PRINCIPLES OF THERMAL DESIGN

CLIMATOLOGY,MBS SPA 2016

Assistant Prof. Rohit Kumar

CONTENTS

• The focus is on the understanding of thermal quanitites like – Heat,– specific heat, – latent heat,– Heat flow rate,– Conductance (& resistance),– Transmittance,– Sol air temperature,– Solar gain factor.

TEMPERATURE

• Outward appearance of the thermal state of the body‐ A symptom rather than a physical quantity. (Unit: degC or degree Celsius)

• Degree of hotness of a body.

– If day time temperature: 36 degCNight time temp. is        12 degCDiurnal range is 24 degC

HEAT

• A form of energy, appearing as molecular movement in substances or as ‘radiant heat’, a certain wavelength band of electromagnetic radiation in space (700 to 10,000nm). 

• Unit: Energy : Joules (J)– Length: metre (m)– Mass: kilogramme (kg)– Time: second (s)– Speed: m/s– Acceleration: m/sq.s

HEAT

• Force: A push or a pull upon an object resulting from the object’s interaction with another object.– Acceleration caused in unit mass of body.– Unit: kg m/sq.s or Newton

• Work: If unit force acting over unit mass of a body moves it over unit length.– Unit: kg sq.m/sq.s or Joule

• Energy: Ability to do work/Potential to carry out work. – Property of objects which can be transferred to other objects or converted to different forms but cannot be destroyed

– Unit: kg sq.m/sq.s or Joule

SPECIFIC HEAT

• Specific heat of a substance is the amount of heat energy necessary to cause unit temperature increase of a unit mass of the substance.

• Higher the specific heat of a substance, the more heat it will absorb for a given increase in temperature.

• Water has the highest specific of all common substances at 4187 J/kg degC. 

• Thermal capacity is the product of its mass and specific heat of its material.

LATENT HEAT

• Latent heat of a substance is the amount of heat energy absorbed by unit mass of the substance at change of state (from solid to liquid to gaseous) without any change in temperature.

• For water, latent heat is:– Of fusion (0 deg ice to 0 deg water)     335 KJ/kg– Of vaporization at 100 deg  2261 KJ/kg– Of evaporation at around 20 deg 2400 KJ/kg

HEAT FLOW

• Heat energy tends to distribute itself evenly until a perfectly diffused uniform thermal field is achieved.

• It tends to flow from high temperature to lower temperature in the following ways:– Conduction– Convection– Radiation

• The motive force is the temperature difference between the two zones or areas considered.

HEAT FLOW RATE• Rate of heat flow is measured in Watts.• Power is the ability to do work in unit time. It is the rate of energy expenditure

• Unit is J/s or Watt– 1 hp (metric) = 735.5 W– 1 Btu/h = 0.293 W– 1 ton of regfrigeration = 3516 W

• A ton of refrigeration is the cooling power of 1 ton (American ‘short’ ton of 2000 lb) of ice melting in 24 hours.– 1 ton= (2000 x 144)/24=12000 Btu/h = 12000 x 0.293 W = 3516 W

UNITS

• Length: m• Time: s• Mass: kg• Speed: m/s• Acceleration: m/sq.s• Force: kg m/sq.s (Newton)• Work/Energy : kg sq.m/sq.s (Joule)• Power: kg sq.m/cu.s: J/s : (Watt)• Specific/Latent heat: J/Kg degC• Conductivity: Watt/m degC• Conductance: Watt/ sq.m degC

• Conductivity= k‐value• Resistivity= 1/k• Conductance= C• Resistance= R=1/C• R=b/k (Where b is thickness of the material and 1/k is resistivity)

BRIEF

CONDUCTIVITY

• Heat flow in conduction takes place through bodies in direct contact, the molecular movement constituting the flow of heat.

• The rate at which such molecular movements spreads varies with different materials and is described as a property of materials  called thermal conductivity (k‐value)

• Thermal conductivity (or 'k‐value') is defined as the rate of heat flow through unit area of unit thickness of the material, when there is a unit temperature difference between the two sides. 

