pumps compressorslecture notes (1)

7/27/2019 PUMPS CompressorsLecture Notes (1)

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MOMENTUM TRANSFER

Professor: Dr. Merlinda A. Palencia

Chemical Engineering Dept.

Adamson University

Pumps, Compressors and Blowers

Introduction

Fluids are moved by pumps, fans, blowers, and compressors. These use work to increase the

mechanical energy of a fluid, which in turn can increase the flow rate (velocity), pressure, or

elevation of the fluid. Definitions overlap, but broad categories can be defined -- thecharacterization is based on the phase of the fluid, the flow capacity, and the required pressure

change (head).

Liquids are typically moved by pumps. Gases are moved by fans (large volume, small pressure

difference), blowers (large volume, moderate pressure difference), or compressors (large

pressure differences). Specialized equipment is also used to produce vacuums in process

systems.

A. Pumps

There are two main categories of pumps -- positive displacement and centrifugal. The choice is

based on the liquid to be pumped and the desired head and capacity.

Types of Pump

1. Centrifugal Pumps

Centrifugal pumps are probably most common in industrial applications. They may be built in a

very large number of materials. Capacity ranges up to 6000 gpm are common, as are heads to600 feet, all without special drivers. Performance drops off significantly when handling viscous

fluids or when air or vapor are present in the liquid.

For a given head and capacity, centrifugal pumps tend to be smaller and lighter than other types,

hence costs are lower.

2. Positive Displacement Pumps

Positive displacement pumps operate by trapping a fixed volume of liquid then releasing it to ahigher pressure by means of a piston or rotary gear.


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Reciprocating Pumps

Reciprocating pumps use a piston, plunger, or diaphragm to raise the pressure of a liquid. Thepumping chambers are surrounded by one-way valves so that liquid can only move in from the

low pressure side and out from the high pressure side. They are classed as "single acting" if fluid

is moved only on the downstroke, or "double acting" if fluid is moved by both sides of thepiston.

Because of the mechanism, these pumps produce a pulsating flow; but since flow is independentof head, they can be used to produce large pressure changes.

Reciprocating pumps are no longer common in most industrial installations. They are best for

low volume, high head applications (up to 50000 psi). They cannot be used when pulsating flowis a problem.

Diaphragm pumps are a sub-class of reciprocating pump. The pumping chamber is separated

from the moving parts by a flexible diaphragm. Their chief advantage is that the fluid beingpumped never comes in contact with the mechanism and eliminates leakage; thus they are goodfor toxic or very expensive liquids. They cannot produce large head differences.

Rotary Pumps

Rotary pumps use a gear, lobe, screw, cam, or vane to compress liquid. Liquid enters through agap between the rotating element and pump wall at a low pressure where it is trapped. Then, as

the element rotates, it squeezes the liquid out through a one-way valve on the opposite side of the

casing.

Typically, rotary pumps are used in high head, low flow applications. They are good for highviscosity and low vapor pressure fluids. The fluid pumped must be "lubricating"; solids cannot

be present. A key difference from centrifugal pumps is that discharge pressure variation has littleeffect on capacity.

Rotary pumps are common in laboratory settings because they have constant displacement at aset speed, and so can be used as metering pumps. Rotary pumps are also extremely common in

fluid-power applications.


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Head

The two key quantities in every pump design are the capacity (flow rate, typically in gpm) and

total headdeveloped by the pump. This information must be provided by the process engineer.

"Head" is just a way of expressing pressure -- specifically, in terms of the height of a column ofliquid that would produce the same pressure. Pressures measured in "mmHg" should be familiar

to anyone who has had a chemistry course, but there is no reason why mercury should be the

only fluid used. In practice, pressure measurements in inches or feet of water are also very

common. Many (most?) pump manufacturers use head units in their information, so make sureyou know how to use them.

Consider a mechanical energy balance written on a pump, standing alone. The only frictionallosses are those within the pump itself. These can be accounted for using an efficiency term

applied to the work.

The remaining terms represent the pressure, static or potential head, and velocity or kinetic head and

can be summed to create total headterms. If you consider the units on each of these terms, you will see

that they all represent the energy per unit mass of a contribution. The work done by the pump can then

be expressed in terms of the efficiency and total head.

