API 661 Roundtable AirCooled Heat Exchanger Performance

1
API 661 Roundtable Air-Cooled Heat Exchanger
Performance
2005 API SpringRefining Equipment Standards
Meeting- SCHTE -April 19, 2005
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Roundtable Ground Rules

• Never agree on or recommend the use (or the
  non-use) of a specific item or manufacturer
• Be sure that all statements about a product or
  manufacturer are factual and correct
• Do not advertise, promote, or disparage
  proprietary products or processes
• Do not estimate future prices or costs or supply
  and demand from which prices or costs might be
  extrapolated.
• Discussions must not
• Damage a supplier's competitive position
• Inhibit any purchaser from selecting any quality
  level chosen
• Establish any barriers for entry of any supplier
  into the field.

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Agenda

• 0900 - 0930 Equipment Design Overview
• 0930 - 1000 Performance Testing
• 1000 - 1030 Air Side Fouling/Cleaning
• 1030 - 1045 Break
• 1045 - 1115 Performance Enhancements
• 1115 - 1145 Fans

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Roundtable Panel

• Buddy Kluppel (Hudson Products)
• Paul Harte (Shell Canada)
• John Nesta (Fluor Canada)

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Equipment Design Overview

• Air-cooled heat exchangers (ACHEs) are used
  extensively in the refining, natural gas,
  petro-chemical, and power industries
• Used in high temperature fluid applications which
  would result in high skin temperatures in
  cooling water exchangers
• Used in areas where cooling water is not
  available or preferred as a cooling medium
• Make up a large percentage of plant plot space
• Reliable performance can be critical for plant
  productivity
• In many cases, represent bottlenecks to plant
  production either directly when cooling process
  streams, or indirectly when providing cooling to
  prime movers (such as compressors).

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Common Types of Air Coolers

• Induced Draft
• Used more in hot climates to mitigate solar
  radiation effects (plenum protects bundle)
• Advantage in low MTD applications (reduced hot
  air recirculation potential)
• Better airside flow distribution
• Limits on air exhaust temperature.
• Forced Draft
• Most widely used type
• Lower capital and maintenance costs
• Easier access to clean fins
• Requires less horsepower
• Fans are not exposed to hot exhaust air
• Can be more prone to hot air recirculation.

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Hot Air Recirculation

• High air approach velocities to air coolers
• Low exit hot air velocities (forced draft)
• Air coolers placed too close to each other in the
  downwind direction
• Air coolers placed in front of downwind
  obstructions
• Air coolers placed at different elevations near
  each other
• Indiscriminate plot plan mixing of forced and
  induced draft air coolers
• Inadequate analysis of plot plan layout in view
  of prevailing summer wind direction
• Placing air coolers with close temperature
  approaches on the leeward side.

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Hot Air Recirculation
Forced Draft
Induced Draft
Induced draft unit will always have less hot air
recirculation potential than forced draft, due to
its higher exhaust velocity !
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Hot Air Recirculation
For a single air cooler, under no wind, potential
for hot air recirculation can be estimated by 1
Exit Air Velocity Head
Inlet Air Velocity Head
With steady wind
Ø Negative pressure on down-wind side of air
cooler
1. Hot Air Recirculation by Air Coolers A.Y.
Gunter and K.V. Shipes, Hudson Products
Corporation, 1971.
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Incorrect and Correct Layouts
Ensure that there are no obstructions on the
down-wind side.
?
?
Ensure adjacent coolers are mounted with exhaust
section at the same elevation. Avoid mixing
forced and induced draft where possible.
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Incorrect and Correct Layouts
?
?
Arrange coolers from lowest to highest approach
?T considering direction of prevailing wind
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Header Box Construction

• Plug Header
• Most commonly used type
• Up to 20,700 kPag (3000 psig) pressure rating

• Cover Plate Header
• Used in high fouling services
• Bundle width limitations for ease of removal,
  increased sealing capability
• Limited to lower pressure designs, typically lt
  2400 kPag (350 psig).

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Split Headers

• Used to provide thermal restraint relief for tube
  bundles
• Top rows thermally expand more than bottom rows
• API 661/ISO 13706 states split headers, U-tubes,
  or other restraint relief shall be employed when
  the fluid ?T from inlet to outlet of a multi-pass
  bundle exceeds 110ºC (200ºF)
• FEA normal evaluation method
• Where design temperature gtgt normal operating
  temperature, ensure any high temperature upset
  cases are considered.

