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Page 1: Hampson, J 2013, Electrotechnology Practice: Section 6

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to accompany

Section 6

Electrical Heating

Page 2: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

Heat is the energy transferred by virtue of a difference in temperature between a hot body (physicists use the term ‘systems’) and a cooler body. Heat can only be identified at the moment it crosses from one body to another. It is a momentary occurrence.

When a hot body is in thermal communication with a cooler body, heat energy is transferred until both bodies become stable or are in equilibrium at the same temperature.

Page 3: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

Heat as energy transfer

A refrigerator is an excellent example of heat transference. A refrigerator uses a liquid refrigerant that evaporates readily into a gas.

Rapid evaporation of the refrigerant in the evaporator (freezer) causes effective cooling thereby enabling the gas to remove large amounts of heat.

The heat energy from the food is transferred to the cold circulating gas.

Page 4: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

A compressor is used to suck the gas from the evaporator and recycle the refrigerant through the system. The gas is compressed and discharged into a condenser (black tubes on the back of the refrigerator).

The condenser dissipates the heat energy gained by the gas in the evaporator to the surrounding air and allows the gas to return to liquid form. The liquid is then returned via a capillary tube back to the evaporator thereby completing the cycle.

Page 5: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

Page 6: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

In SI units the unit of heat is the same unit that is used for other mechanical and electrical forms of energy, the joule (J).

The first law of thermodynamics is often called the Law of Conservation of Energy.

This law states that energy can be changed from one form to another but it cannot be created or destroyed.

The total amount of energy and matter in the universe always remains constant.

Page 7: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

The second law of thermodynamics states that heat can never pass on impulse from a colder to a hotter body.

As a result of this fact, natural processes that involve heat energy transfer must have one irreversible direction. This law also predicts that the heat energy spontaneously disperses if not hindered.

The term given to the measurement of the dispersal (before and after effect) is called entropy.

Page 8: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

The third law of thermodynamics states that if the thermal motion (kinetic energy) of molecules could be stilled a state called absolute zero would occur.

Absolute zero is a thermodynamic temperature of 0 Kelvin or –273.15 °Celsius.

Page 9: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

Page 10: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating and heat energy

Modes of heat transfer

There are three modes of heat transfer:

conduction

convection

radiation.

Page 11: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Modes of heat transfer

Conduction

Conduction is the transfer of thermal energy between two solid materials in direct contact.

The rate of the transference depends upon the temperature difference of the two solid materials, the thermal conductivity of each material and the distance through which the heat is dissipated.

Page 12: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Modes of heat transfer

Page 13: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Modes of heat transfer

Convection

This is based on the principle that a hot fluid (liquids and gases) is less dense than a cold fluid.

When a cold fluid is heated the cool molecules gain kinetic energy and move away from each

other.

Page 14: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Modes of heat transfer

Page 15: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Modes of heat transfer

Radiation, unlike conduction and convection, does not need a transporting medium such as a molecule to exist. It is an electromagnetic wave like light but with a longer wavelength.

It is transmitted at the speed of light through any transparent medium including a vacuum.

It can be emitted from the source surface, reflected off a smooth surface or absorbed through the surface.

Page 16: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Modes of heat transfer

Page 17: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Temperature

Temperature is a property of a substance and is not a holistic measurement of the substance’s internal energy.

It defines the direction of heat flow, which is always away from the body that is at the higher temperature and to the body that is at the lower temperature.

Temperature is the hotness or coldness of a substance and is the measure of the average kinetic energy of the molecules of a substance.

Page 18: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Temperature

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Temperature

Temperature scale

The fundamental unit of temperature measure in the Système International d’Unités (SI) has been defined as the Kelvin (K). Zero K (0 K) is chosen as absolute zero (temperature at which no thermal activity occurs), while the freezing point of water is 273.16K and the boiling point is 373.15 K.

Page 20: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Temperature

Page 21: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Temperature

The triple point of water is the most important defining thermometric fixed point used in the calibration of thermometers that accurately obey known laws.

The use of this defined point means that temperature standards around the world can be accurately equivalent.

Page 22: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Temperature

Page 23: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Specific heat capacity

When the heat energy absorbed by a body increases, the temperature of the body increases.

The quantity of increase depends on the mass of the body.

