gas movers and pumps

8/17/2019 Gas Movers and Pumps

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Pumps, fans, blowers, and compressors are all devices that move fluids across an adverse pressure difference, i.e., from a

region of lower pressure to a region of higher pressure. The fluid may require the higher pressure to overcome frictional

losses in subsequent piping, to participate in a high-pressure operation such as a chemical reactor, or to serve as the

drive medium in a hydraulic or pneumatic system. Or the objective may lie on the inlet side of the device where it is

desired to maintain vacuum in some region, in which case the pressure on the outlet side may simply be atmospheric. In

any of these cases there may or may not be a change in net velocity.

The two broad categories of fluid to be moved are liquids and gases. iquids are moved by pumps! gases are moved by

fans, blowers, and compressors. The main differences between the moving of liquids and the moving of gases are that

gases undergo significant changes in volume and temperature if the rise in pressure is appreciable . "long with liquids

and gases, there are more comple# fluid media, such as liquid$gas mi#tures, liquids that partially vapori%e, gases that

condense, slurries that consist of a liquid containing solid particles, and gas$particulate mi#tures, all of which may

require special handling or special equipment.

&luid movers fall into two general types' (inetically driven and positive displacement. )inetically driven devices impart

internal velocity to the fluid, and then convert this momentum to pressure at the e#it. Positive-displacement devices trap

incremental volumes of lower pressure fluid and transport it forcibly into the region of higher pressure.

Gas movers

&our types of device are used for moving gases. *epending on the pressure range and pressure change a gas may be

moved by a+ acuum pump

+ &an

+ lower

+ ompressor

Vacuum pumps create an internal %one of low pressure into which the gas in a region of sub-atmospheric pressure is

induced to flow. " single stage of vacuum pumping discharges to atmospheric pressure. /ith multiple stages, the

discharge of intermediate stages is to sub-atmospheric pressure.


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arge fans are usually centrifugal, operating on e#actly the same principle as centrifugal pumps. &an impellers are

mounted inside light sheet-metal casings. learances are large and discharge heads low from 012 to 0322 mm water

gauge. 4ometimes, as in ventilating fans, nearly all the added energy is converted into velocity energy and almost none

into pressure head. One difference between pumps and gas equipments recogni%e the effect of pressure and temperature

on the density of the gas entering the machine. 5as equipment is ordinarily rated in terms of standard cubic meters. "

volume in standard cubic meters is that measured at a specified temperature and pressure regardless of the actual

temperature and pressure of the gas to the machine.

Blowers and Compressors

/hen the pressure on a compressible fluid is increased adiabatically, the temperature of the fluid also increases. The

temperature rise has a number of disadvantages. ecause the specific volume of the fluid increases with temperature, the

wor( required to compress a unit mass of fluid is larger than if the compression were isothermal. 6#cessive temperatures

lead to problems with lubricants, stuffing bo#es and material of construction. The fluid may be one that cannot tolerate

high temperatures without decomposition.

&or a given gas, the temperature ratio increases with increase in the compression ratio. In blowers with a compression

ratio below about 1 or 7, the adiabatic temperature rise is not large, and no special provision is made to reduce it. In

compressors, however, where the compression ratio may be as high as 02 or more, the temperature rise becomes

e#cessive. "lso, since actual compressors are not frictionless, the heat from friction is also absorbed by the gas, and

temperatures well above the isentropic temperature are attained. ompressors are therefore cooled by jac(ets through

which cold water or refrigerant is circulated. &or multistage compression, inter-stage coolers are used. Often, an after-

cooler is used to cool the high pressure gas from the final stage.


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Fans generally add only a small amount of pressure to a gas, generally no more than 82 (Pa 9:2 in. of water;. &luid

compressibility can be ignored in the calculations. Typically, fans pull vapors from a slightly sub-atmospheric region and

discharge to atmosphere or they pull from the atmosphere and discharge to a space that is slightly above atmospheric

pressure. &or e#ample, in the former case, they may be removing unwanted vapors! in the latter, they may be supplying

fresh air .

Blowers and compressors impart significant positive pressure to gases. 4uch devices may have several stages, where the

suction pressure of the first is atmospheric and that of subsequent stages is higher. In these devices it is necessary to ta(eaccount of change in density with pressure and also the heat evolved by wor( done on the gas.


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Centrifugal compressors

entrifugal compressors are dynamic machines in which the rapidly

rotating impeller accelerates the gas 9&igure >;. The process flow

propagates from a#ial to radial 9perpendicular to shaft centerline; into

a stationary diffuser converting velocity to pressure.

