Heat Transfer
5. Applications of Heat Transfer
Learning outcomes
- I can identify situations where conduction, convection, and radiation occur together.
- I can explain how insulation reduces heat transfer.
- I can describe how heat transfer is used in buildings, cooking, and engineering.
- I can explain how evaporation can cool objects and living organisms.
- I can evaluate methods of improving thermal efficiency in practical applications.
Applications of Heat Transfer
Heat Transfer in Real Systems
In most real situations, conduction, convection, and radiation occur at the same time.
The dominant method depends on:
- the materials involved
- whether liquids or gases can move
- temperature differences
- surface properties
- the surrounding environment
For example, when cooking food in an oven:
- conduction transfers energy through the baking tray and through the food
- convection transfers energy through moving hot air
- radiation transfers energy from the hot heating elements and oven surfaces
Understanding how these processes work together allows engineers to either increase heat transfer when it is useful or reduce it when energy needs to be conserved.
Thermal Efficiency
Thermal efficiency describes how effectively a system uses transferred thermal energy for its intended purpose.
In many systems, some energy is transferred to places where it is not useful.
For example, when heating a house:
useful transfer: heating the rooms
unwanted transfer: thermal energy escaping through walls, windows, doors and the roof
Improving thermal efficiency usually means:
- increasing useful energy transfer
- reducing unwanted energy transfer
- reducing energy consumption
- reducing operating costs
The exact solution depends on the purpose of the system.
A saucepan should transfer energy quickly into food, while a thermos should transfer energy as slowly as possible.
Insulation
Thermal insulation reduces the rate of unwanted heat transfer.
Insulation does not completely stop energy transfer. Instead, it makes the transfer occur more slowly.
Effective insulation may reduce:
- conduction
- convection
- radiation
Different methods are required for each mechanism.
Insulation and Conduction
Conduction can be reduced by using materials with low thermal conductivity.
Common thermal insulators include:
- fibreglass
- foam
- wool
- wood
- plastic
- cork
- trapped air
Many insulating materials contain numerous small air pockets.
Air is a poor thermal conductor, so trapping air can greatly reduce conduction.
The air must remain relatively still because moving air could establish convection currents.
Insulation and Convection
Convection requires the movement of a fluid.
It can therefore be reduced by preventing air or other fluids from circulating.
Examples include:
- trapping air inside foam
- using several layers of clothing
- sealing gaps around windows
- using lids on containers
- creating narrow air spaces
A large open air space can allow convection currents to develop.
Many small trapped air pockets are generally much more effective because they restrict the movement of air.
Insulation and Radiation
Radiative heat transfer can be reduced using reflective surfaces.
Shiny metallic surfaces reflect much of the infrared radiation reaching them.
They are therefore used in:
- vacuum flasks
- reflective building insulation
- emergency blankets
- some spacecraft
- thermal barriers
A reflective surface is particularly useful when radiation is an important heat-transfer mechanism.
Heat Transfer in Buildings
Buildings provide an excellent example of conduction, convection and radiation operating together.
A heated building can lose thermal energy through:
Conduction: through walls, windows, floors and the roof.
Convection: through moving air, ventilation and gaps around doors or windows.
Radiation: from warm surfaces toward cooler surroundings.
Reducing these transfers can significantly improve a building's thermal efficiency.
Roof and Wall Insulation
Warm air inside a building can transfer energy to walls and ceilings.
Insulating materials slow the conduction of energy through these structures.
A thicker insulating layer generally provides greater thermal resistance.
Materials such as:
- fibreglass
- mineral wool
- cellulose
- rigid foam
are commonly used because they have low thermal conductivity.
Good insulation means heating or cooling systems do not need to operate as frequently to maintain a comfortable indoor temperature.
Double and Triple Glazing
Windows can be an important route for heat transfer.
A double-glazed window contains two panes of glass separated by a sealed gap.
The gap usually contains air or a gas such as argon.
The gas has relatively low thermal conductivity and is trapped so that large convection currents cannot develop.
