Movement of Thermal Energy: Heat Flow Three methods Conduction Convection

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Transcript Movement of Thermal Energy: Heat Flow Three methods Conduction Convection

 Movement of Thermal Energy: Heat Flow
 Three methods
 Conduction
 Convection
 Radiant
 Most heat transfer has some combination of all three
occurring at the same time
 Conduction
A method by which heat is transferred from a warmer
substance to a cooler substance by molecular collisions.
Direct contact.
 Convection
The transfer of thermal energy from a fluid flowing over a
solid object- [Air is a fluid!]
 Radiation
A method by which heat can be transferred through
objects and empty space. Electromagnetic.
CONDUCTION
 Liquid - Liquid - Pouring cold cream into coffee
 Liquid - Gas - Ocean and Atmosphere
 Gas - Gas – Cold and warm weather systems mixing
 Solid - Solid – Touch a hot pot on a stove
Photo © Kevin Kennefick 2001
 Contact Area
 Type of Material i.e. Cast Iron vs. Stainless Steel
 Temperature Difference
 Distance heat must travel
CONVECTION
The transfer of thermal energy from a fluid flowing over a
solid object- [Air is a fluid!]
 But, air is a relatively poor conductor of heat
 A solid object = dense arrangement of molecules
 Liquid = less dense arrangement
 Gas = least dense arrangement of molecules
 Transferring heat using a gas is inefficient
 Must pass a lot of molecules over an object to equal the carrying
capacity of a denser material.
 In a closed room cool air will settle to the bottom while
warm air will rise
 Warm air rising through a heat register
 Bowl of soup – Hot liquid in the center moves to the
cooler outside where it drops and is reheated at the
center and the cycle continues.
What is required of convection to occur:
 Air Flow = Pressure Difference + Path (Hole)
Temperature
Wind
Exhaust/Mechanicals
Temperature is typically the dominant effect
RADIATION
A method by which heat can be transferred through
objects and empty space. Electromagnetic.
The transfer of thermal energy or heat that is in
direct line of sight of the object being heated.
 The sun’s heat
 A bonfire
 Warm soil on a cool night
 Surface area
 Temperature difference
 Type of material
Emittance
Reflectivity
 Emittance (or emissivity), refers to the ability of a
material’s surface to give off radiant energy. All
materials have emissivities ranging from zero to one.
The lower the emittance of a material, the lower the
heat radiated from its surface.
 Reflectance (or reflectivity) refers to the fraction of
incoming radiant energy that is reflected from the
surface. Reflectivity and emissivity are related and
effect each other inversely.
 For example, aluminum with a reflectance of 0.97 has
an emittance of 0.03
Material Surface
Emittance
Asphalt
0.90 - 0.98
Aluminum foil
0.03 – 0.05
Brick
0.93
Fiberglass
0.80 – 0.90+
Glass
0.95
Steel
0.12
Wood
0.90
And
Energy Efficiency
As humans we try to maintain a body
temperature of 98.6° F
 Three Mechanisms
 Heat generated within the body
 Heat gained from surroundings
 Heat lost to surroundings
We shiver to
generate heat
We sweat to
Give off heat
We get goose bumps
Blood Flow
 Decreases to hands and feet in winter
 Increase in summer to encourage heat loss
To be comfortable humans must loose heat at the same
rate as it is produced or gained.
 Air temperature
 Air Speed
 Humidity
 Mean radiant temperature
Each has a direct influence on heat loss or gain to the
human body
 Air Temperature - This affects temperature
differences between the body and the surroundings,
consequently affecting the rate of heat loss or gain by
convection.
Air Speed - This affects the rate at which
the body loses heat by convection.
 An air temperature of 35°F and a wind speed of 20
miles/hour combine to give a wind chill temperature of
11.2°F.
 Air speed is also very important during summer when
the body is trying to lose heat to maintain comfort.
Humidity - Affects the rate at which the
body loses heat by evaporation. During hot
weather, high humidity increases discomfort
by making it more difficult to evaporate
perspiration into the air.
 The Chill Factor can be a direct cause of discomfort
 A lesser noticed effect of unbalanced forced air systems is
inducing increased infiltration
 Due to pressure imbalances and duct leaks
 Heating the air in a room does a relatively poor job of
heating solid objects
 Those objects in the room at a temperature lower than one’s body
act to rob the body of heat (through radiation), requiring higher
room temperatures to offset that effect
Mean Radiant Temperature (MRT) - MRT is the average
surface temperature of the surroundings with which the
body can exchange heat by radiant transfer.