Power

All pumps must have a driverto supply power. Typically, drivers are electric motors or steam

turbines; gas engines may be used in remote locations. As a general rule, motors are single speed

devices -- variable speed motor drivers are expensive -- while turbines can be operated atvariable speed by the addition of a governor.

The power requirement of a pump depends on the total head developed and the mass to bepumped per unit time. It is calculated by multiplying the shaft work term, Ws, by the mass

flowrate.


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When considering power requirements, be sure you know what you are trying to find.

Do you want the power delivered to the fluid (Pf), sometimes called the "work","water", or "liquid" horsepower? If so, the pump efficiency isn't needed.

Do you want the power supplied to the pump (PB), usually called the brakehorsepower (bhp)? This will be a smaller number since it accounts for leakage and

friction losses. To get it, just include the efficiency(;defined as LHP/BHP).

If you want the power supplied to the driver, PD ,you need to include the driver andcoupling efficiencies (D)as well.

Pump efficiencies typically range between about 65 and 80%.

Driver efficiencies are higher, at 80 to 90%.

Cavitation and NPSH

As liquid moves into a pump, there is a pressure drop due to the effects of the entrance, frictionin the suction piping, etc. Although the pressure is soon increased, if the pressure drops below

the vapor pressure of the fluid being moved, the liquid may vaporize. The bubbles that form

cause a volume increase and "choke" the pump. Then, as the pressure is increased by thepumping action, the bubbles implode, creating shockwaves that can pit and erode the equipment.

This phenomena is called cavitation and can severely damage the pump. Cavitation also causes

serious noise and vibration problems.

To prevent cavitation, it is important that the pressure within the pump suction be compared to

the vapor pressure of the liquid. The difference between the total suction head at the suction

flange and the vapor pressure of the liquid is called theNet Positive Suction Head, or NPSH.NPSH is typically stated in feet of liquid.

As a process engineer, then, one must determine the "available NPSH" and compare it to the"required NPSH" of the pump. If available NPSH is greater than or equal to required NPSH, the

pump should not cavitate. Required NPSH is calculated by the pump manufacturer and is often

plotted on the performance curve. McCabe, Smith, and Harriott (p. 191) report that requiredNPSH ranges from about 5 ft for small centrifugal pumps to 50 ft for very large pumps. For


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large, high energy pumps (i.e. boiler feed pumps), Welch (p. 849) recommends NPSHA be 1.5 to

2.0 times the NPSHR.

You should always calculate the available NPSH if:

the pump is installed above the liquid level

the pump draws from a tank under vacuum the liquid has a high vapor pressure the suction line is unusually long the plant is at high altitude (reduced atmospheric pressure)

The calculation is:

(

) [

]

where

hsv is the available NPSH hpsa is the suction surface absolute pressure; usually atmospheric pressure

for an open tank or the absolute pressure above the liquid for a closed tank

hss is static suction head -- the height of the liquid surface above the pumpcenterline

hfs is friction head loss between the liquid surface in the suction tank andthe pump suction flange

hvpa vapor pressure of the liquid Pa = Absolute pressure at surface of reservoir Pv = vapor pressure hfs = friction at suction line

All quantities are used in head units. The first three terms sum to the total suction head.


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Centrifugal Pumps

A centrifugal pump increases liquid pressure by increasing its velocity by means of a rotating

impeller. Liquid enters at the center of the impeller, is accelerated by the impeller vanes, andleaves through the side of the pump casing.

Performance Curves

There is a trade-off relationship between the capacity (flow rate) of a centrifugal pump and thehead it can add to a fluid. Commonly the relationship is represented by a performance curve.

These plot head vs. flow rate as function of impeller speed (one curve for each speed measured).

Since efficiency and power requirements are also functions of capacity, many manufacturers

include them on performance curve plots. Power requirements depend on the density of the fluid

being pumped, and so typically must be multiplied by the specific gravity of the fluid.

Pump efficiency is the ratio of the "energy imparted by the pump" to the "energy required by thepump". This tells us how much energy is lost by the pump mechanism. If flow were completelyfrictionless, a centrifugal pump would be 100% efficient; however, the world isn't that ideal.

The total head generated by a centrifugal pump is limited by the attainable rotational speed.Multistage pumps, using several impellers in series, can be used to obtain larger total heads.