Triple glazing adds another pane and insulating gap.
Some energy-efficient windows also have special coatings that reduce radiative heat transfer.
Draught Proofing
Small gaps around:
- doors
- windows
- pipes
- vents
can allow warm indoor air to escape and cooler outdoor air to enter.
This increases energy transfer through air movement.
Draught proofing seals unnecessary gaps and reduces this exchange.
However, buildings still require appropriate ventilation for indoor air quality and moisture control.
Thermal efficiency therefore involves balancing several practical requirements.
Reflective Building Materials
Buildings exposed to strong sunlight can gain substantial energy through radiation.
Light-coloured or reflective roofs can reflect more incoming solar radiation and reduce heat absorption.
This can reduce the amount of energy needed for air conditioning in hot climates.
The best building design therefore depends partly on climate.
A design intended to retain thermal energy in a cold environment may differ from one intended to reduce solar heating in a hot environment.
Cooking
Cooking requires thermal energy to be transferred into food efficiently and safely.
Different cooking methods use different combinations of heat-transfer mechanisms.
Cooking on a Stove
Consider a metal saucepan containing water.
Energy travels through several stages:
stove → saucepan → water → food
Conduction
The metal pan conducts energy from the stove.
Convection
Heated water rises while cooler water sinks, establishing convection currents.
Radiation
Hot stove surfaces and the pan emit infrared radiation.
The cookware is deliberately designed to manage these transfers.
Why Cookware Uses Different Materials
The base of a saucepan should conduct energy efficiently.
Materials such as:
- aluminium
- copper
- steel
are therefore commonly used.
The handle has a completely different purpose.
We want to reduce:
hot pan → hand
Handles may therefore contain:
- plastic
- wood
- silicone
- other insulating materials
One object can therefore contain both a thermal conductor and thermal insulator, depending on the purpose of each part.
Convection Ovens
A convection oven contains a fan that circulates hot air.
Moving hot air increases energy transfer between the air and food.
This can:
- reduce cooking time
- produce more even heating
- reduce large temperature differences within the oven
Forced convection is also widely used in engineering whenever faster heat transfer is desirable.
Lids and Thermal Efficiency
Placing a lid on a cooking pot can improve efficiency.
A lid reduces:
- convection from the water surface
- evaporation
- escape of warm water vapor
Less energy is transferred to the surroundings.
Therefore, less energy may be required to maintain the desired cooking temperature.
This is a simple example of improving thermal efficiency without changing the heat source.
Evaporation and Cooling
Evaporation can be deliberately used to cool objects and organisms.
During evaporation, higher-energy particles are more likely to escape from the surface of a liquid.
The remaining particles have a lower average kinetic energy.
Therefore:
evaporation → energy removed → cooling
Sweating
Humans use evaporative cooling to regulate body temperature.
Sweat glands release water onto the skin.
As the water evaporates, energy is required to separate the water molecules and allow them to enter the gas phase.
This energy comes partly from the skin.
The skin therefore loses thermal energy and cools.
Sweating works most effectively when evaporation occurs rapidly.
Why Humidity Affects Cooling
High humidity reduces the rate of evaporation because the surrounding air already contains substantial water vapor.
As a result:
high humidity → slower evaporation → less evaporative cooling
This is one reason hot, humid conditions can feel particularly uncomfortable.
Moving air can have the opposite effect.
Wind or a fan removes humid air close to the skin and replaces it with less humid air.
This can increase evaporation.
Evaporative Cooling in Animals
Many animals also use evaporation for temperature regulation.
Dogs, for example, have relatively limited sweating compared with humans.
Instead, they can increase evaporation from moist surfaces of the mouth and respiratory system by panting.
Some animals use other strategies involving:
- sweating
- panting
- licking their bodies
- spreading water across body surfaces
All of these can increase evaporative heat loss.
Evaporative Coolers
An evaporative cooler deliberately evaporates water to cool air.
Warm, dry air passes across water or wet material.
Water evaporates and absorbs energy.
