Chemical Plant Equipment & Systems - Topic 3-Heat Transfer Equiment R1

8/9/2019 Chemical Plant Equipment & Systems - Topic 3-Heat Transfer Equiment R1

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CLC122:

CHEMICAL PLANTEQUIPMENT & SYSTEMS

Topic 3: Heat Transfer

Equipment

1



Topic 3.1: Modes of Heat Transfer

• Transfer of heat to and from process fluids is an essential partof most chemical processes in chemical plants.

• Heat is a type of energy called thermal energy and can be

transferred (moved) by three main modes: Conduction

Convection

Radiation.

• During heat transfer, thermal energy always moves in thedirection of Hot to Cold:

HOT COLD2



• Temperature is a measure of how hot an object is.

• Heat transfer only takes place when there is a temperaturedifference. The heat energy flows from a hotter (highertemperature) area to a cooler (lower temperature) area.

• Different materials need different amount of energy to increasetheir temperature by the same amount.

To increase thetemperature of 1 kg

of water by 1°C,

requires 4200 J.

To increase thetemperature of 1 kg

of copper by 1°C,

requires 390 J.


Water

Copper

3



• Water and copper require different amounts of energy becausethey have different values for a property called specific heatcapacity.

• Specific heat capacity is the amount of thermal energy requiredto increase the temperature of 1 kg of a material by 1°C.

• Thus, the specific heat capacity for water is 4200 J/kg°C; andfor copper is 390 J/kg°C.

• We can use specific heat capacity to calculate how muchthermal energy is needed to raise the temperature of a material

by a certain degree:

Thermal

energy

specific heatcapacity

temperaturechange= mass x x

T C mQ p


4



For example:

Knowing the specific heat capacity of water is 4200 J/kg°C, howmuch energy is needed to increase the temperature of 600 g ofwater by 80°C in a kettle?

• Note: mass = 600 g = 0.6 kg (units consistency)

• Energy (Q) = 0.6 kg x 4200 J/kg°C x 80 °C

= 201,600 J

T C mQ p


5



• Molecules arranged in solid, liquid and gas differently.

• Particles that are very close together can transfer heat energyas they vibrate. This type of heat transfer is calledconduction.

• Conduction is the method of heat transfer in solids but not inliquids and gases.

solid liquid gas

Topic 3.1: Modes of Heat Transfer(Conduction)

6



Conduction in non-metals

7



How do metals conduct heat?

• Metals are good conductors ofheat. The outer electrons of metalatoms are not attached to anyparticular atom. They are free tomove between the atoms.

• When a metal is heated, the freeelectrons gain kinetic energy.

• The free electrons move faster andtransfer the energy through themetal.

• Insulators do not have freeelectrons and so they do notconduct heat as good as metals.

heat

Conduction in metals

8



• Liquids and gases are called fluids as they can both flow andbehave in similar ways.

• When a fluid is heated, the heated fluid particles gain energy, sothey move about more and spread out. The same number ofparticles now take up more space, so the fluid has become lessdense.

heat

less dense

fluid

Topic 3.1: Modes of Heat Transfer(Convection)

9



• Warmer regions of a fluid are lessdense than cooler regions of thesame fluid.

• The warmer regions will rise because they are less dense.

• The cooler regions will sink as theyare more dense.

• This is how heat transfer takesplace in fluids and is called

convection.

• The steady flow between the warmand cool sections of a fluid, such asair or water, is called a convectioncurrent.

Topic 3.1: Modes of Heat Transfer(Convection)

10



Convection in Liquid

11



Convection in Gas

12



Convection is important in fridges

• The freezer compartment is at thetop of a fridge because cool airsinks.

•

The freezer cools the air at the topand this cold air cools the food onthe way down.

• It is warmer at the bottom of thefridge.

• This warmer air rises and so aconvection current is set up insidethe fridge, which helps to keep thefridge cool.

Convection in gas

13



• The Earth is warmed by heat energy from the Sun.

• There are no particles between the Sun and the Earth, so the heat

cannot travel by conduction or by convection.

• The heat travels to Earth by electromagnetic waves. These aresimilar to light waves and are able to travel through empty space(vacuum).

Electromagneticwaves

Topic 3.1: Modes of Heat Transfer(Radiation)

14



• Heat can move by travelling as electromagnetic waves. Theseelectromagnetic waves are like light waves, but with a longerwavelength.