The actual performance of an installed centrifugal pump is determined by its characteristicperformance curve (obtained from the manufacturer) and the resistance vs. flow curve for the

piping network (which must be calculated). The intersection of the two curves is the operating

point, so if both can be expressed mathematically, the problem can be stated as one of

simultaneous equations.

Affinity Laws

Sometimes it is useful to consider operating a pump with a different impeller or at a new speed.

Often, when such a possibility is being evaluated, the pump curve does not show the requiredconfiguration. In these cases, it is possible to estimate the new requirements using appropriate

ratios, called the affinity laws orfan laws:

Capacity (Flow) is directly proportional to impeller speed or impeller diameter


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Head is directly proportional to the square of impeller speed or impeller diameter

Power is directly proportional to the cube of impeller speed or impeller diameter.

The speed relationships are more accurate than the diameter relationships.

NPSH also typically varies as the cube.

Torque (J or ft-lbf): T

T = mass rate [r2] [Vu2]

[r2] = radius of impeller at discharge

[Vu2] = actual velocity in the pump

Power for an ideal pump = P = mass rate [ ] [r2] [Vu2]

= angular velocity

Minimum Flow

All centrifugal pumps are subject to minimum flow requirements to prevent mechanicalproblems due to temperature rise, etc. These are of particular concern for installations where the

pumps are liable to be operated intermittently or "closed in". For small pumps, typical minimum

flow values are 30% of the flow at BEP (best efficiency point). Larger and multistage pumps arelikely to have minimum values closer to 50% of flow at BEP.

Another class of minimum flow problems occurs at the left side of the performance curve. Thisissuction side recirculation. It effectively increases the NPSH required to prevent cavitation.

If you anticipate large turndowns in your pumping system, you probably should considerincluding a spillback or recirculation loop as low flow protection for your pump.


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Specific Speed

The hydraulic performance of a centrifugal pump depends on the shape and proportions of the

impeller. This relationship can be expressed in terms of a dimensionless quantity -- thespecificspeed.

The specific speed for a given impeller is constant, no matter what the rotating speed (prove it to

yourself -- use the affinity laws), and so provides a useful parameter for impeller selection.

A similar dimensionless number, thesuction specific speed, is used to rank a pumps ability tooperate under low NPSH:

This formula is essentially the same as for specific speed except that the required NPSH at max diameter

and best efficiency, hsv, replaces the total head. The higher the suction specific speed the lower the

NPSH required.

Seals, and Stuffing Box

Leakage from pumps can be very dangerous. Seals and gaskets are thus very important. The seal

between the rotating stationary parts is a complicated problem -- astuffing box, so called because

it is typically packed with loose sealing material, is located where the rotating shaft enters the

pump case.

Compressors


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Compressors are used to move gases and vapors in situations where large pressure differences

are necessary.

Types of Compressor

Compressors are classified by the way they work: dynamic (centrifugal and axial) orreciprocating. Dynamic compressors use a set of rotating blades to add velocity and pressure to

fluid. They operate at high speeds and are driven by steam or gas turbines or electric motors.

They tend to be smaller and lighter for a given service than reciprocating machines, and hence

have lower costs.

Reciprocating compressors use pistons to push gas to a higher pressure. They are common in

natural gas gathering and transmission systems, but are less common in process applications.Reciprocating compressors may be used when very large pressure differences must be achieved;

however, since they produce a pulsating flow, they may need to have a receiver vessel to dampen

the pulses.

The compression ratio, pout over pin, is a key parameter in understanding compressors and

blowers. When the compression ratio is below 4 or so, a blower is usually adequate. Higherratios require a compressor, or multiple compressor stages, be used.

When the pressure of a gas is increased in an adiabatic system, the temperature of the fluid mustrise. Since the temperature change is accompanied by a change in the specific volume, the work

necessary to compress a unit of fluid also changes. Consequently, many compressors must be

accompanied by cooling to reduce the consequences of the adiabatic temperature rise. Thecoolant may flow through a jacket which surrounds the housing with liquid coolant. When

multiple stage compressors are used, intercoolerheat exchangers are often used between the

stages.

Dynamic Compressors

Gas enters a centrifugal or axial compressor through a suction nozzle and is directed into thefirst-stage impeller by a set of guide vanes. The blades push the gas forward and into a diffuser

section where the gas velocity is slowed and the kinetic energy transferred from the blades is

converted to pressure. In a multistage compressor, the gas encounters another set of guide vanesand the compression step is repeated. If necessary, the gas may pass through a cooling loop

between stages.