The outgoing air becomes cooler.
These systems can be effective in hot, dry environments but are less effective when humidity is already high.
Refrigerators
A refrigerator is designed to transfer thermal energy from a cold interior to warmer surroundings.
This does not happen spontaneously. Electrical energy is used to operate the system.
A refrigerant circulates through the refrigerator and repeatedly changes pressure and state.
In simplified terms:
inside refrigerator: refrigerant absorbs energy
outside refrigerator: refrigerant releases energy
The coils at the back or underneath a refrigerator can become warm because energy removed from inside the refrigerator, plus energy supplied to the system, is ultimately released into the room.
Heat Sinks and Electronics
Electronic devices generate thermal energy during operation.
Excessive temperature can:
- reduce performance
- shorten component life
- damage electronics
A heat sink transfers energy away from components such as processors.
Heat sinks are often made from aluminium or copper because these materials conduct thermal energy effectively.
Fins increase the surface area available for energy transfer to the surrounding air.
A cooling fan can then increase convection.
The sequence may be:
processor → conduction → heat sink → convection → surrounding air
Radiation also contributes a smaller amount of energy transfer.
Car Engines
Vehicle engines produce large quantities of thermal energy.
Too much heat could damage engine components.
A cooling system therefore transfers thermal energy away from the engine.
Typically:
- Coolant absorbs energy from the engine.
- The heated coolant flows to the radiator.
- Energy conducts through the radiator material.
- Air moves across the radiator.
- Convection transfers energy to the surrounding air.
Radiators contain many thin fins to increase surface area.
This is another example where good engineering means increasing, rather than reducing, heat transfer.
Vacuum Flasks
A vacuum flask is designed to do the opposite: minimize energy transfer.
It uses several methods simultaneously.
Vacuum
Reduces conduction and prevents convection between the walls.
Reflective surfaces
Reduce radiative energy transfer.
Insulating stopper
Reduces conduction and prevents air circulation through the opening.
Narrow neck
Reduces the area through which energy can escape.
A good thermal design therefore targets each possible heat-transfer pathway.
Thermos vs Radiator: Different Goals
Thermal efficiency does not always mean reducing heat transfer.
Consider two devices.
Vacuum Flask
Goal:
minimize heat transfer
Therefore engineers use:
- insulation
- vacuum
- reflective surfaces
Car Radiator
Goal:
maximize heat transfer
Therefore engineers use:
- conductive metals
- large surface area
- flowing coolant
- moving air
The "best" thermal design depends on what the system is supposed to accomplish.
Improving Thermal Efficiency
Engineers can improve thermal performance in several ways.
| Method | Effect |
|---|---|
| Low-conductivity insulation | Reduces conduction |
| Trapped air | Reduces conduction and convection |
| Vacuum | Greatly reduces conduction and prevents convection |
| Reflective surface | Reduces radiative transfer |
| Double/triple glazing | Reduces heat transfer through windows |
| Draught proofing | Reduces unwanted air movement |
| Increased surface area | Increases heat transfer when cooling is desired |
| Fans | Increase forced convection |
| Conductive metals | Increase conduction |
| Lids on containers | Reduce convection and evaporation |
| Evaporation | Provides cooling |
The correct method depends on whether the goal is to retain thermal energy or remove it.
Evaluating a Thermal Design
Suppose an engineer wants to improve the insulation of a house.
Simply choosing the thickest insulation may not automatically be the best solution.
The engineer might consider:
- thermal conductivity
- insulation thickness
- installation cost
- durability
- moisture resistance
- fire safety
- environmental impact
- expected energy savings
A useful solution should therefore be evaluated according to both thermal performance and practical considerations.
Worked Example: Improving a House
A house loses significant thermal energy through its roof and windows.
Possible improvements include:
Roof insulation: reduces conduction.
Double glazing: reduces conduction and convection through the windows.
Reflective coatings: can alter radiative heat transfer.
Draught proofing: reduces unwanted air movement.
