More on methods of reducing heat transfer - thermal conductivity

e.g. in a house and other situations to minimise heat losses

 Whatever 'heat system' we are dealing with, heat energy is always lost.  You can never get from an energy store, a 100% efficient conversion to useful energy.  Therefore, it is of great importance to minimise heat losses and save money in the process! A good example is how to save money in the home or any other building where heating systems of some form are used. This page also describes a simple experiment to investigate the insulation effectiveness of various materials

You may need to be able to use your knowledge and understanding to ...

• Compare ways in which energy is transferred in and out of objects by heating and ways in which the rates of these transfers can be varied.

• Evaluate the design of everyday appliances that transfer energy by heating, including economic considerations eg reducing unwanted heat energy transfers - heat losses cost money!

  • Examples you should be familiar with include radiators and heat sinks.

• Evaluate the effectiveness of different types of material used for insulation, including thermal conductivity (eg U-values - a measure of the rate of heat transfer) and economic factors including payback time.

  • You should have studied examples like loft insulation and cavity wall insulation.

• Evaluate different materials according to their specific heat capacities.

  • The heat specific heat capacity in simple terms is how much energy (J) is needed to heat a specific mass (1 kg) by one degree ^oC.

  • Examples may have studied include the use of water, which has a very high specific heat capacity, oil-filled radiators and electric storage heaters containing concrete or bricks.

• Reminder of particle theory: There is always a net transfer of thermal energy from hot materials to colder ones by ...

  • ... conduction of thermal energy through the bulk of a substance, where higher kinetic energy particles either bump into (liquids or gases) or vibrate against, lower kinetic energy particles, so that thermal energy is transferred.

  • ... convection involves the bulk movement of particles, the hotter higher KE particles in gases or liquids space out more lowering the density of them and so will rise with respect to the surrounding cooler fluid. These convection currents are effectively a 'buoyancy' current because the less dense warmer fluid is trying to float on the cooler more dense fluid.

  • ... infrared - thermal radiation - surface particles of a material at a higher temperature will emit more infrared radiation than a colder material surface. All material surfaces are constantly absorbing and emitting infrared, but there will be net transfer of thermal radiation from a hotter thermal energy store to a cooler one.

Thermal conductivity- Applications of heat transfer science

All materials conduct heat to a greater or lesser degree.

The thermal conductivity of a material is a measure of how efficiently heat is transferred through a material by conduction.

Materials like metals are very good heat conductors and transfer thermal energy very quickly.

Materials like stone, brick, wood and concrete etc. are poor heat conductors and have low thermal conductivities - therefore can be used as good thermal insulation materials.

Thermal conductivity data is particularly important when considering the material required to fulfil a particular application e.g. in heating systems when in one situation you might want good insulation and in another rapid heat transfer.

Clothes and blankets trap pockets of air.

Air is a poor conductor of thermal energy, so the trapped air provides an effective layer of insulation.

Materials like wool fibres, which is also a poor thermal energy conductor, trap the air giving a very effective layer of thermal insulation.

Also, because of the porous nature of the material, little of your body heat is conveyed away by convection.

 

Other material situations

Getting your 'fish and chips' served in layers of paper keeps the food hot because paper is a poor conductor of heat.

Also, if you want to keep your freshly bought ice cream as cool as possible on returning from the supermarket, you can wrap it is old newspaper - which can still be recycled.

Heating and insulating buildings

• U-values measure how effective a material is as an insulator.

  • What is the U-value of a material? What does the U-value mean?

  • The U value of a material is related to its thermal conductivity.

  • The U-value of a material gives a numerical value of how efficient heat is transferred through a material.

  • Materials with high U-values are relatively good conductors of heat - higher thermal conductivity - poorer insulators

  • Materials with low U-values are relatively good insulators - poorer heat conductors - lower thermal conductivity.

  • You probably won't be asked about what a U value is, BUT, you will be asked about relative thermal conductivities and how this affects the materials used in a particular context, which is usually about thermal insulation.

• Conserving energy in the home

  • Typical percentage thermal energy (heat energy) losses from a house:

    • doors 15% - draught excluders, door curtain, double glazed glass panes.

    • floors 15% - thick carpet, even a layer of insulating material included with the concrete base too,

    • roof 25% - loft insulation layers of ceramic wool

    • walls 35% - cavity wall insulation

    • windows 10% - double glazing (two sheets of glass trapping air), curtains drawn in the evening.

      • If all of these methods of insulating your home are applied, your energy bills e.g. oil, gas or electricity, are considerably reduced.

      • More details of insulation methods are discussed below.

  • Methods of reducing the rate of heat energy transfer reduce costs and also in there own small way contribute to reducing global warming by using less fossil fuel based energy supplies.

  • What is effective? What is cost effective? Not always the same! Payback time?

  • Loft insulation - cheap and effective - a thick layer of fibreglass wool or similar material laid all over the loft floor and reduces conduction and convection of heat lost through the roof - payback time a few years.

