TOPIC 2 - HEAT.pptx
- Количество слайдов: 42
HEAT Power. Point® Slides by TH FOO NADZIRAH ZAINORDIN MSc QS [Heriot-Watt, UK], BSc (Hons) QS [IUCTT, Msia]
INTRODUCTION • Thermodynamics is a science dealing with the relationship between heat and other forms of energy. It includes the subjects of heat transfer and psychometrics. • The mechanism by which the building interior thermally interacts with its surroundings, and by which mechanical heating and cooling equipment delivers thermal comfort, is heat transfer. • Using heat transfer theory as a tool, the building envelope can be analyzed for its heat flow characteristics, including its ability to control heat gain and loss by its construction, orientation, and use of particular building materials. • The flow of heat is defined as energy transfer between two regions due to different temperatures.
PRINCIPLES OF HEAT TRANSFER 1. How the heat moves from one place to another is to think of it as the energy of molecules bouncing around. 2. Heat is transferred from one thing to another when the bouncing of the molecules causes nearby, less active molecules to start moving around too. 3. The motion that is transferred from one bunch of molecules to another also transfers heat from the more excited group of molecules to the less excited group. 4. A cold area is just an area with quieter molecules, and therefore with less thermal energy.
5. As long as there is a temperature difference between two areas, heat always flows from a region of higher temperature to a region of lower temperature, which means that it flows from an area of active movement to one of less movement. 6. This tendency will decrease the temperature and the amount of activity in the area with higher temperature, and increase temperature and activity in the area with the lower temperature. 7. When there is no difference left, both areas reach a state of thermal equilibrium, and the molecules bounce around equally.
HEAT TRANSFER MECHANISM RADIATION CONDUCTION CONVECTION
RADIATION • The internal energy that sets molecules vibrating sets up electromagnetic waves. • Electromagnetic energy comes in many forms, including cosmic-ray photons, ultraviolet (UV) radiation, visible light, radio waves, heat, and electric currents, among others. • Infrared (IR) radiation is made up of a range of longer, lower frequency wavelengths between shorter visible light and even longer microwaves. • The sun’s heat is mostly in wavelengths from the shorter, and hotter, end of IR radiation. • Buildings get heat in the shorter IR wavelengths directly from the sun
• Buildings also receive thermal radiation from sun-warmed earth and floors, warm building surfaces, and even contact with human skin, all of which emit irradiation at much lower temperatures and at longer wavelengths • Radiation warms our skin when the sun strikes it, or when we stand near a fire. When we stand near a cold wall or under a cool night sky, radiation cools our skin. • A cold window in a room usually has the greatest effect of draining radiated heat away from our bodies, making us feel colder. • Closing the drapes blocks the heat transfer, and helps keep us warm.
How building material radiate heat? • The ways that building materials interact with thermal radiation that reaches them is of great importance to designers. • Three properties that describe these interactions with radiant heat are reflectance, absorptance, and emittance. • Each of these can be influenced by the interior design of a space.
Refers to the amount of incoming radiation that bounces off a material, leaving the temperature of the material unchanged. Is just the opposite of reflectance. An absorptive material allows thermal energy to enter, raising its temperature; when the sun shines on a stone, the stone becomes warmer. Absorptance Reflectance THE PROPERTIES This ability of a material to radiate heat outward to other objects is called emittance. The amount of energy available for emittance depends upon the amount absorbed, so a highly reflective material would have less absorbed energy to emit. Emittance
CONDUCTION • Conduction is the flow of heat through a solid material, as opposed to radiation, which takes place through a transparent gas or a vacuum. • Molecules vibrating at a faster rate (at a higher temperature) bump into molecules vibrating at a slower rate (lower temperature) and transfer energy directly to them. • The molecules themselves don’t travel to the other object; only their energy does. • When a hot pan comes in contact with our skin, the heat from the pan flows into our skin. When the object we touch is cold, like an iced drink in a cold glass, the heat flows from our skin into the glass. • Conduction is responsible for only a small amount of the heat loss from our bodies. • Conduction can occur within a single material, when the temperature is hotter in one part of the material than in another.
CONVECTION • Convection is similar to conduction in that heat leaves an object as it comes in contact with something else. • In the case of convection, the transfer of heat happens by means of a moving stream of a fluid (liquid or gas) rather than another object. • Our skin may be warmed or cooled by convection when it is exposed to warm or cool air passing by it. Skin absorb heat and we feel cooler. • The same thing happens when we run cold water over our skin. • The amount of convection depends upon how rough the surface is, its orientation to the stream of fluid, the direction of the stream’s flow, the type of fluid in the stream, and whether the flow is free or is forced. • When there is a large difference between the air temperature and the skin temperature, plus more air or water movement, more heat will be transmitted by convection. • Convection can also heat, as well as cool. A hot bath warms us thoroughly as the heat from the water is transferred by convection to our skin. Hot air from a room’s heating system flowing past us will also warm our skin.