Electromagnetic waves act like light waves:

• They can travel through a vacuum.

• They travel at the same speed as light – 300,000,000 m/s.

• They can be reflected and absorbed.

• Electromagnetic waves heat objects that absorb them and are

known as thermal radiation.


15



Radiation emit from surface

16



• All objects emit (give out) some thermal radiation.

• Certain surfaces are better at emitting thermal radiation thanothers.

• Matt black surfaces are the best emitters of radiation.

• Shiny surfaces are the worst emitters of radiation.

white silvermatt

black

best emitter worst emitter


17



Radiation absorb by surface

18



• Thermal radiation heat objects that absorb (take in) them.

• Certain surfaces are better at absorbing thermal radiation thanothers. Good emitters are also good absorbers.

• Matt black surfaces are the best absorbers of radiation.

• Shiny surfaces are the worst absorbers because they reflectmost of the radiation away.

• This is why solar panels used for heating water are covered in a

black outer layer.

best emitter worst emitter

best absorber worst absorber

white silvermatt

black


19



• Heat transfer is an important function of many chemical processes.

• Heat exchangers are equipment that widely used to transfer heatfrom one process to another .

• A heat exchanger allows a hot fluid to transfer heat energy to acooler fluid through conduction and convection. In fired heater , inaddition to conduction and convection, radiation is an important

mode of heat transfer as well.

Topic 3.2: Heat exchangers and theirapplications

20



• The main types of heat transfer equipment used in the chemicalprocess industries are listed below, and they will be introduced in thesubsequent slides.

i. Double-pipe exchanger : the simplest type, used for cooling andheating.

ii. Shell and tube exchangers: used for all applications.

iii. Plate heat exchangers: used for heating and cooling.iv. Air cooled: used as coolers and condensers.

v. Fired heaters: used as charge heaters.


21



Shell and tube heat exchanger

Double pipe heat exchanger

Plate heat exchanger

Air cooled heat exchanger

Fired heaters


22



• The general equation for heat transfer across a surface is:

where Q = heat transferred per unit time, W or J/s

U = the overall heat transfer coefficient, W/m2°C

A = heat-transfer area, m2

ΔTm = the mean temperature difference, the temperaturedriving force,°C

mT UAQ


23



• The overall heat transfer coefficient, U , will depend on the natureof the heat transfer process (conduction, convection,condensation, boiling or radiation), on the physical properties ofthe fluids, on the fluid flow-rates, and on the physical arrangementof the heat-transfer surface.

• During operation, most process fluids will deposit material (fouling) on the heat-transfer surfaces in an exchanger to some extent. The

deposited material will normally have a relatively low thermalconductivity and will reduce the overall coefficient.

• It is therefore necessary to oversize an exchanger to allow for thereduction in performance caused by fouling during operation.


24



• Temperature difference is the driving force by which heat istransferred from a hot fluid to cold fluid.

• General equation for heat transfer across a surface and equationto calculate energy needed to raise the temperature of a fluid canbe used together in heat exchanger design calculation.

mT UAQ T C mQ p and


25



For example:

Calculate the heat transfer surface area of a heat exchangerneeded to raise the temperature of a process stream flowing at100 kg/s from 30 °C to 60 °C. Given that the specific heatcapacity of the process stream is 2500 J/kg°C, the overall heat

transfer coefficient of the heat exchanger is 300 W/m2°C, andthe mean temperature difference is 25 °C

• Heat needed to raise the temperature of a process streamfrom 30 °C to 60 °C is :

• Heat transfer surface needed:

2

2

1000

25300

000,500,7

m

C C m s

J S

J

T U

Q A

T UAQ

m

m

s

J C C kg

J

s

kg

T C mQ p

000,500,7)3060(2500100


26



Double pipe heat exchanger:

• The double pipe heat exchanger incorporates a tube within a tubedesign. Standard shell diameters range from 2” to 6”.

• It can be found with plain tube or externally finned tubes.

• In chemical plants, they are commonly used for high foulingservices and it is economical for smaller duties where the heattransfer area is less than 400 ft2 (40 m2)


27



Finned tube

Double pipe heat exchanger using finned tube



28




• In double pipe heat exchanger, the temperature changes of hotand cold fluid as they pass through a heat exchanger depends onthe flow arrangement.

