chapter (2)

‫الجامعة المستنصرية – كلية الھندسة‬ ‫قســــم الھندســــــــة الميكـــــانيكيـــة‬ ‫محطــــــــــــات طاقــــــــــــــــــــــة‬

‫ )‪CHAPTER (2‬‬ ‫ ‪Steam Power Plant Cycles‬‬ ‫‪ ‬‬

‫‪ ‬أ‪.‬م‪.‬د‪ .‬مشعل عبد االمير‬ ‫‪05‐Oct‐14 ‬‬

‫‪ ‬‬

‫‪ ‬‬ ‫‪ ‬‬

Chapter 2 Steam Power Plant Cycles Simple vapor cycle and its components: Boiler, Turbine, Condenser, Pump, Carnot Cycle for Steam, Disadvantages of Carnot Cycle application, Rankine Cycle and its components: Steam Generator, Turbine, Condenser, Feed Pump, Cycle net work, Work Ratio, Steam Rate, Heat Rate, Efficiencies in Steam Power Plant: Thermal efficiency, Mechanical efficiency, Isentropic efficiency, Example (1), Mean Temperature of Heat Addition, Effect of Superheat, Effect of inlet pressure, Reheat, Example (2), Regeneration, The Stirling Cycle, The ideal regenerative cycle, Type of regenerative feed water heaters, Open type regenerative feed water heaters, Example (3), Closed type regenerative feed water heaters with drain cascaded backward, Closed type regenerative feed water heaters with drain pumped forward, Example (4), Cogeneration: Back pressure Turbine, Pass-Out Turbine, Example (5), Tutorial Sheet 2.

Power Plant Engineering

‫الجامعة المستنصرية – كلية الھندسة‬

TUTORIAL SHEET ‐ 2

Steam Power Plant Cycles ‫قسم الميكانيك – محطات طــــــــاقة‬ 1‐

Draw a simple sketch then show its processes on the (T‐s) and (h‐s) diagrams for each ideal cycle listed in the table below. Cycle

Type

A B

Rankine Rankine

C

Reheat

D

Regenerative

Steam Condition Turbine inlet

Reheat Outlet

Condenser

4 bar, Dry 60 bar, 410 Co 100 bar, 410 Co 60 bar, 410 Co

‐ ‐

Direct Contact FWH

0.035 bar 0.07 bar

Drain Cascaded Backward ‐ ‐

Drain Pumped Forward ‐ ‐

60 bar, 410 Co

0.07 bar

‐

‐

‐

‐

0.07 bar

‐

‐

20 bar

‐ ‐

10 bar

E

Regenerative

60 bar, 410 Co

‐

0.07 bar

20 bar

‐

10 bar

F

Regenerative

60 bar, 410 Co

‐

0.07 bar

‐

20 bar

10 bar

G

Regenerative

60 bar, 410 Co

‐

0.07 bar

20 bar

10 bar

‐

H

Regenerative

60 bar, 410 Co

‐

0.07 bar

‐

‐

I

Regenerative

60 bar, 410 Co

‐

0.07 bar

20 bar

‐

20 bar 10 bar 2 bar 2 bar

60 bar, 410 Co

‐

20 bar

2 bar

‐

0.07 bar

20 bar

10 bar

2 bar

25 bar, 410 Co

0.07 bar

‐

‐

‐

12 bar 2 bar 0.5 bar 0.5 bar

J

Regenerative

10 bar 0.07 bar

‐

10 bar

K

Regenerative

L

Reheat‐ Regenerative

60 bar, 410 Co 100 bar, 410 Co

M


100 bar, 410 Co

25 bar, 410 Co

0.07 bar

N


100 bar, 410 Co

25 bar, 410 Co

0.07 bar

12 bar 2 bar ‐

O


100 bar, 410 Co

25 bar, 410 Co

0.07 bar

12 bar

40

Closed FWH

12 bar 2 bar 2 bar

0.5 bar 0.5 bar





Draw all the details of the (T‐s) diagram of the ideal steam power plant layout shown in the figure below.

3‐

For the following steam cycles that are listed in the table below, find (a) in (kj/kg) (b) in (kj/kg) (c) in (kj/kg) (d) (e) S.S.C in (kg/kW.hr) and (f) moisture fraction at the end of the turbine process. Show the results in tabular form with your comments. Cycle

Type

A B

Ideal Rankine, Neglect WP 75% Assume Ideal Rankine Ideal Rankine Ideal Rankine Reheat to 600 Co at maximum I.P to limit end moisture to 15% Reheat to 600 Co at maximum I.P to limit end moisture to 15% but with 75% Single open feed water heater at 110 Co Two open feed water heaters at 90 Co and 135 Co Two closed feed water heaters with drain cascaded backward at 90 Co and 135 Co

C D E F G H I J

Boiler Outlet

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Condenser Pressure 10 bar, Saturated 1 bar 10 bar, Saturated 1 bar 10 bar, Saturated 10 bar, 300 Co 160 bar, 600 Co 160 bar, 600 Co 160 bar, 600 Co

