Electrical Heating


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Electrical Heating

  1. 1. Electrical heating
  2. 2.  Learning Outcome 1: Heat and temperature, heat capacity and heat transfer. Learning Outcome 2: Methods used to control heating in various situations Learning Outcome 3: The processes and techniques used for water, space and industrial process heating. Learning Outcome 4: AS3000:2007 Wiring Rules requirements. Learning Outcome 5: Possible causes of malfunction in electric heating equipment and the tests required to diagnose faults Revision 01 2
  3. 3. Heat and temperature  What is the difference between Heat and Temperature? Heat is a measure of the total kinetic energy of the molecules or atoms in a body. ◦ The quantity of energy stored is measured in Joules ◦ Symbol – J  Temperature is a measure of the degree of movement of the random oscillations of the molecules. Alternatively, it can be defined as a measure of the hotness of a body.  No movement = No temperature. (ie. Absolute Zero)  If a body is not storing heat its temperature is absolute zero. Revision 01 3
  4. 4. Electrical Heating Transfer of HeatHeat is transferred from a hotter region to a colder region
  5. 5. Electrical Heating Heat is EnergyEnergy (W)in Joules (J) Power in Watts (W) Time in seconds (s)
  6. 6. Electrical Heating Temperature Scales The common Some countries use the temperature FAHRENHEIT scalescale is CELSIUS Water boils at 100oC Water boils at 212oF Ice melts at 0oC Ice melts at 32oF
  7. 7. Electrical Heating Temperature Scales The temperaturescale used in science and Water boils at 373K engineering is theabsolute KELVIN scale (K) Zero Kelvin (0K) is “Absolute One Kelvin “degree” Zero” is equal to and is equivalent to One Celsius “degree” Zero Kelvin is “Absolute -273oC Zero” NO heat content; Ice melts at 273K NO molecular motion. The “degree” symbol o is NOT used with the Kelvin scale
  8. 8. Electrical HeatingTemperature Scales To convert Fahrenheit to Celsius:
  9. 9. Electrical Heating Temperature Scales To convert Celsius to Fahrenheit:
  10. 10. Electrical Heating Temperature Scales To convert Kelvin to Celsius:
  11. 11. Electrical Heating Temperature Scales To convert Celsius to Kelvin:
  12. 12.  Kelvin ◦ 0K absolute zero ◦ 273.15K ice point water ◦ 373.15K steam point of water ◦ Note 100 degrees between ice and steam Celsius ◦ -273.15OC absolute zero ◦ 0° C ice point water ◦ 100° C steam point of water ◦ Note 100 degrees between ice and steam Revision 01 12
  13. 13.  The ability of a substance to store heat. If equal masses absorb equal amounts ofthermal energy (heat), different substances show a different temperature increase. Revision 01 13
  14. 14. Electrical Heating Specific Heat Capacity Specific Heat Capacity is the amount of heat energy required to change the temperature of one kilogram of a material through ONE KELVIN (or degree C)Absolute Heat Energy (J) Specific Heat Capacity(J/kg.K) Mass (kg) Temperature change (K or oC)
  15. 15. • Solids ( J/kg°C ) • Liquids ( J/kg°C ) • Gases ( J/kg°C )• Iron 450 • Water 4180 • Steam 1970• Copper 390 • Methanol 2550 • Oxygen 910• Aluminium 900 • Ethanol 2480 • Nitrogen 1040 • Antifreeze 2380 • Dry air ~1000• Gold 130 • Benzene 1720 • Hydrogen 14300• Glass 840 • • Human body 3470 Freon11 870• NaCl 880• Ice 2090• Wood 1680• Sand 820 These are just examples only• Diamond 500• Concrete 880 Revision 01 15
  16. 16.  Q = m x c x (t 2-t 1)◦ Where:◦ Q = Quantity of heat◦ m = mass in kg◦ c = specific heat capacity (tables)◦ t 2 – t 1 change in temperature Revision 01 16