• In parallel flow (Cocucurrent flow) case, both fluid enter at thesame end of exchanger and exit at the other .


29




• The cold fluid (lowercase t’s) experience a temperature increase,and the hot fluid (capital T’s) a temperature decrease. Thetemperature difference varies from T1-t1 at the inlet to T2-t2 at the

outlet.• The cold fluid exit temperature (t2) cannot exceed the hot fluid exittemperature (T2).


30




• In the counter-flow case, the fluids flow in opposite directions.This is much better than the parallel flow for most applications.

• The cold fluid exit temperature (t2) can exceed the hot fluid exit

temperature (T2) when there is sufficient heat transfer area.

• Counter-current flow maximizes temperature differences betweenshell-side and tube-side fluids.


31



• When using the general equation for heat transfer in double pipe heatexchangers calculation, as the temperature difference between thecold and hot fluid vary continuously from one end of the exchanger tothe other , a log mean average temperature difference (LMTD) is used

for ΔTm.

And

)( LMTDUAQ


differenceretemperaturminalLesser te LTTD

differenceretemperaturminalgreater teGTTD

ln

Where

LTTD

GTTD LTTDGTTD LMTD

32



For example: If hot fluid enters the exchanger at 90 °C exit at 60 °C,and cold fluid enters the exchanger at 40 °C exit at 55 °C.

If they are in parallel flow, the LMTD is:

If they are in counter-flow, the LMTD is:

C LMTD 6.19

5

50ln

55090 °C 60 °C55 °C40 °C

90 °C 60 °C

40 °C65 °C

50 °C 5 °C

25 °C 20 °C

C LMTD

4.22

20

25ln

2025

Note: Counter flow has a higher LMTD due to maximize temperature differences


33



• For example:

ΔTm for Counter current flow:

∆=∆2 − ∆1

ln(∆2∆1

)

=50℃ − 40℃

ln(5040)

= 44.8℃


ΔT1 = (100 – 60)°C = 40°C

ΔT2 = (70 – 20)°C = 50°C

34



ΔTm for Cocucurrent flow:

∆=∆2 − ∆1

ln(∆2∆1)

=80℃ − 30℃

ln(8030)

= 50.9℃


ΔT1 = (70 – 40)°C = 30°CΔT2 = (100 – 20)°C = 80°C

35



Shell and Tube exchanger:

• The shell and tube exchanger is the most commonly used type ofheat-transfer equipment used in the chemical industry.

•

The advantages of this type of heat exchanger are:1. The configuration gives a large surface area in a small volume.

2. Good mechanical layout (a good shape for wide pressurerange operation).

3. Uses well-established fabrication techniques.

4. Can be constructed from a wide range of materials.

5. Easily cleaned.

6. Well-established design procedures.


36




• Essentially, a shell and tube exchanger consists of a bundle oftubes enclosed in a cylindrical shell. The ends of the tubes arefitted into tube sheets, which separate the shell-side and tube-side

fluids.• Baffles are provided in the shell to direct the fluid flow and supportthe tubes. The assembly of baffles and tubes is held together bysupport rods and spacers.


37

Tie Rods Spacers



38




• Tube layout (arrangement) in tube bundle is typically, 1 in tubes ona 1.25 in pitch or 0.75 in tubes on a 1 in pitch.

• Triangular layouts give more tubes in a given shell. However,

triangular pitch makes mechanical cleaning of shell side notpossible. Hence not suitable for high shellside fouling services.

• Square layouts give cleaning lanes with close pitch.

Flow

pitch

Triangular

30o

Rotated

triangular 60o

Square

90o

Rotated

square 45o


39



Square layout Triangular layout

Flow Flow



40




• Flow in the tube side of the shell and tube heat exchangers canhave one or more tube pass through the shell as shown below.

One tube pass per shell

Two tube pass per shellFlow inside tubes

Two tube pass per shell


41




• Shell and tube heat exchangers can be connected in variety ofways. The two most common are series and parallel.

• Series arrangement is used where there are large heat transfer

surface requirements. Parallel arrangement is used where thereare needs to reduce pressure drop through exchangers.


42




During design, following guidelines can be used to allocate fluids inshell and tube heat exchangers.

