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ALCORCON ENGINEERING REVIEW CENTER
Cebu Main: 4th^ floor Coast Pacific Downtown Center, Sanciangko St, Cebu City Tel #(032) 254- 33 - 84 Manila: 3rd^ floor JPD Bldg 1955, C M Recto corner N. Reyes St, Sampaloc, Manila Tel # (02) 736- 4438
POWER PLANT ENGINEERING – DAY 1
I. THERMODYNAMICS
Thermodynamics is the study of heat and work. A pure substance is a working substance whose chemical composition remains the same even if there is a change in phase. (most common example is water) An ideal gas is a working substance which remains in gaseous state during its operating cycle and whose equation of state is PV = mRT. (most common example is air)
PROPERTIES OF WORKING SUBSTANCE
1. PRESSURE
Pressure , in general, is the ratio of force per unit area. Gage pressure is the pressure reading from the gage pressure instrument which is higher or lower than the atmospheric pressure. Vacuum pressure is a pressure that is less than atmospheric pressure. Pressure = , KN/m^2 , lb/in^2 Absolute pressure (Pabs) = Gauge pressure + Atmospheric Pressure Pabs = Pg + Patm A. Pressure Conversions 1 atm = 0 kPag = 101.325 kPa = 14.7 psi = 29.92 in Hg = 760 mm Hg B. Hydrostatic Pressure, P P = w h where: w = density of fluid = (SG x ww) h = height of fluid ww = density of water = 1000 kg/m^3 = 9.81 KN/m^3 = 62.4 lb/ft^3 = 1 kg/li Area Force P Ih Pabs Pabs Pgage Pvac Iatmospheric Above atmospheric Below atmospheric
P h
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2. TEMPERATURE
Temperature is the degree of hotness or coldness of a substance or body. A thermometer is an instrument used to measure the temperature of a body or a substance. A pyrometer is an instrument used to measure high temperature gases.
A. Relation between ° C and ° F scales
°C = 5/9 (F – 32) °F = 9/ 5 C + 32
B. Absolute temperatures: °K = °C + 273 °R = °F + 460 C. Temperature Difference: D°C = 5/9 D°F D°F = 9/5 D°C D°C = D°K D°F = D°R D. Absolute-Zero Temperature = - 273 oC = - 460 oF
3. DENSITY, SPECIFIC VOLUME AND SPECIFIC GRAVITY:
Density is defined, in thermodynamic terms, as the mass per unit volume of a substance Specific volume is the volume per unit of mass in a substance. It is also the reciprocal of density. Specific gravity is the ratio of the density of a substance to the density of a standard (water for liquid and air for gas). A. w = B. v = C. SG = If two fluids are mixed together: wm = vm = SGm = SGm = specific gravity of mixture wm = density of mixture
4. INTERNAL ENERGY(U)
Internal energy is the heat energy due to the movement of molecules within the brought about its temperature. v
V
m
w
m
V
Densityofwater Densityofany fluid 1 1 2 2 1 2 1 2 1 2 m/v m / v m m V V m m
1 2 1 1 2 2 1 2 1 2 m m m/v m / v m m
V V
water m w w
This file is only for viewing and printing. II. CONSERVATION OF MASS AND ENERGY The law of conservation of mass states that the total mass is a constant. This means that the total mass entering to a system is equal to the total mass leaving.
FORMS OF ENERGY
1. Potential Energy, P - is an energy produced due to the change in elevation.
P = m g h DP = P 2 - P 1 = mg(h 2 - h 1 ) where: m = mass of the body h = height or elevation Unit Analysis: (SI Unit – English Unit) , kJ, Btu kW, Btu/min m Not known (m = 1 kg/kg, 1lb/lb) kg, lb kg/s, lb/min h m, ft m, ft m, ft
2. Kinetic Energy, KE - is an energy produced due to the mass and velocity.
where: m = mass W = weight v = velocity g = acceleration due to gravity
3. Work, W - is the product of the displacement of the body and the component of the force in the
direction of the displacement. Work = Force x Distance Note: 1. Work done by the system is positive (out from the system)
- Work done on the system is negative (entering to the system)
5. Heat, Q - heat is formed due to the temperature difference.
Q = mcp(t 2 - t 1 ) where: m = mass cp = specific heat t = temperature Note: 1. Heat is positive when heat is added to the system.
