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ITK-233Termodinamika Teknik

Kimia I

3 SKS

6 – Production of Power from Heat & Refrigeration

The Steam Power Plant: Rankine Cycle

Boiler

Condenser

Turbine

Pump

Ws

Qout

Qin

1

2

3

4

The processes: > 1 2 Reversible adiabatic pumping > 2 3 Isobaric heating & evaporation > 3 4 Reversible adiabatic expansion red: irreversible > 4 1 Constant pressure,constant temperature ondensation

T

S

3

41

2

P3

P4

P2 = P3

P1 = P4

Problem 8.4

Steam enters the turbine of a power plant operating on the Rankine cycle at 3300 kPa and exhausts at 50 kPa. To show the effect of superheating on the performance of the cycle, calculate the thermal efficiency of the cycle and the quality of the exhaust steam from the turbine for turbine-inlet steam temperature of 450, 550, and 650 oC.

Pump efficiency = 85%Turbine efficiency = 80%Sketch the process in a T-S and a P-H diagram

Problem 8.11

A power plant operating on heat from a geothermal source uses isobutane as the working medium in a Rankine cycle. Isobutane is heated at 3400 kPa (a pressure just a little below its critical pressure) to a temperature of 140oC, at which conditions it enters the turbine. Isentropic expansion in the turbine produces superheated vapor at 450 kPa, which is cooled and condensed to saturated liquid and pumped to the heater/boiler. If the flow rate of isobutane is 75 kg/s, what is the power output of the cycle and what are the heat transfer rates in the heater/boiler and cooler/condenser? What is the thermal efficiency if the cycle?

The turbine and the pump have an efficiency of 80%.The vapor pressure of isobutane:

068,274C/t775,2606

57100,14kPa/Pln osat

9

Suatu steam power plant beroperasi berdasar siklus Rankine tidak ideal. Kondisi opeasi steam power plant tersebut, adalah sebagai berikut: boiler bekerja pada tekanan konstan 1015 psia dengan temperatur uap air yang dihasilkan 1022 oF kondenser juga bekerja secara isobar pada 2,9 psia, sedangkan turbin dan pompa bekerja secara adiabatik dengan efisiensi masing-masing sebesar 75%. Dengan kondisi operasi demikian, daya yang mampu dihasilkan adalah 100 MW.

Evaluasi kondisi steam keluar turbin!Hitung laju steam yang dibutuhkan (dalam kg/jam)!Hitung laju transfer panas di boiler dan condenser!Hitung efisiensi termal dari plant tersebut!Gambarkan siklus yang dialami oleh uap air pada plant

tersebut dalam diagram T-S dan dalam diagram P-H.

The Steam Power Plant

Internal Combustion Engine:The Otto Cycle – The Gasoline Engine

The processes: > 0 1 Intake at constant pressure > 1 2 Adiabatic compression of fuel/oil mixture > 2 3 Ignition: rapid combustion at constant volume > 3 4 Adiabatic expansion of combustion products > 4 1 Constant volume air rejection

D

Cc V

Vr:rationcompressio

6 - 8

Example 4.20

Consider an air-standard Otto cycle operating on 5 kg of air with inlet conditions of 80 kPa & 37oC. A compression ratio of 10 is used and 500 kJ of heat is added during ignition.

Determine Q, W, P, & T for each step of the process and the overall efficiency of the process.

γ =1.4CV=20.93 J/mol.K

Problem 4.24

An air-standard Otto cycle operates with a compression ratio of 8, inlet conditions of 60oF & 14.7 psia, and a heat addition of 1200 Btu/lb air. The fuel and air in stoichiometric proportions gives off 1200 Btu/lb of mixture on combustion.

Determine the thermal efficiency of the cycle and the maximum temperature & pressure in the cycle.

10

Suatu siklus Otto udara-ideal menyerap panas sebesar 1500 J/mol dari panas hasil pembakaran bahan bakar. Tekanan dan temperatur awal kompresi adalah 1 bar dan 30 oC, dan tekanan pada akhir langkah kompresi adalah 5 bar. Asumsikan udara sebagai gas ideal dengan tetapan Laplace, = 1,4.

Berdasarkan uraian proses siklus Otto di atas:a. Hitung efisiensi termal mesin Otto tersebutb. Tentukan rasio kompresi (rC) siklus Otto udara-ideal tersebut!

Internal Combustion Engine:Diesel Engine

The diesel engine differs from the Otto engine primarily in that the temperature at the end of compression is sufficiently high that the combustion is initiated spontaneously.

For the same compression ratio, the Otto engine has a higher efficiency. The diesel engine operates at higher compression ratio, and consequently higher efficiency.

A

Be

D

Cc

VV

r:ratioansionexp

VV

r:rationcompressio

13 - 17

Problem 4.25

An air-standard Diesel cycle operates at a compression ratio of 14, inlet conditions of 60oF & 14.7 psia, and a maximum temperature of 2000 R.

Determine the thermal efficiency of the cycle.

Problem 8.13

An air-standard Diesel cycle absorbs 1500 J/mol of heat. The pressure & temperature at the beginning of the compression step are 1 bar & 20oC, and the pressure at the end of the compression step is 4 bar.