• Put dirty stream on the tube side - easier to clean inside the tubes.• Put high pressure stream in the tubes to avoid thick, expensiveshell.

• When special materials required for one stream due to for examplecorrosivity, put that stream in the tubes to avoid expensive shell.

• Cross flow over tubes gives higher heat transfer coefficients thanflow in tubes, hence put fluid with lowest coefficient on the shellside.

• If no obvious benefit, try streams both ways and see which givesbest design.


43



Reboilers

• Reboilers are group of shell and tube heat exchangers in whichthe mixed phase flow is important. It is closely associated withdistillation tower operation.

• The major types of reboliers are thermosiphons, kettle, andinternal (stab-in) reboilers.

Thermosiphons Kettle Internal


44



Reboilers - Horizontal thermosiphon

• A fundamental difference between a vertical and horizontalthermosiphon arrangement is fluid assignment.

• For a horizontal thermosiphon, the boiling liquid is placed on the shell

side. Horizontal arrangement allows reboiler to be installed closer tograde which is easier for tube bundle cleaning. However, large plotarea is needed for tube bundle pulling.

• Tube side inlet should be on the top of exchanger and outlet on thebottom. In this way, the density increase as hot fluid cooled will not

affect flow.

Horizontal Thermosiphons Vertical Thermosiphons


45



Reboilers - Vertical thermosiphon

• In vertical thermosiphon, the heating liquid is in the shell side andthe boiling liquid in the tubes.

• For vertical thermosiphon, the tube side has to be single pass.

• Vertical thermosiphons are often attached right to the tower andhave a short process return line. Piping is therefore simple andinexpensive. However, tube bundle cleaning is more difficult.

Vertical Thermosiphons


46



Reboilers - Kettle

• Kettle reboiler is a type of horizontal exchanger. It typically has amulti-pass tube bundle located along the bottom and an over sizeshell providing excess space above the bundle for vapor liquiddisengagement.

• A vertical dam keeps the tubes covered with liquid at all times.Non vaporized liquid flows over the dam into a holding area andwithdrawn as distillation tower bottom product under level control.

Kettle


47



Reboilers – Internal

• Internal reboilers are inserted directly into distillation tower , therebyeliminating the shell and process piping costs.

• They are useful where only moderate or small heat transfer areasare required.

Internal reboiler


48



Plate heat exchangers (PHE)

• PHE are high heat transfer surface exchanger.

• They consist of a series of gasketed plates, sandwiched togetherby two end plate and compression bolts.

• The channels between the plates are designed to creaseturbulent flow so high heat transfer coefficients can be achieved.


49



Plate heat exchangers (PHE)

• As hot fluid enters the hot inlet port on the fixed end cover of PHE,it is directed into alternating plate sections by a common discharge

header.

• As cold fluid enters the counterflow cold inlet port, it is directed intoalternating plate sections. Cold fluid moves up the plates while hotfluid flows down across the plates.

• Heat energy is transferred through the walls of the plates byconduction and into the liquid by convection.


50



Air cooled heat exchangers

• They compose of a structured matrix of plain or finned tubesconnected to an inlet and return header .

• Rectangular tube bundle contains several rows of tubes on atriangular pitch arrangement. The hot fluid entering at the top rowof the bundle and air flowing vertically upward through thebundle.


51



Air cooled heat exchangers

• Air cooled heat exchanger are used when the use of cooling wateris impractical due to high process temperatures.

•

The two most common types of Air cooled heat exchanger use inchemical plants are:

1. Forced-draft type, where air is pushed across the tube bundle.

2. Induced-draft type, where air is pulled through the bundle.

Force draft Induced draft


52



Week 7 Class Test (Tue during lecture)

•Topic 1, Topic 2, Topic 3.1

•Test duration: 1 hour

•TEN (10) questions in Section A – Fill in the blank

•THREE (3) questions in Section B – calculationsand explanations

• Answer all questions

•Equations for calculating pump Liquid Horsepower

and capacity of a rotary pump will be given• All other equations will not be given

•Unit conversion factors will be given if needed

53



• A fired heater or furnace is a direct-fired heat exchanger that uses theflame and hot gases (flue gas) ofcombustion to raise the temperatureof a process fluid flowing through

coils of tubes that run along the insidewalls and roof of the heater .

• The combustion happen in burners and the hydrocarbon fuel used forburners can be in gaseous or liquidform.

