Process Heat Transfer

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Process Heat TransferProcess Heat Transfer

The Cause and Effect of Various The Cause and Effect of Various Design ConceptsDesign Concepts

2““Expect many Expect many enjoyable experiencesenjoyable experiences!”!”David M. ArmstrongDavid M. Armstrong

““Expect many Expect many enjoyable experiencesenjoyable experiences!”!”David M. ArmstrongDavid M. Armstrong

Exchanger VariablesExchanger VariablesExchanger VariablesExchanger Variables

• Fouled surface area

• Non-condensible gases

• Flooded surface area

• Variable process inlet and outlet temperatures

• Variable process flow rates

• All of these change the BTU demand on the heater, changing the pressure and temperature of the heat transfer media

Fouled Surface AreaFouled Surface AreaFouled Surface AreaFouled Surface Area

• Fouled surface area decreases the heat transfer efficiency of the tube bundle

• This inherently causes adjustments in the pressure and/or temperature of the heat transfer media being supplied to the exchanger

Fouled Surface AreaFouled Surface AreaFouled Surface AreaFouled Surface Area

• Resulting in more surface exposed to the transfer media in a level control system. This will increase the BTU transfer rate.

• Higher delivery pressure from the inlet control valve decreases the efficiency of the heat exchanger. Higher pressure lacks the same latent heat content of lower pressure. Energy consumption will increase, while production levels remain unchanged.

Non-Condensible GasesNon-Condensible GasesNon-Condensible GasesNon-Condensible Gases

• Presence of non-condensibles’ occupies valuable steam space

• A reduction of viable heat transfer area can result due to the insulating properties

• Promotion of carbonic acid formation is inherent

• Excessive amounts can inhibit drainage

Flooded Surface AreaFlooded Surface AreaFlooded Surface AreaFlooded Surface Area

• Promotes corrosion and fouling

• Can develop into water hammer

• Controls process temperature by decreasing available surface area for heat transfer (Level Control)

• Typically causes process outlet variations

Variable Process Inlet & Outlet Variable Process Inlet & Outlet TemperaturesTemperatures

• Changes the BTU exchange rate required or (Delta T)

• These variable temperatures can increase or decrease exiting pressure based on condensing rate of the heater

• Will promote flooding on low exchange rate demand

Variable Process Flow RatesVariable Process Flow RatesVariable Process Flow RatesVariable Process Flow Rates

• Variable flows will change BTU demand on the exchanger

• Higher flow rates will increase the surface area needed, raising or lowering the outlet pressure based on available surface area

• Lower flow rates will decrease surface area needed, raising or lowering the outlet pressure based on available surface area

Control OptionsControl OptionsControl OptionsControl Options

• Level Control

• Steam Control

Level ControlLevel ControlLevel ControlLevel Control

• Level control systems flood exchangers to reduce the amount of useable surface area for BTU transfer

• Exchangers run flooded due to the control valve on the condensate outlet, modulating to maintain the desired process outlet temperature

Steam ControlSteam ControlSteam ControlSteam Control

• Allows the exchanger to run at the lowest possible steam pressure, which maximizes energy efficiency due to latent heat content

• Less energy consumed for the same amount of product produced

Process Design SummaryProcess Design SummaryProcess Design SummaryProcess Design Summary

• Utilize all of the surface area

• Eliminate corrosion and fouling by keeping the exchanger dry

• Eliminate non-condensibles

• Optimize the design by using the lowest pressure steam, to gain more latent heat content per pound

Operating CharacteristicsOperating Characteristics

Step 1. During filling, the steam or air inlet and check valve on pumping trap outlet are closed. The vent and check valve on the inlet are open.

Open CheckValve

Steam/Air In - Closed Steam/Air Out - Open

Closed Check Valve

FillingFilling

Begin PumpingBegin Pumping

Step 2. Float Rises with level of condensate until it passes trip point, and then snap action reverses the positions shown in step one.

Check ValveClosed

Steam/Air In - Open Steam/Air Out - Closed

Open Check Valve

Step 3. Float is lowered as level of condensate falls until snap action again reverses positions.

End PumpingEnd Pumping

Steam/Air - In Steam/Air - Closed

Closed CheckValve

Open Check Valve

Step 4. Steam or air inlet and trap outlet are again closed while vent and condensate inlet are open. Cycle begins anew.