Compressor Work

To evaluate the work requirements of a compressor, start with the mechanical energy balance. In

most compressors, kinetic and potential energy changes are small, so velocity and static headterms may be neglected. As with pumps, friction can be lumped into the work term by using an

efficiency. Unlike pumps, the fluid cannot be treated as incompressible, so a differential equation

is required:


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Evaluation of the integral requires that the compression path be known - - is it adiabatic,

isothermal, or polytropic?

uncooled units -- adiabatic, isentropic compression complete cooling during compression -- isothermal compression large compressors or incomplete cooling -- polytropic compression

Before calculating a compressor cycle, gas properties (heat capacity ratio, compressibility,molecular weight, etc.) must be determined for the fluid to be compressed. For mixtures, use an

appropriate weighted mean value for the specific heats and molecular weight.

Adiabatic, Isentropic Compression

If there is no heat transfer to or from the gas being compressed, the porocess is adiabatic andisentropic. From thermodynamics and the study of compressible flow, you are supposed to recall

that an ideal gas compression path depends on:

This can be rearranged to solve for density in terms of one known pressure and substituted into

the work equation, which then can be integrated.

The ratio of the isentropic work to the actual work is called the adiabatic efficiency (or isentropic

efficiency). The outlet temperature may be calculated from


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Power is found by multiplying the work by the mass flow rate and adjusting for the units andefficiency.

Isothermal Compression

If heat is removed from the gas during compression, an isothermal compression cycle may beachieved. In this case, the work may be calculated from:

Isothermal work will be less than the adiabatic work for any given compression ratio and set of

suction conditions.

The ratio of isothermal work to the actual work is the isothermal efficiency.

Isothermal paths are not typically used in most industrial compressor calculations.

Polytropic Compression

Most of the time, real compressors are neither isenthalpic nor isothermal. Instead a polytropic

cycle is followed. In this case:

This equation is the same as for adiabatic compression, except that the polytropic compression

exponent n replaces the heat capacity ratio.

Thepolytropic efficiency is defined as the ratio of polytropic work to actual work.


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The temperatures and pressures are related by:

A value forn must be found from the suction and discharge conditions:

Polytropic efficiencies are typically higher than adiabatic efficiencies for a given service. The

efficiencies for the various compression paths are directly related.

Thus the actual work can be calculated using any path, if the appropriate efficiency is known.

From these equations, it is clear that centrifugal compressors are very sensitive to inletconditions, including temperature and pressure. One that is less obvious, but important, is

molecular weight. This is a particular problem for mixtures of light cases, where a small changein a heavy contaminant can significantly alter the molecular weight of the gas entering the

compressor.

Performance Characteristics

The inlet volumetric flow, head, speed, efficiency, and power of a dynamic compressor areinterrelated. The relationship is called the performance characteristic of the compressor and is

generally plotted up by the manufacturer. These curves are very valuable when analyzing the

performance of an existing compressor.


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Theaffinity lawsapply to compressors as well as pumps and so can be used to evaluate minor

changes in the machine.

The edges of the compressor curves represent possible trouble spots. For a given speed, there is a

peak head on the left of the curve. This is called thesurge line and represents the point where the

flow drops enough to become unstable and pulsate. Surge can do serious damage to a machineand surge prevention sytems are very important.

At the other end is thestonewallpoint, where flow is a maximum and head a minimum. At thispoint, the flow chokes as the impeller cannot accept any more volume.

Fans & Blowers

Fans and blowers move large volumes of gas, typically through fairly large ducts. Fans producevery small pressure differences (inches of water), while blowers produce differences up to about2 atm. Both are rotary devices.

Most fan and blower calculations usestandard cubic feetorstandard cubic meters.

Large fans are usually centrifugal, and basically work the same as a centrifugal pump. Since fansproduce very little pressure change, it is typically safe to use incompressible fluid equations

when modeling the system.

Blower efficiencies are between 0.55 and 0.90.