• The primary modes of heat transfer ina fired heater are radiation andconvection; however, heat must passthrough the tube walls by conduction to be absorbed by the fluid flowing

inside the tubes.

Topic 3.3: Fired Heaters

54

Radiation& Convection

Conduction

Convection



Combustions• Combustion is a rapid chemical reaction that occurs when theproper amounts of fuel and oxygen in the air come into contactwith an ignition source and release heat and light.

• Fired heaters use combustion reaction of hydrocarbon fuel in

burners to provide heat for heating of process fluids.

• Complete combustion occurs when the hydrocarbon fuel and airare in correct proportions, and all the fuel is converted to waterand carbon dioxide. Complete combustion is desirable duringfired heater operations. CxH2y + (x+y/2)O2 ---------> xCO2 + yH2O


56



Incomplete combustion:

• Incomplete combustion can occurs in fired heaters wheninadequate oxygen exist, caused by insufficient supply of air to theheaters. This leads to presence of unburned fuel and production of

carbon monoxide in the heaters.

• Incomplete combustion is undesirable as it wastes fuel, unburnedfuel and carbon monoxide produced can cause after burning whichcan lead to wall, tubes and stack damage, scaling on tubesexternal by carbon black particles can reduce heat transfer , andflame impingement can occur due to elongated flame resulted frominsufficient oxygen.


57



Flame impingement:

• Flame impingement refers to the burner flames touching the tubes and the wall of the furnace. Generally, in any heater, the flameshould be as short as possible to give uniform temperature in the

heater and prevent flame.

• Flame impingement on the tubes causes local overheating ofprocess fluid flowing through the tubes. For hydrocarbon fluids,decomposition and coke formation will occur inside the tube andcan lead to mechanical failure of tube wall.


58

High tube wall temperature aspoor heat removal by fluid flowinginside tube due to coke formedinside tube

lead to



Flame impingement:

After burning

• After burning is the delayed combustion of air and unburned fuel

in the upper section of the heater due to air leakage at thatsection. After burning can cause damage to the walls, tubes andstack .

Flame impingement


59



60

After Burningcan occur if there isair leaking in atconvection section

Incomplete combustionresults in unburned fuelin flue gas.



Theoretical air and Excess air

• In heater operations, theoretical air refers to exact amount of air,based on chemical reaction equation, that will completely convert

all the fuel to water and carbon dioxide.

• To assure completion combustion, amount of air supply to theheater burners must be above the theoretical air , the mount of airexceeded the theoretical air is call “excess air” which is expressed

as % excess.


61

CxH2y + (x+y/2)O2 ---------> xCO2 + yH2O



Theoretical air and Excess air

• Excess air increase the amount of oxygen and the probability ofcombustion of all fuels. Typical excess air to achieve good heateroperations are 10 – 15 vol% excess for natural gas, 15 – 20 vol%excess for fuel oil.

• In theory, the most efficient combustion is achieved by keeping theexcess air as low as possible. However, low excess air is limited

by:Reaching incomplete combustion

Obtaining poor heat distribution among burners

Establishing an unstable and long flame pattern


62



Heating value of fuels

• Different fuels release different amount of heat energy as they areburned. The heat energy referred to as the net heating value (orcalled low heating value, LHV) is measured in Btu per volume orper mass or per mole. For example, hydrogen has the LHV of 274Btu/ft3, 51596 Btu/lb whereas CH4 has a LHV of about 909 Btu/ft3,

21522 Btu/lb.


63



Fire Heater Sections:• Fired heaters come in a variety of shapes and sizes, havedifferent tube arrangements and feed inlets, burn differenttypes of fuels and have different burner design. However, theydo have sections in common which are:

1. Radiant section

2. Shield section

3. Bridgewall

4. Convection section

5. Breeching

6. stack


64



Radiant section:

It is where heater fire box located. Firebox contains burners and it’s wall is lined with a refractory layer that reflects heat back into theheater . The radiant tubes, either horizontal or vertical, are locatedalong the walls of the firebox and receive radiant heat directly from

the flame and refractory wall. Heat is transferred by radiation fromflame and wall, convection by flue gas, and by conduction throughthe tube wall to process fluid flowing inside the tubes. Sight doorsare provided at firebox wall for operators to check flame pattern andfirebox conditions.


