2. 4. Temperatures to prevent equipment failure or reduced...

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Why this module is important

The transfer of heat is necessary for control of:

1.A fluid

temperature and/or its

composition and phase

2.The rate of

mass transfer between phases

3.The rate of chemical reactions

4.Temperatures to

prevent equipment failure

or reduced service life.

a. Direct

b. Indirect

a. Air-cooled heatexchangers(ACHE)

b. Cooling towers

c. CombinationAir-Water


a. Pipe-in-pipe

b. Shell-and-tube

c. Plate Exchangers

d. Brazed Aluminum(Plate-Fin)

e. Printed Circuit,Spiral

f. Coil and SpecialTypes for SpecificServices

1. Fluid-Fluid 2. Fired Heaters(Direct and Indirect)

CoolersUtilizing Air

Heat Transfer Equipment Overview Core ═══════════════════════════════════════════════════════════════════════════════════

©PetroSkills, LLC. All Rights Reserved. _________________________________________________________________________________________________________

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Heat exchangers normally cost less per unit of energytransferred than any other type of energy equipment

Compromises on exchanger size can significantly increase thecost of companion equipment in many instances

Since heat loads vary with flowrates, some flexibility must beprovided

The majority of less-than-satisfactory exchanger installations areas much the fault of the customer as it is the vendor



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Types of Heat Exchangers and Their Common Applications in Oil and Gas

Processing Facilities

Heat Transfer EquipmentOverview Core

This section will cover the following learning objectives:

Learning Objectives

Identify types of heat exchangers and common applications in oiland gas processing facilities

This section will cover the following learning objective:



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Heat Exchanger Types and Applications




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Direct

Indirect

Air-cooled heat exchangers (ACHE)

Cooling towers

Combinationair-water

Types of Heat Transfer Equipment

Shell-and-tube

Compact• Pipe-in-pipe• Plate exchangers• Brazed aluminum

(plate-fin)• Printed circuit

Spiral• Coil• Special types for

specific services

Fluid-FluidFired Heaters(Radiant Heat

Transfer)

CoolersUtilizing Air



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Shell and Tube Heat Exchangers

Most common and most versatile exchanger type

Can handle a wide range of fluids – single phase and multiphase

Robust construction

Variety of fabrication materials possible

Heavy

Relatively large footprint

Expensive

Compact Heat Exchangers

Applications in the oil and gas industry have grown significantly over the past 20-30 years

Relative to shell and tube exchangers:• Smaller size and weight

• Decreased temperature approach and higher efficiency

• Lower cost, especially when expensive materials of construction are required

Several types of compact exchangers:• Plate

• Brazed Aluminum Plate-Fin (BAHX)

• Printed Circuit (PCHE)

• Core-and-Kettle

• Pipe-In-Pipe

• Pipe Coils



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Fired Heaters

Fired heaters are the most common type for high temperatures

Direct Fired Heaters • Two types:

– Combustion in a fire box, process fluid flows through the tubes(used for larger duties)

– Combustion in a fire tube usually immersed in the process fluid(used for smaller duties)

Indirect Fired Heaters• Typically a fire tube heater, fire tube immersed in heat transfer fluid

– Water used as heat transfer fluid in lower temperature applications

– Molten eutectic salts used in higher temperature applications

• Heat transfer to the process fluid occurs in a second tube bundle, which is immersed in the heat transfer fluid

Coolers Utilizing Air

Air-Cooled Heat Exchangers (ACHE)• Most popular in onshore facilities

• Not as popular offshore due to large footprint

• Low environmental impact

Cooling Towers• Air supplies cooling by evaporating water

• Water is the heat transfer fluid

• More efficient than air coolers, but

• Higher CAPEX and environmental impact

• Not widely used in upstream and midstream applications

• More popular in refineries and chemical plants

Combination Air-Water



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This section has covered the following learning objectives:

Learning Objectives

This section has covered the following learning objective:

Identify types of heat exchangers and common applications in oil and gas processing facilities



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Heat Transfer Mechanisms and Parameters Affecting Heat Transfer Coefficient



