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
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BADI Year 3
John Errington MSc
Used for single materials
A low k-value indicates the material is a useful insulator
Stainless Steel k = 16
Glass Wool k = 0.038
Used for composite constructions e.g. walls and roofs
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.
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.
A thermostat turns the compressor off when the fridge is cold enough, and back on when it warms up
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.
If one junction is heated and the other cooled the contact potentials change. Now a small but useful voltage can be generated.
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.
One junction cools down
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.