CONDUCTIVITY & RESISTIVITY

• The unit of measurement isW/m degC.• Its value varies between 0∙03 W/m degC forinsulating materials and up to 400 W/m degCfor metals.

• The lower the conductivity, the betterinsulator a material is.

• Resistivity is the reciprocal of this quantity (1 /k)measured in units of: m degC/W.

• Better insulators will have higher resistivity values.

CONDUCTANCE & RESISTANCE

Whilst conductivity and resistivity are propertiesof a material, the corresponding properties of abody of a given thickness are described asconductance (C), or its reciprocal resistance (R).

C = 1/RConductance is the heat flow rate through a unitarea of the body when the temperaturedifference between the two surfaces is 1 degC.The unit of measurement isW/m² degC.

Resistance of a body is the product of its thickness(b) and the resistivity of its material:

R = b x 1/ k = b/kIt is measured inm² degC/W.

MULTILAYER BODY

• The resistance of a multi layer body of different materials will be the sum of resistances of individual layers.

• The conductance (C) can be found by finding its total resistance (R) and taking its reciprocal:

• C=1/RNote that the conductances are not additive, only the resistances.

SURFACE CONDUCTANCE

• Along with the body, the surface of a material offers a resistance as well, where a thin film of air separates the body from the surrounding air: Surface or thin film resistance.

• Surface conductance is taken to be ‘f’, so surface resistance will be taken as 1/f.

• It includes convective and radiant components of the heat exchange at surfaces.

OVERALL AIR TO AIR RESISTANCE

• If the heat flow from air to one side on one side, through the body, to air on the other side is considered, both surface resistances must be taken into account. 

• The overall air‐to‐air resistance is taken to be sum of the body’s resistance and the surface resistance (s) , i.e. te internal surface resistance and te external surface resistance.

OVERALL AIR TO AIR RESISTANCE

CAVITIESIf an air space or cavity is enclosed within abody, through which the heat transfer isconsidered, this will offer another barrier tothe passage of heat.

It is measured as the cavity resistance (Rc) whichcan be added to the other resistancesdescribed above.

CONVECTIONIn convection, heat is transferred by the bodily movementof a carrying medium, usually a gas or a liquid.

The rate of heat transfer in convection depends on threefactors:

temperature difference (difference in temperature ofthe medium at the warmer and cooler points)

the rate of movement of the carrying medium in termsof kg/s or m3/s

the specific heat of the carrying medium in J/kg degC orJ/m3 degC

These quantities will be used in ventilation heat loss orcooling calculations.

In radiation heat transfer, the rate of heat flow depends on thetemperatures of the emitting and receiving surfaces and oncertain qualities of these surfaces: the emittance andabsorbance.

Radiation received by a surface can be partly absorbed and partlyreflected: the proportion of these two components is expressedby the coefficients absorbance (a) and reflectance (r).

The sum of these two coefficients is always one: a + r = 1Light coloured, smooth and shiny surfaces tend to have a higherreflectance.

For the perfect reflective theoretical white surface: r = 1, a = O.The perfect absorber, the theoretical 'black body', would have thecoefficients: r = 0, a = 1.

RADIATION

• U VALUE• SOL AIR TEMPERATURE• SOLAR GAIN FACTOR

TRANSMITTANCE/ U value

• The reciprocal of air‐to‐air resistance is the air to air transmittance or U‐value.

• U=1/R• This is the quantity most often used in building heat loss and heat gain problems, as its use greatly signifies the calculations.

• U value of common construction materials are provided in appendix 5.4. (Koenigsberger)

U‐ VALUE

• A U value is a measure of heat loss in a building element such as a wall, floor or roof. 

• It can also be referred to as an ‘overall heat transfer co‐efficient’ and measures how well parts of a building transfer heat. 

• This means that the higher the U value the worse the thermal performance of the building envelope. (Less insulation)

U‐ VALUE

• A low U value usually indicates high levels of insulation. 

• They are useful as it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials.

