electrical heating
TRANSCRIPT
G008B Part BElectrical heating
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
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G008B Part B Electrical heating
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.
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Heat and temperature
Electrical HeatingTransfer of Heat
Heat is transferred from a hotter region to a colder region
Electrical HeatingHeat is Energy
Energy (W)in Joules (J) Power in Watts (W)
Time in seconds (s)
Electrical HeatingTemperature Scales
The common temperature
scale is CELSIUSWater boils at 100oC
Ice melts at 0oC
Some countries use the FAHRENHEIT scale
Water boils at 212oF
Ice melts at 32oF
Temperature Scales
The temperaturescale used in science and
engineering is the absolute KELVIN scale (K)
Water boils at 373K
Ice melts at 273K
Zero Kelvin is “Absolute Zero”
NO heat content;NO molecular motion.
Zero Kelvin (0K) is “Absolute Zero”
and is equivalent to
-273oC
One Kelvin “degree”is equal to
One Celsius “degree”
The “degree” symbol o is NOT used with the Kelvin scale
Electrical Heating
Temperature Scales
To convert Fahrenheit to Celsius:
Electrical Heating
Temperature Scales
To convert Celsius to Fahrenheit:
Electrical Heating
Electrical HeatingTemperature Scales
To convert Kelvin to Celsius:
Temperature ScalesTo convert Celsius to Kelvin:
Electrical Heating
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
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Temperature scales
The ability of a substance to store heat.
If equal masses absorb equal amounts of
thermal energy (heat), different substances
show a different temperature increase.
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Heat capacity
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)
Mass (kg)
Temperature change (K or oC)
Specific Heat Capacity(J/kg.K)
• Solids ( J/kg°C ) • Iron 450 • Copper 390 • Aluminium 900 • Gold 130 • Glass 840 • NaCl 880 • Ice 2090• Wood 1680• Sand 820• Diamond 500• Concrete 880
• Liquids ( J/kg°C )• Water 4180 • Methanol 2550 • Ethanol 2480 • Antifreeze 2380 • Benzene 1720 • Human body 3470
• Gases ( J/kg°C )• Steam 1970• Oxygen 910• Nitrogen 1040• Dry air ~1000• Hydrogen 14300• Freon11 870
These are just examples only
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Specific heat capacity
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
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Calculations
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
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Heat transfer
Electrical Heating
Heat Transfer - CONDUCTION
Electrical HeatingHeat Transfer - CONVECTION
Electrical Heating
Heat Transfer - RADIATION
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
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Thermal conductivity
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.
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Thermal conductivity
Two basic types:◦ Open Loop Control◦ No actual control of the amount of
heat
◦ Closed Loop Control◦ Control over the amount of heat
(temperature)
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Heat control
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
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Open loop
Three heat switching
◦ Example:
Most old Urns Electric blankets (almost all) Some stoves in caravans
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Three heat switching
Electrical Heating
Heat Control – 3-Heat Switch
Electrical Heating
Heat Control – 3-Heat Switch
Electrical Heating
Heat Control – SimmerstatThe SIMMERSTAT is an OPEN CYCLE temperature control commonly used with stoves.
Heating Load
Active
Neutral
Contacts
Internal heater element
Operating Bimetal
Compensating Bimetal
Pivot
Main ContactsMain Contacts
Heater element + bi-metal stripHeater element + bi-metal strip
AdjustmentAdjustment
Magnet (to give snapaction switch)
Magnet (to give snapaction switch)
Aux. SwitchAux. Switch
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Simmerstat
Control of heating
Examples: Oven thermostat and element Toilet cistern water level control Car cruise control Almost all industrial processes HWS Electric Iron Electric frypan
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Closed loop control
Electrical Heating
Heat Control – Open/Closed Cycle
Electrical Heating
Heat Control – Open/Closed Cycle
Electrical Heating
Heat Control – Open/Closed Cycle
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
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 strip's shape or an additional mechanism adds 'hysteresis' to the thermostat's characteristics
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Thermostat
Control of heating
2. Bimetal disk. This is similar to (1) but the bimetal element is in the shape of a concave disk (like the “clicker” play toy). These are not common in adjustable thermostats with brad spans, but are the usual element in an over-temperature switch.
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Thermostat
Electrical HeatingHeat Control – Thermostats
Bimetal Disc Thermostat
This thermostat has contacts operated by a cupped bimetal disc.
At a pre-set temperature, the disc snaps the contacts open.
When the disc cools to a preset value, disc returns and the contacts snap closed.
Bimetal StripBimetal Strip
Electric Iron Thermostat
MIMS type elementMIMS type element
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Control of heating
Thermostat
Thermal Cut-out(with manual reset)
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Two Hot Water System Thermostats
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3. Fluid operated bellows. These are not that common in small appliances but often found in refrigerators, air conditioners, stoves, and so forth. An expanding fluid (alcohol is common) operates a bellows which is coupled to a set of movable contacts. As with (1) and (2), hysteresis may be provided by a spring mechanism.
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Thermostat
Electrical HeatingHeat Control – Thermostats
Capillary Tube Thermostat
Bellows Rod moves to operate contacts
Bulb with volatile liquid
Capillary TubeBellows or Diaphragm
Control of heating
Control of heating
Control of heating
Electrical Heating
Heat Control – ThermostatsBi-Metal Thermostat
Support Stem
Mounting Flange & Screw Thread
Helical Bi-Metal Strip
Brazed to Rod
Invar RodBrazed to Stem
Mercury Switch
Bimetal Coil
Bimetal Coil thermostatBimetal Coil thermostat
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Thermostat
Electrical Heating
Heat Control – ThermostatsExpanding Tube Thermostat
Brass Tube
Invar Rod
Tube Brazedto Rod
Tube Brazedto Support
Retaining Clips
Tube Expands/Contracts
Rod Free End Moves to operate contacts
ElectricalContacts
Operating rodOperating rod
The operating rod has a differentexpansion rate than the tube
enclosing it.
