bahan kuliah compressors 1

8/19/2019 Bahan Kuliah Compressors 1

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COMPRESSOR

Lecture Hand Out

Mechanical Engineering Department

Brawijaya University1015


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I. INTRODUCTION

1.1. Description

Definition.

Mechanism to compress or pressurize gas, air, or vapour. The ratio between outletpressure and inlet pressure is Compression ratio (CR).

Since gas is compressible fluid, while pressure increases at the outlet, the volume reduces,and the temperature increases according to thermodynamic equation Boyle-Gay-Lussac,

P1.V1/T1 = P2.V2/T2

They are used a lot in engineering in:

- Jet engine

- Gas turbine system

- Refrigeration system

- To compress gases in pneumatic system

- Pipeline transport


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1.2. Compressor Types


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1.3. Thermodynamic Analysis of Compressor

Compressor is a thermodynamic system, that pressurize gas to higher pressure and results in

lower volume and higher temperature. Compressor is an open thermodynamic system, where

mass of the gas continously enter the system and simultanously exit from the system.Compressor works in steady flow basis, where mass into the system equals to the mass out.

Thermodynamics Open System


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Applying conservation energy principle,

)2

(2

)2

(1 2

2

221

2

11 gz

V h

net gz

V h

net mW mQ

A number of thermodynamic devices such as pumps, fans, compressors, turbines, nozzles,

diffusers, and heaters operate with one entrance and one exit. The steady-state, steady-flow

conservation of mass and first law of thermodynamics for these systems reduce to

)](2

)([ 122

122

12 z z g V V hhmW Q

With one direction of heat and work, the equation may be rewritten as,

For compressor, the differences of kinetic energy and elevation at inlet and outlet usually

very small and could be neglected,


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Compressors and fans are essentially the same devices. However, compressors operate

over larger pressure ratios than fans. If we neglect the changes in kinetic and potential

energies as fluid flows through an adiabatic compressor having one entrance and one exit,

the steady-state, steady-flow first law or the conservation of energy equation becomes,

]0)(2

0)([ 12

2

1

2

212

z z g V V

hhmW Q

)[ 12 hhmW

)()( 12 hhmW in

)( 12 hhmW in

or,

Equation of State in compressor

Pv = RT, or PV = mRT

These equation of state could be applied for steady state liquid flow, as incompressor in time rate,

P.V/t = m/t RT , or

Where, V/t = , volumetric flow rate [m3/sec]

m/t = , mass flow rate [kg/sec]

T RmV P ...

V

m


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The ideal compressor works in isentropic process, and the relation between the

compressor property P, V, and T,

The isentropic work is the minimum possible work that the adiabatic compressor

requires; therefore, the actual work is greater than the isentropic work. Since

efficiencies are defined to be less than 1, the compressor efficiency ( c) is defined

as the ratio between isentropic work and actual work.


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Compressor Horse Power

- Hydraulic or Gas Horse Power (GHP), [kW]- Shaft or Brake Horse Power (BHP), [kW]

- GHP = (r.Q/t.g.H)/1000

- BHP = GHP/hc = (r.Q/t.g.H)/(hc.1000)

Where, r = density of gas [kg/m3]Q/t = volume flow rate [m3/sec]g = gravitation [m/sec2]H = head [m]hc = efisiensi kompresor

EXAMPLE 5 –6 Compressing Air by a Compressor Air at 100 kPa and 280 K is compressed

steadily to 600 kPa and 400 K. The mass flow rate of the air is 0.02 kg/s, and a heat lossof 16 kJ/kg occurs during the process. Assuming the changes in kinetic and potentialenergies are negligible, determine the necessary power input to the compressor.


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P.R. 1.

P.R. 2. Carbon dioxide enters an adiabatic compressor at 100 kPa and 300 K at a rate of 0.5 kg/sand leaves at 600 kPa and 450 K. Neglecting kinetic energy changes, determine (a) the volume flow

rate of the carbon dioxide at the compressor inlet and (b) the power input to the compressor.


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P.R. 1.

P.R. 2.


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bahan kuliah compressors 1

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