65



Bridgewall: This is the area between radiantsection and shield section. Several importantmeasurements are normally made atbridgewall, e.g. bridgewall temperature is thetemperature of the flue gas after the radiant

heat is removed by the radiant tubes andbefore it enters the convection section.

Measurement of the draft (negative pressureinside heater) is also important since thisdetermines how well draft in the heater is set

up.

This is also the ideal place for flue gasoxygen and ppm (parts per million)combustibles measurement.


66



Shield Section: This is above the radiantsection, and contains two rows of bare tubes(without fins or studs) which shield theconvection section tubes from the directradiant heat.


67



Convection section: This section is located in the upper part of theheater , immediately above the shield section. At this section, fluegas heat is transferred to the tubes by convection and throughtubes wall by conduction. The tubes are usually finned type orstudded type to increase heat transfer area.


68



Breeching: The transition from the convection section to the stack iscalled breeching. Measurement of stack emissions for compliancepurposes is normally made at breeching.

Stack: As the flue gas contains combustion by products such asNOx, SOx, stack is necessary to discharge flue gas at height in theatmosphere such that they cannot endanger personnel and thatpollution requirements are met.

Stack Damper : Stack damper at the stack inlet permits adjustmentof heater drafts.

Stack Damper


69



Draft

• Draft is defined as the negative pressureof the flue gas measure at any pointwithin the heater .

• Negative pressure or draft occursbecause the hot flue gas within theheater is less dense than thesurrounding atmospheric air .

• The greater the difference in densities,the greater the draft or negativepressure within the heater.


70



Draft

• The difference in densities causes air toflow into the heater through the burnersor through other openings, and the hotflue gas to flow out of the heater .

• In this manner the movement of cooloxygen containing air through the heaterbecome continuous.

• The control of draft or flow of air throughthe heater is by damper placed at heaterstack inlet In process plants, stackdampers resemble huge butterfly valvesand require only one quarter turn to be

100% open or close.


71



Flow of air into heater causes by draft

Cool AirCool Air Hot flue gas

Low Pressure

High Pressure


72



Burners:

• Air enters through a muffler whichdampens the noise from the burners.

•

The plenum (windbox), distributes air tothe burner throat and dampens thenoise from the firebox.

• The burner tile is a refractory piece thatshapes and stabilizes the flame.

• One or more burner tips are used toinject fuel into the air stream. A smallpilot burner provides an ignition sourcefor the main burner.


73



• Burners can be classified by their flame shape as well, the twomost common flame shapes are round and rectangular .

Round Flame Burners Rectangular Flame Wall-Fired Burners


74



Burners:

• Many refinery and chemical plants useboth liquid and gas fuel for the heaters.For such heaters, combination gas/oilburners will be used.

• Firing a liquid fuel requires atomizationof the liquid. Breaking liquid fuel intosmaller droplet allow fast surfacevaporization, providing the required gasphase for mixing with the air during

combustion.

Oil Gun Atomizationof liquid fuel

Atomization steam


75



Burners:

• Atomization of liquid fuel is done by oilgun. Most oil guns have a concentrictube design in which oil flow through theinner tube while the atomizing medium,

steam, flow through the annular between the inner and outer tube. Fuelatomizers provide proper mixingbetween the oil and steam in oil burners


76

Atomization in Oil Gun



Flame patterns:

Pilot Flame Oil Firing Gas Firing


77



Pilot Burners:

• Pilot burners are small, independentlycontrolled burners that act as an ignitionsource for the larger process burners.

• Pilot burners are predominantlypremixed. The mixer which premixes theair and fuel, is located externally to theburner housing, so the combustion airfor the pilot burners is typically ambientair.

Premix gas pilot


78



Flame out protection:• To prevent forming of explosive air/ fuelmixture inside heater , flameoutprotection is required for each burner ina heater. This is usually accomplished

by using gas-fired continuous pilot.• The main function of a continuous pilotis to ensure a safe source of ignitionalways available for re-ignition of mainburner.

•

For such pilot burner, pilot flameoutdetection is accomplished by usingflame rod. In the event that pilot flame isout, a safety system will shut off the flowof fuel to both the main and pilot burner .Pilot fuel

Main burner fuel

shut off valve


Chemical Plant Equipment & Systems - Topic 3-Heat Transfer Equiment R1

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