the working substance

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THE WORKING

SUBSTANCE



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



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.



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



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



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



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



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.



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:



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.

the working substance

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