2. Characteristics & Measurement of Temperature, Pressure, Velocity, Flow

8/13/2019 2. Characteristics & Measurement of Temperature, Pressure, Velocity, Flow

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Measurement of Temperature:Optical pyrometry identifies the temperature of a surface by its color, or moreprecisely the color of the radiation it emits. A schematic of an optical pyrometer isshown in Figure 8.31. A standard lamp is calibrated so that the current flowthrough its filament is controlled and calibrated in terms of the filamenttemperature. Comparison is made optically between the color of this filament andthe surface of the object whose temperature is being measured. he comparatorcan be the human eye.!ncertainties in the measurement may be reduced by appropriately filtering theincoming light.Corrections must be applied for surface emissi"ity associated with the measuredradiation# uncertainties "ary with the s$ill of the user, and generally are on theorder of %&C. 'eplacing the human eye with a different detector e(tends the rangeof useful temperature measurement and reduces the random uncertainty.he major ad"antage of an optical pyrometer lies in its ability to measure hightemperatures remotely. For e(ample, it could be used to measure thetemperature of a furnace without ha"ing any sensor in the furnace itself. Formany applications this pro"ides a safe and economical means of measuring high

temperatures.

Measurement of Pressure:A pressure transducer is a de"ice that con"erts a measured pressure into amechanical or electrical signal. he transducer is actually a hybrid sensor)transducer. he primary sensor is usually an elastic element that deforms ordeflects under the measured pressure relati"e to a reference pressure. *e"eralcommon elastic elements used, as shownin Figure +.+, include the ourdon tube,bellows, capsule, and diaphragm. A secondary transducer element con"erts theelastic element deflection into a readily measurable signal such as an electrical"oltage or mechanical rotation of a pointer. here are many methods a"ailable toperform this transducer function, and we e(amine a few common ones.-eneral categories for pressure transducers are absolute, gauge, "acuum, anddifferential.


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hese categories reflect the application and reference pressure used. Absolutetransducers ha"e a sealed reference ca"ity held at a pressure of absolute ero,enabling absolute pressure measurements.-auge transducers ha"e the reference ca"ity open to atmospheric pressure andare intended to measure abo"e or below atmospheric pressure or both./ifferential transducers measure the difference between two applied pressures.0acuum transducers are a special form of absolute transducer for low)pressuremeasurements.ressure transducers are subject to some or all of the following elemental errors2resolution, ero shift error, linearity error, sensiti"ity error, hysteresis, noise, anddrift due to en"ironmental temperature changes. lectrical transducers are alsosubject to loading error between the transducer output and its indicating de"ice.4oading errors increase the transducer nonlinearity o"er its operating range.5hen this is a consideration, a "oltage follower can be inserted at the output ofthe transducer to isolate transducer load.


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Measurement of Fluid Flow Velocity:he /oppler ffect describes the phenomenon e(perienced by an obser"erwhereby the fre6uency of light or sound wa"es emitted from a source that istra"eling away from or toward the obser"er is shifted from its original "alue andby an amount proportional to its speed. 7ost readers are familiar with the changein pitch of a train as heard by an obser"er as the train changes from approachingto receding. Any radiant energy wa"e, such as a sound or light wa"e, e(periences

a /oppler effect. he effect was recognied and modeled by Christian ohann/oppler 918:3;18%3


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pro"ides a ready emission source that is monochromatic and remains coherento"er long distances. As a mo"ing particle suspended in the fluid passes throughthe laser beam, it scatters light in all directions. An obser"er "iewing thisencounter between the particle and the beam percei"es the scattered light at afre6uency, fs2 where fiis the fre6uency of the incident laser beam and f/ is the/oppler shift fre6uency. !sing "isible light, an incident laser beam fre6uency is onthe order of 1:1>. For most engineering applications, the "elocities are suchthat the /oppler shift fre6uency, f/, is on the order of 1:3to 1:B>. *uch a smallshift in the incident fre6uency can be difficult to detect in a practical instrument.An operating mode that o"ercomes this difficulty is the dual)beam mode shown inFigure. =n this mode, a single laser beam is di"ided into two coherent beams ofe6ual intensity using an optical beam splitter. hese incident beams are passedthrough a focusing lens that focuses the beams to a point in the flow. he focalpoint forms the effecti"e measuring "olume 9sensor< of the instrument.articles suspended in and mo"ing with the fluid scatter light as they pass throughthe beams. he fre6uency of the scattered light is that gi"en by 6uation +.%:e"erywhere but at the measuring "olume. here, the two beams cross and theincident information from the two beams mi(, a process $nown as optical

