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Heat transfer

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introduction to heat transfer & HVAC

Veröffentlicht in: Ingenieurwesen
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Heat transfer

  1. 1. HEAT TRANSFER by Aruna c.p
  2. 2. CONTENTS  INTRODUCTION  SPECIFIC HEAT CAPACITY  CONDUCTION  CONVECTION  RADIATION  THERMAL CONTACT RESISTANCE  HVAC  INTRODUCTION  HISTORY  BASIC–AC SYSTEM  ECONOMIZER CYCLE  CHOSING AN AIR CONDITIONING SYSTEM  REFRIGERATION
  3. 3. INTRODUCTION  Heat always moves from a warmer place to a cooler place.  Hot objects in a cooler room will cool to room temperature.  Cold objects in a warmer room will heat up to room temperature.
  4. 4. WHY WE NEED HEAT TRANSFER A thermodynamic simply tells us how much amount of heat transfer from one equilibrium state to another equilibrium state.
  5. 5. Heat transfer plays major rule design of many devices,such as radiators, solar collectors, various components of power plants, even space craft.
  6. 6. HOW DOES ENERGY AFFECT MATERIALS? Do different materials need the same amount of energy to increase their temperature by the same amount? To increase the temperature of 1 kg of water by 1°C, requires 4200 J. To increase the temperature of 1 kg of copper by 1°C, requires 390 J. Water and copper require different amounts of energy because they have different values for a property called specific heat capacity. It is the amount of energy required to increase the temperature of 1 kg of a material by 1°C. So, the specific heat capacity for water is 4200 J/kg°C and for copper is 390 J/kg°C.
  7. 7. WHAT IS SPECIFIC HEAT CAPACITY? The specific heat capacity of a material is the amount of energy required to raise 1kg of the material by 1°C. It can be used to work out how much energy is needed to raise the temperature of a material by a certain amount: Energy = mass *specific heat capacity* temperature change  Energy is measured in joules (J).  Mass is measured in kilograms (kg).  Temperature change is measured in °C.  Specific heat capacity is measured in J/kg°C.
  8. 8. SPECIFIC HEAT CAPACITY EXAMPLE Using the specific heat capacity of water (4200 J/kg°C), how much energy is needed to increase the temperature of 600 g of water by 80°C in a kettle? Note: mass = 600 g = 0.6 kg Energy = mass *specific heat capacity* temperature change energy = 0.6 x 4200 x 80 = 201 600 J
  9. 9. HEAT TRANSFER MODES  Conduction  Convection  radiation
  10. 10. WHAT IS CONDUCTION? How are the particles arranged in a solid, a liquid and a gas? solids liquids gases Conduction is the transfer of energy from more energetic particles of a substance to adjacent less energetic ones result of interaction between particles.
  11. 11. FOURIER LAW In 1822 Fourier postulated that the rate of heat transfer is proportional to the temperature gradient present in a solid.
  12. 12. THERMAL CONDUCTIVITY
  13. 13. CONVECTION What happens to the particles in a liquid or a gas when you heat them? The particles spread out and become less dense. Convection heat transfer
  14. 14. Fluid movement Cooler, more d____, ense fluids sink through w_____, armer less dense fluids. In effect, warmer liquids and gases r___ up. ise Cooler liquids and gases s___. ink
  15. 15. TYPES OF CONVECTION  forced convection ex: pump  Natural convection ex: wind
  16. 16. HOW DOES HEAT TRAVEL THROUGH SPACE? The Earth is warmed by heat energy from the Sun. How does this heat energy travel from the Sun to the Earth? infrared waves There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection. The heat travels to Earth by infrared waves. These are similar to light waves and are able to travel through empty space.
  17. 17. WHAT ARE INFRARED WAVES? Heat can move by travelling as infrared waves. These are electromagnetic waves, like light waves, but with a longer wavelength. This means that infrared waves act like light waves:  They can travel through a vacuum.  They travel at the same speed as light – 300,000,000 m/s.  They can be reflected and absorbed.
  18. 18. EMISSION EXPERIMENT Four containers were filled with warm water. Which container would have the warmest water after ten minutes? Shiny metal Dull metal Dull black Shiny black shiny metal The __________ container would be the warmest after ten minutes because its shiny surface reflects heat radiation _______ back into the container so less is lost. The ________ dull black container would be the coolest because it is the best at _______ emitting heat radiation.