Suppose roof insulation is inexpensive and greatly reduces energy loss, while replacing every window is much more expensive for a smaller improvement.
The insulation may provide the better cost-to-benefit ratio.
Engineering decisions often involve such trade-offs rather than simply choosing the technology with the greatest theoretical performance.
Common Mistakes
Mistake: "Insulation stops heat transfer."
Insulation reduces the rate of heat transfer. It does not normally eliminate it.
Mistake: "Thermal efficiency always means preventing heat transfer."
Sometimes efficient design requires increasing heat transfer, as in radiators and heat sinks.
Mistake: "A vacuum stops all heat transfer."
A vacuum greatly reduces conduction and convection but electromagnetic radiation can still travel through it.
Mistake: "A fan makes air cold."
A fan mainly moves air. This can increase convection and evaporation, increasing the rate at which energy leaves a warm surface.
Mistake: "Sweat cools the body simply because sweat is cold."
The major cooling effect occurs when sweat evaporates.
Mistake: "Only one type of heat transfer occurs at a time."
Conduction, convection and radiation commonly occur simultaneously.
Key Terms
Thermal efficiency — measure of how effectively a system uses or controls thermal energy for its intended purpose.
Thermal insulation — materials or methods used to reduce the rate of unwanted heat transfer.
Thermal conductivity — measure of how effectively a material conducts thermal energy.
Conduction — energy transfer through particle interactions without bulk movement of matter.
Convection — energy transfer through the bulk movement of a fluid.
Thermal radiation — energy transfer by electromagnetic waves.
Evaporation — escape of particles from the surface of a liquid into the gas phase.
Evaporative cooling — cooling caused when higher-energy particles escape during evaporation.
Heat sink — device designed to transfer thermal energy away from a component.
Radiator — device designed to transfer thermal energy efficiently to its surroundings.
Double glazing — two panes of glass separated by an insulating gas layer.
Draught proofing — sealing unwanted gaps to reduce air movement and energy transfer.
Forced convection — convection enhanced by devices such as fans or pumps.
Reflective insulation — material designed to reduce radiative heat transfer by reflecting electromagnetic radiation.
Key Takeaways
- Conduction, convection and radiation commonly occur together in real thermal systems.
- Insulation reduces unwanted heat transfer rather than completely stopping it.
- Low-conductivity materials reduce conduction.
- Trapped air can reduce both conduction and convection.
- Reflective surfaces can reduce radiative heat transfer.
- Building insulation, glazing and draught proofing can reduce energy consumption.
- Cooking systems deliberately use conduction, convection and radiation to transfer energy into food.
- Lids can improve cooking efficiency by reducing convection and evaporation.
- Evaporation cools surfaces because higher-energy particles escape from the liquid.
- Sweating uses evaporation to help regulate body temperature.
- High humidity reduces the effectiveness of evaporative cooling.
- Heat sinks and radiators are designed to increase heat transfer.
- Vacuum flasks are designed to decrease heat transfer.
- Improving thermal efficiency requires identifying the important heat-transfer pathways and choosing appropriate methods to control them.
- Practical engineering decisions must also consider factors such as cost, safety, durability and environmental impact.
Check Your Understanding
- Give an example in which conduction, convection and radiation occur at the same time.
- Why does trapped air make a useful thermal insulator?
- Explain how double glazing reduces heat transfer through a window.
- Why can reflective surfaces improve thermal insulation?
- Explain why a saucepan might have a metal base but a plastic handle.
- How does a convection oven increase the rate of heat transfer to food?
- Why does sweating cool the human body?
- Why is evaporative cooling less effective in humid conditions?
- Explain how a heat sink and fan work together to cool a computer processor.
- A designer can choose between making an insulating wall thicker or using a material with lower thermal conductivity. Explain how either change could reduce heat transfer.
- Why does putting a lid on a saucepan usually improve its thermal efficiency?
- Compare a vacuum flask and a car radiator. Explain why one is designed to minimize heat transfer while the other is designed to maximize it.