I t was midsummer, the heat rippling above the macadam roads, cicadas screaming out of the trees, and the sky like pewter, glaring. The days were the same day, like the shallow mud-brown river moving always int he same direction but so low you couldn't see it.
Except for Sunday: church in the morning, then the fate Sunday newspaper, the color comics, and newsprint on your fingers. Rhea and Rhoda Kunkel went flying on their rusted old bicycles, down the long hill toward the railroad yard, Whipple's Ice, the scrubby pastureland where dairy cows grazed. They'd stolen six dollars from their own grandmother who loved them. They were eleven years old; they were identical twins; they basked in their power.
You just wouldn't say the names that way. Not even the teachers at school would say them that way. We went to see them in the funeral parlor where they were waked; we were made to. The twins in twin caskets, white, smooth, gleaming, perfect as plastic, with white satin lining puckered like the inside of a fancy candy box.
And the waxy white lilies, and the smell of talcum powder and perfume. The room was crowded; there was only one way in and out. Rhea and Rhoda were the same girl; they'd wanted it that way. Only looking from one to the other could you see they were two. The heat was gauzy; you had to push your way through like swimming. On their bicycles Rhea and Rhoda flew through it hardly noticing, from their grandmother's place on Main Street to the end of South Main where the paved road turned to gravel leaving town.
That was the summer before seventh grade, when they died. Death was coming for them, but they didn't know. They thought the same thoughts sometimes at the same moment, had the same dream and went all day trying to remember it, brining it back like something you'd be hauling out of the water on a tangled line.
We watched them; we were jealous. None of us had a twin. Sometimes they were serious and sometimes, remembering, they shrieked and laughed like they were being killed. They stole things out of desks and lockers but if you caught them they'd hand them right back; it was like a game.
There were three floor fans in the funeral parlor that I could see, tall whirring fans with propeller blades turning fast to keep the warm air moving. Strange little gusts came from all directions, making your eyes water. By this time Roger Whipple was arrested, taken into police custody. No one had hurt him. He would never stand trial; he was ruled mentally unfit and would never be released from confinement.
He died there, in the state psychiatric hospital, years later, and was brought back home to be buried--the body of him, I mean. His earthly remains. Rhea and Rhoda Kunkel were buried in the same cemetery, the First Methodist.
The cemetery is just a field behind the church. In the caskets the dead girls did not look like anyone we knew, really. They were placed on their backs with their eyes closed, and their mouths, the way you don't always look in life when you're sleeping. Their faces were too small. Every eyelash showed, too perfect.Wavesurfer cursor plugin
I stared and stared. What had been done to them, the lower parts of them, didn't show in the caskets. Roger Whipple worked for his father at Whipple's Ice. In the newspaper it stated he was nineteen.A heat exchanger is a system used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air.
Another example is the heat sinkwhich is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant. There are three primary classifications of heat exchangers according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side.
In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is the most efficient, in that it can transfer the most heat from the heat transfer medium per unit mass due to the fact that the average temperature difference along any unit length is higher. See countercurrent exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.
What is Heat Treatment Processes?
For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence. The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined.
In most simple systems this is the " log mean temperature difference " LMTD. Double pipe heat exchangers are the simplest exchangers used in industries.Tsc cannot find module csstype
On one hand, these heat exchangers are cheap for both design and maintenance, making them a good choice for small industries. On the other hand, their low efficiency coupled with the high space occupied in large scales, has led modern industries to use more efficient heat exchangers like shell and tube or plate.
However, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basics to students as the fundamental rules for all heat exchangers are the same.
Shell and tube heat exchangers consist of a series of tubes which contain fluid that must be either heated or cooled.
A second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers: There can be many variations on the shell and tube design.
Typically, the ends of each tube are connected to plenums sometimes called water boxes through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes. Fixed tube liquid-cooled heat exchangers especially suitable for marine and harsh applications can be assembled with brass shells, copper tubes, brass baffles, and forged brass integral end hubs. Another type of heat exchanger is the plate heat exchanger.
These exchangers are composed of many thin, slightly separated plates that have very large surface areas and small fluid flow passages for heat transfer.
Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. In HVAC applications, large heat exchangers of this type are called plate-and-frame ; when used in open loops, these heat exchangers are normally of the gasket type to allow periodic disassembly, cleaning, and inspection. There are many types of permanently bonded plate heat exchangers, such as dip-brazed, vacuum-brazed, and welded plate varieties, and they are often specified for closed-loop applications such as refrigeration.