heterodyne. he outcome of this mi(ing is a separation of the incident fre6uencyfrom the /oppler fre6uency. A stationary obser"er, such as an optical photodiode,focused on the measuring "olume, sees two distinct fre6uencies, the /oppler shiftfre6uency and the unshifted incident fre6uency, instead of seeing their sum. =t is asimple matter to separate the much smaller /oppler fre6uency from the incidentfre6uency by filtering.

Selection of Velocity Measuring Methods*electing the best "elocity measuring system for a particular application in"ol"esa number of factors that an engineer needs to weight accordingly21. 'e6uired spatial resolution. 'e6uired "elocity range

3. *ensiti"ity to "elocity changes only. 'e6uired need to 6uantify dynamic "elocity%. Acceptable probe bloc$age of flow@. Ability to be used in hostile en"ironmentsB. Calibration re6uirements8. 4ow cost and ease of use5hen used under appropriate conditions, the uncertainty in "elocity determinedby any of the discussed methods can be as low as 1D of the measured "elocity,although under special conditions4/A methods can ha"e an uncertainty one order of magnitude lower.itot)*tatic ressure 7ethods


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he pressure probe methods are best suited for finding the mean "elocity in fluidsof constant density. 'elati"e to other methods, they are the simplest andcheapest method a"ailable to measure "elocity at a point. robe bloc$age of theflow is not a problem in large ducts and away from walls.Fluid particulate bloc$s the impact ports, but aspirating models are a"ailable forsuch situations.hey are subject to mean flow misalignment errors. hey re6uire no calibrationand are fre6uently used in the field and laboratory ali$e.hermal Anemometerhermal anemometers are best suited for use in clean fluids of constanttemperature and density. hey are well suited for measuring dynamic "elocitieswith "ery high resolution. >owe"er, signal interpretation in strongly dynamic flowscan be complicated 91B,%ot)film sensors are less fragile and less susceptibleto contamination than hot)wire sensors. robe bloc$age is not significant in largeducts and away from walls. hermal anemometers are 18: degrees directionallyambiguous 9i.e., flows from the left or right directions gi"e the same outputsignal


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urbine meters ma$e use of angular momentum principles to meter flowrate. =n a typical design, a rotor is encased within a bored housing through whichthe fluid to be metered is passed. =ts housing contains flanges or threads for directinsertion into a pipeline. =n principle, the e(change of momentum within the flowturns the rotor at a rotational speed that is proportional to the flow rate. 'otorrotation can be measured in a number of ways. For e(ample, a reluctance pic$upcoil can sense the passage of magnetic rotor blades, producing a pulse trainsignal at a fre6uency that is directly related to rotational speed. his can bedirectly output as a 4 pulse train, or the fre6uency can be con"erted to ananalog "oltage. urbine meters offer a low)pressure drop and "ery good accuracy.A typical instrumentGs systematic uncertainty in flow rate is :.%D 9+%D< with aturndown of :21. he measurements are e(ceptionally repeatable ma$ing themeters good candidates for local flow rate standards. >owe"er, their use must berestricted to clean fluids because of possible fouling of their rotating parts. heturbine meter rotational speed is sensiti"e to temperature changes, which affectfluid "iscosity and density and therefore H1. *ome compensation for "iscosity"ariations can be made electronically. he turbine meter is "ery susceptible toinstallation errors caused by pipe flow swirl, and a careful selection of installationposition must be made.