Learning Objectives

Describe heat transfer mechanisms: conduction, convection and radiation

Define heat transfer coefficient and describe the primary parameters that affect its value




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Heat Transfer Mechanisms

Heat is energy transferred as a result of a temperature difference

Conduction• Transfer of thermal energy through a substance due to a

temperature gradient

Convection• Transfer of thermal energy due to bulk fluid motion caused by the

presence of a temperature gradient and its effect on fluid properties

Relative magnitude (1-Lowest and 4-Highest):

Radiation• A hot body radiates heat that may be absorbed, reflected or

transmitted to a colder body

1.Conduction

2.Natural

Convection

3.Laminar Forced

Convection

4.Turbulent Forced

Convection

t1 2

( )(L )

ln( / ) / (2 )W

o i

kQ T T

r r

Tube Wall Conduction Heat Transfer

(k)(DrivingForce)

ResistanceForceQ



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Combined Convection & Conduction Heat Transfer

convectionconduction

4o

1

ln(r / 1)

21 i

i i oW oh A

rT

A

T

k

Q

L h

convection

Overall Heat Transfer Coefficient, Uo

Tube (Ao πDoL andAi πDiL)

oln(D / D )1 1

2o o i

o i i W o

D D

U hD k L h

1 ln ⁄2

1 1



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Fouled Overall Heat Transfer Coefficient, Uo

oln(D / D )1 1

2 o o i o

i oo i i W o i

D D Df f

U h D k L h D

Temperature Gradient Through a Fouled Pipe Wall



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Fouling Factors

Avoid using fouling factors as an arbitrary safety factor

Empirical and estimated from actual operating data

Fouling is dependent upon the velocity of the fluid in theexchanger

Rule of Thumb: The fouling factors should not contribute morethan 20% excess area to the HEX design

If fouling factors have been specified, vendors typically provideheat exchanger data sheets with Uclean and Uservice performance

Clean overall heat transfer coefficient Uclean will be greater thanservice overall heat transfer coefficient Uservice

Overall Heat Transfer Coefficients

Correlations like theseare not recommendedfor design calculations,but are useful forplanning or scopingstudies

Calculation of overallheat transfer coefficientUo from the equation onthe previous slides maybe necessary

Values of thermalconductivity, k, and filmheat transfer coefficient,h, are required



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Effect of Velocity on Performance

Fluid velocity has a significant effect on exchanger performance

As velocity increases, h increases and therefore Uo increases; for agiven heat exchanger area, the heat exchanger duty increases asvelocity increases

hi ∝v0.8For flow inside a tube:

ho ∝v0.6For flow outside a tube:

Effect of Velocity on Pressure Drop

As the fluid velocity increases, the pressure drop also increases

∆P∝v1.8For flow inside a tube:



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Learning Objectives


Describe heat transfer mechanisms: conduction, convection and radiation

Define heat transfer coefficient and describe the primary parameters that affect its value



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Estimating Exchangers’ Heat Transfer Area



Learning Objectives

Describe the rate equation used to calculate heat transfer area

Describe the “effective temperature difference” and explain how it affects heat transfer area

Estimate heat transfer surface area required for a heat exchanger application




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Rate Equation

∆Teff is the effective temperature difference in the exchanger

For conventional exchanger configurations and no phase change, it can be estimated from a simple average equation

For more complex exchangers and a phase change in one or both of the fluids, it can be estimated by dividing the exchanger into sections and using numerical integration

The smaller the effective temperature difference, the more surface area required

∆Teff

Tem

pera

ture

Heat Transferred, Q(a)

HotEnd

T2 ColdEnd

T1

Heat Transfer Equation

Where: Q = heat transfer rateUo = overall heat transfer coefficientA = Heat exchanger area

teff = Effective temperature difference

Tem

pera

ture


HotEnd

T2

ColdEnd

T1

Rate Equation

∆Teff is the effective temperature difference in the exchanger

For conventional exchanger configurations and no phase change, it can be estimated from a simple average equation

For more complex exchangers and a phase change in one or both of the fluids, it can be estimated by dividing the exchanger into sections and using numerical integration

