Calculating Indoor Temperature Under Different Roof Materials - Assignment Example

THERMAL RESISTANCE Course: Tutor: Institution: Location of the Institution: Date: 27/4/2015 Thermal Resistance Thermal Resistance in Roofing Materials Thermal resistance is the quality of an object to resist conduction of heat, which is monitored by deviation in temperature. It is measured using R –value, which basically refers to the degree of insulation. (Wikipedia, Web, 22 April, 2015) It can be measured by various means, one of which is the use of an infra- red thermometer. (Pg.3 Energy Independence Community, 2012) It is a constant in every given object or specific material, in which case it is referred to as the Specific thermal resistance. It can also be specific to a particular component within an object or material. In this case it is known as Absolute thermal resistance. This is the variation in temperature across a given structure, when a unit or fraction of heat energy passes through it. The duration of the heat transmission is measured as time used in units. Thermal resistance elevates as the insulating layer is increased. (Wikipedia, Web, 22 April, 2015) There are various factors considered when looking for appropriate roofing material, thermal resistance being one of them. Some of the materials used for this purpose include: Polyethylene, Aluminum, Cement and Iron. Materials manufactured from Polyethylene are as a result of two main processes of polymerization. That is, low pressure and high pressure polymerization. Properties of polyethylene materials are dependent on the grade; which varies by density and distribution of molecular weight. When solids are transmitted in other forms such as gaseous and liquid, abrasion of the internal wall of polyethylene products occurs. This is as a result of friction between the product’s walls and particles being transmitted. These products tend to have a higher level of abrasion resistance as compared to ductile iron and cement. Polyethylene is used for insulating purposes in roofing structures. (Vinidex Pty Ltd, 2013) Aluminum is second in popularity of use world- wide, after Iron. It is a very good conductor of heat and is also widely used for its great ability to conduct electricity. Used in the manufacture of roofing materials due to its thermal property, aluminum also reflects radiated heat well. It forms an oxide after being exposed to oxygen. This layer of oxide protects it from corrosion, as it self- repairs. (Sapa Profiles UK Ltd. 2015), (www.aluminiumdesign.net), (Web, 22 April, 2015) Cement is also used for roofing purposes. It is used in a large number of buildings, due to its strength. It reduces the movement of heat which in turn reduces variations in temperature. That is, buildings or roofing made with cement as a component tend to have more stable and cooler temperature ranges. It is fire- resistant and acts as a barrier that prevents fire from spreading. (Thermal Properties of Cement, Web, 22 April, 2015) Another material used in the roofing industry is Iron. It is the most used metal, constituting ninety- five percent of metal production globally. It exists in a wide range of oxidation states (-2 to +6). However, ferrous (+2) and ferric (+3) compounds are the most common. Iron also oxidizes in air to result in Hydrated Iron oxides, popularly known as rust. Iron, upon exposure to higher temperatures, results in changes from one state to the other; and becomes magnetic at 770 ° C. It is ranked sixth in a list of abundant elements universally. (Wikipedia, 2015) Outdoor temperature refers to the intensity of solar radiation on the roof and atmospheric or air temperature. Indoor temperature on the other hand, refers to sensible heat emitted from persons, lights, machines and also due to transfer from the outdoor to an inward enclosed space. Air flow results from hot indoor and cold or cooler outdoor temperature difference. Loss of heat through roofing and other parts of a building is due to temperature difference and materials’ thermal conductivity. (Pg.8, EMS 10 Heating, Ventilating and Conditioning) In Australia, a large percentage of land is arid but rich in mineral ores. Australian Outback is rich in various metal and ores, which makes it a popular mining region. “Outback” refers to an area of land in Australia that is sparsely populated and has little or erratic rains. The elements found in this area include aluminum, gold, gold deposits, as well as zinc ores. It is spread over a vast region, making mining a great economic activity. Argyle Diamond mine, is the largest producer of diamonds in the world, with a production rate of thirty- three percent of the global