The Principles of Heat Transferand Microporous Insulation

Heat Transfer

Heat transfer from hot regions to cold regions takes place whenever there is a temperature difference.

The rate at which heat flows from hot to cold depends on many factors, but there will always be some flow.

The shape of the temperature gradient is defined by the conditions (hot and cold face temperatures, thermal conductivity of barrier, heat capacity and thickness of material etc.).

Hot face Cold face

Temperaturegradient

Temp

Hot face temp

Cold face temp

Steady State ConditionsFor any system at steady state:

heat entering system + heat generated by system = heat leaving system

Good insulator Poor insulator

Transient Conditions

T = 10 minsRadiator started but heat has not

reached cold side of wall

T = 0Both sides of wall at ambient temp

T = 30 minsCold side of wall

warming up

T = 60 minsSteady state

achieved

During transient conditions the system sees a net gain or loss in heat

Heat Transfer Mechanisms

The transfer of heat from a region of higher temperature to a region of lower temperature occurs by three basic mechanisms.

Conduction in a solid, a liquid, or a gas is the movement of heat through a material by the transfer of kinetic energy between atoms or molecules.

Convection in a gas or a liquid is the bulk movement of fluid caused by the tendency for hot areas to rise due to their lower density.

Radiation is the dissemination of electromagnetic energy from a source. This does not require any intervening medium and occurs most efficiently through a vacuum.

Generally, all three mechanisms work simultaneously, combining to produce the overall heat transfer effect. The thermal conductivity of a material is a physical property which describes its ability to transfer heat.

Control of Heat Transfer

Heat transfer is controlled by using an insulation material which has a low thermal conductivity.

To be truly effective, it must act as a barrier to all mechanisms of heat transfer.

mICROTHERm is a microporous thermal insulation with unusual properties which other common insulation materials do not have.

mICROTHERm approaches the lowest theoretically possible thermal conductivity – even lower than that of still air.

All the physical properties of mICROTHERm are designed to provide maximum resistance to the transfer of heat across it.

Thermal Conductivity of Solids, Liquids and Gases

1 10 100 1000 10000 100000 1000000

Thermal Conductivity (mW/mK)

Metals

Other solids

Liquids

Gases

Microtherm

Microtherm VIP

Conduction

All materials transfer heat by conduction as their component atoms or molecules exchange energy through collisions.

Solids are the most effective conductors of heat. Although the atoms in a solid have fixed positions, they constantly vibrate and interact with their neighbours. In hot areas the atoms vibrate more strongly, so the interactions tend to pass energy to cooler regions resulting in heat flow. Some solids conduct heat much better than others, depending on the way the atoms are bonded together.

Liquids are generally less good conductors of heat than solids. The interactions are weaker than in solids and this makes energy transfer less efficient.

Gases are very poor conductors of heat. The atoms or molecules are widely separated and interact rarely compared to solids and liquids.

Heat Flow in Conduction

Steady state unidirectional heat flux is defined by the Fourier Law and given by the equation:

q = - k dT dx

q = heat flux in W/m 2

k = thermal conductivity in W/m.KdT = temperature difference in Kdx = distance across section in m

Conduction Through a Heated Rod

HOT(lots of vibration)

COLD(not much vibration)

Heat travelsalong the rod

Microporous Insulation –Control of Solid Conduction

Solid conduction is minimised by:

• Using solids with low intrinsic thermal conductivity.

mICROTHERm insulation is formed largely from amorphous silica particles with a low intrinsic TC of 1.4 W/mK.

• Having a low ratio of solid particles to void space.

mICROTHERm insulation is about 90% void space.

• Using very fine particles to increase the path length of solid conduction across the material.

The amorphous silica in mICROTHERm insulation has a fundamental particle size of 5-25 nanometers.

Gaseous Conduction

Gaseous conduction at atmospheric pressure is much less efficient than solid conduction because interactions between molecules are less frequent. However, it is still an important heat transfer mechanism.

When a gas molecule collides with another gas molecule, energy is exchanged. This is an efficient way of transferring heat through a gas.

Gaseous-Solid Heat Transfer

When a gas molecule hits a solid, energy is not transferred efficiently from the gas to the solid. Instead, the gas molecule bounces off retaining its energy.

A good insulator will minimise gaseous conduction by ensuring that a maximum number of collisions a gas molecule undergoes are with solid surfaces instead of with other gas molecules.

Microporous Insulation –Control of Gaseous Conduction

Due to the extremely small particle size of the silica particles (5-25 nm), the pore size in mICROTHERm meets this criterion.

However, up to 80% of thermal conductivity through mICROTHERm at room temperature occurs by this mechanism. This is still much less than other insulation materials.

The average distance travelled by a molecule between collisions with other gas molecules is defined as the mean free path of the gas.To effectively prevent gaseous collisions the average pore size should be smaller than the mean free path of the gas (60 nm in air at 0°C). This is the definition of a microporous insulation.

