Transcript Energy, Work and Heat

Lesson 3
ENERGY, WORK, AND HEAT
• DEFINE the following:
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Heat
Latent heat
Sensible heat
Units used to measure heat
• DEFINE the following thermodynamic properties:
– Specific enthalpy
– Entropy
Energy
• The capacity of a system to perform work or produce heat
• Categories
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Potential Energy
Kinetic Energy
Specific Internal Energy
Specific P-V Energy
Enthalpy
Work
Heat
Entropy
Potential Energy
• Energy of position
• PE = mgz/gc
where
PE = potential energy (ft-lbf)
m = mass (lbm)
z = height above some reference level (ft)
g = acceleration due to gravity (32.17 ft/sec2)
gc = gravitational constant = 32.17 ft-lbm/lbf-sec2
Kinetic Energy
• Energy of motion
• KE = mv2/2gc
where:
KE = kinetic energy (ft-lbf)
m = mass (lbm)
v = velocity (ft/sec)
gc = gravitational constant = 32.17 ft-lbm/lbf-sec2
Specific Internal Energy
• Internal energy per unit mass
• Equal to total internal energy (U) divided by the
total mass (m).
u = U/m
where:
u = specific internal energy (Btu/lbm)
U = internal energy (Btu)
m = mass (lbm)
• Internal energy includes
– Energy due to rotation, vibration, translation, and interactions among
molecules
– Can not be measured or evaluated directly
Specific P-V Energy
• Energy per unit mass.
• Equals the total Pressure times the Volume of a liquid
divided by the total mass m
• Also equals the product of the pressure P and the specific
volume v, and is written as Pv
Pv = PV/m
where:
P = pressure (lbf/ft2)
V = volume (ft3)
n = specific volume (ft3/lbm) V
m
m = mass (lbm)
Enthalpy
• A property of a substance, like pressure, temperature,
and volume,
• Cannot be measured directly
• Normally given with respect to some reference value.
• Usually used in connection with an "open" system
problem in thermodynamics
• Specific enthalpy (h)
h = u + Pv
where
u is the specific internal energy (Btu/lbm)
P is the pressure of the system (lbf/ft2)
v is the specific volume (ft3/lbm) of the system.
Work
• Energy in transit
• Not a property of a system – it is a process
done by or on a system
• Defined as the action of a force on an object
through a distance
W = Fd
where:
W = work (ft-lbf)
F = force (lbf)
d = displacement (ft)
Heat
• Like work, is energy in transit
• Occurs at the molecular level as a result of a
temperature difference
• Denoted by the letter Q
• Heat transferred per unit mass denoted by q
q = Q/m
where:
q = heat transferred per unit mass (Btu/lbm)
Q = heat transferred (Btu)
m = mass (lbm)
Heat (continued)
• Sensible Heat - The heat added to or removed
from a substance to produce a change in its
temperature
• Latent Heat - The amount of heat added to or
removed from a substance to produce a change
in phase.
– Latent Heat of Fusion - the amount of heat added or
removed to change phase between solid and liquid.
– Latent Heat of Vaporization - the amount of heat
added or removed to change phase between liquid
and vapor
Heat Capacity
• The ratio of the heat (Q) added to or removed
from a substance to the change in
temperature (ΔT) produced
• Denoted by Cp
• Specific heat (cp) is heat capacity of a
substance per unit mass
• Applies when the heat is added or removed at
constant pressure
Heat Capacity (continued)
Cp = Q/ΔT
cp = Q/mΔT
cp = q/ΔT
where:
Cp = heat capacity at constant pressure (Btu/°F)
cp = specific heat at constant pressure (Btu/lbm-°F)
Q = heat transferred (Btu)
q = heat transferred per unit mass (Btu/lbm)
m = mass (lbm)
ΔT = temperature change (°F)
Entropy (S)
• A property of a substance
• Quantifies the energy of a substance that is no longer available to perform
useful work
ΔS = ΔQ/Tabs
Δ s = Δq/Tabs
where:
Δ S = the change in entropy of a system during some process (Btu/°R)
ΔQ = the amount of heat transferred to or from the system during the process
(Btu)
Tabs = the absolute temperature at which the heat was transferred (°R)
Δs = the change in specific entropy of a system during some process
(Btu/lbm -oR)
Δq = the amount of heat transferred to or from the system during the process
(Btu/lbm)
Power
• Power - The time rate of doing work.
• Units are energy per unit time
– English system - ft-lbf/sec or ft-lbf/hr
– Horse Power - hp
– British thermal units per hour - Btu/hr
– Electrical units - watts (W) or kilowatts (kW)
Energy and Power Equivalencies
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1 ft-lbf = 1.286 x 10-3 Btu = 3.766 x 10-7 kW-hr
1 Btu = 778.3 ft-lbf = 2.928 x 10-4 kW-hr
1 kW-hr = 3.413 x 103 Btu = 2.655 x 106 ft-lbf
1 hp-hr = 1.980 x 106 ft-lbf
1 Joule = 778 ft lbf/Btu
1 ft-lbf/sec = 4.6263 Btu/hr = 1.356 x 10-3 kW
1 Btu/hr = 0.2162 ft-lbf/sec = 2.931 x 10-4 kW
1 kW = 3.413 x 103 Btu/hr = 737.6 ft-lbf/sec
1 hp = 550.0 ft-lbf/sec