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HEAT TRANSFER by Aruna c.p

Author: aruna-c-p

Post on 22-Nov-2014

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introduction to heat transfer & HVAC

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• 1. HEATTRANSFERbyAruna c.p

2. CONTENTS INTRODUCTION SPECIFIC HEAT CAPACITY CONDUCTION CONVECTION RADIATION THERMAL CONTACT RESISTANCE HVAC INTRODUCTION HISTORY BASICAC SYSTEM ECONOMIZER CYCLE CHOSING AN AIR CONDITIONING SYSTEM REFRIGERATION 3. INTRODUCTION Heat always moves from awarmer place to a cooler place. Hot objects in a cooler roomwill cool to room temperature. Cold objects in a warmer roomwill heat up to roomtemperature. 4. WHY WE NEED HEATTRANSFERA thermodynamic simplytells us how much amount ofheat transfer from oneequilibrium state to anotherequilibrium state. 5. Heat transfer plays major rule design of manydevices,such as radiators, solar collectors, variouscomponents of power plants, even space craft. 6. HOW DOES ENERGY AFFECT MATERIALS?Do different materials need the same amount of energy to increase theirtemperature by the same amount?To increase thetemperature of 1 kgof water by 1C,requires 4200 J.To increase thetemperature of 1 kg ofcopper by 1C, requires390 J.Water and copper require different amounts of energy because they havedifferent values for a property called specific heat capacity.It is the amount of energy required to increase the temperature of 1 kg ofa material by 1C.So, the specific heat capacity for water is 4200 J/kgCand for copper is 390 J/kgC. 7. WHAT IS SPECIFIC HEAT CAPACITY?The specific heat capacity of a material is the amount of energy required toraise 1kg of the material by 1C.It can be used to work out how much energy is needed to raise thetemperature of a material by a certain amount:Energy = mass *specific heat capacity* temperaturechange 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/kgC. 8. SPECIFIC HEAT CAPACITY EXAMPLEUsing the specific heat capacityof water (4200 J/kgC), how much energyis needed to increase the temperature of600 g of water by 80C in a kettle?Note: mass = 600 g = 0.6 kgEnergy = mass *specific heat capacity* temperaturechangeenergy = 0.6 x 4200 x 80= 201 600 J 9. HEAT TRANSFER MODES Conduction Convection radiation 10. WHAT IS CONDUCTION?How are the particles arranged in a solid, a liquid and a gas?solids liquids gasesConduction is the transfer of energy from more energetic particles of asubstance to adjacent less energetic ones result of interaction betweenparticles. 11. FOURIER LAWIn 1822 Fourier postulated that the rate of heat transfer is proportional tothe temperature gradient present in a solid. 12. THERMAL CONDUCTIVITY 13. CONVECTIONWhat happens to the particles in a liquid or a gas when you heat them?The particles spread out andbecome less dense.Convection heat transfer 14. Fluid movementCooler, more d____, ensefluidssink through w_____, armerlessdense fluids.In effect, warmer liquids andgases r___ up.iseCooler liquids and gases s___.ink 15. TYPES OF CONVECTION forced convectionex: pump Natural convectionex: wind 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?infraredwavesThere are no particlesbetween the Sun and theEarth, so the heat cannottravel by conduction or byconvection.The heat travels to Earth byinfrared waves. These aresimilar to light waves andare able to travel throughempty space. 17. WHAT ARE INFRARED WAVES?Heat can move by travelling as infrared waves.These are electromagnetic waves, like light waves, but with alonger 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. EMISSION EXPERIMENTFour containers were filled with warm water. Whichcontainer would have the warmest water after ten minutes?Shiny metalDull metalDull blackShiny blackshiny metalThe __________ container would be the warmest after tenminutes because its shiny surface reflects heat radiation_______ backinto the container so less is lost. The ________ dull blackcontainerwould be the coolest because it is the best at _______ emittingheatradiation. 