Radiant heat transfer to and from the body is quite
apparent when sitting near a fireplace (high MRT) or
large cold window area (low MRT).
72°F
70°F
68°F
59°F
64°F
Mean Radiant Temperature: = 67 °F
72°F
70°F
59°F
59°F
64°F
Mean Radiant Temperature: = 63 °F
 Comfort is achieved by either increasing the ambient
temperature or by raising the mean radiant
temperature of an environment.
 A higher radiant temperature means that people
become comfortable with a lower ambient
temperature and the reverse is also true.
In general for every 1 degree F that MRT drops, the
air temperature must be raised about 1.4 degrees F
to achieve comfort conditions.
How can you raise the MRT?
 Close blinds and curtains
 Solar Film on windows
 Seal heat leaks
 Low-E Windows
 Insulated exterior doors
 Dry Bulb 73°
 Relative Humidity 50%
In the zone
 Dry Bulb Temp. 78°
 Relative Humidity 70%
 Dry Bulb Temp. 78°
 Relative Humidity 70%
 Requires a wind speed
of 250 FPM
(250*60)/5280
MPH = 2.84
 Dry Bulb Temp. = 50°F
 Relative Humidity 55%
 Dry Bulb Temp. = 50°F
 Relative Humidity 55%
BTU/Hour = 250
Cooling:
CONDUCTION
Heating:
CONDUCTION
CONVECTION
RADIATION
CONVECTION
RADIATION
 The basic measure of heat
 The amount of heat needed to raise one pound of
water one degree Fahrenheit
BTU =
A kitchen match contains about 1 BTU of Heat Energy
 1 Kilowatt-hour electricity = 3,413 Btu
 1 cubic foot of natural gas = 1,025 Btu
 1 gallon fuel oil = 138,700 Btu
 1 gallon kerosene = 135,000 Btu
 1 ton of coal = 27,000,000 BTU
 1 gallon LPG = 91,000 Btu
 1 pound LPG = 21,500 Btu
 Power is theINSTANTANEOUS use of energy
 Think of it as POTENTIAL use, whether it is running or
not (engine, light bulb)
 Btu/h
 Watts, Kilowatts (Watts = Volts x Amps)
 the amount of voltage across a circuit x the current through the circuit
 Energy is USE of power over TIME (heat energy)
 Btu/h x hours = Btu
 Watts x hours = Watt hour (Kilowatt x h = kWh)
R VALUE
 R-Value is the measure of resistance to heat flow
through the defined material. The higher the R-Value
the less heat will transfer through the wall, making the
system more energy efficient.
 It’s the most common unit of measure for describing
insulation performance
 It’s the inverse of U-Value
 It represents “Resistance” to heat flow
 R-Value can be added (in thermal path)
 But can’t be averaged over area
U-Value:
 Rate of heat energy (Btu) flowing through 1 SF of
material, per hour, per 1°F temp. difference
 Basis of heat loss calculations
 U-Value is “one over” R–value (U=1/R)
 Smaller U-Value means lower heat loss
 Larger U-Value means higher heat loss
Windows and doors are rated in “U”
U-Values can be averaged over surface areas
but cannot be added in thermal path
SIDING (SEE ELEVATION)
15# BLDG. PAPER (OR TYVEK)
1/2" RATED SHEATHING
2 X 6 STUDS @ 16" O.C.
R-21 BATT INSULATION
1/2" GYPSUM BD.
 Bevel Siding R .80
 ½” Plywood Sheathing R.63
 Wood Studs R1/In.
FLOOR FINISH
1/2" PART. BD. UNDERLAY
3/4" PLYWOOD SUBFLOOR
R-25 BATT INSULATION
CRAWLSPACE
6 MM BLACK "VISQUEEN"
 Insulation R21
 Drywall R .45
8"
2 X 6 P.T. MUDSILL WITH
1/2" A.B. @ 48" O.C.
SLOPE
8"
1'-6"
8"
1'-4"
TYP. WALL SECTION
SCALE: 3/4" = 1'-0"
4" PERFORATED DRAIN
TILE (TYP. WHERE REQ.)
