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    Thermal Properties of Materials

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    suniljaya
    This document discusses thermal properties of materials and thermal performance of buildings. It covers topics like specific heat capacity, volumetric specific heat, thermal capacity, latent heat, heat flow rate, conductivity, resistivity, resistance, air to air heat transfer, windows, heat loss calculation, ventilation and infiltration loss rate, gains through windows, nominal heat gains from occupants, effective use of roof insulation, emittance and absorptance of polished surfaces, resistive insulation, and examples of roofs with different insulation configurations. The key thermal properties discussed are specific heat capacity, conductivity, resistivity, resistance, and latent heat. Different methods of heat transfer and calculation of heat loss and gains are also summarized.

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    Thermal Properties of Materials

    Uploaded by

    suniljaya
    27 views30 pages
    This document discusses thermal properties of materials and thermal performance of buildings. It covers topics like specific heat capacity, volumetric specific heat, thermal capacity, latent heat, heat flow rate, conductivity, resistivity, resistance, air to air heat transfer, windows, heat loss calculation, ventilation and infiltration loss rate, gains through windows, nominal heat gains from occupants, effective use of roof insulation, emittance and absorptance of polished surfaces, resistive insulation, and examples of roofs with different insulation configurations. The key thermal properties discussed are specific heat capacity, conductivity, resistivity, resistance, and latent heat. Different methods of heat transfer and calculation of heat loss and gains are also summarized.

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    Thermal properties of materials

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    This document discusses thermal properties of materials and thermal performance of buildings. It covers topics like specific heat capacity, volumetric specific heat, thermal capacity, latent heat, heat flow rate, conductivity, resistivity, resistance, air to air heat transfer, windows, heat loss calculation, ventilation and infiltration loss rate, gains through windows, nominal heat gains from occupants, effective use of roof insulation, emittance and absorptance of polished surfaces, resistive insulation, and examples of roofs with different insulation configurations. The key thermal properties discussed are specific heat capacity, conductivity, resistivity, resistance, and latent heat. Different methods of heat transfer and calculation of heat loss and gains are also summarized.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    27 views30 pages

    Thermal Properties of Materials

    Uploaded by

    suniljaya
    This document discusses thermal properties of materials and thermal performance of buildings. It covers topics like specific heat capacity, volumetric specific heat, thermal capacity, latent heat, heat flow rate, conductivity, resistivity, resistance, air to air heat transfer, windows, heat loss calculation, ventilation and infiltration loss rate, gains through windows, nominal heat gains from occupants, effective use of roof insulation, emittance and absorptance of polished surfaces, resistive insulation, and examples of roofs with different insulation configurations. The key thermal properties discussed are specific heat capacity, conductivity, resistivity, resistance, and latent heat. Different methods of heat transfer and calculation of heat loss and gains are also summarized.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 30

    Thermal properties of materials

    and thermal performance of


    buildings
    Specific heat capacity (Cp)

    • Specific heat capacity gives the relationship between


    heat and temperature
    • It is the amount of heat energy required to cause a unit
    temperature increase of a unit mass of the substance
    • It is measured in J/kgK

    Volumetric specific heat


    • Volumetric specific heat is a similar quantity.
    • However, it is based on a unit volume. The unit is
    J/m3K.
    Thermal capacity

    • Thermal capacity is a property of a given body


    • It is the amount of heat required to cause a unit temperature
    increase of that body
    • It is a product of its mass and specific capacity of its material
    • The unit is kg x J/kgK or J/K

    Latent heat
    • The latent heat of a substance is the amount of heat absorbed at a
    change of state by a unit mass of that substance without change in
    temperature; It is measured in J/kg

    • Latent heat of fusion (ice to water) at 0oC = 355 kJ/kg


    • Latent heat of evaporation at 100oC = 2261 kJ/kg
    • Latent heat of evaporation at 18oC = 2400 kJ/kg

    • At a change in the reverse direction, the same amount of heat is


    released.
    Heat flow rate (Q)
    • Heat flow rate is generally measured in Watt (W) or J/s.

    • Conduction
    •
    • Conduction can take place within a body or between bodies in contact. It can be thought of as spreading
    of molecular movement. The magnitude of heat flow will depend on the following:

    1. Cross sectional area A through which the heat can flow. This is taken perpendicular to the direction of flow.
    2. Thickness of the body (b)
    3. The temperature difference ΔT between the two points considered
    4. A property of the material known as conductivity (k value)

    • Conductivity

    • Conductivity is given as the rate of heat flow through a unit area of body of unit thickness, with a unit
    temperature difference between two sides. The unit is W/mK.

