and it’s a hot, humid summer day.  The air already contains a lot of moisture, so it can’t absorb as much sweat from your body as it would in a drier environment.  As a result, your sweat doesn’t evaporate so well.  It lingers on your skin, keeping you miserable.  Now introduce a summer breeze.  The increase of air flow across your skin that it produces serves the same purpose as an electric fan in your home.  They both make you cooler by increasing the surrounding air flow, thereby making more air available to contact your skin, and increasing the sweat evaporation process.

     In the study of psychrometry, mechanical engineers learn about the thermodynamic properties of moist air.  Then they use these properties to analyze conditions and design processes which deal with moist air, things like air conditioning systems and dehumidifiers.

     Let’s return for a moment to that air conditioner example that we used in our discussion of Thermodynamics in Mechanical Engineering, Part III. This is shown in Figure 1 below.  Psychrometry would be used here, too.  For example, when you are determining how much heat must be removed from the warm, humid air inside your home by the evaporator coil inside your air conditioner.  Knowing how much heat must be removed is one of the first steps to designing a system which is properly sized and works efficiently in order to keep you comfortable.

 

refrig1

Figure 1 – A Simple Refrigeration Cycle

 

     Psychrometric calculations can get pretty involved, and our discussion is meant to provide only a brief overview, but suffice it to say that their basic function is to set up a mass and energy accounting system that adheres to the principles of the First Law of Thermodynamics.  In other words, energy and mass going into a system has to add up to energy and mass coming out.

     Now, let’s return to our discussion on psychrometry in relation to the design of the air conditioning system of Figure 1.  Let’s focus on the evaporator coil from this system, as shown in Figure 2.  This coil is contained in a duct along with a blower.  The air sucked into the evaporator coil from the room has water vapor mixed into it.  The pure air part and the water vapor part each contain heat energy.  Our bodies perceive that heat energy as warm, humid air.  As that humid air is cooled by the evaporator coil, much of the water vapor condenses out of it as liquid moisture, which is then drained out of the air conditioner.  What’s left is a cooler mixture of air and greatly reduced water vapor.  This mixture then leaves the evaporator coil and is sent back into your home from the duct by way of a blower, resulting in a more comfortable environment for you.

 

evap1   

Figure 2 – An Evaporator Coil In An Air Conditioning Unit 

 

     So, using the First Law of Thermodynamics, the heat accounting system for the air conditioner looks like this:

 Qevaporator =

      (Qair + Qwater vapor)going in – (Qair + Qwater vapor + Qcondensed moisture)going out

where, “Qevaporator” is the heat energy removed by the evaporator coil, “Qair” is the heat energy contained in the air, “Qwater vapor” is the heat energy contained in the water vapor, and “Qcondensed moisture” is the heat energy contained in the condensed moisture drained out of the air conditioner.   By the way, the letter “Q” is often used to denote heat in thermodynamics.

     To solve for the equation above, one has to first consider what the pressure, temperature, and relative humidity of the air will be in the room when the air conditioner is first turned on.  We must next determine what the desired pressure, temperature and relative humidity should ideally be once the conditioned air leaves the evaporator coil on its journey back into the room.  In other words, we need to know the conditions we are starting out with in order to know where we want to end up, comfort-wise.  Once these parameters are known, thermodynamic formulas are used to calculate how much heat must be removed by the evaporator coil.  Now the air conditioning equipment can be designed with a large enough evaporator coil, with sufficient refrigerant flowing through it, and a large enough blower to efficiently perform the task of keeping us cool.

     This concludes our tour of the world of thermodynamics.  Next week we’ll begin our discussion of an area of mechanical engineering known as fluid mechanics, which is the study of the force, pressure, and energy of both stationary and moving fluids.  We’ll see how a hydraulic car jack works, how water flows through pipes, and how airplane wings lift a plane into the sky.

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