HEAT EXCHANGE IN BUILDINGS

TERMINOLOGIES Thermal conductivity: is the rate of heat flow through a unit area of unit thickness of the material for a unit temperature difference across the material. It is measured in W/mK (watts per kelvin meter). The higher the k or λ value, the better the thermal conductivity. Good insulators will have as low a value as possible. This makes them poor insulators. The k or λ value for any material will become higher with an increase in temperature.

TERMINOLOGIES Thermal conductivity:

TERMINOLOGIES Thermal conductivity:

TERMINOLOGIES Thermal resistance – R - value, is a product of thermal conductivity and thickness. The greater the material thickness, the greater the thermal resistance.

TERMINOLOGIES Coefficient of heat transmission - U-Value: U-value is a measure of the transmission of heat through a pre-determined area of the building fabric — this being 1m2. The unit measurements are therefore W/m2K (watts persq uare metre kelvin) and describe the heat transfer, in watts, through a square metre of a building element (such as a wall, floor or roof). Defined as the ratio between the density of heat flow rate q,W/m2, through the construction and the temperature difference between the ambient temperatures on both sides

TERMINOLOGIES Coefficient of heat transmission - U-Value: For a construction with n layers the U-Values are calculated as the reciprocal of the sum of R values (1/total sum of R values):

TERMINOLOGIES Coefficient of heat transmission - U-Value:

TERMINOLOGIES Steady state heat flow: Steady state means that the temperatures of the system do not vary with time. Heat transfer processes can be quantified in terms of appropriate rate equations. These equations may be used to compute the amount of energy being transferred per unit time. For heat conduction, the rate equation is known as Fourier’s law.

TERMINOLOGIES Fourier’s law.

H EAT EXCHANGE EQUATION like the human body, the building can also be considered as a defined unit. 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.

H EAT EXCHANGE EQUATION

Qi = Internal heat gain (though lighting and mechanical fixtures, human bodies in the given volume of space), Qs = Solar heat gain, Qc = Conductive heat exchange through materials(wall, roof, window) Qv = Convective heat exchange by ventilation, Qm = Heating or cooling by mechanical means (HVAC Systems used.) Qe = Cooling due to Evaporation

H EAT EXCHANGE EQUATION Qi + Qs ± Qc ± Qv ± Qm ‐ Qe = 0 Qi = Internal heat gain (though lighting and mechanical fixtures in the given volume of space), Qs = Solar heat gain, Qc = Conductive heat exchange through materials, Qv = Convective heat exchange by ventilation, Qm = Heating or cooling by mechanical means (HVAC Systems used.) Qe = Cooling due to Evaporation

CONDUCTION (QC) Conduction Heat flow rate through a wall of a given area can be described by an equation: If Heat loss from a building is considered when To Ti.

CONDUCTION (QC) Question one: The wall of the house 7m wide and 6m high is made from 0.3m thick brick with k=0.6W/mK. The surface temperature on inside wall is 16deg C and that on outside is 6deg C. find the heat flux through the wall and the total heat loss through it. Question two: assume a wall with a U-Value of 4.5 W/m² K and a surface area of 10 m². If the outside temperature was 30°C and the inside was 25°C, calculate the total heat gain due to conduction through the wall.

CONDUCTION (QC) Question three: Calculate outer temperature T2 of the wall in deg C

CONDUCTION (QC) Question four: Calculate the heat loss through the window

CONDUCTION (QC) Question five:

CONDUCTION (QC) Question six:

CONDUCTION (QC) Question seven:

CONDUCTION (QC) Question eight:

CONVECTION (Q V )  In convection, heat is transferred by the bodily movement of a carrying medium, usually a gas or a liquid. This movement may be self- generating, due to thermal forces alone (temperature differences, thus different densities, causing convection currents).  Rate of heat transfer in convection depends on three factors. 1. Temperature difference between warmer and cooler points of the medium 2. The rate of movement of the carrying medium in terms of kg / s or m3/s 3. The specific heat of the carrying medium.

CONVECTION (Q V )

 In convection, heat is transferred by the bodily movement of a carrying medium, usually a gas or a liquid. This movement may be self- generating, due to thermal forces alone (temperature differences, thus different densities, causing convection currents).  Rate of heat transfer in convection depends on three factors. 1. Temperature difference between warmer and cooler points of the medium 2. The rate of movement of the carrying medium in terms of kg / s or m3/s 3. The specific heat of the carrying medium.

CONVECTION (Q V ) If the number of air changes per hour is given by ‘N’, then;

RADIATION  Radiation received by a surface can be partly absorbed and partly reflected.  The proportion of these two components is expressed by the coefficient of absorbance (a) and reflectance (r).  Always, a + r = 1  For a light coloured reflective surface, a = 0, r = 1  For a perfect absorber, a lack body, a = 1, r = 0