Plate heat exchangers also differ in the types of plates that are used, and in the configurations of those plates. When compared to shell and tube exchangers, the stacked-plate arrangement typically has lower volume and cost.
Another difference between the two is that plate exchangers typically serve low to medium pressure fluids, compared to medium and high pressures of shell and tube. A third and important difference is that plate exchangers employ more countercurrent flow rather than cross current flow, which allows lower approach temperature differences, high temperature changes, and increased efficiencies.Heat treatment is defined as a combined process of heating and cooling of metal to change the physical and mechanical properties of a material.
Heat treatment is being used to homogenize the cast metal alloy to enhance their work-ability in the very high temperature, to change the micro-structure in such a way as to achieve the desired mechanical properties. To perform the heat treatment process, safety is the first priority of the person who is dealing with it, because in the process large amount of heat releases, if anything goes wrong may result in very serious problems.
The heat treatment process is performed in the furnace and ovens where the temperature is changing as per the requirement and metal onto the process has to perform, apart from this the gases are used to control the atmosphere for the particular process of heat treatment.
When the metal comes in contact with the atmosphere then there is the possibility that metal can react with atmosphere and can involve in the chemical reaction. In the atmosphere, many gases and moisture is present that can affect the process of heat treatment that is why before performing any heat treatment process atmosphere of the particular space has to be maintained.
It reduces the effect of oxidation on the components which is being treated. Heat treatment is associated with increasing the strength of the material, but it is not only associated with the strength. It changes the manufacturing aspects also such as improve machining, formability, and when the operation gets over material restore it ductility on cooling.
The parameters that affect the composition and materials properties of the metal are as follows. Homogenization is a general treatment process when the material goes for operation than before the actual treatment start homogenization is performed to maintain the equal temperature throughout the material being treated. As the word says that, first the material is being heated at a very high temperature and then slowly cooled. It is one of the heat treatment processes which is used to increase the ductility of the material and reduce the hardness.
When the hardness and the ductility of material get changes it results in the reduction of dislocation in the crystal structure of a material. The material is being heated at the prefixed temperature, hold for a certain time then Start cooling slowly at room temperature.
If the material is steel, it is carried out by heating the steel just above the critical temperature of steel i. It is the heat treatment process in which the material is being heated above the degree Celsius to complete the austenitization. Once the material reaches austenitization stage then it is cooled in the presence of air to obtain the fine pearlite, it has good hardness and ductility.
Normalizing process is used for the ferrous material to enhance the mechanical properties of the material. Carburising surface heat treatment process, which is performed on the surface of the material in order to increase the hardness and wear resistance of the metal.Time Required: 3 hours three minute class periods. Most curricular materials in TeachEngineering are hierarchically organized; i. Some activities or lessons, however, were developed to stand alone, and hence, they might not conform to this strict hierarchy.
Related Curriculum shows how the document you are currently viewing fits into this hierarchy of curricular materials. The fan inside this CPU is one example of why heat is so important to engineering and the design of engineered systems, as well as our everyday lives. Understanding heat transfer is essential knowledge for the engineering of mechanical, chemical and biological systems. Design of internal combustion engines, air conditioning and heating systems, chemical and biological reactors and even clothing technology requires an understanding of heat transfer.
Design of insulating materials for homes, buildings and even beverage containers also requires an understanding of heat transfer. Each TeachEngineering lesson or activity is correlated to one or more K science, technology, engineering or math STEM educational standards.
In the ASN, standards are hierarchically structured: first by source; e. Make observations to provide evidence that energy can be transferred from place to place by sound, light, heat, and electric currents. Grade 4. Do you agree with this alignment? Thanks for your feedback! Alignment agreement: Thanks for your feedback! Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
Grades 6 - 8. View aligned curriculum. With the help of simple, teacher-led demonstration activities, students learn the basic physics of heat transfer by means of conduction, convection and radiation. They also learn about examples of heating and cooling devices, from stove tops to car radiators, that they encounter in their homes, scho Students learn about the nature of thermal energy, temperature and how materials store thermal energy.
