Download presentation

Presentation is loading. Please wait.

Published byJackeline Sobie Modified over 3 years ago

1
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

2
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

3
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

4
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

5
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.

6
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.

7
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.

8
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.

9
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.

Similar presentations

Presentation is loading. Please wait....

OK

Matter and Energy.

Matter and Energy.

© 2017 SlidePlayer.com Inc.

All rights reserved.

Ads by Google

Ppt on current economic issues in india Ppt on advertisement of honda city Ppt on agriculture robotics Ppt on introduction to object-oriented programming encapsulation Ppt on water resources engineering Download ppt on bhuj earthquake 2001 Ppt on power generation by speed breaker specification Ppt on scientific calculator project in java Ppt on voice based web browser Ppt on schottky diode leakage