The smaller the effective temperature difference, the more surface area required

∆Teff

Tem

pera

ture


HotEnd

T2 ColdEnd

T1

Heat Transfer Equation

Where: Q = heat transfer rateUo = overall heat transfer coefficientA = Heat exchanger area

teff = Effective temperature difference



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Effective Temperature Difference Schematics


Example: oil being cooled

with water



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with water


with water




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Example: chiller in a gas processing

facility


Example: gas-gas exchanger



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Effective Temperature Difference SchematicsExample: gas being cooled by a multicomponent

refrigerant, or side reboiler on a de-methanizer using

feed gas as the heat source

Log Mean Temperature Difference

Assumptions:1. The heating and cooling curves are

linear

2. The physical properties of the fluids do not significantly change in the exchanger

ln



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• The minimum approach may occur at the “hot” end or the “cold” end of the exchanger depending on the application.

• The minimum approach may also occur inside the exchanger.

• Smaller values of T2 decrease utility costs (power and fuel) because there is less “lost work” in the heat transfer process.

• As T2 decreases, Teff decreases and the required heat transfer area increases.

• This increase can be significant as Teff approaches zero.

The minimum temperature approach is an economic choice

Suggested Approach Temperatures

The minimum temperature approach is an economic choice

Suggested Approach Temperatures



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Energy Balance

In a fluid exchanger, the energy balance for each fluid reduces to H = Q

The Q of one fluid = the Q of the other fluid if one ignores heat losses to, or heat gains from, surroundings

Heat loss or gain is normally considered to be zero in exchanger heat balances

*Cp can be used when no phase change

occurs.

T1m12

1

m34

T33

T44

T2

2

∗

∗



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Learning Objectives


Describe the rate equation used to calculate heat transfer area

Describe the “effective temperature difference” and explain how it affects heat transfer area

Estimate heat transfer surface area required for a heat exchanger application



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Shell and Tube Exchanger Types and Their Applications



Learning Objectives

Describe shell and tube exchanger types and applications




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Shell and Tube Exchangers

TEMA: Tubular ExchangerManufacturers Association

TEMA defines three classesof mechanical standards:

• Class R

• Class B

• Class C

Designation of exchangershown below: AKT

(Courtesy Tubular Exchanger Mfgrs. Assn. )

Fixed Tubesheet, Straight Tube

Advantages:• Lowest cost of any TEMA type, especially type NEN

• Provides the maximum surface area for a given shell and tube diameter

• Can be constructed with multiple tube passes to optimize tube velocity

Disadvantages:• Shell side can only be cleaned by chemical methods• Differential thermal expansion

Most common application of this exchanger type: gas-gas exchanger

Type BEM

Expansion joint



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Fixed Tubesheet, Straight Tube

This TEMA type is the simplest design (others include BEM, AEM, NEN)

The tubesheet is welded to the shell

The heads are either bolted to the tubesheet or, in the NEN design,welded to the tubesheet

Type BEM

Expansion joint

Removable Bundle, Floating Head w/Internal Split Ring

This TEMA Type is widely used for applications requiring frequent tubebundle removal for inspection and cleaning

Advantages:• Floating head design allows for differential thermal expansion between the

shell and tube bundle• Inside of shell can be inspected and cleaned

• Tubes can be mechanically cleaned (square layout only)

• Less expensive per unit of surface area than pull-through designs

Type AES

Split backing ring



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Removable Bundle, Floating Head w/Internal Split Ring

Disadvantages:• Higher maintenance than pull-through designs: shell cover, split backing ring

and floating head cover must be removed to pull tube bundle

• More expensive than fixed tubesheet or U-tube types

Applications:• Those requiring frequent tube bundle removal for inspection and cleaning

• For large differential temperatures between the shell and tube fluids

Type AES

Split backing ring

Removable U-Tube Bundle

This TEMA Type is widely used for applications requiring frequent tubebundle removal for inspection and cleaning