  19. 19. ABSORPTION EXPERIMENT Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes? dull black The __________ container would be the warmest after ten minutes because its surface absorbs heat _______ radiation the best. The _________ shiny metal container would be the coolest because it is the poorest at __________ heat radiation. absorbing Shiny metal Dull metal Dull black Shiny black
  20. 20. RADIATION QUESTIONS Why are houses painted white in hot countries? White reflects heat radiation and keeps the house cooler. Why are shiny foil blankets wrapped around marathon runners at the end of a race? The shiny metal reflects the heat radiation from the runner back in, this stops the runner getting cold.
  21. 21. STEADY V/S TRANSIENT HEAT TRANSFER Steady; no change with time Transient: change with time
  22. 22. THERMAL CONTACT RESISTANCE The thermal contact resistance will be greater for rough surfaces because an interface with rough surfaces will contain more air gaps whose thermal conductivity is low. The thermal contact resistance can be minimized by applying • a thermal grease such as silicon oil • a better conducting gas such as helium or hydrogen • a soft metallic foil such as tin, silver, copper, nickel, or aluminum thermal contact resistance is significant and can even dominate the heat transfer for good heat conductors such as metals, but can be disregarded for poor heat conductors such as insulations.
  23. 23. 25 THERMAL CONTACT RESISTANCE Temperature distribution and heat flow lines along two solid plates pressed against each other for the case of perfect and imperfect contact.
  24. 24. HVAC
  25. 25. INTRODUCTION Heating, Ventilating and Air Conditioning, HVAC, is a huge field. HVAC systems include a range from the simplest hand-stoked stove, used for comfort heating, to the extremely reliable total air-conditioning systems found in sub marines and space shuttles.
  26. 26. HISTORY In 1851 Dr. John Gorrie was granted U.S. patent 8080 for refrigeration machine. Cooling the New York Stock Exchange, in 1902, was one of the first comfort cooling systems. Comfort cooling was called “air conditioning
  27. 27. SCOPE OF MODERN HVAC  Greenhouse gas emissions and the destruction of the earth’s protective ozone layer are concerns that are stimulating research  Energy conservation is an ongoing challenge to find novel ways to reduce consumption in new and existing buildings without compromising comfort and indoor air quality.
  28. 28. AIR CONDITIONING “air conditioning,” when properly used, now means the total control of temperature, moisture in the air (humidity), supply of outside air for ventilation, filtration of airborne particles, and air movement in the occupied space. There are seven main processes required to achieve full air conditioning 1. Heating 2. Cooling 3. Humidifying 4. Dehumidifying 5. Cleaning 6. Ventilation 7. Air movement
  29. 29. BASIC AIR-CONDITIONING SYSTEM
  30. 30. ECONOMIZER CYCLE
  31. 31. CHOOSING AN AIR-CONDITIONING SYSTEM Building design; if there is very little space for running ducts around the building, an all-air system may not fit in the available space Location issues;  site conditions  peak summer cooling conditions  summer humidity  peak winter heating conditions  wind speeds  sunshine hours  typical snow accumulation depths Utilities: availability and cost; The choice of system can be heavily influenced by available utilities and their costs to supply and use
  32. 32. …CONT Indoor requirements and loads;  The thermal and moisture loads  Outside ventilation air  Zoning Client issues; Buildings cost money to construct and to use. Therefore, the designer has to consider the clients’ requirements both for construction and for in-use costs
  33. 33. REFRIGERATION Heat flows in direction of decreasing temperature, i.e., from high-temperature to low temperature regions. The transfer of heat from a low-temperature to high-temperature requires a refrigerator and/or heat pump
  34. 34. COP
  35. 35. IDEAL VAPOR COMPRESSION REFRIGERATION CYCLE Vapor compression cycle
  36. 36. REFERENCES 1. Yunus A. Cengel “Heat and mass transfer” McGraw-Hill Education(India) Pvt Limited, 2007. 2. www.board works.com. 3. ASHRAE Learning Institute’s Fundamentals of HVAC&R e-Learning System .
  37. 37. THANK YOU

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