0.1 bar 0.1 bar 0.1 bar 0.1 bar 0.1 bar

10 bar, Saturated 10 bar, Saturated 10 bar, Saturated

0.1 bar 0.1 bar 0.1 bar





5‐

6‐

7‐

8‐

In an ideal steam cycle, steam at (20 bar, 360 Co) is expanded in a turbine to (0.08 bar). It then enters a condenser where it is condensed to saturated liquid water. Then the pump feeds back the water into the boiler. Find per kg of steam (a) (b) (c) if 80% , find the percentage reduction in and . Ans. (a) 969.61 (kj/kg) (b) 32.5% (c) 20.1 % (d) 20.1 % Neglect the feed pump term, compare the Rankine cycle efficiency of a high pressure steam plant operating at (80 bar) with that of a low pressure plant at (40 bar). In both cases, the maximum temperature is (400 Co) and the condenser pressure is (0.07 bar). Explain your result briefly. Also find the ratio of the heat rejected from the condensing plants. Assume that the turbines are to produce the same power. Ans. 0.391, 0.357, 0.83 (H.P/L.P) An ideal reheat cycle works between pressures of (120 and 0.035 bar). The steam is superheated to (570 Co) and after expansion to the dry saturated state is reheated to (500 Co). Calculate the thermal efficiency of the cycle taking account of the feed pump work. Ans. 0.453 A steam turbine is supplied with steam at (40 bar, 450 Co) and exhausted at (0.035 bar). Neglecting the feed pump term, find the ideal Rankine cycle efficiency of the plant, and compare it with the ideal efficiency which would be obtained if regenerative feed water heating were employed. Assume the use of open heaters and (a) One bleeding point (3 bar), (b) Three bleeding points at (15, 3, and 0.5 bar). Ans. 0.391, (a) 0.173 kg, 0.415, (b) 0.114, 0.083, 0.08 kg, 0.429 Saturated steam at (30 bar) enters a high pressure turbine and expands isentropically to a pressure at which its dryness fraction is (0.841%). At this pressure, the moisture is extracted and returned to the boiler via feed pump. The remainder, assumed to be dry steam, is expanded isentropically to (0.04 bar) in a low pressure turbine, and the condensate is returned to the boiler via a second feed pump. Calculate the cycle efficiency when the isentropic efficiency of each turbine and feed pump is (0.8). What is then the dryness fraction at the exit of each turbine? Ans. 0.357, 0.291, 0.88, and 0.871.

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Steam Power Plant Cycles ‫قسم الميكانيك – محطات طــــــــاقة‬ A steam turbine gets its supply of steam at (70 bar), (450 Co). After expanding to (25 bar) in high pressure stages, it is reheated to (420 Co) at the constant pressure. Next, it is expanded in intermediate pressure stages to the extraction point for a direct‐contact feed water heater to an appropriate minimum pressure such that part of the steam bled at this pressure heats the feed water to a temperature of (180 Co). The remaining steam expands from this pressure to a condenser pressure of (0.07 bar) in the low pressure stage. The isentropic efficiency of the high pressure stage is (78.5%), while that of the intermediate and low pressure stages is (83%) each. Draw a simple sketch of the cycle and show its processes on the (T‐s) and (h‐s) diagrams, then determine the following. (a) The minimum pressure at which bleeding is necessary. (b) The quantity of the steam bled per kg of flow at the turbine inlet. (c) The thermal efficiency of the cycle. (Neglect pumps work). Ans. (a) 10 bar, (b) 0.206 kg/kg steam flow at the turbine inlet, (c) 35.92%. 10‐ In an ideal reheat cycle, steam initially at (150 bar, 550 Co) expands in a turbine to a condenser pressure of (0.1 bar), and the moisture in the condenser inlet is (5%). Determine (a) The reheat pressure, (b) The cycle efficiency, and (c) The steam rate. Ans. (a) 13.5 bar, (b) 43.65%, (c) 205 kg/kW.hr. 11‐ The net power output of an ideal reheat‐regenerative cycle is (80 MW). Steam enters the HP turbine at (80 bar) and (500 Co), and expands till it becomes saturated vapor. Some of the steam then goes to an open feed water heater and the balance is reheated to (400 Co), after which it expands in the LP turbine to (0.07 bar). Determine (a) The reheat pressure, (b) The steam mass flow rate to the HP turbine, and (c) The thermal efficiency. Ans. (a) 6.5 bar, (b) 58.4 kg/s, and (c) 43.7%. 12‐ The net power output of an ideal reheat‐regenerative cycle is (100 MW). Steam enters the HP turbine at (90 bar) and (550 Co). After expansion to (7 bar), some of the steam goes to an open feed water heater and the balance is reheated to (400 Co), after which it expands in the LP turbine to (0.07 bar). Determine (a) The steam mass flow rate to the HP turbine in (ton/hr), (b) The total pump work, and (c) The thermal efficiency. Ans. (a) 256. 186 (ton/hr), (b) 9.767 (kj/kg), and (c) 44.36%.

9‐

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