Advantages:• Allows for differential thermal expansion between the shell and the tube bundle

as well as for individual tubes• Inside of shell can be inspected and cleaned

• Less costly than floating head designs

• Removable tube bundle• Capable of withstanding thermal shock applications

Type CFU



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Removable U-Tube Bundle

Disadvantages:• U-tubes cannot be mechanically cleaned

• Individual tubes are difficult to replace

• Single tube passes or true countercurrent flow is not possible• Tube wall thickness in the U-bend is thinner than in straight portion of tubes

Applications:• Oil, chemical and water heating applications

Type CFU

Other Designs

Pull through floating heads (TEMA Type T)

• Easier maintenance than S type because there is no backing ring

• Lower surface area per shell diameter than S and U types

Outside packed floating head (Type P) and externally sealed floatingtubesheet (Type W)

• Not suitable for most oil and gas applications because of limited integrityof sealing mechanism and the flammability and toxicity of fluids

Shell (Types G, H, J and X)

• Type G and H shells are often used in low pressure drop applications,such as thermosiphon reboilers

• J-type (divided flow) shells shorten the shellside fluid flow path; these areoften used in low pressure-drop applications

• X-type (crossflow) shells are also used in very low pressure-drop servicessuch as condensers; multiple inlet and outlet nozzles can be used



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Tubes

60°Triangular

30°Rotated

Triangular

90°Square

45°RotatedSquare

The most common tube diameter is19 mm [3/4 in]

Triangular is the most common layout

Larger tubes are easier to clean andare sometimes used in severe foulingservices

A square layout is preferred inremovable tube bundle applications,because it is easier to clean

Both triangular and square layoutscan be rotated to achieve moredesirable performance

Baffles

Baffles support the tube bundle andincrease the heat transfer coefficient byforcing the shell-side fluid to traverse thetube bundle several times

The baffle cut is expressed as a fraction ofinside shell diameter; typical baffle cutsrange from 0.2 to 0.35

The opening is often called the bafflewindow, and should provide roughly thesame flow area as the crossflow areabetween the baffles

The distance between the baffles istermed the baffle pitch; it typically rangesfrom 20-50% of the shell diameter

More baffles result in higher shell sideheat transfer coefficient and pressure drop

The most common type is the singlesegmental baffle

Baffle cut

Baffle window



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Fluid Placement

Shell-Side

1. Viscous fluid

2. Fluid having the lowerflowrate

3. Boiling fluid

4. Condensing fluids intotal condensers

5. Fluid having loweravailable pressure drop

Tube-Side

1. Toxic and lethal fluid

2. Corrosive fluid

3. Fouling fluid

4. High temperature fluid

5. High pressure fluid

6. Fluid requiring inhibitorinjection

7. Partially condensingfluid



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Learning Objectives


Describe shell and tube exchanger types and applications



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Compact Heat Exchangers and Fired Heaters



Learning Objectives

Describe compact heat exchangers and fired heaters




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Gasketed Plate Heat Exchangers (PHE)

(Courtesy of Tranter)

Commonapplications includesea water-coolingmedium exchangeand crude oilcoolers offshore

Onshore, they havebeen used in lowpressure fluid-fluidapplications, suchas lean-rich amineand lean-rich glycolexchangers

Advantages Disadvantages

1. Compact with a small footprint 1. Limited operating temperature range

2. Cost effective 2. Limited operating pressure range

3. Plates can be added, removed orrearranged in the plated rack for differentprocess operating requirements

3. Gasket materials may not be compatiblefor fluids

4. Can easily be taken apart for cleaning 1 4. Gasket maintenance

5. Fluid leakage due to damage gasket isexternal and is easily detected

1 Gasketed PHEs are easy to disassemble and clean, however care must be taken to prevent damage to the gaskets during disassembly. Some companies may prefer to purchase a spare plate pack to allow plate pack replacement when maintenance is required. The fouled plate pack is then sent to the vendor for cleaning and maintenance.