The heart of a centrifugal compressor is its impeller 9sometimes calledthe wheel;. The principle of operation is similar for centrifugal

compressors, blowers, and fans and the impeller design is similar to

that of a fan. 4ince the gas leaving the impeller has significant velocity

the casing design employs a diffuser 9static vanes; to reduce velocity

and gain static pressure, as shown in &ig. 8>. 9This technique is also

sometimes used with centrifugal pumps.;

entrifugal compressors, li(e centrifugal pumps, at a given speed and throughput generate a constant head rather than

constant pressure. /hen the same compressor is operating with a different gas, the compressor will produce the same

head but not the same differential pressure. &or this reason, low molecular weight gases 9less than 02; are not practically

handled by centrifugal devices because to achieve a reasonable pressure, the required head is too large for any

industrially available centrifugal compressor! a reciprocating compressor is normally used instead.

Two-lobe blower


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entrifugal compressors are e#tremely popular because most are close to being oil-free . "lthough oil that is used in the

compressor can create an aerosol, the special sealing systems used in most centrifugals reduce oil contamination to very

low levels. entrifugals are also popular because of the very large capacities that are possible with a single compressor,

combined with fairly high pressures with multiple stages. entrifugals are economically attractive when flows are high,

and they are the only choice in many high-flow= pressure situations.

If other compressors are available that will meet the same pressure =flow requirements, centrifugals are preferred when

the process requirement allows for a fi#ed pressure ratio and requires oil-free gas. If centrifugal and reciprocating

compressors have similar costs, the centrifugal is selected for its reliability advantages and lower life-cycle costs. It

should be (ept in mind that centrifugal units often have higher installed costs than reciprocal and screw compressors for

the same pressure =flow range, and they are infle#ible to changes in pressure ratios, i.e., their capacities drop

significantly as the discharge pressure rises.

Axial Compressors

"#ial compressors, as shown schematically in &ig. 8?, operate with a rotor fitted with successive rows of blades, whichmove the gas forward. etween the rows of rotating blades are rows of static blades, which remove swirl and (eep the

flow a#ial. The space between rotor and barrel becomes progressively smaller, causing the gas to speed up and acquire

(inetic energy. The blades are aerodynamically shaped to achieve ma#imum thrust and minimum drag. "#ial

compressors are typically ?$02@ more efficient than centrifugal compressors.


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Kinetically Driven, !ectors

The gas-driven ejector is the non-rotary member of the (inetically driven family. 6jectors continue to be a common

means of creating and maintaining vacuum, chiefly because of their low capital cost and the absence of moving parts .

They are used not only for vacuum but in other applications where it is desired to combine two streams of different

pressures into a single stream of intermediate pressure 9the devices are sometimes call thermo-compressors;.

The principle is to create a %one of very low pressure by raising a high-pressure motive stream 9e.g., steam; to supersonic

velocity. 4uction gas or vapor is drawn into this %one where it combines with the motive fluid. The mi#ed stream, also at

supersonic velocity, is slowed down and its pressure recovers to a level intermediate between that of the motive and

suction streams.

" two-stage ejector system is shown in &ig. 12, in this case using a pre-condenser and an inter-condenser to reduce the

vapor load, as can be done for a condensable motive gas li(e steam. The method of control is shown, whereby a AAbleedBB

stream of e#ternal or higher pressure gas is allowed into the suction stream. 6jectors are sometimes configured up to si#in series.

4team jet ejectors offer a simple, reliable, low-cost way to produce vacuum. They are especially effective in the chemical

industry where an on-site supply of the high-pressure motive gas is available. 6jectors are considered an alternative to

mechanical vacuum pumps for a number of reasons'

+ Co source of power is required other than the motive gas!

+ecause they have no moving parts, they are reliable vacuum producers!

+They are easy to install, operate and maintain.

The two major functions of ejectors are follows'

"#ermocompressors

Thermocompressors are ejectors applied to recompressing spent steam and process fluids. They are used in applications

where it is desired to combine two streams of different pressures into a single stream of intermediate pressure .


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$acuum %roducers& 6jector-based systems are particularly appropriate as primary vacuum producers, particularly

where motive steam is almost always available. They are applied in processes such as crystalli%ation, deaeration, drying,

cooling, high vacuum distillation and deodori%ation.

6jector systems range from the simple, single-ejector stage to very comple# systems with as many as si# ejectors in

combination with intercondensers.


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!ector 'peration& In operation, a high-pressure motive gas enters the steam chest at low velocity and e#pands through

the converging-diverging no%%le. This results in a decrease in pressure and an increase in velocity.