They discuss the difference between conduction, convection and radiation of thermal energy, and complete activities in which they investigate the difference between temperature, thermal energy and Students learn the scientific concepts of temperature, heat and the transfer of heat through conduction, convection and radiation, which are illustrated by comparison to magical spells found in the Harry Potter books.
Students explore heat transfer and energy efficiency using the context of energy efficient houses. They gain a solid understanding of the three types of heat transfer: radiation, convection and conduction, which are explained in detail and related to the real world.Class 11 chemistry chapter 1 numericals pdf
A familiarity with basic concepts about energy and its different forms, as well as a basic understanding of temperature. Raise your hand if you ever put on a jacket? Or turned on a heater? Or melted an ice cube in your hand? You probably appreciate heat on a cold day.We have seen in previous chapters that energy is one of the fundamental concepts of physics.
Heat is a type of energy transfer that is caused by a temperature difference, and it can change the temperature of an object. As we learned earlier in this chapter, heat transfer is the movement of energy from one place or material to another as a result of a difference in temperature.Ubakka libambo baixar
Heat transfer is fundamental to such everyday activities as home heating and cooking, as well as many industrial processes. It also forms a basis for the topics in the remainder of this chapter. We also introduce the concept of internal energy, which can be increased or decreased by heat transfer. We discuss another way to change the internal energy of a system, namely doing work on it.
Thus, we are beginning the study of the relationship of heat and work, which is the basis of engines and refrigerators and the central topic and origin of the name of thermodynamics. A thermal system has internal energy also called thermal energywhich is the sum of the mechanical energies of its molecules. As we saw earlier in this chapter, if two objects at different temperatures are brought into contact with each other, energy is transferred from the hotter to the colder object until the bodies reach thermal equilibrium that is, they are at the same temperature.
No work is done by either object because no force acts through a distance as we discussed in Work and Kinetic Energy.
These observations reveal that heat is energy transferred spontaneously due to a temperature difference. Heat is a form of energy flow, whereas temperature is not. Incidentally, humans are sensitive to heat flow rather than to temperature. Since heat is a form of energy, its SI unit is the joule J. Another common unit of energy often used for heat is the calorie caldefined as the energy needed to change the temperature of 1.
Also commonly used is the kilocalorie kcalwhich is the energy needed to change the temperature of 1. Since mass is most often specified in kilograms, the kilocalorie is convenient. It is also possible to change the temperature of a substance by doing work, which transfers energy into or out of a system.
This realization helped establish that heat is a form of energy. James Prescott Joule — performed many experiments to establish the mechanical equivalent of heat — the work needed to produce the same effects as heat transfer.
In the units used for these two quantities, the value for this equivalence is. It helped establish the principle of conservation of energy. Gravitational potential energy U was converted into kinetic energy Kand then randomized by viscosity and turbulence into increased average kinetic energy of atoms and molecules in the system, producing a temperature increase.
Increasing internal energy by heat transfer gives the same result as increasing it by doing work. Temperature and internal energy are state variables.
To sum up this paragraph, heat and work are not state variables. Incidentally, increasing the internal energy of a system does not necessarily increase its temperature.
An example is the melting of ice, which can be accomplished by adding heat or by doing frictional work, as when an ice cube is rubbed against a rough surface. We have noted that heat transfer often causes temperature change. Experiments show that with no phase change and no work done on or by the system, the transferred heat is typically directly proportional to the change in temperature and to the mass of the system, to a good approximation.
Below we show how to handle situations where the approximation is not valid. The constant of proportionality depends on the substance and its phase, which may be gas, liquid, or solid. We omit discussion of the fourth phase, plasma, because although it is the most common phase in the universe, it is rare and short-lived on Earth. We can understand the experimental facts by noting that the transferred heat is the change in the internal energy, which is the total energy of the molecules.Workers who are exposed to extreme heat or work in hot environments may be at risk of heat stress.
Exposure to extreme heat can result in occupational illnesses and injuries. Heat stress can result in heat stroke, heat exhaustion, heat cramps, or heat rashes. Heat can also increase the risk of injuries in workers as it may result in sweaty palms, fogged-up safety glasses, and dizziness. Burns may also occur as a result of accidental contact with hot surfaces or steam. Workers at risk of heat stress include outdoor workers and workers in hot environments such as firefighters, bakery workers, farmers, construction workers, miners, boiler room workers, factory workers, and others.