Gasketed PHEs Advantages and Disadvantages



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Shell and Tube Plate Heat Exchanger

Passes One shell side / four tube side One / one

Materials All carbon steel Carbon steel frame / 316 SS plates

Design Pressure / Temperature

689 kPa / 121⁰C [100 psi / 250⁰F] 689 kPa / 121⁰C [100 psi / 250⁰F]

Overall “U” 392 W/m2⁰C [58 Btu/hr‐ft2‐⁰F] 1 908 W/m2⁰C [336 Btu/hr‐ft2‐⁰F]

Required Surface 366 m2 [3940 ft2] 59 m2 [635 ft2]

Pressure Drop Hot side: 145 kPa [21 psi]Cold side: 34 kPa [5 psi]

Hot side: 67 kPa [9.7 psi]Cold side: 57 kPa [8.2 psi]

Size 0.91 m diameter / 7.32 m length[3 ft diameter / 24 ft length]

1.52 m length / 0.91 m width[5 ft length / 3 ft width]

Weight 10 796 kg [23 800 lb] 1740 kg [3837 lb]

Plot space required 1.22 m x 18.29 m [4 ft x 60 ft] 1.52 m x 1.83 m [5 ft x 6 ft]

Quoted price ratio 1.0 0.33

Ratio of cost/surface area 1.0 2.02

Example: Shell & Tube vs. Plate Heat Exchanger

Schematic of a Semi-Welded PHE

Used for refrigeration applications (condensing and boiling) on thewelded side of the exchanger

(Courtesy ITT Industries)

Ring gasket

Aggressive fluid

Non-aggressive fluid

Field gasket

Cassettes

Welded plateflowunits are ideal for handling aggressive fluids



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Block Style Fully Welded PHE

Used for TEG dehydration (lean/rich exchanger), aminesweetening, and fractionation tower condenser

The block style fully welded PHE is used frequently, where leakscould be hazardous to personnel or the environment

(Courtesy Alfa Laval)Support

Lowerhead

Gasket

Baffle

Heat transferplate pack

UpperheadPanel

Girder

Welded PHEs Advantages and Disadvantages

The advantages and disadvantages of welded plate exchangersare similar to the gasketed and semi welded PHE, with exceptionsprovided in this table:


1. Higher design temperature 1. Mechanical cleaning moredifficult, and impossible on sometypes

2. Higher design pressure

3. Reduced or eliminated gasketrequirements



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Basic Components of a Brazed Aluminum Heat Exchanger

(Courtesy Chart Heat Exchangers)

Plate-Fin Heat Exchanger Brazed aluminum

plate-fin heat exchangers (BAHX) are frequently used in low temperature gas processing service

Applications:

• Deep NGL recovery

• Nitrogen rejection

• Air separation units

• Helium recovery

• LNG production

• Refrigeration

Plate Fin Exchanger (BAHX)

Composed ofalternating layers ofcorrugated fins andflat separator sheetscalled parting sheets

Each fluid pass in acore has theappearance of asection of the wall ofa cardboard box

Number of layers,type of fins, stackingarrangement, andstream circuiting willvary



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BAHX Advantages and Disadvantages

Advantages• Compact, lightweight and

efficient (25 times more surfacearea per unit weight than anequivalent shell and tubeexchanger)

• Can combine multiple fluids andduties (cold box)

• Cost effective, especially forclean gases and lighthydrocarbon liquids

• Minimum design temperature is4 K [-269 C, -452 F]

• Can achieve temperatureapproach of 1 C [2 F]

Disadvantages• Mechanical cleaning

difficult/impossible

• Mercury corrosion

• Vulnerable to fire

• Maximum pressure ≈ 100 barg[1450 psig]

• Limited size

• Vulnerable to temperaturecycling fatigue

• Complex design procedure

Core and Kettle Exchanger

Advantages overshell and tubeexchangers:• Significantly higher

heat transfer surfacearea per unit volume

• Temperatureapproaches of 1 °C[2 °F]

Used as chillers andcondensers in gasprocessing and LNGplants with boilingrefrigerant as thecooling medium

(Courtesy Chart Heat Exchangers)



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Printed Circuit Exchanger (PCHE) Stacked Plates

Printed circuit exchangers (PCHEs) were introduced in the oil andgas industry in the early 1980s

PCHEs are constructed from flat metal plates into which flowchannels have been milled or chemically etched

Passages are typically 1-2 mm [0.04-0.08 in] deep







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Two or more fluids can be accommodated in the core

