Presentation on theme: "Temperature and Heat Loss The following topics are covered in this presentation: Calculation of U-values Heat loss from the building envelope Use of U-values."— Presentation transcript:
Temperature and Heat Loss The following topics are covered in this presentation: Calculation of U-values Heat loss from the building envelope Use of U-values in heat loss calculations
Temperature and Heat Loss Solution: The total thermal resistance of a component is given by: R total = R si + R materials + R airspace + R so where R total is the total thermal resistance R si is the thermal resistance of the inside surface R materials is the thermal resistance of the materials R airspace is the thermal resistance of the cavity R so is the thermal resistance of the outside surface 1. Calculate the U-value of a) single-glazed window b) double-glazed window. Given: Thickness of glass = 4 mm Thermal conductivity of glass = 1.02 W/mK Thermal resistance of surfaces: 0.12 (inside); 0.06 (outside) Thermal resistance of 20 mm wide airspace = 0.18 m 2 K/W
Temperature and Heat Loss a)Single-glazed window: R total = R si + R glass + R so (no airspace in this case) = 0.1839 m 2 K/W U-value = 1/R = 5.44 W/m 2 K
Temperature and Heat Loss b) Double-glazed window: R total = R si + R glass + R airspace + R glass + R so = 0.12 + 0.0039 + 0.18 + 0.0039 + 0.06 = 0.3678 U-value = 1/R total = 2.72 W/m 2 K
Temperature and Heat Loss D W Wall – 35% 10% 15% Floor – 15% 25% The figure above shows the typical values of heat loss from an un- insulated house.
Temperature and Heat Loss HEAT LOSS FROM A BUILDING: Heat loss from a building is determined in two parts; the first part is the FABRIC HEAT LOSS and the second is called HEAT LOSS BY VENTILATION. Fabric Heat Loss: The unit of U-value is: W/m 2 K This can be written as U = W/m 2 K After transposition, W = U × m 2 × K Replacing m 2 by A (area) and K by T (temperature difference): W = U × A × T where W is the heat loss in Watts. For determining the heat loss from an element/component we need to know its U-value, the surface area and the difference between the inside and outside temperatures. 1.
Temperature and Heat Loss The heat loss due to ventilation occurs when the warm air of a room is replaced by cold, but fresh, air from outside. Rate of heat loss due to ventilation = where C v is the volumetric specific heat capacity of air V is the volume of the room N is the number of air changes per hour T is the difference between inside and outside air temperatures Total rate of heat loss = Fabric heat loss + Ventilation heat loss 2.
Temperature and Heat Loss EXAMPLE: Find the total heat loss from a room; the U-values and surface areas of the different elements are given in the Table. The inside and outside air temperatures are 20 ºC and 0 ºC respectively. The ground temperature is 5 ºC. Take Cv Cv = 1212 J/m 3 K; N = 2 per hour; V = 150 m3m3 Element U-value Area Walls0.35 68.0 Doors2.2 3.3 Windows1.9 11.7 Floor0.25 55.0 Roof0.25 55.0 Temp. Diff. 20 15 20 Rate of heat loss 0.35x68x20 = 476.0 145.2 444.6 206.3 275.0 Total = 1547.1 W Fabric heat loss: 1.
Temperature and Heat Loss Heat loss due to ventilation: Heat loss = = 2020.0 W Total Heat Loss = 1547.1 + 2020.0 = 3567.1 W or 3567.1 J/s (1W = 1J/s) 2.