ARCH 215 Lecture Notes - Lecture 14: Latent Heat, Horsepower, Cool Air
8/25
BTU = British Thermal Unit
1kJ = 0.9478 BTU
1st law - nuclear power somehow breaks it
2nd law - disorder
3rd law - everything eventually comes to a halt
temperature is not the only measure of heat
sensible heat - dry heat and linear
X → 2X twice as much heat
surface area doubles → twice as much heat content
1 BTU::1lbs water::1 Fahrenheit increase
concrete has ⅕ the required heat to raise temperature compared to water
⅀radiation → i.e. starlight
no direct contact, no air, nothing required
latent heat → phase changes
i.e. ice → water → vapor; temperature stays until phase changes
boiling point 212 F, freezing 32 F
Conduction = a flow of heat through solid material; surface touching; requires a temp difference;
heat flows from high to low
Convection = a transfer of heat by moving stream of fluid (air or water)
● warm floor heats air instantly → hot air expands and becomes less
dense and raises → automatically circulates causing its own
conduction
● hot air ceiling → heats air and air stays above to keep an insulation
blanket. keeps floor cool; cold floor → warm air stays up because it
is less dense than the cool air below
Radiation = transfer of heat by electromagnetic waves through space
Latent heat = melts or freezes, evaporates or condenses
● can’t measure latent heat with temperature b/c it involves change of state
conductivity (k) = how fast(a rate) heat can be conducted within object per inch/thickness
conductance (C) = thickness of material per inch
thermal resistance = reciprocal of conductance; insulating value
resistivity (R) = how much material resist heat
find more resources at oneclass.com
find more resources at oneclass.com
resistance (R) = value for specific things; reciprocal of conductance
U-value = assembly of all present materials’ relationships; sum of all/one over the sum; 1/⅀R;
defines the overall conductive capacity of the building component
thermal capacity (Cp) = massive structure of building can act as a heat sink. think about water
tank analogy (the bigger, the deeper, the longer time to let water out); massive material =
larger time lag
R= x/k; x is thickness of material
U-value = 1/R
time delay and storage ≠ insulation
time lag = rapidly vs slowly; defines the conductive performance of a material in a building;
affects by Cp
1’ thick adobe = 10 hours delay
1’ thick common brick = 10 hours delay
1’ heavyweight concrete = 8 hours delay
1’ wood = 20 hours delay
1” concrete/stone = 1 hour delay
heat will flow from higher outdoor temp to lower indoor temp during the day, but part of the heat
will be stored in the mass and transmitted to the outdoors at night when the temp is lower.
polystyrene: 10 times resistance of concrete’s
high thermal storage: metal and concrete
low thermal storage: styrofoam and dry air
Ex.
what flow rate is required to heat 50 lbs of water from 60F to 70F in one hour?
50 lb x delta T
50 lb x (70-60F)
50 lb x 10F
rate = one hour/per hour;
specific heat for water = 1
Mean Radiant Temperature (MRT) = room wall
temperature average
Absorptance (radiation that is absorbed by the
material), reflectance (radiation that bounces
off the material), emittance (radiate through
surface; metal has low emittance all others
find more resources at oneclass.com
find more resources at oneclass.com
have high emittance), transmittance (radiation passes through material)
White plaster
low absorptivity 0.07 (7%) means bounces off light easily
high emissivity 0.9 (90%) means releases heat
Solar Panel
high absorptivity
low emissivity
Glass
transparent to short waves,
opaque to long waves → warmer inside
than outside
i.e. Iron panel heated surface and
hand right next to it → radiation
i.e. candle melting (change of phase
solid → liquid) → radiation and
latent heat
Horsepower (hp): 1hp = 746 W
8/27
U- value = relates to conduction
Temperatures
human body = 98.6 F
human skin = 92 F
brain damage = 110 F
pass out = 107 F
nauseous = 105 F
sweat = 100 F
shivering goosebumps = 95 F
lose power of speech = 85 F
irreversibly cold for body = 72 F
lowest temperature with recovery possible = 60 F
human at rest = 100 W
Thermal Comfort
find more resources at oneclass.com
find more resources at oneclass.com