Diffusion bonding is a welding process in which the plates arecompressed together and heated to just below the meltingtemperature of the material

Printed Circuit Heat Exchanger

(Courtesy Heatric)

To complete exchangerconstruction, fluidheaders and nozzlesare welded to the coreto direct the fluids tothe appropriatepassages

Design pressures arevery high, up to 50MPa [7280 psig]

The most commonservice for PCHEs aredischarge coolers andgas-to-gas exchangersin offshoreenvironments



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PCHE Advantages and Disadvantages

Advantages

• Compact, lightweight andefficient

• Size and weight < 25% ofshell and tube heatexchanger

• Good for very high pressure≈ 700 bar and clean, non-fouling fluids

• No pressure relief required

Disadvantages

• Small (2 mm) flow passagesso plugging can be an issue

• Cannot mechanically clean

• Not suitable for highviscosity liquids

• Susceptible to thermalstress failure in temperaturecycling services

Finned Tubes and Pipe-in-Pipe Heat Exchangers

This pipe-in-pipe exchanger may be advantageous for relativelylow heat loads, where one stream is a gas or viscous liquid or forrelatively small exchangers operating at high pressure

All fins shown are on the outside of the tubes, but they also canbe used inside – the shape and style vary widely

Single Tube with Fins

Multi-Tube with Fins



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Pipe-in-Pipe Advantages and Disadvantages

Pipe-in-pipe exchangers are configured such that the fluid’s flow is truecounter-current

The upper economic limit of these exchangers is a UA of 79 kW/°C[150 000 Btu/hr-°F]

With corrosive fluids, erosion-corrosion may be enhanced due toimpingement and turbulence problems

Advantages

• True counter-current flow

• Good for viscous liquids

• Good for high pressure

• Can easily be enlarged orreduced in size by adding orremoving a single tube unit

Disadvantages

• Limited in size, good forsmall heat loads

• Fins increase ∆P, difficult toclean

Examples of Coil Wound Heat Exchangers

Very popular forLNG service

Minimum resistanceto flow

Maximum surfacearea per unit weightand volume

Manufactured fromaluminum

Multiple tubebundles to handleseveral fluids

(Courtesy Linde Engineering)



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Indirect Fired Heaters

Typically a fire tube heater in which the fire tube is immersed inheat transfer fluid

Process fluid circulates through a second heat transfer coilimmersed in the heater transfer fluid

Water bath - applications < 100 °C [212 °F]• Natural gas line heaters and natural gas heaters upstream of

pressure let-down stations

Molten eutectic salt bath > 260 °C [500 °F]• Regeneration-gas heaters in small dry-desiccant dehydration

systems

• Crude oil and condensate stabilizer reboilers

Example of an Indirect Fired Heater

Used to heat oil and gas in production operations, where the heatloads are not large



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Primary Applications of Fired Heaters

Direct Fired Heaters (Q = 3 to 100 MW [10 to 340 MMBtu/hr]):• Combustion in a fire box, process fluid flows through the tubes

• Typically used in high heat duty applications– Boilers

– Still bottom heaters in lean-oil plants

– Regeneration-gas heaters in dry-desiccant dehydration systems

– Hot oil (or other heat transfer fluid) heaters

– Oil heaters upstream of oil dehydration units

– Crude oil or condensate stabilizer reboilers

• Combustion in a fire tube usually immersed in the process fluid

• Typically used in smaller heat duty applications– Small boilers

– Glycol reboilers

– Heater treaters in oil dehydration applications

– Reboilers in small amine systems

Common Direct Fired Heater Types in Oil and Gas Processing



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Learning Objectives


Describe compact heat exchangers and fired heaters



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Process Cooling Methods and Air-cooled Heat Exchangers (ACHE)



Learning Objectives

List the four primary process cooling (heat rejection) methods

Describe why air-cooled heat exchangers are so frequently used,key operating parameters, and the difference between induceddraft and forced draft designs




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Process Cooling

In all processes, heat must be rejected to ambient (heat sink)

Methods:1. Once-Through Cooling Water (Direct Cooling)

2. Cooling Towers

3. Indirect Heating Medium

4. Air-Cooled Heat Exchangers (ACHE)

Applications include:1. Compressor aftercoolers

2. Refrigeration condensers

3. Reflux condensers

4. Steam condensers

PFD of a Direct Cooling Water System

Used in offshore facilities and onshore facilities located near a large body of water, e.g. sea, lake, river, et. al.