  17. 17.  Heat moves from high to low temperature levels. The rate of heat transfer is partly dependant on the difference between the two temperature levels. 3 types of heat transfer  Conduction  Convection  Radiation Revision 01 17
  18. 18. Electrical HeatingHeat Transfer - CONDUCTION
  19. 19. Electrical HeatingHeat Transfer - CONVECTION
  20. 20. Electrical HeatingHeat Transfer - RADIATION
  21. 21.  Thermal conductivity is the material’s ability to transmit heat by conduction. Depends on four factors: ◦ Type of material ◦ Length of transfer path ◦ Cross-sectional area of path ◦ Temperature difference Revision 01 21
  22. 22.  The frame of a motor is designed to conduct the heat from the windings (centre of motor) to the surface and then dissipate the heat to the environment. The frame of a Hot Water Service is designed to ensure the heat is trapped in the centre of the Service. Revision 01 22
  23. 23.  Two basic types:◦ Open Loop Control◦ No actual control of the amount of heat◦ Closed Loop Control◦ Control over the amount of heat (temperature) Revision 01 23
  24. 24. Examples: On-Off control of a switch Set the car throttle in one position for a trip… Simmerstat on stoves to control the hotplates O/H fan speed control Fixed position of valve regardless of changes to flow requirements Garden sprinkler Electric toaster Microwave oven: Power setting. Time setting Electric Blanket Revision 01 24
  25. 25.  Three heat switching ◦ Example:  Most old Urns  Electric blankets (almost all)  Some stoves in caravans Revision 01 25
  26. 26. Electrical HeatingHeat Control – 3-Heat Switch
  27. 27. Electrical HeatingHeat Control – 3-Heat Switch
  28. 28. Electrical Heating Heat Control – Simmerstat The SIMMERSTAT is an OPEN CYCLE temperature control commonly used with stoves. Active ContactsCompensating Bimetal Pivot Operating Bimetal Heating Load Internal heater Neutral element
  29. 29. Heater element + bi-metal strip Main Contacts Magnet (to give snap action switch)Adjustment Aux. Switch Revision 01 29
  30. 30. Examples: Oven thermostat and element Toilet cistern water level control Car cruise control Almost all industrial processes HWS Electric Iron Electric frypan Revision 01 31
  31. 31. Electrical HeatingHeat Control – Open/Closed Cycle
  32. 32. Electrical HeatingHeat Control – Open/Closed Cycle
  33. 33. Electrical HeatingHeat Control – Open/Closed Cycle
  34. 34. Electrical Heating Heat Control – Thermostats A THERMOSTAT is a Closed-Cycle Control that:• SENSES the output temperature• COMPARES it with the pre-set value• VARIES or SWITCHES the input energy
  35. 35.  Four types are typically found in appliances. The first three of these are totally mechanically controlled: ◦ 1. Bimetal strip. When two metals with different coefficients of thermal expansion are sandwiched together, the strip will tend to bend as the temperature changes. In a thermostat, the bimetal strip operates a set of contacts which make or break a circuit depending on temperature. In some cases the strips shape or an additional mechanism adds hysteresis to the thermostats characteristics Revision 01 36
  36. 36. 2. Bimetal disk. This is similar to (1) but the bimetal element is in theshape of a concave disk (like the “clicker” play toy). These are notcommon in adjustable thermostats with brad spans, but are the usualelement in an over-temperature switch. Revision 01 38
  37. 37. Electrical Heating Heat Control – Thermostats Bimetal Disc ThermostatThis thermostat has contacts operated by a cuppedbimetal disc.At a pre-set temperature, the disc snaps the contactsopen.When the disc cools to a preset value, disc returns andthe contacts snap closed.