    • The fibre glass or other 'woolly' material traps air which is a very poor heat conductor i.e. a very good insulator.

  • Thick walls made from a good insulating material with a low thermal conductivity.

    • The thicker the wall, the lower the rate of heat transfer, a slower rate of cooling, better heat retention due to a lower thermal conduction rate.

    • Good insulating materials (poor heat conductors), irrespective of thickness, are brick, stone and breeze blocks etc.

  • Cavity wall insulation is another technique for heat retention - insulating foam injected between two brick walls (inner wall and outer wall) - reduces heat loss by conduction and convection across the walls - quite costly, payback time a few years.

    • Air is a poor conductor so the trapped air provides an effective layer of insulation and because of the porous nature of the foam, heat cannot be conveyed away by convection.

    • Foam blocks for cavity wall insulation, or any other thermal insulation situation, can be produced with a thin layer of shiny foil (e.g. aluminium) on the outer surface of the block.

      • This reduces heat loss by thermal radiation because the infrared waves are reflected back towards the house interior rather than being absorbed or emitted.

    • Sometimes the gap is just left with just air in, a sort of brick/breeze block equivalent of double glazing windows, but still considerably reduces heat loss mainly by conduction because air has a very low thermal conductivity.

  • Hot water tank jacket - cheap and effective - a jacket of lagging of a foam filled plastic cover reduces conduction and radiation heat losses - quick payback time.

  • Double glazing of glass windows - expensive, longer payback time - insulating air is trapped between the glass panes a few mm apart, so reducing heat losses mainly by conduction.

  • Draught-proofing - cheap and effective draught excluders, a few years payback time - strips of foam or plastic around door frames, thick curtains across the windows - all of these measures reduce heat loss from the house mainly by convection i.e. warm air in rooms brushing against cold surfaces like windows or warm air moving to colder unused parts of the house.

  • Pipes - hot water pipes can be covered in insulation to minimise heat losses by conduction and convection - often foam which traps an insulating layer of air.

    • You can paint pipes white to minimise loss by infrared radiation.

    • Central heating hot water pipes

      • Making pipes as short as possible, means the water spends less time in them, reducing heat losses before the water reaches the radiators.

      • Making pipes as wide as possible means a smaller proportion of hot water is contact with the surface of the pipe from which heat is conducted away to the surrounding air.

• The most effective methods of insulation give you the biggest annual savings of heat energy, but the most cost-effective methods tend to be the cheapest. For double glazing and cavity wall insulation you need to think long-term to get your money back.

A simple experiment to investigate the insulating effectiveness of various materials.

• This is an investigation into thermal insulation - comparing, indirectly, the thermal conductivity of different materials..

• How can we get a simple comparative measure of how effective a material is in reducing the rate of heat transfer?

• Below is illustrated a simple experiment to get a comparative thermal conductivity value for a solid material.

• also to use heat02.gif

• The basic idea is to use a container e.g. a copper calorimeter filled with a fixed amount of water and covered with a lid to seal the system and minimise heat losses from the surface of the water.

  • Its quite a good idea to use a copper container because it is a good conductor of heat - copper has a relatively high thermal conductivity - so its a good test of an insulating material to keep the temperature of the water as high as possible for as long as possible.

  • The left side of the diagram represents the control i.e. no insulation around the copper calorimeter.

  • The right side illustrates the insulated copper calorimeter.

  • The lid must have a hole in it to allow the thermometer to be suspended in the bulk of the water.

  • The lid is essential to avoid heat losses to the outside air, it can be just a thick piece of card.

  • The copper vessel, water, thermometer and insulation constitutes the system and we are investigating heat loss from this system.

  • The copper vessel is quite good to use because its made of a good thermal conductor just like an immersion cylinder in a house - which of course needs insulating to keep the water hot over long periods of time.

  • Therefore any decent insulating material should be quite effective in reducing the rate of heat loss i.e. rate of temperature fall.

  • You can experiment with materials like carpet, bubble wrap, cotton wool, polystyrene etc. which are poor heating conducting non-metallic materials that also trap air to a greater or lesser degree.

  • When comparing the effectiveness of insulating materials, for a fair test you should use the same thickness of material.

• The hot water can be quickly produced in an electric kettle and measure out a fixed amount eg 100 cm³ into the copper vessel.

  • You may need to use a plastic measuring cylinder for very hot water at 80-90^oC to avoid risk of cracking the glass, or use water at say around 50^oC with a Pyrex measuring cylinder, either way take great care!

• It is important not only to keep the water volume constant (constant mass), but also the same starting temperature.

• You can gather the temperature-time data in several ways.

• (i) A single temperature-time reading (which should be of course repeated): You take the initial temperature and then allow the system to cool for a fixed time for each material of e.g. 20 minutes and remeasure the temperature to get the temperature fall in a given time,

  •  or for a fixed given temperature fall, measure the time taken to do so.