Repeat FillingRepeat FillingSteam/Air In - Closed Steam/Air Out - Open

Closed Check Valve

Open Check Valve

Pump Trap ApplicationsPump Trap Applications

Process Heat ExchangerProcess Heat Exchanger with 100% Turndown Capabilitywith 100% Turndown Capability

SteamContro lValve

Therm ostaticA ir Vent

HeatExchanger

ProcessIn let

Reservoir

Therm ostaticA ir Vent

Pum p Trap

F&TTrap

M otiveSteam

CondensateReturn

Vacuum Reboiler Construction Comparison Vacuum Reboiler Construction Comparison

Hydrocarbon Knockout Drum/Separator Hydrocarbon Knockout Drum/Separator

Equaliz ingVent L ine

G asIn le t

G asO utle t

SeperationCham ber

ExpandedReservoirP ip ing

Pum p Trap

To Reclam ationDestination

N itrogen orInert G asM otive S upply

Flare Header Drain Flare Header Drain

Flash Vessels Flash Vessels

M otiveSteam

AirVent

Pum p Trap

Reservoir

F lashTank

SteamOutlet

CondensateInlet

Steam Turbine Casing Steam Turbine Casing

Pump Trap ApplicationsPump Trap ApplicationsPump Trap ApplicationsPump Trap Applications

• Process Heat Exchangers

• Liquid Separators

• Sumps

• Vacuum Systems

• Condensate Drum – Flash Tanks

• Vented Systems

• Closed Loop Applications

Understanding and Benefiting from Understanding and Benefiting from Equipment StallEquipment Stall

Q = U Q = U A A T TQ = U Q = U A A T T

Q = Design Load (BTU/Hr)

U = Manufacturer’s Heat Transfer Value (BTU/ft2/°F/Hr)

A = Heat Transfer Surface Area (ft2)

T = (Ts – T2) Approaching Temperature (°F)

Ts = Operating Steam Temperature (°F)

T2 = Product Outlet Temperature (°F)

What is wrong with this application?What is wrong with this application?

Effects of “Effects of “StallStall””Effects of “Effects of “StallStall””

• Inadequate condensate drainage

• Water hammer

• Frozen coils

• Corrosion due to Carbonic Acid formation

• Poor temperature control

• Control valve hunting (system cycling)

• Reduction of heat transfer capacity

Factors Contributing to “Factors Contributing to “StallStall””Factors Contributing to “Factors Contributing to “StallStall””

• Oversized equipment

• Conservative fouling factors

• Excessive safety factors

• Large operating ranges

• Back pressure at steam trap discharge

• Changes in system parameters

Finding “Finding “StallStall””Finding “Finding “StallStall””

Where does Stall occur??• Air heating coils

• Shell & tube heat exchangers

• Plate & frame heat exchangers

• Absorption chillers

• Kettles

• Any type of heat transfer equipment that has

Modulating Control

What is the “Stall” Solution?What is the “Stall” Solution?What is the “Stall” Solution?What is the “Stall” Solution?

• Use a bigger steam trap?

• Use a vacuum breaker?

• Implement a safety drain?

• Install a Posi-Pressure system?

• Use an electric pump?

Keys to OperationKeys to OperationKeys to OperationKeys to Operation

• How quick it can fill: This is dictated by head pressure & inlet pipe and check valve size

• Vent/Equalization: Vent connection must always be in vapor space

• Pump Out: Motive vs. back pressure and gas used

VocabularyVocabularyVocabularyVocabulary

Filling Head: Distance between the top of the pump and the bottom of the receiver or reservoir pipe

Vocabulary Vocabulary (Continued)(Continued)Vocabulary Vocabulary (Continued)(Continued)

Receiver/Reservoir Pipe: This is a temporary holding place to store condensate while the pump is in the pump down cycle. The receiver/reservoir pipe is designed and sized to prevent condensate from backing up into the system.