19. ABSORPTIONEXPERIMENTFour containers were placed equidistant from a heater. Whichcontainer would have the warmest water after ten minutes?dull blackThe __________ container would be the warmest after tenminutes because its surface absorbs heat _______ radiationthe best.The _________ shiny metalcontainer would be the coolest because it isthe poorest at __________ heat radiation.absorbingShiny metalDull metalDull blackShiny black 20. RADIATIONQUESTIONSWhy are houses painted white in hotcountries?White reflects heat radiation andkeeps the house cooler.Why are shiny foil blankets wrappedaround marathon runners at the end of arace?The shiny metal reflects theheat radiation from therunner back in, this stopsthe runner getting cold. 21. STEADY V/S TRANSIENT HEAT TRANSFERSteady; no change withtimeTransient: change withtime 22. THERMAL CONTACT RESISTANCEThe thermal contact resistance will be greaterfor rough surfaces because an interface withrough surfaces will contain more air gapswhose thermal conductivity is low.The thermal contact resistance canbe minimized by applying a thermal grease such as silicon oil a better conducting gas such ashelium or hydrogen a soft metallic foil such as tin, silver,copper, nickel, or aluminumthermal contact resistance is significant and can even dominate theheat transfer for good heat conductors such as metals, but can bedisregarded for poor heat conductors such as insulations. 23. 25THERMAL CONTACT RESISTANCETemperature distribution and heat flow lines along two solid platespressed against each other for the case of perfect and imperfect contact. 24. HVAC 25. INTRODUCTIONHeating, Ventilating and Air Conditioning, HVAC, is a huge field. HVAC systemsinclude a range from the simplest hand-stoked stove, used for comfort heating,to the extremely reliable total air-conditioning systems found in sub marinesand space shuttles. 26. HISTORYIn 1851 Dr. John Gorrie wasgranted U.S. patent 8080 forrefrigeration machine.Cooling the New York Stock Exchange, in1902, was one of the first comfort coolingsystems. Comfort cooling was called airconditioning 27. SCOPE OF MODERN HVAC Greenhouse gas emissions and the destruction of the earths protectiveozone layer are concerns that are stimulating research Energy conservation is an ongoing challenge to find novel ways toreduce consumption in new and existing buildings without compromisingcomfort and indoor air quality. 28. AIR CONDITIONINGair conditioning, when properly used, nowmeans the total control of temperature,moisture in the air (humidity), supplyof outside air for ventilation, filtration ofairborne particles, and air movementin the occupied space.There are seven main processes required toachieve full air conditioning1. Heating2. Cooling3. Humidifying4. Dehumidifying5. Cleaning6. Ventilation7. Air movement 29. BASIC AIR-CONDITIONING SYSTEM 30. ECONOMIZER CYCLE 31. CHOOSING AN AIR-CONDITIONING SYSTEMBuilding design;if there is very little space for running ducts around the building, an all-airsystem may not fit in the available spaceLocation issues; site conditions peak summer cooling conditions summer humidity peak winter heating conditions wind speeds sunshine hours typical snow accumulation depthsUtilities: availability and cost;The choice of system can be heavily influenced by available utilities and theircosts to supply and use 32. CONTIndoor requirements and loads; The thermal and moisture loads Outside ventilation air ZoningClient issues;Buildings cost money to construct and to use. Therefore, the designerhas to consider the clients requirements both for construction and for in-usecosts 33. REFRIGERATIONHeat flows in direction of decreasing temperature, i.e., from high-temperature tolow temperature regions. The transfer of heat from a low-temperature to high-temperaturerequires a refrigerator and/or heat pump 34. COP 35. IDEAL VAPOR COMPRESSION REFRIGERATIONCYCLEVapor compression cycle 36. REFERENCES1. Yunus A. Cengel Heat and mass transfer McGraw-HillEducation(India) Pvt Limited, 2007.2. www.board works.com.3. ASHRAE Learning Institutes Fundamentals of HVAC&Re-Learning System. 37. THANK YOU