Electrochlorinator

To Water Injection or Overboard

Course Filter

Sea Water Lift Pumps

To Utility Users

Cooling Loads



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Advantages and Disadvantages

Once-Through Cooling Water

Heat Sink Temperature: water ambient temperatureHeat Transfer Fluid: waterApproach: 5-10 °C [9-18 °F] (process fluid to heat sink temperature)

Advantages: 1) Simplicity, low capital cost2) Typically gives lowest process temperatures3) Water is heat transfer fluid4) Exchangers are small, minimizing footprint5) Water is less susceptible to ambient temperature fluctuations6) Less equipment than an indirect cooling system7) Potentially lower CAPEX than indirect systems, especially for only a few cooling loads

Disadvantages: 1) Limited availability, e.g., desert applications2) Temperature limits on water returned to environment3) Water is usually corrosive and fouling, this can be a significant problem for systems thatuse sea water4) Freezing5) Water can be contaminated by process fluid creating environmental discharge issues7) Low sea water temperature may cause hydrate problems in the process8) Sea water systems require corrosion resistant metallurgy, which is often titanium

Once-Through Cooling Water

Advantages

Disadvantages

PFD of an Indirect Cooling Medium System

Used in offshore facilities

Cooling Loads

Cooling Medium CoolersCooling Medium

Reciric Pump

Centrifugal Pump

Cooling Medium Expansion Drum



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Indirect Cooling Medium

Heat Sink Temperature: Ambient water temperatureHeat Transfer Fluid: Ambient waterApproach*: 3-5 °C [6-9 °F] on ambient water / water heat exchangerApproach*: 5-10 °C [9-18 °F] on process heat exchangers

Advantages: 1) Less equipment exposed to ambient water, which is corrosive and fouling2) Allows a wider range of heat exchanger options3) Allows the use of less aggressive cooling medium, such as glycol / watermixtures4) Less susceptible to ambient temperature fluctuations

Disadvantages: 1) Requires additional larger heat exchanger and circulation pumps2) Environmental limits on temperature of ambient water return3) Sea water / water can be contaminated with indirect cooling medium4) Freezing in some locations

* process fluid to heat sink temperature

Indirect Cooling Medium

Advantages

Disadvantages

PFD of a Cooling Tower System

Cooling towers are seldom used in oil and gas processing applications

Large supply of ambient water not necessary

Warm WaterReturn

Air

Blowdown

Make-up Water

Cooled Waterto Process

Cooling Loads

Cooling Water PumpsCooling Tower



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Cooling Towers

Heat Sink Temperature: wet-bulb air temperatureHeat Transfer Fluid: waterApproach*: 15-20 °C [27-36 °F]

Advantages: 1) Water is heat transfer fluid2) Exchangers are small, minimizing footprint3) Large supply of ambient water not necessary4) Lower process temperatures achievable than air cooling

Disadvantages: 1) High capital and operating cost2) Make-up water supply required3) Chemicals are necessary to treat water for corrosion, scaling, algae, etc.4) Cooling water blowdown disposal5) Freezing

* process fluid to heat sink temperature

Cooling Towers

Advantages

Disadvantages

The Two Basic Types of ACHEs

ACHE are the most popular in onshore facilities

Offshore, ACHEs are sometimes used on shallow water installationsbut are seldom if ever used in deeper water

(Courtesy the Rainey Corp.)

Fan Ring

Air Plenum Chamber

Nozzles

Driver

Drive Assembly

Induced Draft

Headers

Tube Bundle

Fan

Fan Ring

Air Plenum Chamber

Nozzles

DriverDrive

AssemblyForced Draft

Headers

Tube Bundle

Fan

Supporting Structure



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Induced Draft versus Forced Draft

(Courtesy the Rainey Corp.)

