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Khairul Nizar Ismail
EAT 257
7 March 2013
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Objectives Find R and U values for building compenent
Select appropriate indoor and outdoor design
condition Calculate room and building heat transfer losses
Determine room and building heating loads
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The Energy Equation (First Law of
Themodynamics) The first Law of Themodynamics is a principle that
may be stated in various ways for instance energy canneither be created nor destroyed or there isconservation of energy in nature
This principle is used extensively in the HVACindustry especially when stated as an energy balance :
The change in total energy in a system equals theenergy added to the system minus the energy removedfrom the system
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Energy Equation The energy balance can be expressed as an equation,
called the Energy Equation:
Ech
= Ein
- Eout
Ech= Change in stored energy
Ein = Energy added (entering) the system
Eout= Energy removed from (leaving) the system
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Example 1A hot water heating convector in a office is supplying
4000 BTU/hr of heat. Heat is being transferred fromair to the outdoors at rate of 6500 BTU/hr. What willhappen in the room? What size electric heater shouldthe office temporarily use to solve the emergency?
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Solution 1Ech= EinEout
Ech= 4000 BTU/hr 6500 BTU/hr
Ech= - 2500 BTU/hr
The negative sign means the room air energy isdecreasing
The room temperature will be dropped
A solution is to install an electric heater that will makeup the heat loss of 2500 BTU/hr
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Cont Solution 1 The capacity of an electric heater is nomarlly
expressed in watts (W) or kilowatts (KW) rather thenBTU/hr
The heater should therefore have the followingcapacity
3410 BTU/hr = 1000 watts
2500 BTU/hr x 1000 watts = 733 watts3410 BTU/hr
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Example 2A classroom has 1000 watts of lighting and some small
motors with a total output of 10 HP. All of the energyin the lighting and from the motors is converted intoheat. What is the increase in enthalpy of the classroomair from the sources?
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Solution 2 The energy added to the classroom air will increse its
enthalpy
Applying the Energy Equation & converting all units toBTU/hr
Ech= EinEout
Ech= 1000 watts x 3.41 BTU/hr + 10 HP x 2545 BTU/hr - 0
1 watts 1 HPEch= 28, 860 BTU/hr
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Defination Conduction- is the form of heat transfer through a
body that occurs without any movement of the body, itis result of molecular or electron action
Convection- is the form of heat transfer that resultsfrom gross movement of liquids or gases
Thermal Radiation- is the form of heat transfer that
occurs between two separated bodies as a result of ameans called electromagnetic radiation, sometimescalled wave radiation
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Heat Conduction
Key Question:
How does heat passthrough differentmaterials?
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Heat Transfer The science of how heat flows is called heat transfer.
There are three ways heat transfer works: conduction,convection, and radiation.
Heat flow depends on the temperature difference.
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Thermal Equilibrium Two bodies are in thermal
equilibriumwith each other
when they have the sametemperature.
In nature, heat alwaysflowsfrom hot to cold until
thermal equilibrium isreached.
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Heat Conduction Conductionis the transfer of heat through materials
by the direct contact of matter.
Dense metals like copper and aluminum are very goodthermal conductors.
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Heat ConductionA thermal insulatoris a material that conducts heat
poorly.
Heat flows very slowly through the plastic so that thetemperature of your hand does not rise very much.
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Heat Conduction Styrofoam gets its
insulating ability bytrapping spaces of airin bubbles.
Solids usually are betterheat conductors than
liquids, and liquids arebetter conductors thangases.
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Heat Conduction The ability to conduct heat
often depends more on thestructure of a material than
on the material itself. Solid glass is a thermal
conductor when it isformed into a beaker or
cup.
When glass is spun intofine fibers, the trapped airmakes a thermal insulator.
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Thermal Conductivity The thermal conductivityof a material describes how
well the material conducts heat.
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Thermal
Conductivity
Heat conduction insolids and liquidsworks by transferringenergy through bonds
between atoms ormolecules.
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There are three ways heat transferworks: conduction, convection,
and radiation
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Rate of Heat Transfer The rate at which heat is conducted through any
material depends on three factors:
1) The temperature difference across which the heatflows
2) The area of the surface through which heat is flowing
3)The thermal resistance (R) of the material to heat
transfer
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Heat transfer equation Q = 1 x A x TD
R
Q = heat tranfer rate (BTU/hr)
R = thermal resistance of material
A = surface area through which heat flow
TD = temperature difference across heat flow
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Example 3 Determine the heat transmission coefficient for
building wall section:
Face brick : 0.44
Concrete : 1.11
Insulation : 11.0
Drywall : 0.32
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Solution 3 U-Factor = 1/R
U-factor = 1/ 0.44+1.11+11.0+0.32
U-factor = 1/12.87
U-factor = 0.077 BTU/h/sq ft/F
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Example 4 Determine the heat loss through the wall described in
Example 3. The wall is 30 feet wide by 12 feet high. Theheating design indoor air is 70 F and heating design
outdoor air is 5 F
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Solution 4 Q = U x A x T
Q = 0.077 BTU/hr/sq ft/F x 360 sq ft x 65 F
Q = 1801.8 BTU/hr
The heat loss through the wall 1801.8 BTU/hr
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Load calculator
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