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Li(uid Ring compressor

iquid Fing compressors utili%e squirrel cage fan

type impeller which is placed eccentric inside a tube

9&igure 7;. " compatible liquid is introduced into

the chamber along with the gas to be compressed.

ecause of the centrifugal force and the shape of the

internal cavity, the liquid forms an eccentric shape producing regions of changing volume. The liquid

must be separated from the compressed gas after the

compression process and recirculated.


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)uction Lift and Cavitation

" common concern with most pumps is the possibility of vapori%ation in the pump inlet. Cot only does the appearance of

a gaseous phase reduce the capacity of the pump but the possible subsequent collapse of bubbles as pressure recovers

9AAcavitationBB; may severely erode surfaces in the device. ecause this consideration is common to almost all pumps.

This pressure has the specific designation of net positive suction head—available or CP4Da and is defined as the

difference between the pressure at the pump inlet and the vapor pressure of the liquid at the temperature at the pump inlet.

If the suction pressure is only slightly greater than the vapour pressure, some liquid may flash to vapour inside the pump,a process called cavitation, which greatly reduces the pump capacity and causes severe erosion. If the suction pressure is

actually less than the vapour pressure, the re will be vapourisation in the suction line, and no liquid can be drawn into the

pump. To avoid cavitation, the pressure at the pump inlet must e#ceed the vapour pressure by a certain value, called the

net positive suction head 9CP4D;.The value of CP4D increases with pump capacity, impeller speed and discharge

pressure. &or a pump ta(ing suction from a reservoir, li(e that shown in &ig.?.3, the available CP4D is calculated as,

&or the special situation where the liquid is practically non-volatile 9 pvG2;, the friction negligible 9h

fs=2;, and the pressure

at station aH atmospheric, the ma#imum possible suction lift can be obtained by subtracting the required CP4D from the barometric head. &or cold water, this ma#imum suction lift is about 17 ft 902.7 m;


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avitation is a phenomenon that adversely affects the performance of a centrifugal pump and it must be avoided during

normal operation. The onset of cavitation in a pump, at any given speed and flow rate, is brought about by a particular

combination of temperature and pressure at the pump suction flange. The absolute total head is called the Net Positive

Suction Head or NPSH . The letter P tells us that CP4D, by definition, can never be a negative number.

Vapour Pressure is the pressure acting on a body of liquid $ in slurry pumping mostly water $ at which the liquid boils at

a particular temperature. y varying the pressure, we can ma(e the liquid boil at virtually any temperature' the lower the

pressure, the lower the boiling temperature. This e#plains why at altitudes, high above sea level, water boils at below

0222 and food ta(es longer to coo(.

"t atmospheric pressure and on the point of boiling, tiny spheres of water convert to vapour bubbles thereby e#panding

their original volumes 0:22 times. If the vapour bubbles then move to a %one of higher pressure, they immediately

implode, with considerable force, bac( to their original liquid volumes . In an open vessel, all these implosions simply

dissipate quietly through the boiling liquid surface into the surrounding ambient. In a closed vessel on the other hand, theimplosions generate loud, localised pressure shoc(s, which cause intermolecular crac(s on internal metallic surfaces ,

gradually dislodging small solid particles and finishing up with sponge-li(e cavities. This is cavitation damage and the

process causing it is cavitation. This type of damage does not usually occur on rubber surfaces because there are no inter-

crystalline boundaries and rubbers simply absorb the shoc(s.

/ater passing through a centrifugal pump is similarly subjected to low and high pressure %ones. The lowest pressure

e#ists at the eye of the impeller. If this pressure falls below the vapour pressure, local boiling ta(es place, generating

masses of tiny vapour bubbles within the liquid just past the leading edges of the pumping vanes. These bubbles implodeand can cause damage as soon as they are swept downstream to %ones of higher pressures $ only to be replaced

immediately with new ones.

The continuous procession of new vapour bubbles produces what appears to be a stationary cloud of vapour at the

impeller eye, throttling the flow of water . The end effect is a drop in flow rate and of total head D and a reduction in

pump performance, which is $ as much as any surface damage $ the reason why a pump should operate under conditions

sufficiently free from cavitation.


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CP4Da and is defined as the difference between the pressure at the pump inlet and the vapor pressure of the liquid at the

temperature at the pump inlet, both pressures e#pressed as head 9meters; of liquid. In the e#ample the available CP4D

9m; is equal to

The ris( of cavitation in systems can be reduced or prevented by'

+ owering the pump compared to the water level - open systems.