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Measurement of Speed, Torque & Power:or6ue and power are important 6uantities in"ol"ed in power transmission inrotating machines li$e engines, turbines, compressors, motors and so on. or6ueand power measurements are made by the use of a dynamometer. =n adynamometer the tor6ue and rotational speed are independently measured andthe product of these gi"es the power.9i< or6ue 7easurement2I ra$e ArrangementI 4oad electrically ; ngine dri"es a generatorI 7easure shear stress on the shaft9ii< 7easurement of rotational speed2I achometer ; 7echanical /e"iceI Jon contact optical rpm meteror6ue measurement2 ra$e drum dynamometer 9the rony bra$e


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lectric generator as a dynamometer2An electric generator is mounted on the shaft that is dri"en by the power de"icewhose output power is to be measured as shown in Figure. he stator that wouldnormally be fi(ed is allowed float between two bearings. he loading of thedynamometer is done by passing the current from the generator through a ban$of resistors. he power generated is again dissipated as heat by the resistor ban$.=n practice some cooling arrangement is needed to dissipate this heat. he statorhas an arm attached to it which rests on a platform balance as shown in thefigure. he stator e(periences a tor6ue due to the rotation of the rotor due toelectromagnetic forces that is balanced by an e6ual and opposite tor6ue pro"idedby the reaction force acting at the point where the tor6ue arm rests on theplatform balance. he tor6ue on the shaft is gi"en by the reading of the platformbalance multiplied by the tor6ue arm.7easurement of rotational speed2A mentioned earlier the measurement of power re6uires the measurement ofrotational speed, in addition to the measurement of tor6ue. 5e describe belowtwo ways of doinjg this.achometer ; 7echanical /e"icehis is a mechanical method of measuring the rotational speed of a rotating shaft.he tachometer is mechanically dri"en by being coupled to the rotating shaft. herotary motion is either transmitted by friction or by a gear arrangement 9as shownin Figure


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Hnowing the dri"e gear speed ratio one may calibrate the angular position on thedial in terms of the rotational speed in rpm.

Jon contact optical rpm meter2his is a non contact method of rotational speed of a shaft. >owe"er it re6uires awheel with openings to be mounted on the rotating shaft. he opticalarrangement is shown in Figure. he arrangement is essentially li$e that was usedfor chopping a light beam in applications that were considered earlier. he

fre6uency of interruption of the beam is directly proportional to the rpm of thewheel, that is usually the same as the rpm of the shaft to be measured and thenumber of holes pro"ided along the periphery of the wheel. =f there is only onehole the beam is interrupted once e"ery re"olution. =f there are n holes the beamis interrupted n times per re"olution. he rotational speed of the shaft is thuse6ual to the fre6uency of interruptions di"ided by the number of holes in thewheel. he 4/ photo detector wheel assembly is a"ailable from suppliers as aunit readily useable for rpm measurement.>ence the ower is obtained by EKJL@:

Measurement of Humidity:For finding the moisture contents of air, dry bulb and wet bulb temperaturemeasurements are essential. =t is easy to measure humidity of air under ambientconditions by swinging a sling psychrometer with hand. he airLgas whose wetbulb is to be measured must ha"e a "elocity of % to 8 mLs o"er the wetted bulb. =nthe hot air ducts usually such "elocities are a"ailable. =f not, a portion of the gasflow can be directed to the bulb. he usual wet bulb thermometer has a wic$dipped in water which is close to wet bulb temperature when the temperature ishigh and the relati"e humidity is low the wet bulb is 6uite low and e"aporationfrom the wic$ is too rapid. hree possible approaches can be used to determinewet bulb under such conditions.

1. A long stem mercury in glass thermometer of up to 11: deg. C range with :.%deg. C graduations can be co"ered with a absorbent and clean cotton wic$ andheld in the hot humid air stream while watching the temperature rise. hetemperature will rise rapidly and stabilie at the wet bulb temperature whichshould be careful ly noted immediately. After this stage the temperature will againstart rising when the thermometer must be withdrawn 6uic$ly. his direct methodis suitable for temperatures up to about 3%: deg. H in wet bulb and clean air.. A sample of the gas must be di"erted from the main stream and cooled butcondensation must be a"oided. 5et and dry bulb both then should be measuredfrom this sample.


