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BULDING CLIMATOLOGY - LECTURE NOTES ON HEAT for SEM 3

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- 1. CLI MATE & THE BUI LT ENVI RONMENT HEATHEAT climatology
- 2. ▪ TEMPERATURE - definition ▪ Unit
- 3. TEMPERATURE Temperature is actually not a physical quantity but it can be thought of as a symptom-as the outward appearance of the thermal state of a body. If energy is conveyed to a body, the molecular movement within that body is increased and it appears to be warmer. ▪ Temperature is measured by the Celsius scale. ▪ A position on this scale, i.e. the temperature of an object is donated as: o C but an interval or difference in temperature is: deg C.
- 4. ▪ HEAT ▪ SPECIFIC HEAT ▪ LATENT HEAT ▪ THERMAL CAPACITY
- 5. heat Heat is 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 10000 nm). As such, it is measured in general energy units: joules (J). Specific heat of a substance is the amount of heat energy necessary to cause unit temperature increase of a unit mass of the substance. It is measured in: J/kg degC. 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 or liquid to gaseous) without any change in temperature. It is measured in: J/kg. Thermal capacity of a body is the product of its mass and the specific heat of its material. It is measured as the amount of heat required to cause unit temperature increase of the body, in units of J/degC.
- 6. Heat Flow: ▪ CONDUCTION ▪ CONVECTION ▪ RADIATION ▪ Unit
- 7. 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 zones, by any or all of the following ways: ▪ Conduction ▪ Convection ▪ Radiation The 'motive force' of heat flow in any of these forms is the temperature difference between the two zones or areas considered. The greater the temperature difference, the faster the rate of heat flow. The rate of heat flow is measured in Watts (W). In most practical applications, the multiple of watt 'kilowatt' (kW), will be used. (1 kW = 1000 W)
- 8. ▪ CONDUCTIVITY ▪ RESISTIVITY ▪ CONDUCTANCE ▪ RESISTANCE
- 9. CONDUCTI VI TY & RESI STI VI TY 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. The unit of measurement is W/m degC. Its value varies between 0·03 W/m degC for insulating materials and up to 400 W/m degC for metals. The lower the conductivity, the better insulator 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.
- 10. CONDUCTANCE & RESI STANCE Whilst conductivity and resistivity are properties of a material, the corresponding properties of a body of a given thickness are described as conductance (C), or its reciprocal resistance (R). C = 1/R Conductance is the heat flow rate through a unit area of the body when the temperature difference between the two surfaces is 1 degC.The unit of measurement is W/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/k It is measured in m² degC/W.
- 11. MULTI LAYER BODY If a body consists of several layers of different materials, its total resistance will be the sum of the resistances of the individual layers. The conductance of such a multilayer body (C) can be found by finding its total resistance (R) and taking its reciprocal: Rb= R1 + R2 + R3 = b1/k1 + b2/k2 + b3/k3 = Σ b/k Cb = 1/ Rb = 1/ Σ b/k Note that the conductances are not additive, only the resistances.
- 12. SURFACE CONDUCTANCE In addition to the resistance of a body to the flow of heat, a resistance will be offered by its surfaces, where a thin layer of air film separates the body from the surrounding air. This is the surface or film-resistance. It is denoted as 1/f (m² degC/W), f being the surface or film-conductance (W/m² degC).
- 13. OVERALL AI R- TO- AI R RESI STANCE The overall air-to-air resistance (Ra) is the sum of the body's resistance and the surface resistances: Ra = 1/fi + Rb + 1/fo Where, 1/fi= internal surface resistance, Rb = resistance of the body, 1/fo = external surface resistance, all resistance values in m² degC/W.
- 14. transmittance (u-value) The reciprocal of the overall air-to-air resistance (Ra) is the air-to-air transmittance or U-value. U = 1 / Ra Its unit of measurement is the same as that of conductance - W/m² degC. This is the quantity most often used in building heat loss and heat gain problems.
- 15. CAVI TI ES If an air space or cavity is enclosed within a body, through which the heat transfer is considered, this will offer another barrier to the passage of heat. It is measured as the cavity resistance (Rc) which can be added to the other resistances described above.
- 16. convection In convection, heat is transferred by the bodily movement of a carrying medium, usually a gas or a liquid. The rate of heat transfer in convection depends on three factors: temperature difference (difference in temperature of the medium at the warmer and cooler points) the rate of movement of the carrying medium in terms of kg/s or m3 /s the specific heat of the carrying medium in J/kg degC or J/m3 degC These quantities will be used in ventilation heat loss or cooling calculations.
- 17. radiation In radiation heat transfer, the rate of heat flow depends on the temperatures of the emitting and receiving surfaces and on certain qualities of these surfaces: the emittance and absorbance. Radiation received by a surface can be partly absorbed and partly reflected: the proportion of these two components is expressed by the coefficients absorbance (a) and reflectance (r). The sum of these two coefficients is always one: a + r = 1 Light coloured, smooth and shiny surfaces tend to have a higher reflectance. For the perfect reflective theoretical white surface: r = 1, a = O. The perfect absorber, the theoretical 'black body', would have the coefficients: r = 0, a = 1.