The specific heat of a body is the amount of heat required per kilogram of the body to raise the temperature by one degree.

Page 24: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Thermal conductivity

Thermal conductivity is the flow of internal energy from a region of higher temperature to one of lower temperature by the interaction of the adjacent particles, atoms, molecules, ions or electrons in the intervening space.

Page 25: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Thermal conductivity

There are four factors that affect the rate at which thermal conductivity occurs.

Temperature

Length

Cross-sectional area (CSA)

Type of material

Page 26: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

In order to manage the thermal communication of a heating appliance some form of control device is required.

This control can be exercised via three forms of control—manual, automatic or programmable.

Page 27: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Manual control

There are several types of manual control devices and their choice depends upon the switching statement the control device executes.

The simplest form of a control device is a single-pole single-throw (SPST) switch.

Page 28: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Manual control

Another manual switch that was developed from the simple manual switch specifically to control two heating elements is the three-heat switch.

These switches have a four-position switching statement—off, low, medium and high.

Page 30: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Automatic control

There are two types of automatic temperature control—thermostatic and an energy regulator device called a simmerstat.

Page 33: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Thermostats

These devices are used whenever a heating system must be maintained within moderately accurate limits. They are designed to hold a thermal load continually at a set-point temperature with very little variation around the set point.

There are three main configurations of thermostats—bi-metal, strut and tube, and the bulb and capillary.

Page 34: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Page 38: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

The bi-metal, the strut-and-tube and the bulb-and-capillary thermostats are known as closed-loop control sensors.

A closed-loop control senses the temperature of the load (called feedback) and opens or closes accordingly.

Page 39: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Simmerstat control (infinite switch)

The simmerstat is a bi-metal switch in which the bi-metal sensing element is activated by the current flow through its own resistance wire element.

This element is wrapped around the sensing bi-metal element and is connected in parallel with the load that the simmerstat is controlling.

Page 40: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Page 41: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Programmable thermostat control

Programmable heat controllers are used when fine accuracy in temperature control is required.

These devices are based on microprocessors and thermistor sensors.

Most of these programmable heat controllers can store and initiate multiple daily temperature settings and they can adjust the turn on and off times of heating and cooling systems as the outside ambient temperature varies.

Page 42: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Programmable thermostat control

There are five essential operating elements that relate with the programmable thermostat—

sensor

input

comparator

output

load.

Page 43: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Page 44: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Page 45: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Testing a thermostat

In order to maintain consistent quality of heat or cold production it is necessary to perform calibrations on process sensors such as thermostats.

A calibration is a matter of qualifying the sensor under test.

By knowing the limitations of the sensor it is possible to maximise the effectiveness of the heating or cooling process loop.

Page 46: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Control of heating

Page 47: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

A water heater is a closed vessel used to supply hot water.

The heat generated by resistance elements heats the water.

The water heater includes all the controls and sensors necessary to prevent water temperatures from exceeding 60–70 °C.

Water heating processes fall into two categories—instantaneous and storage.

Page 48: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

Instantaneous 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.

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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Heating process

Storage water heaters

Storage tank water heaters or heat exchange units are very common.

They store and heat a large volume of water in an insulated (injected polyurethane) copper tank at a thermostatically controlled temperature.

The fully enclosed units are mounted at ground or floor level usually outside of the building.

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

Mains-pressure water heaters

In the case of a mains-pressure unit the cold water mains pipe is connected directly to the copper storage tank via the cold water inlet.

On entering the unit the cold water is heated by an electric resistance element and then rises to the top of the tank.

Turning on a hot water tap allows the heated water to be delivered at the same pressure as the cold water.

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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Heating process

Mains-pressure water heaters

Mains-pressure storage heaters are normally attached to an off-peak electricity meter that usually operates between 10:00 pm to 7:00 am.

When a hot-water tap is opened, cold water enters the tank through the NRV and the PLV and the drop in temperature triggers the thermostat and element at the bottom of the tank.

When the tap is turned off, the heating element continues to carry current until the thermostats temperature setting is realised.

Page 54: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

Low-pressure water heaters

Low-pressure or gravity-fed vented (permanently open to the atmosphere) water heaters store water at a pressure lower than mains pressure.

The pressure is determined by the height (pressure = 10 kPa/m height) at which the unit is mounted with respect to the hot water outlets.

