250 likes | 555 Vues
Heating and cooling. BADI Year 3 John Errington MSc. Definitions. Refrigeration Removing Heat from Cold Region to a Hot Region Specific Heat Amount of heat required to raise the temperature of 1 pound of substance 1F (water is 1 Btu/lb F) Sensible Heat
E N D
Heating and cooling BADI Year 3 John Errington MSc
Definitions • Refrigeration • Removing Heat from Cold Region to a Hot Region • Specific Heat • Amount of heat required to raise the temperature of 1 pound of substance 1F (water is 1 Btu/lb F) • Sensible Heat • Heat that is given off or absorbed by a substance without changing its state • Latent Heat of Fusion • The heat that must be removed from a liquid to change it into a solid or added to go from solid to liquid without any change in temperature • Latent Heat of Vaporization • Amount of heat required to change the state of a substance from liquid to a vapor without a change in temperature
Heating • Combustion • Coal, Oil, Petrol, Natural Gas (non-renewable) • Wood, Alcohols, Biomass, Hydrogen (renewable) • Electricity • Resistive heating • Heat pumps • Solar • Solar collectors for water heating • Fuel Cells • Produce electricity and heat at efficiencies up to 85% as compared to about 30% for combustion in power stations and vehicle engines. • Thermal gain • from windows, lighting, internal equipment and occupants
Cooling & Refrigeration • Heat transfer e.g. ice pack • Induced vaporisation of a liquid • Evaporative cooling • Forced vaporisation by reducing pressure • Peltier effect • Endothermic chemical reactions
Insulation • Volumes to be heated or cooled efficiently need to be insulated • The effectiveness of insulation is measured as a U value, R value, or K value. • A low U or K value indicates good insulation • A high R-value indicates good insulation
K-value Used for single materials • K = Thermal conductivity • How well the material will conduct heat • k = W / m deg C A low k-value indicates the material is a useful insulator Stainless Steel k = 16 Glass Wool k = 0.038
U-value Used for composite constructions e.g. walls and roofs • Thermal transmittance or overall heat transfer coefficient • U = W / m2 deg C • A low U-value ( >>1) indicates the structure is well insulated
R-value • Thermal resistance of a material • R = m2 deg C / W • Used to calculate total thermal transmittance of a structure
Total thermal transmittance of multiple insulating layers • Add together the R-values of all the materials in a construction, plus an allowance for inside and outside air layers. • The reciprocal of this sum is the U-value for the construction
Examples of cooling by vaporisation • When you heat a liquid it evaporates more quickly. • When a liquid evaporates it uses heat. Keep on heating a kettle of water, and the water will never get hotter than 100C. The heat you apply is used up converting the water from liquid to steam.
Evaporative cooling When we are hot we sweat. The evaporation of the moisture cools us down. Q: How can you keep a bottle of wine cool on a picnic? A: Wrap a thin wet cloth around it, put in a draught in the shade. Evaporation cools the cloth and keeps the wine cool.
Energy is needed for a change from liquid to vapour • This energy comes as heat from the liquid, which thus becomes cooler. • Practical refrigerators use substances like ammonia. This is a gas at room temperature and pressure. Compress it and it turns into a liquid. Let it expand and it turns again to gas • Latent Heat of Vaporization: • Amount of heat required to change the state of a substance from liquid to a vapor without a change in temperature
Block diagram of refrigerator / heat pump Expansion valve Insulated box Warm gas Cold gas A thermostat turns the compressor off when the fridge is cold enough, and back on when it warms up Less cold gas Cool liquid Compressor
Work function and contact potential When two dissimilar metals are connected together, a voltage called the “contact potential” is produced. It’s not easy to observe, because however you connect the wires, the voltages cancel out. It happens because some metals lose electrons more easily than others. Reactive metals (Lithium, Magnesium, Aluminium, Zinc, Iron) lose electrons easily, while less reactive metals (platinum, rhodium, chromium, copper) do not easily lose electrons. The energy needed to remove an electron from a material is called it’s work function. Copper Iron Copper V
Seebeck effect - thermocouples Hot junction If one junction is heated and the other cooled the contact potentials change. Now a small but useful voltage can be generated. Copper Iron Copper Cold junction V
Peltier effect The other junction heats up If the same thermocouple has a current passed through it the reverse effect occurs. One junction heats up and the other cools down. It’s important to use good conductors because Joule heating (W = V * I) also occurs and this can easily exceed the Peltier effect cooling. Copper Iron Copper One junction cools down
Applications Thermocouples are frequently used to measure temperatures in the range of -100C to +1200 C. Semiconductor junctions have higher junction potentials and give bigger voltages. They can be used to generate electricity, as thermo-generators. Peltier coolers are used for spot cooling for microscope slides and computer chips.