Open System ConfigurationOpen System Configuration

Closed System ConfigurationClosed System Configuration

Open System

Open SystemOpen SystemOpen SystemOpen System

Advantages:• Drain multiple pieces of equipment

• Can use Air or Steam for pump trap operation

• Easiest to understand

Disadvantages:• Lose valuable flash steam

• Must run a potentially expensive atmospheric vent line

• Size the pump trap based total design load

• Must compete with electric pumps

Closed System

Closed SystemClosed SystemClosed SystemClosed System

Advantages:

• No flash steam loss

• No need to run long expensive vent lines

• Use a smaller pump than in a open system*

• Return condensate hotter

Disadvantages:

• Dedicated pump for a single piece of equipment

• More complex

• Cannot use air as motive force

Pump Sizing / Receiver SizingPump Sizing / Receiver SizingPump Sizing / Receiver SizingPump Sizing / Receiver Sizing

Pump Sizing

• Determine head available from equipment (distance from equipment outlet to grade)

• Select either closed loop or vented design (Note: If multiple sources of condensate, vented system must be used to prevent short circuiting)

Pump Sizing

• Determine maximum pumping load

• Calculate maximum back pressure (including lift)

• Determine motive pressure and gas to be used (use capacity correction factor if using a medium other than steam)

Pump Sizing

• Check and specify head pressure (distance from bottom of receiver/reservoir to top of selected pump)

• Make sure to use capacity correction if more or less head is available than standard catalog dimension

Pump Sizing

• Calculate maximum flash rate & needed vent size – if vented system

• Determine and size reservoir – if closed loop system

• Size downstream F&T trap if needed for closed loop system

Vented Receiver SizingVented Receiver SizingVented Receiver SizingVented Receiver Sizing

lb/hr kg/hr in mm in mm in mmup to

75 34 4 102 36 914 1-1/2 40

150 68 6 152 36 914 2 50

300 136 9 229 36 914 2-1/2 65

600 272 10 254 36 914 3 75

900 408 12 300 36 914 4 100

1200 544 16 405 36 914 6 150

2000 907 20 508 36 914 8 200

Vented Receiver Sizing for Open Systems

Flash SteamReceiver Diameter

Receiver Length

Vent Line Diameter

Note: When draining from a single or multiple pieces of equipment in an “open” system, a vented receiver should be installed horizontally above and ahead of the pump trap. In addition to sufficient holding volume of the condensate above the fill head of the pump trap to hold the condensate during the pump trap cycle, the receiver must also be sized to allow enough area for flash steam and condensate separation.

in mm in mm in mm in mm in mm in mmlb/hr 2 50 3 75 4 100 6 150 8 200 10 250up to

ft m ft m ft m ft m ft m ft m

500 227 4 1.2 2-1/2 0.7 1-1/2 0.4

1,000 453 4-1/2 1.4 2 0.6 1-1/2 0.4

1,500 680 7 2.1 3 0.9 2 0.6

2,000 907 9 2.7 4 1.2 2-1/2 0.7

2,500 1,134 11 3.4 5 1.5 3 0.9 1-3/4 0.5

3,000 1,360 13-1/2 4.1 6 1.8 3-1/3 1.1 2 0.6

4,000 1,814 18 5.5 8-1/2 2.6 5 1.5 2-1/2 0.7

5,000 2,268 10 3.0 6 1.8 3 0.9 1-1/2 0.4

6,000 2,722 12 3.7 7 2.1 3-1/2 1.1 2 0.6

7,000 3,175 14-1/2 4.4 8-1/2 2.6 4 1.2 2 0.6

8,000 3,629 16-1/2 5.0 9-1/2 2.9 4-1/2 1.4 2-1/2 0.7 1-1/2 0.4

9,000 4,082 11 3.4 5 1.5 3 0.9 2 0.6

10,000 4,536 12 3.7 5-1/2 1.7 3 0.9 2 0.6

11,000 4,990 13 4.0 6 1.8 3-1/2 1.1 2 0.6

12,000 5,443 14 4.3 6-1/2 2.0 4 1.2 2-1/2 0.7

Inlet Reservoir Pipe Sizing for Closed Systems

Length of Pipe

Condensate Load

Reservoir Pipe Diameter

Closed Loop Receiver SizingClosed Loop Receiver SizingClosed Loop Receiver SizingClosed Loop Receiver Sizing

Note: When draining from a single piece of equipment in a closed loop system, to achieve maximum energy efficiency a reservoir should be installed horizontally above and ahead of the pump trap. Sufficient reservoir volume is required above the filling head level to hold condensate during the pump trap discharge cycle. The chart above shows the minimum reservoir sizing, based on the condensate load, to prevent equipment flooding during the pump trap discharge cycle.