+ Increasing the system pressure - closed systems.

+ 4hortening the suction line to reduce the friction loss.

+ Increasing the suction lineBs cross-section area to reduce the fluid velocity and thereby reduce friction.

+ "voiding pressure drops coming from bends and other obstacles in the suction line.+ owering fluid temperature to reduce vapour pressure


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" pump is any device that transfers a liquid from a region of lower pressure into one of higher pressure. This movement

may or may not be accompanied by a change of velocity but that effect is incidental. 5enerally it is possible to ignore

changes of density e#cept for great changes in pressure or for liquids containing a gaseous phase. In pumps, the density

of the fluid is both constant and large. Pressure differences are usually considerable, and heavy construction is needed .

4ome considerations in choosing a pump are+ Pressure rise to be effected

+ iquid flow rate

+ Fange of flow rates

+ Fequired accuracy of flow rate

+ iquid viscosity

+ 4uction-side pressure

The positive-displacement family of pumps is so named because there is a direct connection between pump action and

liquid motion, with no reliance on an uncertain conversion between (inetic energy and pressure. )inetic energy plays, at

most, a subsidiary role in the action of these devices. 4ome of them are primarily used for moving highly viscous liquids,

where it would be difficult to generate (inetic energy in the first place. 4ome are used for developing high pressure,

which would require e#tensive staging in a (inetically driven device. 4ome are used to achieve high accuracy of liquid

delivery rate with no need for a flow meter to monitor the rate.

To transport a liquid through pipes energy has to be fed to the liquid. The energy is needed to overcome the dynamic

friction losses in the pipe. "lso energy is needed to compensate differences in level between the beginning and the end ofa pipe 9lift energy;. asically a pump is a piece of equipment to feed energy to a liquid flow. Pumps, fans, blowers and

compressors comprise the largest group of energy-absorbing turbomachines with which the mechanical engineer might

wor(. Two types of pumps can be distinguished'

Pumps capable of lifting water from one free surface to another' open pumps or "rchimedean screws 9fig. 1.0;.Pumps capable of feeding energy to water in combination with a closed pipe' centrifugal or impeller pumps.


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Arc#imedean screw

The "rchimedean screw is used in situation that large quantities of water have to be pumped from one free surface level

to another with a level difference of a few meters. " typical use of "rchimedean screws is drainage of polder areas to

pump out large volumes of storm water 9fig. 1.:;. The screw can also be used if water is polluted with debris as wood,

plants and other floating objects. The capacity of the screw is dependant of the head D 9difference between fluid

surfaces;, the slope of the screw with the hori%ontal, the diameter of the screw * and the diameter of the casing d, the

number of blades and the pitch 4 and the rotating speed of the screw n.


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" positive displacement pump wor(s following the principle of figure 1.>. 4everal types of displacement pumps are

available all wor(ing following the principle that a fi#ed amount of fluid is JencapsulatedK and pushed to the pressure side

of the pump. "nother e#ample is shown in figure 1.?. *isplacement pumps are mostly used for AdifficultB fluids li(e very

high viscous fluids or for applications a high pressure is needed.


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entrifugal pumps consist of one or more impellers, attached to a rotating shaft and

surrounded by a casing . &luid enters through a suction pipe into the eye of the

impeller and is JthrownK outward through the action of centrifugal force. " volute

sometimes augmented by diffuser vanes collects the discharged fluid converting part of

the velocity head into pressure head. The impeller is fitted with guide vanes or blades

that convert the energy of rotation into velocity and pressure head and guide the flow.

entrifugal pumps operate at relatively high speeds and are usually direct connected to

the prime mover. They are compact, have no internal rubbing parts, possess high

reliability, and can move fluids containing solids. They can handle high volumes and

relatively high pressures with impellers in series on a single shaft or with pumps

connected in series. One disadvantage of the centrifugal pump is that it is not self-

priming! the casing must be filled before pump action can begin.

"n important consideration in the design of centrifugal pumps is to ensure that the pressure throughout the flow field remains above the vapor pressure of the liquid. If

this condition is not met, the liquid will vapori%e and form bubbles that subsequently

collapse releasing enormous energy and causing pitting, erosion, noise and a reduction

in efficiency.

Running in series or parallel

entrifugal pumps operate within ranges of head and velocity. Operating outside of these ranges may require using aspecialty pump. Other options for handling high-head or high-flow applications include using pumps in series or parallel.

/hen running in series, the heads are added, and the total capacity is equal to that of the pump with the smallest capacity .