    • Resistivity (r)
    •
    • Resistivity is the reciprocal of conductivity.
    • Note: -ity ending implies the property of a material
    • -ance ending refers to some property of a defined body
    Resistance
    • Resistance of a body is the product of its thickness and resistivity of the material.
    •
    • R = b x r = b/k (m mK/W) or m2k/W
    •
    • This property is used to measure the performance of an insulating material and is
    often referred to as R-value.
    •
    • For a multi-layer building element, the resistance of layers are additive. For a material
    with three layers,
    • Rbody = R1 + R2 + R3
    Air to air heat transfer
    • When the heat transfer is taking place between air
    to air, it is necessary to take account of resistance of
    two surfaces. These are called the following:

    • Rse - external surface resistance


    • Rsi - internal surface resistance
    • Surface resistance for high emissivity material
    (emissivity greater than 0.8) is given in the table and
    it can be seen that it will depend on the type of
    element (wall, roof, floor, etc.) and the surface
    roughness
    • Air to air resistance is Ra-a
    Windows
    • From the thermal point of view, in cold climates, window is the
    weakest element of any reasonably constructed building envelope
    • Windows can be single glazed, double glazed or can have special
    arrangements for better efficiency
    Heat loss calculation under steady state

    • The heat loss generally occurs in a winter situation


    where the outdoor temperature remains below the
    desirable indoor temperature
    • The heat loss rate consists of two components
    • Qc is the conduction loss rate
    • Qv is the ventilation and infiltration loss rate that
    occur when the outdoor cold air replaces the indoor
    heated air
    • The total heat loss Q = Qc + Qv

    • It may be useful to calculate the specific heat loss


    rate q where qc = ∑A.U in W/K.
    Ventilation and infiltration loss rate (Qv)

    • The ventilation rate is normally specified as the number of complete air


    changes that take place per hour (ACH)
    • For a typical new house with closed windows and also proper
    construction, the number of air changes per hour could be about 0.5 (it
    takes 2 hours for the air to be completely replaced by new, incoming air)
    • The energy needed to raise one cubic meter of air through one Kelvin is
    0.33 Watt-hours; this means that heat capacity per cubic meter is
    0.33Wh m-3K-1
    • If the volume of the space is, V, and the number of air changes is N, the
    specific heat loss rate qv will be given by

    • qv = 0.33 x V x N (Wh/m3K) x (m3/h)


    • Hence the answer will be in W/K

    • The total specific heat loss rate, q = qc + qv

    • The total heat loss rate = Q = q x ΔT


    Gains due to solar radiation
    • When solar radiation strikes a surface, some of the
    radiation may be reflected, some may be transmitted (if the
    body is transparent or translucent) and the remainder will
    be absorbed
    • The absorbed component will cause the heating effect
    • The sum of the three coefficients, reflectance,
    transmittance and absorptance is always 1
    The gains through windows
    • When there are windows that face the sun path, the solar heat gain Qs
    may be taken into account to reduce the required heating rate
    • For windows, the heat gain is given by A x G x sgf, where sgf is called the
    solar gain factor and G is the solar irradiance
    • The typical values would be 0.76 for 6 mm single glazing and 0.64 for
    clear 6 mm double glazing
    • When 6 mm single glass is used with a reflecting film, it could drop to
    about 0.52
    • When shading devices are available, even for 6 mm clear glass, sgf can
    drop to about 0.16
    Nominal heat gain values from occupants

    • In this house, if the total internal heat gains is about 600 W


    (occupants and appliances), it is possible to calculate the
    amount of heat input needed to maintain the indoor at 21oC

    • Total heat loss rate = 4712.7 W

    • Total gains due to solar radiation = 539.4 W


    • The internal gains = 600 W
    • The amount of heating needed to maintain the indoors at 21oC
    = 4712.7 - 539.4 - 600 = 3573.3 W
    Effective use of roof insulation in warm humid tropical
    climates
    • In warm humid tropical climates, roof is one of the main sources of heat gains
    • The heat gain through the roof can be controlled using reflective and resistive
    insulation
    • The solar radiation gain by the roof will depend on the colour of the roofing
    material
    • When a surface of a body is exposed to solar radiation, which of short wave
    form, the temperature of the body will increase
    • Then, it will start emitting heat in the form of longwave to the nearby objects
    that will at a lower temperature
    • For longwave radiation, the absorptance is equal to emittance
    • For most of the opaque surfaces, emittance is 90% and hence absorptance
    will also be 90%
    • Hence, for long wave radiation, absorptance will not depend on the surface
    colour
    • For polished surfaces, the emittance is a low value
    • Hence, the absorptance of long wave radiation will also be low
    • This is the property that is very effectively used when polished surfaces are
    used as reflective insulation.
    Emittance and absorptance of polished surfaces
    Resistive insulation
    • Resistive insulation act as a medium to slow down the heat flow
    • Air has the lowest thermal conductivity
    • Hence, resistive insulation can be manufactured by capturing air within
    the voids formed by a material with a very low conductivity
    • Examples are expanded polystyrene where 98% can be air with only 2%
    polystyrene forming the bubble
    • It could also be out of expanded polyethylene or mineral wool trapping
    air in a suitable manner
    • Often, resistive insulation can be used combined with reflective
    insulation
    Reflective insulation mounted
    on resistive insulation
    Thickness of resistive
    insulation can be 3mm, 8 mm,
    12 mm, 20 mm or 25 mm
    Resistive thermal insulation
    Roof with ceiling, no-insulation
    Roof with ceiling with single sided reflective insulation
    on resistive insulation
    Roof provided with double sided reflective on resistive insulation
    above the ceiling
    Factory roof provided with double sided reflective on
    thicker resistive insulation, but without a ceiling

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