• U values are important because they form the basis of any energy or carbon reduction standard. 

U‐ VALUE• The U value is defined as being reciprocal of all the

resistances of the materials found in the building element.• The resistance of a building material is derived by the

following formula:

R = (1/k) x d

where k is the conductivity of the building material and d is the material thickness.

• The formula for the calculation of a U value is

U(element) = 1 / (Rso + Rsi + R1 + R2 ...)

where Rso is the fixed external resistancewhere Rsi is the fixed internal resistanceand R1… is the sum of all the resistances of the building materials in the constructional element.

U‐ VALUE

U‐ VALUE EXERCISE

• Compare the U‐value of a south facing normal brick wall and a concrete wall of thickness 230mm each.

• (Refer to Appendix 5.1 and 5.2 of Koenisberger for the supporting data)

• Conductivity= k‐value• Resistivity= 1/k• Conductance= C• Resistance= R=1/C• R=b/k (Where b is thickness of the material and 1/k is resistivity)

• Surface resistance= 1/fi, 1/fo• Overall air to air resistance= R`=R+1/fi+1/fo• Transmittance/U‐Value= U = 1/R`

BRIEF

SOL‐AIR TEMPERATURE

• For building design purposes, it is useful to combine the heating effect of radiation incident on a building with the effect of warm air: sol‐air temperature concept. (Effect of Convection +radiation on building)

• A temperature value is found which would create the same thermal effect as the incident radiation in question and this value is added to air temperature.

SOL‐AIR TEMPERATURE

SOL‐AIR TEMPERATURE

• In cold climate (heat loss situation), a lesser surface conductance would help reducing heat loss.

• In warm climate (heat gain situation), a greater surface conductance would help in reducing solar over‐heating.

• The reason being the incident radiation increases surface temperature far above the air temp. and some heat is dissipated to out door air immediately. Greater the s.c, more heat will be dissipated before it can be conducted away by wall. 

SOLAR GAIN FACTOR

• To consider the combined effects of reflective surfaces and thermal insulation.

• To reduce solar heat gain, a dark, highly absorptive surface with good insulation may be just as effective as a more reflective but less well‐insulated element. 

SOLAR GAIN FACTOR

SOLAR GAIN FACTOR

• The heat flow rate through the construction due to solar radiation expressed as a fraction of the incident solar radiation. 

• As this value can be related to the increase in the inner surface temperature, a performance requirement can be established on the basis of experience, in terms of this solar gain factor.

• Value should not exceed 0.04 in warm humid climate and 0.03 in hot dry part of composite climate when ventilation is reduced.

ANNEXURES

• Thermal resistance (R) and thermal conductance (C) of the materials are reciprocals of one another and can be derived from thermal conductivity (k) and the thickness of the materials.

• Thermal conductanceA measure of the ability of a material to transfer heat per unit time, givenone unit area of the material and a temperature gradient through thethickness of the material.

k-value – Thermal ConductivityThermal conductivity is the time rate of steady state heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area, W/m⋅K.(1)

Where,L – Thickness of the specimen (m)T – Temperature (K)q – Heat flow rate (W/m2)

R-value – Thermal ResistanceThermal resistance is the temperature difference, at steady state, between two defined surfaces of a material or construction that induces a unit heat flow rate through a unit area, K⋅m2/W. According to this definition and Equation 1, Equation 2, therefore, can be obtained.As indicated in Equation 2, the value of the thermal resistance can be determined by dividing the thickness with thermal conductivity of the specimen.(2)

C-value – Thermal ConductanceThermal conductance is the time rate of steady state heat flow through a unit area of a material or construction induced by a unit temperature difference between the body surfaces, in W/m2⋅K. C-value, hence, is the reciprocal of the R-value and can be expressed as Equation (3).(3)

Consequently, the value of the thermal conductance can be calculated by dividing the thermal conductivity with the thickness of the specimen.

BIBLIOGRAPHY

• Koenigsberger, O. H., Manual of Tropical Housing and building, Orient Longman private limited, 1973.