the working substance
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THE WORKING
SUBSTANCE
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DEFINITIONS:
Thermodynamics – is the study of heat and work and those properties of
substance that bear a relation to heat and work
Working Substance – a substance to which heat can be stored and from which
heat can be extracted
a. Pure Substance – a working substance whose chemical composition remains
the same even if there is a change in phase. Ex. Water, ammonia, Freon-12
b. Ideal Gas – a working substance which remains in gaseous state during its
operating cycle and whose equation of state is PV = mRT ; Ex. Air, O2, N2,
CO2
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PROPERTIES OF A WORKING SUBSTANCE:
1. Mass and Weight
Mass – a property of matter that constitutes one of the fundamental physical
measurements or the amount of matter a body contains. Units of mass are in
lbm, slugs, kgm, or in kg.
Weight – the force acting on a body in a gravitational field, equal to the product
of its mass and the gravitational acceleration of the field. Units of weight are inlbf , kgf , N or kN.
2. Volume
Volume – the amount of space occupied by, or contained in a body and is
measured by the no. of cubes a body contains. Units of volume are in ft3,
Gallons, liters, cm3, or m3.
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3. Pressure = force per unit area. Units of pressure are measure in psi, kg/cm2,
kN/m2, or in kPa.
Absolute Pressure = Gauge Pressure + Absolute Atmospheric Pressure
kPaa = kPag + 101.325
Psia = Psig + 14.7
1 Atm Pressure = 0 kPag, 0 psig
= 101.325 kPa= 1.033 kg/cm2
= 29.92 in Hg
= 760 mm Hg
= 14.7 psia
1 bar = 100 kPa
Pressure of Perfect Vacuum = - 101.325 kPag= absolute zero pressure
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4. Temperature
Temperature – the degree of hotness or coldness of a substance
Relations of Temperature Scales, oC and oF:
oC = 5/9 ( oF – 32 )
oF = 9/5oC + 32
Temperature at which molecules stop moving= - 273oC = - 460 oF
Absolute Temperatures:oK = oC + 273oR = oF + 460
Temperature Change or Temperature Difference:
∆oC = 5/9 ∆oF ∆oK = ∆oC
∆oF = 9/5 ∆oC ∆oR = ∆oF
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THERMAL ENERGY VERSUS TEMPERATURE
Thermal Energy is kinetic energy in transit from
one object to another due to temperaturedifference. (Joules)
Temperature is the average kinetic energy of
particles in an object – not the total amount of kinetic energy particles. (Degrees)
Temperature #1 Temperature #2
Heat
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TEMPERATURE SCALES
Scale Freezing
point of water
Boiling point
of water
Celsius 0°C 100°C
Fahrenheit 32°F 212°F
Kelvin 273K 373K
Matter is made up of molecules in motion (kinetic
energy)
An increase in temperature increases motion
A decrease in temperature decreases motion
Absolute Zero occurs when all kinetic energy isremoved from a object 0 K = -273° C
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5. Specific Volume, Density, and Specific Weight
Density, p = Mass kg/m3
Volume
Specific Volume, v = Volume = 1 m3/kg
Mass p
Specific Weight, w = Weight kN/m3
Volume
Specific Gravity of a liquid = Density of Liquid
(Relative Density) Density of Water
Density of water = 1000 kg/m3 = 9.81 kN/m3 = 62.4 lb/ft3
Specific Gravity of a Gas = Density of a Gas(Relative Density) Density of Air
Density of air = 1.2 kg/m3 at 101.325 kPa and 21.1 oC.
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1. A water temperature rise of 18oF in the water cooled condenser is equivalent ino
C to:
A. 7.78oC C. 263.56oK
B. 10oC D. -9.44oC
Solution:
Temperature Change, ∆oC = 5/9∆oF = 5/9(18) = 10oC
2. An iron block weighs 5 N and has a volume of 200 cm3. What is the density of
the block?
A. 988 kg/cu.m C. 2550 kg/cu.m
B. 1255 kg/cu.m D. 800 kg/cu.m
Solution:
Density = Mass = 5N x kg x (100)3cm3
Volume 200 cm3 9.81 N m3
= 2548 kg/m3
EXAMPLES:
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PROBLEM SOLVING:
1. A fluid moves in a steady flow manner between two sections in a flow line. At
section 1: A1 = 10 ft2, v1 = 100 fpm, v 1 = 4 ft3/lb. At section 2: A2 = 2 ft2, p2 =
0.20 lb/ft3. Calculate (a) the mass flow rate and (b) the speed at section 2.
2. Convert the following readings of pressure to kPa absolute, assuming that the
barometer reads 760 mm Hg: (a) 90 cm Hg gage; (b) 40 cm Hg vacuum; (c) 100
psig; (d) 8 in. Hg vacuum, and (e) 76 in. Hg gage.