BUILDING
ASSEMBLY PROBLEM
Calculate the theoretical R-Value of a wall assembly
 Wood Stud 5.5” X 1 = R-5.5
 1/5.5 =.1818 U Value
 Insulation R-21
 1/21 = .048 U Value
 Siding = R-.80
 1/.8 = 1.25 U Value
 ½” Wall Sheathing = R.63
 1/.63 = 1.59
 Dry Wall = R-.45
 1/.45 = 2.22 U Value
Walls are framed 16” On Center (Add the R Values)
 14.5” of the wall = .8 +.63 + 21 + .45 = 22.88
 1/23.02 = .0437U Value
 1.5” of the wall = .8 +.63 +5.5+ .45 + = R7.38
 1/7.38 = .1355 U Value
 Stud + Bevel Siding + 1/2 Sheathing + 1/2” Drywall
 5.5 + .8 +.63 + .45 = 7.38
 1/7.38 = .1355 U Value
 Cavity = Insulation + Bevel Siding + ½” Sheathing +
1/2” Drywall
 21 + .8 +.63 + .45 = 22.88
 1/22.88 = .0437 U Value
 UA Calculation
 UA = (14.5 x .0437) + (1.5 x .1355)/16 = .052 U Value
 R Value of the Wall Assembly = R 19
UA CALCULATIONS
“UA” refers to the U-Value, times the area of a given
component
UA is the heat transfer through that component
Example: 1000 SF of R-11 wall (U-Value = 0.091)
U x A = .091 x 1000 = 91 Btu/hr/°F
U x A x ΔT is the heat transfer at a given temperature
Example: Heat loss at 70°F in and 30°F out
91 x 40 = 3640 Btu/hr
A= Area
U = 1/R





R = 1/U
Btu/h = U * A * Δ T
U = Btu/h/ (A * Δ T)
Btu/h = 3.413 * Watts
U = (3.413 * W )/(A * Δ T)
 R = (A * Δ T) / (3.413 * W)





R = R-Value
U = U-Value
A = Surface Area (Must be in Square Feet)
Δ T = Change in Temperature
W = Watts
The change in the internal energy of a system
is equal to the heat added to the system
minus the work done by the system.
ΔU = Q – W
Δ U = Change in internal energy
Q = Heat added to the system
W = Work done by the system
What conditions do we need for radiant heat
transfer to take place?
 Difference in temperature
 An air gap between objects
 Two objects touch have the same temperature
(Conduction)
NO
 U-FACTOR
 Solar Heat Gain Coefficient
 Visible Transmittance
 Air Leakage
U-FACTOR
The rate of heat loss is
indicated in terms of the
U-Factor of a window
assembly. The insulating
value is indicated by the
R-Value which is the
inverse of the U-Value.
The lower the U-Value
the greater a windows
resistance to heat flow
and the better the
insulating value.
The SHGC is the fraction of
incident solar radiation
admitted through a window.
SHGC is expressed as a
number between 0 and 1.
The lower a windows solar
heat gain coefficient, the
less solar heat it transmits.
The visible transmittance is
an optical property that
indicates the amount of
visible light transmitted.
Theoretical values vary
between 0 and 1, but most
values are between 0.3 and
0.8
Heat loss and gain occur
by infiltration through
cracks in the window
assembly.
Air leakage is expressed
in cubic feet of air passing
through a square foot of
window area.
.3 is recommended for
Oregon
 Glass is coated with silver or tin oxide which allows
visible light to pass through but reflects infrared heat
radiation back into the room.
 Reduces heat loss in cool climates
 Allows visible light to pass through but reflects
infrared heat radiation away from the room, admits up
to 40% less radiant heat from the sun.
 Reduces heat gain in warm climates
The lower the
number the better
the insulating
value
Varies from 0 to
1.0 The higher the
# the more light is
transmitted.
High number for
cold climate. Low
number for warm
climates
The best windows
have air leakage
rating between 0.1
and 0.6 cfm/ft.
Single-Glazed
with Clear
Glass
Single-Glazed
with Bronze
or Gray Tinted
Glass
DoubleGlazed with
High Solar
Gain Low-E
Glass,
Argon/Krypto
n Gas
Triple-Glazed**
with Moderate
Solar Gain
Low-E Glass,
Argon/Krypton
Gas