Temperature = a measurement of stored heat energy but temp. is never transferred, onlyl heat energy is
Heat is sometimes transferred without a change in temp., such as a melting ice cube
sensible heat = heat that causes a change in temp.
latent heat = heat that causes a change in state (like melting ice)
objects at the same temp. can store different amounts of heat, this storage capacity is called specific heat (Cp) which is measure by comparing it to the storage capacity of water.
British Thermal Unit (Btu) = Amount of heat energy required to raise one (1) pound of water by one (1) degree Fahrenheit.
So, specific heat is measured in terms of the number of Btu's req'd to change the temp. of a specific material by one (1) degree Fahrenheit.
Radiation
Radiation = Method by which heat is transferred between two objects not in contact and NOT SHIELDED from each other.
Heat you feel from the sun is through heat transfer by radiation
Radiation always takes place but it usually takes place at a slow rate.
ALL objects radiate at each other, even people standing next to one another.
Wavelength of the radiation is based on the temp. of the object.
Warm things radiate infrared (non-visible)
Really hot objects (like red hot steel) begin to glow in the visible spectrum
Glowing heat:
Red = Hot
Orange = Hotter
White = Hottest
The rate of radiative exchange is based on the surface temp. of the objects, the viewed angle and a property called emissivity.
The Emissivity (ε) of a surface is a property of the material and is usually the same as the absorptivity at any given wavelength.
Simple example is color = Black surfaces have higher emissivities and absorptivities than white or shiny surfaces.
Black heats up rapidly and cools off rapidly. Shiny surfaces heat up more slowly but stay hot longer.
Emissivity and absorptivity are often different between the infrared and visible spectrum.
Selective surfaces = surfaces which have a high absorptivity rate in one wavelength (usually solar) and low emissivity in another (usually infrared).
Foil on the back of some insulation has a very low emissivity which reduces the radiative transfer across the air space inside the wall.
Transmissivity = measure of how easily a material allows radiant energy to pass through it.
Glass has low transmissivity since it does not allow infrared to pass through like in a greenhouse.
Sunlight (a short wavelength and visible portion of spectrum) passes through the glasse heating up materials inside, when the materials heat up (called reradiate) they heat up in the infrared spectrum which cannot pass through the glass and becomes trapped heat energy.
Viewed angle = depends on the size of the surface and your distance from it.
Stand close to a meat freezeer, it occupies a large angle of view and you lise a lot of heat to it.
Stand across the room from it, it occupies a smaller angle of view nd there is consequently much less heat exchange (loss in this case)
Mean Radiant Temperature (MRT) = the average radiant temperature of your surroundings.
Device that measures MRT is called a globe thermometer.
Convection
Convection = the heat exchange process which occurs only in a fluid medium, such as air or liquid.
Hot air rising is an example, it expands when it's hot, which reduces it's density and makes it lighter. The cool, heavier air falls, the warm lighter air rises.
Smoke rises in chimneys since it is warmer.
One Exception, water expands when it gets colder and only just before it freezes.
Convection occurs in rooms all the time, it's the only means of heat transger qhich is strictly directional, it never transfers heat downward, it can transfer horizontally by stirrin air but not as rapidly as upward.
Stagnation = when the top of a space si warmer than the bottom and the hot air rises and stays there.
Stack effect = difference in pressure in a vvertical space (positive or outward at the top and negative or inward at the bottom).
Rising air pushes at the top and it pulls air in behind it , down below.
Can be a significant effect in office towers where elevator shafts act like smokestacks.
Thermal resistance values are different based on orientation of a surface (horizontal or vertical, floor/roof or wall), and based on the direction of the heat flow (up or down).
The orientaition is more critical than the thickness.
The thin film of air which occurs next to a wall also provides a resistance, calculated as the inverse of the so'called film coefficient.
Conduction
Conduction = the heat transfer process which occurs when objects are in direct contact.
Non-directional, it transfers in any direction with contact.
Conduction occurs inside walls, transferring walls from the warmer side to the colder side.
Each material has a different conductivity and resistivity.
Resistivity = inverse of conductivity
Specific thicknesses of materials have a calc'd conductances and resistances.
The resistance (R) is calculated from the thickness (x) and the conductivity (k), using the following formula:
x / k = R
Insulation for example is specified by the letter R followed by a number, i.e. R-19 insulation has a resistance of 19.
A complete wall assembly has a calc'd conductance, which represents all the interactions of the internal materials, including some radiation and convection, this is called the U value.
The U value is the reciprocal of the sum of the resistances (1 divided by the sum):
U = 1 / (R + R + R + ... + R)
(to be continued...)
Study time: 5/12 9:00pm-10:15pm = 1.25 hrs
Conduction Continued...
Latent Heat
Viz. Dict. of Arch. (Ching) = The quantity of heat absorbed or released by a substance during a change in phase at constant temp. and pressure.
e.g., sweat evaporates and cools off the body by absorbing some of the heat by the change of state from liquid to steam, the change of state uses the heat energy.
phase change = when an element changes state like from liquid to solid, liquid to gas, etc., it either stores or releases energy, measured by heat transfer
-32deg F = freezing point of water
212deg F = boiling point of water
Pound of water at 212deg F requires 1,000 btu's to evaporate
study time: 5/24 5pm-5:45pm, 9pm-9:15pm
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