Movement of Thermal Energy: Heat Flow

1
Heat/Energy Transfer

• 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

2
Three Types of Heat Transfer
3
Definitions

• 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.

4

• CONDUCTION

5
Conduction Examples

• 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

6
(No Transcript)
7
Conduction Rate Factors

• Contact Area
• Type of Material i.e. Cast Iron vs. Stainless
  Steel
• Temperature Difference
• Distance heat must travel

8

• CONVECTION

9
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.

10
Convection Examples

• In a closed room cool air will settle to the
  bottom while warm air will rise
• Warm air rising through a heat register

11
Convection Examples

• 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.

12
CONVECTION

• What is required of convection to occur
• Air Flow Pressure Difference Path (Hole)

13
The Stack Effect
14
(No Transcript)
15
Convection
Temperature is typically the dominant effect
16

• RADIATION

17
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.

18
Radiation Examples

• The suns heat
• A bonfire
• Warm soil on a cool night

19
Radiation Rate Factors

• Surface area
• Temperature difference
• Type of material
• Emittance
• Reflectivity

20
Radiation Terms

• Emittance (or emissivity), refers to the ability
  of a materials 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.

21
Radiation Terms

• 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

22
Emissivity or Emittance
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
23
(No Transcript)
24
HUMAN COMFORT

• And
• Energy Efficiency

25
Human Comfort Zone

• 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

26
Human Comfort Zone

• We shiver to
• generate heat

27
Human Comfort Zone

• We sweat to
• Give off heat

28
Human Comfort Zone

• We get goose bumps

29
Human Comfort Zone

• Blood Flow
• Decreases to hands and feet in winter
• Increase in summer to encourage heat loss

30
Thermal Neutrality

• To be comfortable humans must loose heat at the
  same rate as it is produced or gained.

31
Factors Affecting Human Comfort

• Air temperature
• Air Speed
• Humidity
• Mean radiant temperature
• Each has a direct influence on heat loss or gain
  to the human body

32
Factors Affecting Human Comfort

• Air Temperature - This affects temperature
  differences between the body and the
  surroundings, consequently affecting the rate of
  heat loss or gain by convection.

33
Factors Affecting Human Comfort

• Air Speed - This affects the rate at which
• the body loses heat by convection.
• An air temperature of 35F and a wind speed of 20
  miles/hour combine to give a wind chill
  temperature of 11.2F.
• Air speed is also very important during summer
  when the body is trying to lose heat to maintain
  comfort.

34
Factors Affecting Human 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.

35
The Chill Factor

• 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 ones body act to rob the body of heat
  (through radiation), requiring higher room
  temperatures to offset that effect

36
Factors Affecting Human Comfort

• 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).

37
Mean Radiant Temperature
38
Cold Surfaces
39
Comfort

• 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.

40
Mean Radiant Temperature

• 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

41
Bioclimate Chart
42
Example 1

• Dry Bulb 73
• Relative Humidity 50

43
In the zone
44
Example 2

• Dry Bulb Temp. 78
• Relative Humidity 70

45
(No Transcript)
46
Example 2

• Dry Bulb Temp. 78
• Relative Humidity 70
• Requires a wind speed of 250 FPM
• (25060)/5280
• MPH 2.84

47
Example 3

• Dry Bulb Temp. 50F
• Relative Humidity 55

48
(No Transcript)
49
Example 3

• Dry Bulb Temp. 50F
• Relative Humidity 55
• BTU/Hour 250

50
What Does All This Mean?
51
Relative Importance of Usage
52
British Thermal Units

• The basic measure of heat
• The amount of heat needed to raise one pound of
  water one degree Fahrenheit

53
Heat Content of Fuels

• 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

54
Energy and Power

• 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)

55

• R VALUE

56
R-Value Resistance

• 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.

57
R-Value Resistance

• Its the most common unit of measure for
  describing insulation performance
• Its the inverse of U-Value
• It represents Resistance to heat flow
• R-Value can be added (in thermal path)
• But cant be averaged over area

58
U Value Measure of Heat Loss

• U-Value
• Rate of heat energy (Btu) flowing through 1 SF
  of material, per hour, per 1F temp. difference
• Basis of heat loss calculations
• U-Value is one over Rvalue (U1/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

59
R Value in Assemblies

• Bevel Siding R .80
• ½ Plywood Sheathing R.63
• Wood Studs R1/In.
• Insulation R21
• Drywall R .45

60

• BUILDING
• ASSEMBLY PROBLEM
• Calculate the theoretical R-Value of a wall
  assembly

61
R Value Calculation for an 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

62
R Value Calculation for an Assembly

• 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

63
R Value Calculation for an Assembly

• 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

64

• UA CALCULATIONS

65
UA

• 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 ?T is the heat transfer at a given
  temperature
• Example Heat loss at 70F in and 30F out
• 91 x 40 3640 Btu/hr
• A Area
• U 1/R

66

• R 1/U
• Btu/h U A ? T
• U Btu/h/ (A ? T)
• Btu/h 3.413 Watts
• U (3.413 W )/(A ? T)
• R (A ? T) / (3.413 W)
• R R-Value
• U U-Value
• A Surface Area (Must be in Square Feet)
• ? T Change in Temperature
• W Watts

67
1st Law of Thermodynamics

• 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.
• ?U Q W
• ? U Change in internal energy
• Q Heat added to the system
• W Work done by the system

68
Keeping it Real
69
(No Transcript)
70

• 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)

71
Can this product stop radiant heat transfer?

• NO

72
(No Transcript)
73
WINDOWS - 4 Ways to Evaluate

• U-FACTOR
• Solar Heat Gain Coefficient
• Visible Transmittance
• Air Leakage

74
U-FACTOR
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.
75
Solar Heat Gain COEFFICIENT
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.
76
Visible Transmittance
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
77
Air Leakage
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
78
Low-E Windows

• 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

79
High number for cold climate. Low number for warm
climates
The lower the number the better the insulating
value
The best windows have air leakage rating between
0.1 and 0.6 cfm/ft.
Varies from 0 to 1.0 The higher the the more
light is transmitted.
80
Single-Glazed with Clear Glass
81
Single-Glazed with Bronze or Gray Tinted Glass
82
Double-Glazed with High Solar Gain Low-E Glass,
Argon/Krypton Gas
83
Triple-Glazed with Moderate Solar Gain Low-E
Glass, Argon/Krypton Gas
84
(No Transcript)