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Alternati"ely a gas sample that cleans dusty sample and cools the air to dew pointis re6uired. he degree of accuracy for gas wet bulb or dew point temperature ameasurement is M:.%D of the absolute gas temperature reading.

Measurement of Lux:=n doing lighting efficiency wor$, you need to measure light intensity. ?ou alsoneed to $now how to e(press light intensity for selecting lamps and for laying outthe o"erall lighting configuration. !nfortunately, lighting terminology tends to beconfusing and somewhat inconsistent. his brief Jote introduces you to the termsthat the lighting trade uses to communicate about light intensity, and it points outwhich of these terms are important to $now.N4umen is the unit of total light output from a light source. =f a lamp or fi(turewere surrounded by a transparent bubble, the total rate of light flow through thebubble is measured in lumens. 4umens indicate a rate of energy flow. hus, it is apower unit, li$e the watt or horsepower. ypical indoor lamps ha"e light outputsranging from %: to 1:,::: lumens. ?ou use lumens to order most types of lamps,to compare lamp outputs, and to calculate lamp energy efficiencies 9which aree(pressed as lumens per watt


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temperature are o"erfiring of the boiler and switching from low fire to high fire.oiler o"er firing increases stac$ gas temperature, because too much heat isreleased for any gi"en heat e(changer surface of the boiler.=n the case of a switch from low fire to high fire the stac$ gas temperatureincreases because the boiler heat e(changer surface is designed for the high fireoutput and conse6uently has no trouble at all to adsorb the reduced heat input atlow fire. >owe"er the switch from low fire to high fire is a typical case where notonly the stac$ gas temperature goes up, but the e(cess air goes down due to theburner design re6uiring less e(cess air at high firing rates.'ecall the mass flow e6uation for stac$ gasmSG E 91 M AF< in tonsLhour

for one ton of fuel oil per hour.5here E (cess air factor

AF E Air to fuel ratio in $gL$g for stochiometric combustion

Assuming an integral specific heat cSG of the stac$ gas, the energy loss is gi"en as

4*- E cSG 91 M AF< 9stac$ gas ) %< in 7Lton of fuel

he air)to)fuel ratio 9AF< is fuel specific and does not change with . =n the case of

switching from low fire to high fire the stac$ gas temperature goes up and down.=n other words, is a function of , or 9


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adjust the e(cess air le"el you may be able to measure the effect of on the stac$gas temperature. One should be "ery careful when lowering e(cess air at highfire. =t could happen that the burner will be star"ed of air at low fire and a "eryunpleasant bac$ firing and Nrumbling of the boiler will follow. 7ost commonmista$e is to measure O and temperature without $nowing the firing rate 9highfire, low fire, or modulatedigh stac$ gas temperatures and high e(cess air are caused by human failure tota$e care of a boiler. ?our tas$ is to establish the Nas is situation and try tocon"ince management to impro"e the situation.here may be cases such as one or two pass fire tube boilers, where stac$ gastemperatures are high due to insufficient heat e(changer surfaces. *uch cases

as$ for an economic analysis and most li$ely a replacement of the boiler. =n allother cases better house$eeping measures combined with impro"ed monitoring ofstac$ gas temperatures and O le"els will reduce fuel consumption. 7onitoringstac$ gas temperatures is ine(pensi"e, while monitoring O le"els is e(pensi"eand more complicated.

>ow to calculate stac$ lossQ


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!L"S#S $F F'(L

1. Fuel cost sa"ings "ersus fuel sa"ingsAs shown in 4ecture 1 fuel costs are by far the highest cost of most boileroperations. hey account for @:)+: D of total steam costs in most systems.Conse6uently reducing fuel costs is a major objecti"e of any boiler energy audit.As a consultant you are interested to reduce fuel costs e"en if the energyconsumption stays the same or increases. his strategy may conflict with-o"ernment guidelines that subsidies energy conser"ation projects and re6uires acompany to sa"e fuel energy. >owe"er, some energy conser"ation projects sa"eenergy costs but increase energy consumption. A typical e(ample is a fuel switchfrom fuel oil to solid fuels.