- Heat is negative when rejected from the system. III. LAWS OF THERMODYNAMICS
1. Zeroth Law of Thermodynamics
The zeroth law of thermodynamics states that when the two bodies are in thermal equilibrium with the third body, they are in thermal equilibrium with each other and hence are at the same temperature.
mgh
PE =
mh
PE =
kg
kJ
lb
Btu
mv^2 2
KE = m(v v ) 2
D KE = 12 - 22 m(v v ) 2
D KE = 12 - 22
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2. First Law of Thermodynamics
States that one form of energy may be converted into another form. All energy entering = All energy leaving A. P 1 + K 1 + Wf1 + U 1 + W = P 2 + K 2 + Wf2 + U 2 + Q B. W = (P 2 - P 1 ) + (K 2 - K 1 ) + (U 2 - U 1 ) + (Wf2 - Wf1) + Q C. W = DP + DK + DU + DWf + Q But: Dh = DU + DWf D. W = DP + DK + Dh + Q E. Neglecting Potential energy, Kinetic energy & Heat: W = Dh = h 2 - h 1
3. Second Law of Thermodynamics
Kelvin-Planck Statement: “It is impossible to construct a heat engine which operates in a cycle and receives a given amount of heat from a high temperature body and does an equal amount of work”
4. Third Law of Thermodynamics
States that the absolute entropy of a pure crystalline substance in complete internal equilibrium is zero at zero degrees absolute. IV. IDEAL GAS Ideal gas is a substance that has the equation of state:
PV = mRT PV = 8.314 n T
where: P = absolute pressure m = mass of gas V = volume of gas R = gas constant T = absolute temperature n = number of mols
Boyle’s Law: (T = C ) P 1 V 1 = P 2 V 2
Charles’s Law : (P = C and V = C)
BASIC PROPERTIES OF IDEAL GAS
1. Relationship between cp, cv, R and k A. cp = cv + R B. cv = C. cp = D. k = k = 1.4 for cold air k = 1.3 for hot air where: cp = constant pressure specific heat cv = constant volume specific heat k = specific heat ratio R = gas constant For air: cp = 1.0 KJ/kg-K = 0.24 Btu/lb-R cv = 0.7186 KJ/kg-K = 0.171 Btu/lb-R R = 0.287 KJ/kg-K = 53.3 ft-lb/lb-R 2 1 2 1 T
T
V
V
2 1 2 1 T
T
P
P
k 1
R
Rk
c
c
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- A pipe has a diameter of 4” at section AA, and a diameter of 2” at section BB. For an ideal fluid flow, the velocity is given as 1 ft/sec at section AA. What is the flow velocity at section BB? A. 4 ft/sec B. 0.5 ft/sec C. 1.0 ft/sec D. 2.0 ft/sec
CONSERVATION OF ENERGY (FIRST LAW)
- Steam turbine is receiving 1014 lbm/hr of steam, determine the horsepower output of the turbine if the work done by steam is 251 Btu/lbm A. 100 Hp B. 462.7 Hp C. 200 Hp D. 6002.7 Hp
- Steam enters a turbine stage with an enthalpy of 3628 kJ/kg at 70 m/s and leaves the same stage with an enthalpy of 2846 kJ/kg and a velocity of 124 m/s. Calculate the power if there are 5 kg/s steam admitted at the turbine throttle? A. 4597.45 kW B. 3976.55 kW C. 3883.81 kW D. 1675.42 kW
IDEAL GAS
- If the initial volume of an ideal gas is compressed to one-half its original volume and to twice its temperature, the pressure: A. Doubles B. Quadruples C. Remains constant D. Halves
- Find the mass of carbon dioxide having a pressure of 20 psia at 200oF with 10 ft^3 volume. A. 1.04 lbs B. 1.14 lbs C. 1.24 lbs D. 1.34 lbs
- Find the work possessed for a Helium gas at 20oC. A. 609 KJ/kg B. 168 KJ/kg C. 229 KJ/kg D. 339 KJ/kg