HEAT TRANSFER PROPERTIES Thermal conductivity Thermal resistance Thermal transmittance • Measure how good the material is at conducting heat • Measure how good the material is at resisting heat • Measure overall rate of heat loss • Power loss per square meter of surface when the temperature difference between the outside and inside air is 1 0 C
Strategies to Control Heat Flow
Control strategies Why is it necessary to control heat flow? 1. To maintain a constant comfortable temperature within a building 2. Conserve energy and reduces heating or cooling costs 3. Reduces surface condensation which is unsightly, unhealthy and damages decorations / finishes Thermal insulation is required to control heat flow into the building (during summer) or outside the building (during winter) Insulating materials will block the heat transfer between areas at different temperatures
Control strategies Heat flow or transfer can occurs through conduction, convection and radiation. – – Insulate to reduce conduction Control air flow to reduce infiltration/convection Provide shading/select windows to control heat absorbed Provide materials to reflect radiation
Control strategies 1. Control Airflow • Block infiltration and convection pathways at attics and crawlspaces • Seal leaky joints in walls, floors and ceilings • Seal attic openings • Balance pressures within house • Seal ductwork in unconditioned space
Control strategies 2. Control radiation • Radiant barrier in attic • Low-E windows • Infrared-reflective roofing • Shading devices • Roof overhangs 3. Control moisture flow • Moisture through soils / foundations • Moisture through walls • Moisture through doors / windows • Moisture through roofs
Control strategies Heat gain through buildings- example
Control strategies Heat gain / loss through buildings- typical % Summer Winter
Control strategies Heat gain / loss through buildings- example
Insulation • Insulation is the primary defence against heat transfer through the building envelope. • The walls are the most important area to insulate, as they have the largest area. You can check if an existing building’s walls are insulated by removing an electrical outlet cover and looking inside. • Insulation comes in many forms and can be grouped generally as follows: - Rigid preformed materials eg. aerated concrete blocks - Flexible materials eg. fibreglass quilts - Loose fill materials eg. expanded polystyrene granules - Materials formed on site eg. foamed in place polyurethane - Reflective materials eg. aluminium foil
To select the suitable type of insulation, consider properties such as : • • • Strength or rigidity required Moisture resistance Fire resistance Resistance to pests and fungi Compatibility with adjacent materials Harmless to people and environment
• Loose-fill insulation consists of mineral wool fibers, granular vermiculite or perlite, or treated cellulose fibers. It is poured by hand or blown through a nozzle into a cavity or over a supporting membrane above ceilings on attic floors. • Foamed-in-place materials include expanded pellets and liquidfiber mixtures that are poured, frothed, sprayed, or blown into cavities, where they adhere to surfaces. • Foamed-in-place insulation is made of foamed polyurethane. By filling all corners, cracks, and crevices for an airtight seal, foamed insulation eliminates random air leakage, which can account for up to 40 percent of heating energy.
Types of insulation • Environmentally sound foamed insulation made without formaldehyde, chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), or volatile organic compounds (VOCs) is available, and is safe for use by individuals who are chemically sensitive or suffering with allergies or asthma. • Foamed insulation can also reduce levels of airborne sound from plumbing, outside noises, or indoor activities such as home theaters.
Types of insulation • Flexible and semirigid insulation is available in batts and blankets. • Batt ( thin sheet) insulation is made of glass or mineral wool. It comes in various thicknesses and lengths, and in 41 - and 61 -cm (16 - and 24 -in. ) widths, to fit between studs, joists, and rafters in light frame construction. • Batt insulation is sometimes faced with a vapor retarder of kraft paper, metal foil, or plastic sheeting, and is also used for acoustic insulation.
Types of insulation • Rigid insulation comes in blocks, boards, and sheets and is preformed for use on pipes. It is made of plastic, or of cellular glass. • Cellular glass is fire resistant, impervious to moisture, and dimensionally stable, but has a lower thermal-resistance value than foamed plastic insulation. • Foamed plastics are flammable, and must be protected by a thermal barrier when used on the interior surfaces of a building. • Rigid insulation with closed-cell structures, made of extruded polystyrene or cellular glass, is moisture resistant, and may be used in contact with the earth. • Such insulation is often applied to the outside of the building and covered with fabric-reinforced acrylic.
Types of insulation • Sheets and rolls of insulation with reflective surfaces offer a barrier to radiant heat. • Reflective insulation uses material of high reflectivity and low emissivity, such as paper-backed aluminum foil or foil-backed gypsum board, in conjunction with a dead-air space to reduce the transfer of radiant heat.
Insulation materials- examples
Insulation materials- special glass Summer Winter
Locations for insulation installation
Locations for insulation installation
Building Science Diagnostic Tools Blower door testing • In blower door test, the fan is used to pressurize or depressurize the house to a standard test pressure (typically 50 Pascal's). • The fan flow at this test pressure is the amount of air that is leaking into or out of a home at that pressure. • By using the house geometry, this fan flow is typically translated into air changes per hour or equivalent leakage area.
Building Science Diagnostic Tools Blower door testing
Duct Pressure Testing • The duct fan is smaller than the blower door, but is calibrated like the blower door. • When the fan creates a standard pressure difference, the volume (CFM) of air movement is referenced to either the floor area served by the system or the air handler fan capacity. • For example, test results are often given as a percentage of air handler capacity or floor area. • A duct blaster can also be used to identify leak locations so these leaks can be sealed.
Duct Pressure Testing
Infrared Imaging • The least expensive professional unit on the market is a handheld model • Ideal for finding missing or poorly installed insulation and othermal defects. » Image of missing wall insulation
Heat transfer Calculations Heat transfer through materials (conduction) Hc = U x A x ( Ti - To ) or = U x A x ∆T where Hc = heat transfer, W U = thermal transmittance, W/m 2 K (or W/m 2 C) A = surface area, m 2 Ti = inside air temperature, K or C To= outside air temperature, K or C ∆T = temperature difference