The tube bundle is covered The air plenum chamber is above

tube bundle The fan is above the tube bundles

Forced DraftInduced Draft

The tube bundle is not covered The air plenum chamber is below

tube bundle The fan is below the tube bundle

Induced Draft Design


1. Easier to shop, assemble, ship and install 1. More difficult to remove bundles for maintenance

2. Easier to clean underside when covered with lint, bugs and debris 2. High temperature service limited due to effect of hot air on the fans

3. More efficient air distribution over the bundle 3. More difficult to work on the fan assembly, i.e., adjust blades due to heat from bundle, and their location

4. Less likely to be affected by hot air recirculation 4. Fan power is higher due to larger inlet air volumes

5. Quieter

6. Less effect of ambient conditions (sun, rain, snow and hail) since over 60% of the face area of the tube section is covered

7. In the event of fan failure, the natural draft stack effect is greaterthan a forced draft design

Forced Draft Design


1. Easy to remove and replace bundle 1. Less efficient air distribution over bundle section

2. Easier to mount motors or other drivers with short shafts 2. Increased possibility of hot air recirculation due to low discharge velocity from bundle section

3. Lubrication, maintenance, etc. more accessible 3. Exposure of the tube section to ambient weather conditions (sun,rain, snow and hail)

4. With reinforced straight side panes to form a rectangular box type plenum, shipping and mounting is greatly simplified, permitting complete preassembled shop-tested units

4. Low natural draft capability in the event of a fan failure

5. More adaptable to cold climate operation with warm airrecirculation

Induced Draft and Forced Draft Air Coolers



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ACHEs Advantages and Disadvantages

Advantages1. Air readily available

everywhere

2. Low environmental impact

3. Lower maintenance thancooling towers

4. Less fouling than water

5. Mechanically simple andflexible

6. Partial cooling available inthe event of power failure

7. Facility water consumptionrequirements reduced

Disadvantages1. Highest heat sink temperature

compared to other methods

2. Large footprint and equipment size

3. Process fluid freezing in lowtemperature environments

4. Air flow must be free of surroundingobstructions

5. Fan noise

6. Daily temperature variation affectsACHE performance

7. More complex control systemsrequired

8. Winterization of equipment isexpensive

9. Thermal cycling must be limited

ACHE Key Operating Parameters

Type of ACHE

• Induced or Force Draft

Process fluid

• Vapor or liquid coolers

• Condensers

• Properties

• Overall heat transfer coefficient

Air ambient temperature, atmospheric pressure and relative humidity

Air flowrate

Tube design and number of bays

Type of tube fins

Fan arrangement, type and speed

Control of cooled fluid temperature



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Heat Exchangers Specification and Selection

These are some factors you should consider:

1. Do not specify or purchase a heat exchanger withoutconsideration of its effect on the total process.

2. Do not make the capital cost of the heat exchanger a solecriterion for purchase.

3. Acquaint the vendor with details of service and point out thechoice will be made on both initial and operating cost, not initialcapital cost alone.

4. Use realistic pressure drop specifications since this affects sizeand cost. Allow as much pressure loss as economics dictatesfor the actual system and not merely reproduce a standard specthat might not apply.



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Learning Objectives


List the four primary process cooling (heat rejection) methods

Describe why air-cooled heat exchangers are so frequently used,key operating parameters, and the difference between induceddraft and forced draft designs

PetroAcademyTM Gas Conditioning and Processing Core

Hydrocarbon Components and Physical Properties Core

Introduction to Production and Gas Processing Facilities Core

Qualitative Phase Behavior and Vapor Liquid Equilibrium Core

Water / Hydrocarbon Phase Behavior Core

Thermodynamics and Application of Energy Balances Core

Fluid Flow Core

Relief and Flare Systems Core

Separation Core

Heat Transfer Equipment Overview Core

Pumps and Compressors Overview Core

Refrigeration, NGL Extraction and Fractionation Core

Contaminant Removal – Gas Dehydration Core

Contaminant Removal – Acid Gas and Mercury Removal Core



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