  38. 38. Electric Iron ThermostatBimetal Strip MIMS type element Revision 01 40
  39. 39. Thermal Cut-out (with manual reset)Thermostat Revision 01 42
  40. 40. Two Hot Water System Thermostats Revision 01 43
  41. 41. 3. Fluid operated bellows. These are not that common in small appliancesbut often found in refrigerators, air conditioners, stoves, and so forth. Anexpanding fluid (alcohol is common) operates a bellows which is coupledto a set of movable contacts. As with (1) and (2), hysteresis may beprovided by a spring mechanism. Revision 01 44
  42. 42. Electrical Heating Heat Control – Thermostats Capillary Tube Thermostat Bellows or Capillary Tube DiaphragmBellows Rod Bulb with volatilemoves to operate liquidcontacts
  43. 43. Electrical HeatingHeat Control – Thermostats Bi-Metal Thermostat Support Stem Invar Rod Brazed to Stem Brazed to Rod Helical Bi-Metal StripMounting Flange & ScrewThread
  44. 44. Bimetal Coil thermostat Mercury Switch Bimetal Coil Revision 01 50
  45. 45. Electrical Heating Heat Control – Thermostats Expanding Tube Thermostat Retaining Clips Brass Tube Tube Tube Brazed Expands/Contracts to RodRod Free End Tube Brazed Invar RodMoves to operate to Supportcontacts
  46. 46. Expanding tube thermostat Operating rod Rod is welded to the end of the tube The operating rod has a different expansion rate than the tube enclosing it.ElectricalContacts Revision 01 52
  47. 47. Bi-metal helix Expanding rod type Bulb type Revision 01 53
  48. 48. 4. Electronic thermostats. These typically use a temperature controlledresistance (thermistor) driving some kind of amplifier or logic circuit whichthen controls a thyristor or contactor. Revision 01 54
  49. 49.  Note that these terms can only apply to a closed loop system such as thermostats. If there is no feedback, the system cannot have: ◦ Hysteresis ◦ Differential ◦ Sensitivity ◦ Accuracy Revision 01 55
  50. 50.  Sensitivity Is a measure of the change of output to a change of input. A more sensitive thermostat will have a smaller differential. It is a measure of how closely a unit can maintain a given temperature. It is better applied to temperature measuring devices that give an analogue output. A more sensitive device gives a greater change of output to the change of input (temperature). Revision 01 56
  51. 51.  Thermocouples Resistance Temperature Detectors (RTD’s) Diodes and semiconductor IC’s Gas expansion system Mercury expansion system Coiled bimetal strip (see P&N) Radiation Pyrometers Revision 01 57
  52. 52.  Instantaneous Mains pressure - Storage Mains pressure - Heat exchanger Low pressure storage Solar Heat Pump HWSL/O 3.1 Revision 01 58
  53. 53.  Instantaneous or tankless water heaters are small cabinets that heat water on demand or instantly as it passes through the heater. They contain no significant water storage, possessing only up to a 6 litre operating holding. These water heaters only use energy when the hot water outlet is turned on and shut down immediately when the outlet is turned off.
  54. 54.  Mains Pressure HWS: direct heated ◦ Installed at ground level. ◦ Requires a pressure relief system. ◦ Requires an expansion control valve. ◦ New houses require a tempering valve for warm water to the bathroom. Revision 01 61
  55. 55. Mains Pressure HWS -Direct heated Insulation Hot water OutNote: The tank operatesat mains pressure. Cold water In Water Heater + thermostatL/O 3.1 Revision 01 62
  56. 56. If both have the same colour tags, then this wont be a problem1400kPa Revision 01 64
  57. 57. Bottom Cold Water Expansion Valve must be 200kPa lower than the top pressure relief valve.1200kPa Revision 01 65
  58. 58. Revision 01 66
  59. 59. • Hot water (73°C max.) to laundry and kitchen. Pressure Relief Hot Water• Warm water (50°C max.) to Valve Outlet bathroom. (73°C max.)• If major renovations are Tempering Warm Water carried out in the bathroom, Valve Outlet then a tempering valve must (50°C max.) be added.• The house owners can sign a Cold Water form saying they don’t want it Inlet (as only adults will be using Cold Water Cold Water Expansion it), and the plumber will not Valve Tap be responsible for any consequences. Revision 01 67
  60. 60. Heat exchange Storage HWSSmall Storage HWS designed for under sink operation Revision 01 68
  61. 61.  Must be mounted above taps. Low pressure hot water only. More to go wrong. ◦ If float valve sticks… Revision 01 69
  62. 62. Low Pressure HWS Toilet cistern type water level sensor Elementand electrical Cold connection Water In Hot Tank fills from Water Bottom Gravity Out Feed Revision 01 70
  63. 63.  Faults: ◦ Element goes open circuit.  Replace element. ◦ Thermostat either stays on, or stays off  Replace thermostat Revision 01 71
  64. 64. • Solar – Still requires booster element – 8-10 year pay back period – May require extra roof support. – Does the roof face the required direction? Revision 01 72
  65. 65.  In solar systems cold water travels through the roof-mounted solar collector where the water absorbs heat from the sun. Water heating using solar energy occurs during the day and the solar involvement varies significantly throughout the year depending on the climatic conditions. The apparatus of solar heaters includes the solar collector, insulated storage tank and, if required, pump and control valves.