Mean free path of gas molecule

Convection

Convection is heat transfer by bulk movement within a heated fluid such as a liquid or a gas.

Free convection is caused by expansion of fluids when heated, causing hot regions to become buoyant. Circulation occurs as the hot fluid cools and sinks down again.

Free convection systems can be very large and convey massive amounts of heat, for instance in weather systems and the circulation of molten rock inside the Earth.

Radiators make use of convection to transfer the heat from hot materials (water, night

storage bricks) to a room.

Rate of Convective Heat Transfer

In 1701, Newton observed that the rate of cooling of an object was proportional to the difference in temperature between the object and its surroundings and proposed Newton’s Law of Cooling.

q = hc (ts – tf)

Convection is the principal means by which an object loses heat to its surroundings at relatively low temperatures.

q = heat flux in W/m 2

hc= convection heat transfer coefficient in W/m2.K

ts = temperature of hot surface in K

tf = temperature of fluid in K

The Convection Heat Transfer Coefficient hc

The convection heat transfer coefficient hc can be greatly increased by forcing fluid past the hot surface, for instance by using a fan or a pump. This is called forced convection. Heat transfer rates can be increased by a factor of 10-20 in this way.

hc depends on the surface geometry, the fluid motion, the viscosity of the fluid, and several other properties. It can be calculated from first principles, approximated, or taken from standard tables.

Free convection patterns around a hot ball

Boundary Layers in Convection

Fluid motion at velocity v

HOT SURFACE, TEMP = TH

COLD FLUID, TEMP = Tc

Velocity = 0

Velocity = v

Velocity BoundaryLayer

TemperatureBoundaryLayer

Heat transfer at the surface takes place by conduction, not convection, because the fluid velocity is zero. Convection becomes more important away from the surface.

The change in temperature is largest close to the surface. The temperature boundary layer may not be the same thickness as the velocity boundary layer, but the rate of change of temperature depends on the rate of change of fluid velocity.

The Langmuir Equations

Convection rates in air can be calculated approximately using versions of the Langmuir equation;

Qc = 1.9468 (ts – t0)1.25 Free convection

Qc = 1.9468 (ts – t0)1.25 ((v + 0.35) / 0.35)0.5 Forced convection

Qc = Heat transferred by convection in W/m2

ts = temperature of surface in K

t0 = external air temperature in K

v = air velocity parallel to surface in m/s

These equations are valid for temperature differences up to 30 K and air velocities not more than 3 m/s.

Microporous Insulation –Control of Convection

Gaseous convection is easily eliminated as a heat transfer mechanism through all common insulation materials by making the average void space in the structure small enough that convection currents cannot form.The higher the temperature in the material, the smaller the voids need to be.

mICROTHERm has extremely small voids and convection currents cannot form within them even at the highest temperatures the material can withstand.Although convection has little effect on the performance of insulation materials, convection effects must be taken into account in thermal calculations for determining cold face temperatures.

Radiation is heat transfer by the emission of electromagnetic waves which carry energy away from the emitting object.

Radiative heat losses from a surface increase rapidly with temperature as defined by the Stefan-Boltzmann equation.

q = ơ (T4surface - T4

surroundings)

Infra-Red Radiation

Infra-red radiation is the principal mode of heat loss at temperatures above about 100 ºC

q = radiative heat flux in W/m2

= emissivity of the surface (between 0 and 1, depending on the material)ơ = Stefan-Boltzmann constant in W/m2K4

Tsurface = temperature of object in K

Tsurroundings = temperature of surroundings in K

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200

Brick temperature (deg C)

He

at

los

s (

kW

/sq

m)

Radiative

Convective

Heat Losses From Brick byDifferent Mechanisms

Ambient temperature 298 K

Radiation and Temperature

Most heat from an object is radiated in the infra-red region of the spectrum, which is not visible to the human eye. As the material gets hotter, the radiation is emitted at shorter wavelengths; first red, then yellow, then white. At extremely high temperatures the radiation can even be blue or ultraviolet. This is the source of ultraviolet radiation from the sun, which causes sunburn.

Increasing heat

Microporous Insulation –Control of Infra-Red Radiation

mICROTHERm insulation includes thermally stable metal oxide opacifiers of controlled particle size distribution. The particle diameter is sized to be about the same as the wavelength of the incident radiation.The opacifier particles scatter the infra-red radiation and so reduce transmission to the lowest possible levels.

Incident radiation

Scattered radiation

Opacifier particles

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 100 200 300 400 500 600

Mean temperature (deg C)

Th

erm

al c

on

du

ctiv

ity

(W/m

.K)

Aerosil silica 100%

Microtherm

Effect of Opacifier on Performance of Microporous Silica

Opacifiers are the reason for the low slope in the

mICROTHERm thermal conductivity curve.

mICROTHERm and Other High Temperature Insulators

Microporous insulation has the lowest thermal conductivity of any insulating material under atmospheric conditions, even better than that of still air.

At high temperatures the differential

between mICROTHERm and non-microporous insulations becomes very obvious.