• (ii) Multiple readings for a given insulator: You can take intermediate readings of temperature versus time and draw graphs to measure the initial temperature gradient as a measure of the rate of cooling.

  • (i) If using a single temperature-time measurement you must run the experiment for the same length of time or measure the time for the same temperature fall.

  • The smaller the temperature fall the better the insulating properties of the material - the lower its thermal conductivity.

  • Using a single time and temperature measurement, a data table might look like this for a specified fall in temperature.

  • Thermal insulating material (must have a control too)	Time for the temperature to fall e.g. by 20oC (keep constant)	Relative rate of cooling = 1/time
    No insulation - control	5 mins	0.20
    Cotton wool	15 mins	0.07
    Polystyrene foam	17 mins	0.06
    Paper	12 mins	0.08

    • The smaller the 1/time value, the more effective the insulation.

    • The lower the rate of cooling, the more effective the insulator.

    • The above values are fictitious, but make the point about using insulation!

  • Alternatively, for the middle column, you could measure the temperature fall e.g. after 10 minutes and keep this time as a constant.

  • In this case, for the 3rd column, the relative rate of cooling would then be given directly by the value of the temperature fall (initial - final temperature reading).

  • The greater the heat conductivity of the insulating material, the less effective the insulating capacity of the material.

• (ii) You can be more elaborate with the experimental measurement by taking a series of readings every minute or longer time period and then plotting temperature versus time to produce cooling curve graphs.

  • Thermal insulating material	Temperature readings every 10 minutes
    Time (minutes)	0	10	20	30	40	50	60
    No insulation - control	80	60	45	33	30	28	25
    Cotton wool	80	70	62	56	51	47	44
    Polystyrene foam	etc.!
    Paper

  • Again, these are fictitious results!

  • A graph of the results - a series of cooling curves.

  • The graph lines level out as you get nearer and nearer the ambient laboratory temperature.

    • The rate of cooling always slows down with time because the temperature difference between the calorimeter contents and the outside air is decreasing - the thermal temperature gradient is steadily decreasing.

  • The initial steepness of the graph line (a negative gradient) gives you a measure of the thermal energy flow from the system.

  • Typical results from taking multiple temperature measurements with time are shown in the diagram above.

    • I haven't shown the tangent lines to show how to measure the gradient - I'm assuming you know how to do this.

    • With no insulation the temperature falls quite rapidly - steep gradient.

    • With any insulation, however poor, the graph falls less steeply - smaller gradient.

    • With a good insulating material, the negative gradient is the smallest, showing it to be the most effective insulation.

      • The graph line gradients indicate that the good insulating material was about twice as effective as the poor insulating material because its temperature gradient is about half of that of the poor insulating material.

  • the greater the heat conductivity of the insulating material, the less effective the insulating capacity of the material.

• By doing the experiment without an insulating layer you get a sort of baseline value.

• You then repeat the experiment with different solid materials, and, although not always possible or convenient, ideally each insulating layer should be of the same thickness.

  • You will have to poor the water away each time and allow the container to cool down before repeating the experiment with the same amount of water and start temperature.

  • You can attempt to cover all the surface of the 'system' but at the very least wrap the insulating material around the curved side of the copper container, even just this will give valid comparative results.

  • You can also try out different thicknesses of the same material.

• In this experiment you can use sheets of polystyrene foam, thick plastic sheeting (not expanded), sheets of newspaper, layer carpet, wool, cotton wool, bubble wrap, felt, fibre glass etc. i.e. anything you can conveniently wrap around the copper container!

  • You should find that materials trapping air e.g. wool or foam will show smaller temperature falls, better heat insulators than bulk materials like newspaper or thick plastic sheeting.

  • You should also find that the greater the thickness of the material the better the heat retention, so the temperature fall should be roughly inversely to the thickness.

• Here are some typical relative thermal conductivity values for a variety of materials.

  • aluminium 205, brass 109, cast iron 58, steel alloys 16 to 43, stone 1.7, dense brick 1.3, concrete 0.4 to 1.7, common brick 0.6 to 1.0, glass 0.8 to 0.9, water 0.58, lightweight concrete 0.1 to 0.3, non-expanded plastics 0.1 to 0.5, wood 0.14 to 0.19 (timber dependent), insulating brick 0.15, balsa wood 0.05, wool/felt insulation 0.04 to 0.07, fibre glass 0.04, glass wool insulation 0.04, wool blankets 0.04, cotton wool insulation 0.03, plastic foams 0.02 to 0.03

  • I've included a wide variety of materials, even though they obviously will not be used as heat insulators, but a high thermal conductivity is important if you do want to transfer heat energy efficiently e.g. a metal radiator.

  • ====> the lower the thermal conductivity value, the better the insulating effectiveness of the material

  • Note ...

    • the wide difference between metals and non-metals in thermal conductivity

    • the increased insulation effectiveness when air is trapped e.g. glass and glass wool, bulk plastic and expanded plastic eg plastic foams

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