  66. 66.  Flat-plate collectors are the most common collector for domestic water heating. A typical flat-plate collector is an insulated rectangular-type metal box with a transparent cover (similar to a greenhouse) and a black absorber plate.
  67. 67.  The evacuated-tube collectors consist of rows of parallel transparent double glass tubes, each containing an electromagnetic energy absorber and covered with a solar-sensitive coating. Sunlight enters the tube, strikes the absorber and heats the water flowing through the collector.
  68. 68.  Calorifiers are cylinders with an internal coil which allows the use of any type of boiler for hot water production. The calorifier can be either mains-pressure or low- pressure hot water storage systems. A significant amount of heat energy can be transferred to the calorifier, allowing a large production of hot water from a relatively small cylinder.
  69. 69.  Heat pump HWS ◦ More expensive than conventional HWS ◦ Smaller than Solar HWS ◦ Can operate with or without sunshine ◦ Operates as a split system Revision 01 82
  70. 70.  A heat pump water heater absorbs heat from the surrounding environment and pumps the acquired heat energy into a hot water storage tank. The heat pump serves as a heater by absorbing heat from the surrounding environment and pumping it into a closed-system heat-exchanger water storage tank.
  71. 71. The compressor compresses cool refrigeration gas, causing it to become hot, high-pressure refrigeration gasThis hot gas runs through a set of coils so it can dissipate its heat, and it condensesinto a liquid.The refrigeration liquid runs through an expansion valve, and in the process itevaporates to become cold, low- pressure refrigeration gasThis cold gas runs through a set of coils that allow the gas to absorb heat and cooldown the air inside the building A solar heat pump works on the same principle only in reverse i.e the coils carryingthe hot gas are used to heat the water. Revision 01 84
  72. 72.  If the water heater’s thermostat, which controls the resistive heating element, malfunctions the pressurised water in the tank could continue to heat and superheat (beyond 100 °C).This will cause two problems: First, since water expands when heated, the water pressure in the tank will increase as the water is superheated. If the pressure exceeds the vessels maximum pressure threshold the tank could rupture or even explode.
  73. 73.  Secondly, the release of superheated water (water heated above 100 °C up to its critical temperature of 374 °C without boiling) causes the water to burst into steam (1 litre of water can produce about 3 litres of steam), causing a sudden increase in volume and release of energy. Lowering the pressure of water lowers the boiling point. There is less pressure above the water to overcome. The superheated vapour plume expands until its pressure equals that of the surrounding atmosphere.