. *olid fuels deli"ery contracts


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he first 6uestion to as$, is how the client purchases solid fuels and whether thefirm has an option and can choose from "arious suppliers. Firms re6uiring large6uantities of solid fuels should not purchase by ton but rather pay per 7 ofenergy recei"ed. he reason is that solid fuels such as coal, wood, or bagasse maycarry large 6uantities of water or ash. oth solid fuel components are notcontributing any 7 to the energy input to the system. "en worse they willsignificantly lower system efficiency. =n the case of water, additional energy isneeded to e"aporate the water and in the case of large amounts of ash, somecarbon remains in the ash and "aluable chemical energy is discharged throughthe waste bin.urchasing solid fuels based on ahtL1::: 7 can already reduce energy costssignificantly and will impro"e thermal efficiency, because it may result in reducedwater and tramp ash on deli"ery.he second issue that should be clarified prior to an audit, are so calledopportunity costs of solid waste fuels. Firms that generate their own waste 9sugarmills, rice mills, wood and food processing companies< often wrongly argue thatthe fuel is a waste and therefore for free. =n reality no fuel is for free, because offuel processing and benefication costs from the state of Nas recei"ed 9ar< to Nas

fired 9a fired< as well as lost opportunities to use a"ailable e(cess fuel energy forother purposes such as power generation.reparing a table as shown below for the case of wood highlights the issue. =t isassumed that wood costs the same %:: ahtLton independent of its moisturecontent. Any moisture in a fuel will lower the thermal efficiency as indicated incolumn . Of interest are only the costs for the Nuseful energy found in thesteam.

Table 1: Wood (50 % C, 6% H, 44% O, 0% N, 0% S maf)Moisture

%HHV

MJ/tonLHV

MJ/tonSystem

Eta *Baht/ton

Baht/1000 MJ

Baht/1000 MJ useful

: 1+.B1 18.3+ BB.1

%:: %.3B 3.+:

1: 1B.B [email protected] B%.%B

%:: 8.18 3B.+

: 1%.BB 1.3 B3.@

%:: 31.B1 3.:@

3: 13.8: 1.1 B1.1%

%:: [email protected] %:.+

: 11.83 1:.:@ @B.81

%:: .B @.33

%: +.8@ B.+B @3.1

%:: %:.B1 8:.3

@: B.88 %.8+ %@.:

%:: @3.% 113.@

B: %.+1 3.81 .:@

%:: 8.@: 1+.:1

R *ystem efficiency ta$en from 1: tonLhour steam boiler fired with wood, at 8 D O9drygas, for all moisture contentsighly "iscous fuel oil :.31@ 1.8: %utane :.B8 3.:+ 3ropane :.BB 33.1+ 35ood 9:D >O< :.3B3 :.8 85ood 9:D >O< :.3B3 @1.+@ 8agasse 9%:D >O< :.3B3 B8.8% 8Anthracite 98D C, %D>OOigh sulfur contents in li6uid fuels may cause considerable corrosion problemsin heat e(changers and may force an operator to increase stac$ gastemperature to stay abo"e the dew point.

esides the name plate information of the boiler, a fuel specification sheet shouldbe established.

,uel S!eifiation Sheet for :

,uel .eneri 2esri!tion3

,uel bran2 name3

4etailer / su!!lier

5ltimate hemial analysis

as reei6e27 % mass3

# H $ & S -sh 8ater

,uel bou.ht in units of

Hi.her an2 lo9er Heatin.

Value MJ/unit3

HHV LHV


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+rie as reei6e27 Baht/unit

4emars

*ome of the re6uired data may not be a"ailable or must be calculated from

literature data.

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2. Characteristics & Measurement of Temperature, Pressure, Velocity, Flow