Page 55: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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Heating process

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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.

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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.

Page 59: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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.

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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Heating process

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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.

Page 65: Hampson, J 2013, Electrotechnology Practice: Section 6

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Heating process

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Heating process

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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Heating process

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Heating process

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).

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Heating process

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.

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Heating process

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.

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Heating process

Space heating

Commercial and industrial premises and even some residential premises can be cold, draughty and difficult to heat.

Poor insulation, high ventilation, excessive outside air filtration and poor draught control lead to chilly indoor environments and reduce work production.

Space heating offers direct, controllable and economic heat when and where it is required.

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Heating process

Electrical heaters

Panel, column and architrave oil-filled convection-type space heaters provide a wide range of temperature variation with broad surface areas.

An oiled-filled column space heater that uses the convective movement of warm air for quick heating

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Heating process

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Heating process

Floor heating

Heating cables have been developed to meet the specific need for large-area space heating.

These cables have a high temperature capability and low electrical resistance values needed for long circuit lengths.

The cables are located under concrete, ceramic tiling or under soil.

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Heating process

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Heating process

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Heating process

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Heating process

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Heating process

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Heating process

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Heating process

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Heating process

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Heating process

Process heating

Heat plays an indispensable role in a wide variety of manufacturing processes:

cooking, softening

melting, drying

curing and fusing.

Electric process heating can be delivered in exact amounts at precise temperatures and has various methods of delivery.

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Heating process

Resistance heating

Resistance heating has been the most economical means of supplying process heat in many applications.

Generally, resistance heating is an indirect method of heating which converts nearly 100% of the energy in the electricity to heat.

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Heating process

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Heating process

Resistance heating is the most simple and therefore the most widely used method of heat generation.

The most important advantages are wide temperature range, easy method of control, temperature uniformity, flexibility in positioning and design of electric heating elements and low cost.

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Induction heating

Induction heating occurs when a substance is heated from the inside out after being placed within an electromagnetic field.

Industries used induction heating for heat treatment, forging, annealing, soldering, pipe welding, thread rolling, removing electrical varnishes and surface coatings from electrical conductors and metal joining.

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Infrared heating

Infrared heating is produced by electromagnetic radiation that is generated in a heat source (425–2200 °C) by vibration and alternation of molecules.

In industry infrared heating is used to warm food and bake bread and chickens, cure plastics and soften adhesives, dry water- and solvent-based paints and for space heating.

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Dielectric heating

Dielectric heating uses high-frequency electromagnetic waves such as microwaves and radio waves to stimulate molecules in non-conductive substances.

The stimulation causes rapid movement of the substance’s molecules that in turn causes heat to be produced.

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Cooking appliances

Many of today’s appliance manufacturers are striving to make the cooking process faster and more efficient by making available a wide variety of cooking appliances to the consumer.

Many appliances are particular in their application while others can be used for a wide variety of cooking functions.

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Tariffs

With hot water tariffs the purchaser of the energy can select any tariff in the residential, commercial and industrial category range that is appropriate to their requirements and user category.

There are in general three types of tariffs:

on-demand

economy controlled

off-peak controlled.

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Causes of faults in heating equipment

Some faults that develop in heating devices result from loose termination screws which can generate heat and high temperatures at a small location.

The loose termination becomes a small heating unit itself. Insulation degradation with shorting between conductors or to earth is a possible result.

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Causes of faults in heating equipment

Ineffective and loose connections can be diagnosed by measuring the voltage across the connection point.

Mechanical scraped and nicked conductors can reduce the current-carrying effectiveness of a conductor, resulting in localised heating and eventual open-circuiting of the conductor.

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Heating processWiring requirements of heating equipment

AS/NZS 3000:2007 Wiring rules

Areas of importance include electric duct heaters, heating cable systems, appliances producing hot water or steam, cooking appliances, under-carpet wiring systems and main switches.

Tables C1 and C2 of the Wiring rules should also be consulted with respect to maximum demand requirements.

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To determine if an element is defective its resistance value needs to be measured.

The resistance can be determined from power and voltage ratings.

If the resistance measurement on the suspected element is proven to be sound then a check of the voltage is required.

A drop of 10% in voltage will reduce wattage by 20%.

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