References:

Pumps :
http://facstaff.cbu.edu/~rprice/lectures/pump.html#alawshttp://facstaff.cbu.edu/~rprice/lectures/pump.html#alawshttp://facstaff.cbu.edu/~rprice/lectures/pump.html#alawshttp://facstaff.cbu.edu/~rprice/lectures/pump.html#alaws


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1. Karassik, I.J., "Centrifugal Pumps and System Hydraulics", Chemical Engineering, October 4,1982, pp. 84-106.

2. McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (5th Edition),McGraw-Hill, 1993, pp. 188-204.

3. Syska, R.E. and J.R. Birk, Pump Engineering Manual(Fifth Edition), The Duriron Co., Dayton, OH,1980, pp. 9-10, 30-32, 62-68, 88-89, 113-114.

4. Talwar, M., "Analyzing Centrifugal Pump Circuits", Chemical Engineering, August 22, 1983, pp.69-73.

5. Welch, Harry J., ed., Transamerica Delaval Engineering Handbook(Fourth Edition), McGraw-Hill,1983, pp. 8-1 to 8-48.

Compressors:

1. McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering(5th Edition), McGraw-Hill, 1993, pp. 208-212.

2. Welch, Harry J., ed., Transamerica Delaval Engineering Handbook(Fourth Edition),McGraw-Hill, 1983, pp. 9-1 to 9-53.

Fans & Blowers:

1. McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (5th Edition),McGraw-Hill, 1993, pp. 204-208.

Vacuum Equipment

A variety of chemical processes operate at pressures below atmospheric. Most (vacuumdistillation, evaporation, drying) require rough vacuums down to 1 mmHg. Freeze drying

typically requires more vacuum, and some electronics processing requires very high vacuums

(on the order of 10-7

mmHg.

A variety of equipment is available to supply these vacuum needs:

steam jet ejectors liquid ring pumps mechanical pumps

o rotary piston pumpso rotary vane pumps

This list is arranged in order of increasing efficiency and cost (ejectors are cheapest, but least

efficient). Ejectors are probably preferred for systems that are corrosive, contain entrained solids,

or are prone to slugs of liquid; ring pumps are best for high discharge pressures and pumping

condensibles; and mechanical pumps for surges of noncondensibles.


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Vacuum Ejectors

Steam jet ejectors are often used to pull vacuum on surface condensers, evaporators, etc. A high

pressure, motive, fluid (usually steam) enters the ejector chest through a nozzle and then

expands. This converts its pressure energy to velocity. The increased velocity causes reducedpressure, which sucks in and entrains gas from the suction. The diffuser section then

recompresses the mixed steam/gas stream to some intermediate pressure. The exhaust is then

sent to a condenser which quickly condenses the steam at a low pressure and temperature so thatthe volume quickly decreases.

Ejector systems have no moving parts; thus, they are designed for optimum performance at asingle set of conditions.

A key performance measure is the compression ratio: the ratio of the discharge pressure to thesuction pressure (note that the pressure of the motive steam is not included). A single ejectorstage can achieve compression ratios up to 8:1, although values in the 3:1 to 5:1 range are more

typical. The discharge pressure is set by the condenser pressure -- minimum pressure is thecondensing pressure of steam at the vapor outlet temperature.


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Compression ratios can be

increased by using several

stages. In this arrangement,vacuum is pulled on each

condenser by a second

ejector. This results in alower vacuum on the process.

Number of

Stages

Suction

Pressure

(lowest)

1 75 mmHg

2 12 mmHg

3 1 mmHg

4 0.2 mmHg

5 0.02 mmHg6 .002 mmHg

The average compression

ratio for a system is best approximated as the overall compression ratio to the 1/NS power (NS isthe number of stages).

References:

1. Croll, S.W., "Properly Speecify Vacuum Systems", Chemical Engineering Progress,92(1): 48-49, January 1996.

2. N.P. Lieberman and E.T. Lieberman,A Working Guide to Process Equipment, McGraw-Hill, 1997, pp. 185-191.

3. McCabe, W.L., J.C. Smith, P. Harriott, Unit Operations of Chemical Engineering, 5thEdition, McGraw-Hill, 1993, pp. 212-213.

4. Ryans, J.L and S. Croll, "Selecting Vacuum Systems", Chemical Engineering, December14, 1981, pp. 72-90.

R.M. PriceOriginal: 4/3/98

Revised:

Copyright 1998 by R.M. Price -- All Rights Reserved

pumps compressorslecture notes (1)

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