Workers at greater risk of heat stress include those who are 65 years of age or older, are overweight, have heart disease or high blood pressure, or take medications that may be affected by extreme heat. Prevention of heat stress in workers is important. Employers should provide training to workers so they understand what heat stress is, how it affects their health and safety, and how it can be prevented.
Learn how to identify the symptoms and protect yourself from heat stress. Now available in ePub format. The approach of summer is a reminder to us all of the need to recognize, and act to prevent, the harmful effects of excessive heat. Skip directly to site content Skip directly to page options Skip directly to A-Z link. Section Navigation. Minus Related Pages.Geopandas plot a point
Heat-Related Illnesses. Additional Resources. Hazards to Outdoor Workers. Links with this icon indicate that you are leaving the CDC website.basics of heat transfer lecture 1 : Conduction, Convection and Radiation
Linking to a non-federal website does not constitute an endorsement by CDC or any of its employees of the sponsors or the information and products presented on the website.
Cancel Continue.Heatenergy that is transferred from one body to another as the result of a difference in temperature. If two bodies at different temperatures are brought together, energy is transferred—i.
The effect of this transfer of energy usually, but not always, is an increase in the temperature of the colder body and a decrease in the temperature of the hotter body.
1.5: Heat Transfer, Specific Heat, and Calorimetry
A substance may absorb heat without an increase in temperature by changing from one physical state or phase to another, as from a solid to a liquid meltingfrom a solid to a vapour sublimationfrom a liquid to a vapour boilingor from one solid form to another usually called a crystalline transition.
The important distinction between heat and temperature heat being a form of energy and temperature a measure of the amount of that energy present in a body was clarified during the 18th and 19th centuries.
Because all of the many forms of energy, including heat, can be converted into workamounts of energy are expressed in units of work, such as joulesfoot-pounds, kilowatt-hours, or calories.
Exact relationships exist between the amounts of heat added to or removed from a body and the magnitude of the effects on the state of the body. The two units of heat most commonly used are the calorie and the British thermal unit BTU. The calorie or gram-calorie is the amount of energy required to raise the temperature of one gram of water from One BTU is approximately calories.
Both definitions specify that the temperature changes are to be measured at a constant pressure of one atmosphere, because the amounts of energy involved depend in part on pressure.
The calorie used in measuring the energy content of foods is the large calorie, or kilogram-calorie, equal to 1, gram-calories. In general, the amount of energy required to raise a unit mass of a substance through a specified temperature interval is called the heat capacityor the specific heatof that substance. The quantity of energy necessary to raise the temperature of a body one degree varies depending upon the restraints imposed. If heat is added to a gas confined at constant volume, the amount of heat needed to cause a one-degree temperature rise is less than if the heat is added to the same gas free to expand as in a cylinder fitted with a movable piston and so do work.
In the first case, all the energy goes into raising the temperature of the gas, but in the second case, the energy not only contributes to the temperature increase of the gas but also provides the energy necessary for the work done by the gas on the piston. Consequently, the specific heat of a substance depends on these conditions. The most commonly determined specific heats are the specific heat at constant volume and the specific heat at constant pressure. The so-called law of Dulong and Petit was useful in determining the atomic weights of certain metallic elements, but there are many exceptions to it; the deviations were later found to be explainable on the basis of quantum mechanics.
It is incorrect to speak of the heat in a body, because heat is restricted to energy being transferred. Energy stored in a body is not heat nor is it work, as work is also energy in transit. It is customary, however, to speak of sensible and latent heat. The latent heat, also called the heat of vaporizationis the amount of energy necessary to change a liquid to a vapour at constant temperature and pressure.
The energy required to melt a solid to a liquid is called the heat of fusionand the heat of sublimation is the energy necessary to change a solid directly to a vapour, these changes also taking place under conditions of constant temperature and pressure. Air is a mixture of gases and water vapour, and it is possible for the water present in the air to change phase; i. To distinguish between the energy associated with the phase change the latent heat and the energy required for a temperature change, the concept of sensible heat was introduced.
In a mixture of water vapour and air, the sensible heat is the energy necessary to produce a particular temperature change excluding any energy required for a phase change.
Article Media. Info Print Print. Table Of Contents. Submit Feedback. Thank you for your feedback. Introduction Heat as a form of energy Heat transfer. Heat physics. See Article History. Get exclusive access to content from our First Edition with your subscription.
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