- 18. 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. This can be done by using the sol-air temperature concept. Ts =To + [(l x a)/fo] whereTs = sol-air temperature in ˚C To = outside air temperature in ˚C l = radiation intensity inW/m² a = absorbance of the surface fo = surface conductance (outside),W/m2 degC.
- 19. solar gain factor (θ) The solar gain factor is defined as the heat flow rate through the construction due to solar radiation, expressed as a fraction of the incident solar radiation. Its value should not exceed 0.04 in warm-humid climates or 0.03 in the hot-dry season of composite climates, when ventilation is reduced. solar gain factor θ = (a x U) / fo
- 20. heat exchange in buildings Just like the human body, the building can also be considered as a defined unit and its heat exchange processes with the out-door environment can be examined. The thermal balance, i.e. the existing thermal condition is maintained if: Qi + Qs ± Qc ± Qv ± Qm - Qe = 0 If the sum of this equation is less than zero (negative), the building will be cooling and if it is more than zero, the temperature in the building will increase.
- 21. Conduction Conduction heat flow rate through a wall of a given area can be described by the equation: Qc= A x U x ∆T Where, Qc= conduction heat flow rate, inW, A = surface area, in m², U = transmittance value inW/m² degC, ∆T = temperature difference in degC
- 22. Convection Convection heat flow rate between the interior of a building and the open air depends on the rate of ventilation, i.e. air exchange.The rate of ventilation can be given in m³/s. The rate of ventilation heat flow is described by the equation: Qv= 1300 xV x ∆T Where, Qv = ventilation heat flow rate, inW, 1300 = volumetric specific heat of air, in J/m³ degC, V = ventilation rate in m³/s, ∆T = temperature difference in degC If the number of air changes per hour (N) is given the ventilation rate can be found as: V = (N x room volume) / 3600 where 3600 is the number of seconds in an hour.
- 23. Radiation through windows The solar heat flow through windows is given by the equation: Qs = A x l x θ, Where, A= area of the window in m², l = radiation heat flow density inW/m², θ = solar gain factor of window glass.
- 24. ▪ PERIODIC HEAT FLOW ▪ TIME-LAG ▪ DECREMENT FACTOR
- 25. periodic heat flow All the equations and calculation methods seen so far are valid if and only if, both out-door and indoor temperatures are constant. As perfectly static conditions do not occur in nature, the basis of the above methods is the assumption of steady state conditions. In nature the variation of climatic conditions produces a non-steady state. Diurnal variations produce an approximately repetitive 24-hour cycle of increasing and decreasing temperatures. The effect of this on a building is that in the hot period heat flows from the environment into the building, where some of it is stored, and at night during the cool period, the heat flow is reversed: from the building to the environment. As the cycle is repetitive, it can be described as periodic heat flow.
- 26. time-lag & decrement factor The two quantities characterizing this periodic change are the time-lag (or phase shift θ) and the decrement factor (or amplitude attenuation µ). The decrement factor is the ratio of the maximum outer and inner surface temperature amplitudes taken from the daily mean.
- 27. ▪ Controls ▪ MICRO-CLIMATE CONTROL ▪ STRUCTURAL CONTROL ▪ MECHANICAL CONTROL
- 28. Controls The environment immediately outside and between buildings can be influenced by the design of a settlement and by the grouping of buildings to a minor extent. Structural (passive) means of control can provide a further leveling out of the climatic variations, and often even comfort conditions can be achieved by such means. Precisely controlled indoor climate can only be achieved by mechanical (active) controls (the straight line in the figure), but this may not be our aim, and even if it is, with adequate structural controls, the task of mechanical controls is radically reduced and it becomes more economical.
- 29. Structural Controls Heat absorbing glass On opaque surfaces the incident radiation is partly absorbed and partly reflected, a + r = 1 with transparent bodies, it may be absorbed, reflected or transmitted. a + r + t = 1 An ordinary window glass transmits a large proportion of all radiation between 300 and 3000 nm, i.e. both visible light and short-wave infra- red, but very little around and outside the 300 to 3000 nm range. Its transmittance is selective. This selective transmittance can be modified by varying the composition of the glass to reduce substantially the infra-red transmission, whilst only slightly affecting the light transmission. Such a product is referred to as heat absorbing glass.
- 30. Other special glasses Whilst the heat absorbing glasses achieve a selective transmittance by selectivity in absorption, the heat reflecting glass achieves a similar selective transmittance by selectivity in reflection. The glass is coated by a thin film of metal (usually nickel or gold), applied by vacuum evaporation. Such glasses absorb very little heat, therefore the improvement in reducing the total solar gain is far greater, but unfortunately they are still rather expensive. Recently, several types of photo chromatic or light-sensitive glasses have been developed, containing submicroscopic halide crystals, which turn dark when exposed to strong light and regain their transparency when the light source is removed. Their transmittance may thus vary between 74 and 1%. When the technique is more developed and more economical, these glasses may have a future in solar control.

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