CriticalCritical Design Criteria Summary Design Criteria SummaryCriticalCritical Design Criteria Summary Design Criteria Summary

1. Maximum condensate flow from exchangers and reboilers

2. Maximum differential pressure across the system

3. Minimum differential pressure across the system (specifically when clean)

4. Minimum tower height needed to achieve maximum condensate flow rate at minimum differential

5. Maximum motive pressure (steam, air, nitrogen, etc.) available to power pumps

Critical Design CriteriaCritical Design Criteria Summary SummaryCritical Design CriteriaCritical Design Criteria Summary Summary

6. Maximum instantaneous discharge rate for downstream pipe sizing & trap sizing

7. Temperature differential of condensate source vs. condensate header design

8. Piping layout to prevent hydraulic shock

9. Total installed cost savings, including construction, on turnkey jobs

10. Integrity of mechanical design due to the critical nature of the service

11. Minimize potential problems with proper designs

MaximumMaximum Differential Pressure Across Differential Pressure Across the Systemthe System

• Maximum pressure from control valve, including minimal drop

• Minimum drop across exchanger

• Maximum pressure – should tube leak occur

• Elimination of back pressure (bypass to grade)

• Consider fouled surface area

MinimumMinimum Differential Pressure Across Differential Pressure Across the Systemthe System

• Consider maximum percentage of turndown on process flow vs. design flow (plus factor)

• Consider over-surfaced heat transfer area

• Evaluate downstream relief valve settings on condensate side as traps (etc.) fail and pressurize the return system

• Undersized return lines are common in facility expansions. Verify effects of additional flow on pipe velocities and back pressures.

Minimum Head PressureMinimum Head PressureMinimum Head PressureMinimum Head Pressure

• Skirt height on reboilers can be minimized by evaluating discharge capacity needed and setting height accordingly. This should be done early in the job scope as it effects tower construction.

• Additional pump capacity can be achieved by increasing head pressure

Maximum Motive PressureMaximum Motive PressureSteam, Air, NitrogenSteam, Air, Nitrogen

• Ensure stable source with negligible variations

• Install drip station to insure dry gas is always present at motive steam valve (pipers often do not realize it is a dead-end steam line)

Maximum Design PressureMaximum Design PressureMaximum Design PressureMaximum Design Pressure

• Utilization of 2/3 Rule can eliminate relief valves on low pressure side needed for tube rupture cases

• Use of liquid drain traps can eliminate gas discharge into return header

Maximum Instantaneous Discharge RateMaximum Instantaneous Discharge RateMaximum Instantaneous Discharge RateMaximum Instantaneous Discharge Rate

• Pump discharge rate must be used when sizing condensate return leads (use bi-phase flow)

• Pump discharge rate also critical to downstream traps in Pump / Trap combinations

Temperature Differential of Source vs. Temperature Differential of Source vs. HeaderHeader

• Minimize thermal shock by maintaining T of 150°F or less

• When feasible, run separate headers for vacuum temperature condensate

• Vacuum condensate headers can be sized on single phase flow if dedicated solely for vacuum temperature condensate

Piping Layout to Prevent Hydraulic Piping Layout to Prevent Hydraulic ShockShock

• Discharge lead from pumps should be piped into top of return header

• Flow patterns should be continual – no opposing flows

• Check valves should be installed at major elevation changes to disperse hydraulic shock

Pipe SizingPipe SizingPipe SizingPipe Sizing

1. Discharge piping should be based on 2-3 times the normal condensing rate due to instantaneous discharge rate of the pump

2. Minimize elevation changes to prevent hydraulic shock

3. Utilize check valves at main header to minimize backflow

Pipe SizingPipe SizingPipe SizingPipe Sizing

4. Run separate lines for vacuum temperature condensate to minimize thermal shock potential

5. Always calculate the maximum flash rate in return lines

6. Insure adequate pipe and nozzle diameters to facilitate bidirectional two-phase flow

Process Heat Transfer

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