In parallel, the capacities of the pumps are added, and the head of all pumps will be equal at the point where the

discharged liquids recombine. Parallel pumps are used for a variety of reasons, including cost 9two smaller pumps may

cost less than a larger one;, an increase in the si%e of an e#isting plant, or to compensate for a process with varying

capacity. Cote that pumps operated in parallel must have similar head characteristics to avoid potential operating

problems.

entrifugal pump schematic


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Gear pumps

5ear pumps are primarily used for high-viscosity liquids. Two or

more gears trap liquid in the space between the gear teeth and thecasing wall and convey it from inlet to outlet. The simplest gear-type

pump uses a pair of mating gears rotating in an oval chamber to

produce flow. "s the gears rotate, the changing si%e of the chambers

created by the meshing and un-meshing of the teeth provides the

pumping action. "nother design uses an e#ternal rotating ring with

internal gear teeth that mesh with an internal gear as it rotates. "s the

inner gear rotates, the tooth engagement creates chambers of

diminishing si%e between the inlet and outlet positions to create flow.

"ll gear-type pumps have a fi#ed displacement. These pumps are

relatively ine#pensive compared to piston and vane-type pumps with

similar displacements, but tend to wear out more quic(ly and are not

generally economically repairable. 5ear pumps are widely used in the

polymer industries, where viscosities of thousands of pascal-seconds

are encountered and where pressures of tens of megapascals arerequired to force these liquids through pipes and vessels. The concept

of the gear pump is illustrated in &ig.0:.


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)crew %umps

4crew pumps are related to the gear pump in that they

act by pushing liquid along the inner surface of the

casing, in this case the screw barrel. The most common

embodiment is a single screw in a single barrel but

other models ma(e use of two 9or more; screws in

parallel intersecting barrels, where the screws may co-

rotate or counter-rotate. 4crew pumps with a single

screw and those with co-rotating twin screws are not

true positive-displacement pumps because liquid is able

to flow bac( along the screw channels.

4crew pumps can still pump against significant

pressures in spite of the bac(flow tendency. "sviscosity increases the bac(flow tendency decreases,

which is why single-screw and co-rotating twin-screw

pumps are widely used for viscous liquids. The two

types of screw pump are illustrated in &igs. 0> and 0?.

&igure 0L shows a counter-rotating twin-screw pump.

In this diagram the screws are fully !nter"meshing and

as such ma(e the pump truly a positive displacement

one. There is no reverse flow along the screw channels!

rather, the screws form discrete poc(ets of liquid,

which are carried down the barrel to the high-pressure

e#it. The only bac(flow is through clearances between

screws and between screws and wall. ounter-rotating

screws can also be made less than fully inter-meshing,

in which case there is bac(flow in the channels.


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%eristaltic %umps

Peristalsis is the mechanism by which muscular

contractions move materials through various passages

in the body. The peristaltic pump mimics this process

by trapping and moving liquids through a fle#ible

tube. The advantage is that there is no contact between

pump mechanism and the liquid. This type of pump isrestricted to low-pressure applications.

&igure 80 shows the principle.

Comparison of Devices for moving fluids

Positive displacement machines, in general, handle smaller quantities of fluids at higher discharge pressures than

centrifugal machines do. Positive displacement pumps are not subject to air-binding and are usually self-priming . In both positive displacement pumps and blower the discharge rate is nearly independent of the discharge pressure, so that these

machines are e#tensively used for controlling and metering flow. Feciprocating devices require considerable maintenance

but can produce the highest pressures. They deliver a pulsating stream. Fotary pumps wor( best on fairly viscous

lubricating fluids, discharging a steady stream at moderate to high pressures. They cannot be used with slurries. The

discharge line of a positive displacement pump cannot be closed without stalling or brea(ing the pump , so that a bypass

line with a pressure-relief valve is required.

entrifugal machines, both pumps and blowers, deliver fluid at a uniform pressure without shoc(s or pulsations . They runat higher speeds than positive displacement machines and are connected to the motor drive directly instead of through a

gear-bo#. The discharge line can be completely closed without damage. entrifugal pumps can handle a wide variety of

corrosive liquids and slurries. entrifugal blowers and compressors are much smaller for a given capacity than

reciprocating compressors and require less maintenance.

&or producing vacuum, reciprocating machines are effective for absolute pressures down to 02 mm Dg. Fotary vacuum

pumps can lower the absolute pressure to 2.20 mmDg and over a wide range of low pressures are cheaper to operate than

multi-stage steam-jet ejectors.

gas movers and pumps

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