supply. Iron ore in Australia is mined at the Pilbara. (Australian Outback, Wikipedia, 2015) During summer, temperatures get to an all-time high, ranging from a minimum of 16°C to 30°C (86°F). In the extremely dry desert regions, the temperatures rise up to 40° C. The weather patterns in Australia are contrary to those experienced in the Northern Hemisphere. Summer usually runs from December to February yet those in the Northern Hemisphere go through winter at that time of the year. (www.australia.com) Materials popularly used for roofing purposes in Australia are tree bark, Aluminum, Iron and Cement. Iron is readily available as the ore is mined at Pilbara. Aluminum is also mined in the Australian Outback, making it easily accessible for use in the local region. Both are good conductors of heat therefore it is necessary to also employ insulating materials to prevent excessing heating when used as roofing materials. Cement and Polyethylene are very effective in providing insulation “under the roof”. The barks of trees have been used since time immemorial to make roofing material in Australia. Paperback and the bark of Eucalyptus tree are the most popular. It is a convenient way of roofing as it is also cost- effective to the locals. (Vinidex Pty Ltd, 2013), (RIC Good Wood Guide) The diagram below illustrates variations in temperature flow, during a typical summer in mining regions in Australia. Ventilation also releases air Hot air from outdoor flowing inwards (High outdoor temperature) 30°C > Cool air released (Low indoor temperature) < 16°C A temperature difference between inside and outside air results in a “natural draft”, which causes air movement through a building. (Pg. 1)The course or direction taken by the flow is dependent on variation in temperatures. Air density tends to be higher where temperatures are higher. As such, air will move upwards and outward if temperatures are higher indoors. The cooler air from outside, will then flow in through lower sections in a building. (Pg.1) Natural draft is as a result of variations in air density, between the air inside and outside a building. It is expressed as: dpmmH2O = 1000 h (ρo - ρr) / ρh2o          (1) where dpmmH2O = head in millimeter water column (mm H2O) ρo = density outside air (kg/m3) ρr = density inside air (kg/m3) ρh2o = density water (in general 1000 kg/m3) h = height between outlet and inlet air (m) (Pg.2 www.EngineeringToolBox.com) During summer in Australia, the temperature range is as follows: Outside temperature = 30° C Inside Temperature (Pr) = 16° C (www.australia.com) The following calculations on temperature difference can be deduced from the above figures: 1. Po = Density of Outside air (kg/m3) Po = (1.293 kg/m3) (273 K) / ((273 K) + (30° C)) = 352.989 / 303 = 1.165 kg/m3 2. Pr = Density of Inside air (kg/m3) Pr = (1.293 kg/m3) (273 K) / ((273 K) + (16° C)) = 352.989 / 289 = 1.2214 kg/m3 The theoretical temperature difference can be calculated as: dp = g(Po – Pr)h Where: g = Acceleration of gravity 9.81 m/s2 h = Height between outlet and inlet air (m). In this case, 10m dp = pressure loss dp = 9.81(1.165 - 1.2214)10 dp = -5.533 N/m2 This figure derived from the above calculation clearly indicates a drop in the atmospheric pressure. (Pg.3 www.EngineeringToolBox.com) (www.australia.com) Another formula used in calculation involves the use of the actual temperature. This can be illustrated as: p = (1.293 kg/m3) (273 K) / (273 K + t) Where: t = Actual temperature. In this case, 25°C (Hypothetical) p = Density of air p = (1.293 kg/m3) (273 K) / (273 K + 25) p=1.1845 kg/m3 (Pg.3 www.EngineeringToolBox.com) Level of humidity in the air and the ventilation mechanisms present in a building are to be considered when calculating Natural draft. This is due to the fact that calculations are based on dry air. Calculations made using data from an extremely dry or desert area would as such be more accurate as the air is not humid in any way. (Pg.5 www.EngineeringToolBox.com) It is important to note that during summer season, temperature variations are similar to those experienced in dry areas. That is, days are hotter while the nights are usually cold as temperatures drop. (Pg.1- 2 www.EngineeringToolBox.com) (www.australia.com) References Australian Outback, Wikipedia, (2015) 22 April, 2015 EMS 10 Heating, Ventilating and Conditioning RIC Good Wood Guide, 2015 Sapa Profiles UK Ltd. (2015) Web 22 April, 2015 Thermal Properties of Cement, Web 22 April, 2015 Vinidex Pty Ltd, 2013 Wikipedia, Web > 22 April, 2015 http://www.aluminiumdesign.net 22 April, 2015 http://www.australia.com22 April, 2015 http://www.EngineeringToolBox.com 22 April, 2015 Read More