Sunday, 14 August 2011

Physics: Dailylife applications of energy II (elective) - Heat exchange process

To access the document version of this chapter of notes click the "Notes Corner" above.

Electronic cooking
-          Electric hotplate/oven: it heats up food by heating a metal plate or wire such that it emits IR to heat up food inside.
-          Induction cooking: By applying high frequency a.c. on a solenoid, it produces a varying sinusoidal magnetic field which causes large eddy current and heat up metal components like the pot. It's safer since non-metal, like human body, won't be heated.
-          Microwave: it has wavelength about 12 cm in application such that it penetrates into the food well and heat evenly. Using the electric field of electromagnetic waves, it flips polar molecules especially water up and down. The collision increases and turns into random k.e. which rises the temperature.
Heat exchange
By laws of thermodynamics, work is done when removing heat. So QH = QC + W.
In air conditioner, the cooling capacity is defined as Qc/t in W, which shows its performance.
COP, coefficient of performance is defined by COP = QC/W = QC/(QH-QC) which is equal to cooling capacity/input electrical power. It varies from 1.75~3.5. If COP is 2, then 1 J of energy is used to remove 2 J of heat.
Sometimes we don't need to be so cold so temperature monitoring devices is used. It's a bimetallic strip and triggers a circuit to stop the cooling/heating function at a certain temperature.
In buildings, heat exchange between indoor and outdoor environment is considered.
Law of conduction states that the rate of heat exchange is proportional to temperature difference and area, and it is inversely proportional to thickness of material. Mathematically, QC/t = κA(TH-TC)/d, where κ is the thermal conductivity in Wm-1K-1. Note that the temperature refers to the temperature difference of the wall only. It does not imply the temperature difference of air in the two sides. It depends only on the material used. Note that it has similar behavior with resistivity ρ, which means that good conductors of electricity would be good conductors of heat as well.
Define U-value (thermal transmittance) be U = κ/d, then QC/t = UA(TH-TC).
Overall thermal transfer value (OTTV) = rate of heat gain through building envelope avg. in one year/total area of building envelope, or we can say OTTV = Q/At, where Q is measure in one year.
The building envelope, unless specified, means the face surrounding the building and the roof, excluding the ground. In Hong Kong, building is regulated to have OTTV lower than 30Wm-2.
We can use some measures to reduce heat transfer in buildings, like using heat insulating building materials like insulators, double glazing glass, or shading fins, solar control windows which blocks IR or design the orientation well (in Hong Kong, window towards South would not receive sunlight in summer but it receives a lot of sunlight in winter.)

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