  74. 74.  Types: ◦ High Temperature radiators ◦ Low temperature panels and convection units ◦ Thermal storage systems ◦ Heat pumps (reverse cycle air conditioners) Revision 01 88
  75. 75. Types: ◦ Low temperature panels and convection units  Under-carpet / under concrete heaters (MIMS in concrete slab)  Can be operated using cheaper power at night  Blower heaters  Oil filled floor heaters Revision 01 89
  76. 76.  Stoves (ranges): ◦ Four types of cooktops:  Coiled element  Solid element  “Ceramic” cooktop  Induction cooktop Revision 01 90
  77. 77. Revision 01 91
  78. 78. Coiled Element Revision 01 92
  79. 79. Solid element Revision 01 93
  80. 80. Ceramic cooktop Revision 01 94
  81. 81. Revision 01 95
  82. 82. • Stoves: – Wiring: Half the elements with their controls Other half of the elements with their controls A A N Connection Box Revision 01 96
  83. 83. • Microwave ovens bombard food with electromagnetic radiation at 2.45GHz• Water absorbs the energy. The molecules vibrate and get hot.• The oven will dissipate the same energy in the cavity no-matter what. (eg. 800W)• Small quantities will cook faster. Large quantities cook slower.• Metal reflects the microwaves• If a microwave oven is left empty, the microwaves will reflect back into the magnetron and heat it up. This destroys the magnetron. Revision 01 97
  84. 84.  There are four (4) process heating methods available for converting the electric energy to heat energy. 1. Resistance 2. Infra-red 3. Induction 4. Dielectric Revision 01 98
  85. 85. Resistance process heatingAll the heat generated by an element is transferred by eitherconvection or conductionThe elements used may be either wire, strip or solid rods.Typical applications include; duct heaters, furnaces, refrigerators,space heaters, greenhouse heating and trace heating.In all cases their temperatures are controlled by thermostats Revision 01 99
  86. 86.  Infra Red heating: ◦ Spray painting booths for cars Induction Heating: ◦ For directly heating small steel parts. ◦ Similar to locking the rotor of a motor… it gets hot. ◦ Usually the work piece has currents induced in it directly. ◦ Frequencies between 50Hz and 5MHz used. 10 Revision 01 0
  87. 87.  Dielectric Heating: ◦ Used to heat non-conducting material. ◦ If an insulator is placed between two electrode plates, and AC is applied to the plates, the molecules are agitated and heat up. ◦ Used in plywood manufacture ◦ Used to dry breakfast cereal and dog biscuits Electric Arc ◦ Used in the steel industry up to 150 tonnes ◦ Used in glass furnaces. eg. Bradford pink batts. ◦ Arc welders fall in this category. 10 Revision 01 1
  88. 88. Demonstrate knowledge of the possiblecauses of malfunction in electric heatingequipment and skills the testing and faultfinding.  5.1 List the possible causes of faults in a malfunctioning electric heating device/circuit.  5.2 Conduct tests and locate a fault in a malfunctioning electric heating device/circuit. 10 Revision 01 2
  89. 89. Open circuits -physical breaks in the element -breaks in wiringShort circuits -resistance reduced to 0ΩPartial open circuits -loose connections etc 10 Revision 01 3
  90. 90. 10Revision 01 4
  91. 91. Element Testing To test an element for continuity the appliance should first be disconnected from power. After the appliance has been made safe to work on, the element needs to be isolated from the rest of the electrical circuit by removing at least one of the connecting wires. Once that is done, an ohm meter or continuity testers leads can be held against each terminal of the element. The exact resistance of an element is often not important as it will not usually change over its life span except to become totally open (show infinite resistance) when defective or becomes shorted to ground (see below). In case youre curious, a large cooktop surface burner is usually in the area of 27 ohms, a small 45 ohms. A griller elements resistance may be in the area of 20 to 40 ohms depending on its wattage. 10 Revision 01 5
  92. 92. 10Revision 01 6
  93. 93. Short to EarthAn element can also become partially shorted to ground.While this may not be enough to create a dead short andcause the element to fail outright, it can create a shockhazard. To test an element for a short to ground, anohmmeter should be set on its highest ohm scale (1K or10K) and tested from one of the elements terminals to theelements metal sheath. It may be necessary to rub theouter element surface with the meter probe to make agood contact. If anything other than infinite resistance isshown, replace the element. 10 Revision 01 7
  94. 94. 10Revision 01 8
  95. 95. Heat damaged 10 Revision 01 9