Rod is weldedto the end of
the tube
Rod is weldedto the end of
the tube
Expanding tube thermostatExpanding tube thermostat
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Thermostat
Bi-metal helix
Bulb typeExpanding rod type
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4. Electronic thermostats. These typically use a temperature controlled resistance (thermistor) driving some kind of amplifier or logic circuit which then controls a thyristor or contactor.
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Thermostat
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
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Terms
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).
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Terms
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
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Other temperature measuring devices
Instantaneous Mains pressure - Storage Mains pressure - Heat exchanger Low pressure storage Solar Heat Pump HWS
L/O 3.1
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Water heaters
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.
Instantaneous heaters
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.
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Storage HWS
Water Heater + thermostat
Cold water In
Hot water Out
InsulationMains Pressure HWS
-Direct heated
Note: The tank operates at mains pressure.
L/O 3.1
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Storage HWS
1400kPa
If both have thesame colour tags,then this wont be
a problem
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Pressure relief valve (at top)
Bottom Cold WaterExpansion Valve
must be200kPa lower thanthe top pressure
relief valve.
1200kPa
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Cold water expansion valve (at bottom)
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Tempering valve
• Hot water (73°C max.) to laundry and kitchen.
• Warm water (50°C max.) to bathroom.
• If major renovations are carried out in the bathroom, then a tempering valve must be added.
• The house owners can sign a form saying they don’t want it (as only adults will be using it), and the plumber will not be responsible for any consequences.
Pressure ReliefValve
Hot WaterOutlet
(73°C max.)
Warm WaterOutlet
(50°C max.)
Tempering Valve
Cold WaterExpansion
Valve
Cold WaterTap
Cold WaterInlet
Pressure ReliefValve
Hot WaterOutlet
(73°C max.)
Warm WaterOutlet
(50°C max.)
Tempering Valve
Cold WaterExpansion
Valve
Cold WaterTap
Cold WaterInlet
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Tempering valve
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Heat exchange Storage HWS
Small Storage HWS designedfor under sink operation
Must be mounted above taps.
Low pressure hot water only.
More to go wrong.◦ If float valve sticks…
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Storage HWS
HotWaterOut
HotWaterOut
Toilet cistern typewater level sensor
Low Pressure HWS
GravityFeed
ColdWater
In
ColdWater
In
Tank fills fromBottom
Tank fills fromBottom
Elementand electricalconnection
Elementand electricalconnection
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Storage HWS
Faults:◦ Element goes open circuit.
Replace element.◦ Thermostat either stays on, or stays off
Replace thermostat
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Storage HWS
• Solar
– Still requires booster element
– 8-10 year pay back period
– May require extra roof support.
– Does the roof face the required direction?
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Other water heaters
Solar water heaters
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.
Flat-plate collectors
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.
Evacuated-tube collectors
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.
Heating process
Calorifiers
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.
Heat pump HWS
◦ More expensive than conventional HWS◦ Smaller than Solar HWS◦ Can operate with or without sunshine◦ Operates as a split system
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Other water heaters
Heat pump
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.
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The compressor compresses cool refrigeration gas, causing it to become hot, high-pressure refrigeration gas
This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid.
The refrigeration liquid runs through an expansion valve, and in the process it evaporates to become cold, low- pressure refrigeration gas
This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building
A solar heat pump works on the same principle only in reverse i.e the coils carrying the hot gas are used to heat the water.
Safety precautions 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.
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.
Types:
◦ High Temperature radiators◦ Low temperature panels and convection units◦ Thermal storage systems◦ Heat pumps (reverse cycle air conditioners)
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Space heating
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
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Space heating
Stoves (ranges):
◦ Four types of cooktops: Coiled element Solid element “Ceramic” cooktop Induction cooktop
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Cooking
Revision 01 91
Stoves
Coiled Element
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Cooking
Solid element
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Cooking
Ceramic cooktop
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Cooking
Revision 01 95
Induction cook top
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Cooking
• Stoves:– Wiring:
NA A
Half the elementswith their controls
Other half of the elementswith their controls
Connection Box
• 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.
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Microwave ovens
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
Process Heating
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Resistance process heating
All the heat generated by an element is transferred by either convection or conduction The 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
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.
Revision 01 100
Process heating
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.
Revision 01 101
Process heating
Demonstrate knowledge of the possible causes of malfunction in electric heating equipment and skills the testing and fault finding.
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.
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Learning outcome 5
Open circuits -physical breaks in the element -breaks in wiring
Short circuits -resistance reduced to 0Ω
Partial open circuits -loose connections etc
Revision 01 103
Possible faults in heating elements
The insulation between the resistance wire and the outer coating can also deteriorate. When this occurs the inner resistance wire can come into contact with the outer sheath causing a short to ground. The force of such a short can blow a hole right through the outer coating, melting it and sometimes damaging cookware being used at the time
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Revision 01 105
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 tester's 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 you're curious, a large cooktop surface burner is usually in the area of 27 ohms, a small 45 ohms. A griller element's resistance may
be in the area of 20 to 40 ohms depending on its wattage.
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An element can also become partially shorted to ground. While this may not be enough to create a dead short and cause the element to fail outright, it can create a shock hazard. To test an element for a short to ground, an ohmmeter should be set on its highest ohm scale (1K or 10K) and tested from one of the element's terminals to the element's metal sheath. It may be necessary to rub the outer element surface with the meter probe to make a good contact. If anything other than infinite resistance is shown, replace the element.
Short to Earth
Revision 01 108
Heat damaged
Revision 01 109
Thermostat