Compressors and Expanders

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PUMPS



Pumping action : Cause increase in the elevation,

velocity and pressure of liquid

Main purpose to provide energy to move liquids from

one place to another.

Common application is to increase the pressure ofliquid.



THEORECTICAL POWER

REQUIREMENT

F is molar flow rate

V is molar volume

P is pressure

Due to smaller liquid molar volume, pumprequires less power than compressor for the same

molar flow rate and increase in P

Normally the outlet T of liquid increase only

slightly ( Based on Heuristic 43)



PUMP TYPE : CENTRIFUGAL, JET

AND ROTARY PUMP



PUMP CHARACTERISTICS

Capacity (Q) in gpm or ft3 /hr

Pump head (H) in ft or m

For negligible ∆V and Z and constant ,∆

d and s refer to discharge and suction



PUMP CHARACTERISTICS

For liquid water,

1. Head of 3000ft correspond to P of 1300 psi∆

2. Head of 20000ft correspond to P pf 8680 psi∆



HEURISTIC 37

For heads up to 3200 ft and flow rates in the

range 10-5000gpm, use centrifugal pump

For high heads up to 20000ft and flow rate up to

500gpm, use reciprocating pump

Less common are axial pumps for heads up to 40

ft for flow rates in the range of 20-100000gpm

and rotary pumps for heads up to 3000ft for flow

rate in the range 1-1500gpm



HEURISTIC 38

For liquid flow, we need to include the

following when determining the required

pumping head :

1. A pipeline pressure drop of 2psi/100ft ofpipe

2. A control valve pressure drop of at least

10psi

3. A pressure drop of 4 psi per 10ft rise in

elevation.



HEURISTIC 39

Estimate the theoretical horsepower (THp) for

pumping liquid using :

THp = (gpm)(pressure increase, psi)/1714



CENTRIFUGAL PUMP

Widely used (90%)

Impeller mounted on shaft and connected to a

motor

Operated at a particular rotational rateN(normally 1750 or 3450 rpm)

The flow rate can be varied by adjusting the

opening of a valve on the pump discharge line.Typical maximum head for a single stage is

500ft.

By using multiple stages, heads as high as

3200ft can be achieved.



CENTRIFUGAL PUMP :

CHARACTERISTIC CURVE

Manufacture will provide characteristic curve for

pumping water for each model

Correction has to be made for other fluid

A plot of P(brake horsepower) vs Q and Efficiencyvs Q are also provided.



CHARACTERISTIC CURVES



CHARACTERISTIC CURVE

Normally pump head (H) decreases as Q increasesBrake horsepower (P) increases with Q increases

The pump will operate at points on the

characteristic curves

Ideally the operating point should be at the point

of maximum efficiency

For a particular pumping task, the required head-

volumetric flow rate point must lie below the H-Qcurve

The control valve will be used to adjust the pump

head to the required head.



CHARACTERISTIC CURVE : EFFECT

OF N(RPM) ON H-Q CURVE



AFFINITY LAWS : APPROXIMATE A

H-Q CURVE AT DIFFERENT N

Q2=Q1(N2 /N1)

H2=H1(N2 /N1)2



CHARACTERISTIC CURVE : EFFECT OF

IMPELLER DIAMETER (D) ON H-Q CURVE



AFFINITY LAWS : APPROXIMATE A

H-Q CURVE AT DIFFERENT N

Q2=Q1(D2 /D1)

H2=H1(D2 /D1)2



CHARACTERISTIC CURVE :

VISCOSITY



CAVITATION

Occur within the pump when the pressure fall

below the vapor pressure of the liquid.

Vaporization will produce bubbles and may

collapse violently against surfaces at high

pressure region, causing erosion and damage to

the impeller.

Low the frequency and flow rate

Do not use when liquids are closed to bubble

point



NET POSITIVE-SUCTION HEAD

(NPSH)

The tendency for cavitation is measured based on

a quantity called ‘required NPSH’

It is specified by manufacturer for each pump and

expressed as a head(ft)

Available NPSH should be larger than required

NPSH in order to avoid cavitation

Available NPSH is calculated as follow,



POSITIVE DISPLACEMENT PUMP

Essentially metering pump

Deliver Q independent of the required pump head

The pump head is limited by the Hp of the driver

Flow rate change is by a bypass or with a speedchanger on the motor

Higher efficient than centrifugal pump due to

less friction

Cavitation is not usually a concern



POSITIVE DISPLACEMENT PUMP :

RECIPROCATING PUMPS

Three types : Piston Pump, Plunger Pump,

Diaphragm Pump

Pumping action causes pulsations leading to flow

and pressure fluctuation

Suitable for hazardous and toxic solid due to

absence of seals for the flexible diaphragm pump.



POSITIVE DISPLACEMENT PUMP :

ROTARY PUMPS

Two types : gear pumps and screw pumps

Design with tight tolerances to avoid binding and

excessive wear

Best suited for liquid with high viscosityFlow rates are more steady than reciprocating

pump but less than centrifugal pump.



COMPRESSORS AND

EXPANDERS

Gas compressor (including fans and blowers) aredesigned to increase the velocity and/or pressure of

gases rather than liquids.

Most compressor systems are designed to prevent liquid

from entering the compressor and to avoid condensation

in the compressor.

A fan mainly increases the kinetic energy of the gas with

a discharge of no more than !" of a suction

pressure.

A blower increases the pressure head more than the

velocity head# with compression ratio of not more than $.

A compressor increases the velocity head very little# witha compression ratio of greater than $



CENTRIFUGAL COMPRESSORS

%entrifugal fans# blowers# and compressors are widely used in chemical

processes because they are produce a continuous flow# are relatively small

and are free of vibration.

&ecause gases are compressible# the temperature difference between thecompressed gas and feed gas is significant at even moderate compression

ratios and may limit the compression ratio possible in a single stage.

'owever# the need for multiple stages in centrifugal compressors is usually

dictated instead by impeller rotationrate limitations# which limit the

compression ratio that can be achieved.







POSITIVE-DISPLACEMENT COMPRESSORS

ositive displacement fans# blowers# and compressors are similar in

action to positivedisplacement pumps and include reciprocatingcompressors# twoor threelobe blowers# and screw compressors.

'owever# with gases# the almost vertical characteristic curves bend

to the left more than for liquids because of the greeter tendency for

slip.

Lobed blower



EXPANDERS

*+panders also known as turboe+panders and e+pansion turbine

,ften used in place of valve to recover power from a gas when its

pressure must be decreased.

At the same time# the temperature of the gas is reduced# and often

the chilling of the gas is more important than the power recovery.

Most common is the radialflow turbine as shown in figure below.



COMPRESSOR AND EXPANDER MODELS IN

SIMULATORS

-wo methods can be used to take efficiency when calculating powerrequirements for compressors# whether they are centrifugal#

reciprocating or screw

olytropic method

0 1constant during compression

2sentropic method

s3-# 4 1 s3 -$#isentropic# $4

s molar entropy

-# -$ temperature at inlet and outlet $

# $ pressure at inlet and outlet $



ẅisentropic 1 5 (h$# isentropic 6 h)

5 molar gas flow ẅisentropic isentropic power

1 1ᵑ

s 1 1ᵑ

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Compressors and Expanders

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