Assuming air to be ideal gas for which Cp = 7/2 R and Cv = 5/2 R, what are the compression ratio and the expansion ratio of the cycle?

11

Suatu mesin diesel bekerja berdasar siklus Diesel udara-ideal menyerap panas sebesar 1500 J/mol dari panas hasil pembakaran bahan bakar. Tekanan dan temperatur awal kompresi adalah 1 bar dan 30 oC, dan tekanan pada akhir langkah kompresi adalah 5 bar. Asumsikan udara sebagai gas ideal dengan tetapan Laplace, = 1,4. a. Hitung efisiensi siklus (dalam %)!b. Tentukan rasio kompresi (rC) dan rasio ekspansi (re) siklus Diesel udara-ideal tersebut!

Brayton Cycle – The Gas Turbine

The advantages of internal combustion engine & turbine are combined in the gas – turbine system.

Problem 4.26

An air-standard Brayton cycle operates at pressure ratio of 4 across the compressor with inlet air entering the compressor at 60oF & 14.7 psia.

The maximum cycle temperature is 2000 R and 1200 lb/min of air flows through the cycle.

Determine the work of the compressor, the work of the turbine, ant the thermal efficiency of the cycle.

Example 2.4

Consider a simple gas turbine system with air entering the compressor at 14.7 psia, 60 F and exhausting at 150 psia. The maximum cycle temperature is 2000 F at the turbine inlet.

Calculate the cycle efficiency and net work per pound of air, using:

Compressor internal efficiency 0.85Turbine internal efficiency 0.88Average constant pressure specific heat 0.25

Btu/lb.RSpecific heat ratio 1.4

Refrigeration– The Reverse Heat Engine

Refrigeration implies the maintenance of a low temperature below that of the surroundings. This requires continuous absorption of heat at a low temperature level.

Refrigeration is best known for its use in the air conditioning of buildings and in the treatment, transportation, and preservation of foods and beverages.

It is also finds large-scale industrial application, viz. in the manufacture of ice and the dehydration of gases. Application in petroleum industry include lubricating oil purification, low-temperature reactions, and separation of volatile hydrocarbons. A closely related process is gas liquefaction, which has important commercial applications.

Refrigeration– The Reverse Heat Engine

The coefficient of performance:

CH

CC

QQQ

WQ

net workeraturelower temp at the absorbed heat

COP

Refrigeration– The Vapor Compression Cycle

The coefficient of performance:

43

12C

HHHH

WQ

net workeraturelower temp at the absorbed heatCOP

Refrigeration– The Vapor Compression Cycle

The coefficient of performance:

23

12C

HHHH

WQ

net workeraturelower temp at the absorbed heat

COP

Property of HFC-134a = CF3CH2F

Pressure – Entalphi Diagram of HFC-134a = CF3CH2F

Choice of Refrigerant

The irreversibilities inherent in the vapor-compression cycle cause the COP of practical refrigerators to depend to some extent on the refrigerant.

Characteristics such as toxicity, flammability, cost, corrosion properties, and vapor pressure in relation to temperature are of great importance in the choice of refrigerant.

In order that air cannot leak into the refrigeration system, the vapor pressure of refrigerant at the evaporation temperature TC should be greater than atmospheric pressure.

On the other hand, the vapor pressure at the condenser temperature TH = TS should not be unduly high because of the expensive high pressure equipment.

The Most Common Refrigerant

CFC-11 : CCl3FCFC-12: CCl2F2

HCFC-123 : CHCl2CF3

HFC-134a : CF3CH2FHFC-125 : CHF2CF3

Two StageRefrigeration

Example 9.1

A refrigerated space is maintained at 10oF, and cooling water is available at 70oF. Refrigeration capacity is 120000 Btu/hr. The evaporator & condenser are of sufficient size that a 10oF minimum-temperature difference for heat transfer can be realized in each. The refrigerant is HFC-134a.

a. What is the value of COP for a Carnot refrigerator?

b. Calculate COP and the rate of circulation of refrigerant if the compressor efficiency is 80%

11

Sebuah sistem refrijerasi menggunakan metana sebagai refrijeran dengan laju alir 1000 kg/jam. Metana berupa cair jenuh memasuki JT valve, diekspansikan secara isentalpi sampai tekanannya menjadi 0,8 MPa. Metana uap jenuh dikompresi secara adiabatik-ireversibel dalam sebuah kompresor sehingga diperoleh kondisi 3,5 MPa dan 240 K. Sedangkan penguapan, desuperheating dan pengembunan berlangsung pada tekanan konstan. Dari deskripsi sistem refrijerasi di atas:

Gambarkan siklus refrijeran dalam diagram P-H metana!Hitung efisiensi kompresor (dalam %)!Daya kompresi yang dibutuhkan (dalam kW)!Tentukan COP siklus tersebut!Hitung kapasitas refrijerasi (dalam ton-refrijerasi)!Tentukan kualitas uap (dalam %) dan temperature (dalam oC)

uap metan meninggalkan JT valve!