FEA Model
Split 1
Split 2
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Bundle w/o Adequate Thermal Restraint Relief
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Fin Selection (Recommended Temperature Limits
per API 661/ISO 13706 Annex A)

• Extruded Fins
• Limited to Process Temp. of 300ºC (570ºF).
• Best for corrosive atmospheric environments.

• Embedded Fins
• Limited to 400ºC (750ºF)

• Footed Fins
• Limited to 130ºC (270ºF)
• Lowest cost
• Least effective in maintaining bond and
  efficiency over time.

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Suggested Roundtable Questions

• What is the roundtable group experience with hot
  air recirculation with Forced draft? Induced
  draft? Has any field testing been completed?
• What are some retrofits that can be done to
  prevent hot air recirculation with Forced draft?
  Induced draft?
• Any problems with older units without split
  headers?
• What do users, contractors specify for fin
  selection temperature normal process inlet
  temperature, design temperature, upset
  temperatures?

17
Performance Testing

• Cannot determine from simple U value calculations
  if a performance problem is due to air side or
  process side, as two air side variables (flow,
  exit temperature) are not usually known from
  plant instrumentation.
• Detailed air side measurements may be required to
  troubleshoot an air cooler that is
  underperforming
• Testing is rigorous and time consuming
• Air flow and exit temperatures are difficult to
  measure and vary depending on the location of
  measurement
• Units with internal or external recirculation
  systems further complicate inlet air temperature
  measurement

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Test Methods

• ASME PTC-30 is the industry standard for
  performance tests of air cooled heat exchangers.
  
• AIChe published method also available.
• Provides uniform methods and procedures for
  testing the thermodynamic and fluid mechanical
  performance of air-cooled heat exchangers
• Covers forced draft, induced draft, natural
  draft, and fan assisted natural draft air
  coolers
• Covers horizontal, vertical, or inclined tube
  bundle orientations.
• Expected uncertainty level is 2-5.

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Measurements

• Airflow
• Air temperatures ambient, entering, and exit
• Air-side pressure differential
• Fan driver power
• Atmospheric pressure
• Wind velocity
• Process fluid temperatures
• Process fluid flow rate
• Process fluid composition

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Airflow Measurements

• traverse of air velocities over a selected area
  with a propeller or rotating vane anemometer.
• Should be measured in an unobstructed area with
  highest airflow (i.e. across the fan ring).
• 30 sec minimum time intervals for individual
  (averaged) readings.
• May be necessary to correct the readings for
  yaw, since the direction of airflow may not be
  normal to the plane of the area surveyed. ? 5º
  angle, no correction required.

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Airflow Measurement
Fan ring is broken down into a set of concentric
(equal area) segments. Measurements averaged to
estimate actual airflow.
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Fan Ring Air Flow Measurement Points
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Air Temperature

• Inlet Temperature
• Forced - traverse at 150 mm (6 in.) below fan
  ring.
• Induced - 300 mm (12 in.) below bundle.
• Exit Temperature
• Forced
• -traverse required across a rectangular grid
  over the exhaust section of the bundle (20
  pts. minimum at 15DT above bundle).
• -average value must be weighted with air flow
  measurements.
• Induced
• - traverse at 150 mm (6 in.) above fan ring.
• Forced draft is more time consuming to test than
  induced draft as airflow measurements are
  required on both inlet and exhaust side of the
  bundle!

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Measurements Points Ta, Te, Sp(From AIChe
Performance Test Guideline)
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Additional Measurements

• Fan Driver Power
• Measure actual voltage and amperage to calculate
  Hp for comparison against predicted values.
• Airside Differential Pressure
• Measurements at high velocity area between fan
  and tube bundle.
• Measurements at low velocity area (above bundle
  for forced draft, below for induced).
• Process data
• Flow, T, P from plant instrumentation,
• Composition, physical properties from a process
  simulator, or sampling.
• Check tube side delta P, if possible, for
  evidence of tube side fouling.

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Calculations

• Heat and Material Balance
• Both process side (Qp) and air-side (Qa) heat
  loads must be calculated.
• Percent error
• lt 15 difference is considered acceptable by
  PTC-30.
• Corrected LMTD.
• U value

Qp Qa Qp Qa
x 200
Q A x MTD
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Suggested Roundtable Questions

• What has been the industry experience in using
  these procedures?
• Have reliable results been achieved using these
  test procedures?
• What types of measurement tools are commonly used?

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Air Side Fouling Cleaning
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Air Side Fouling Cleaning

• Fouling / plugging of fins due to dirt, oily
  residue, debris, tree fluff, bird droppings,
  scale, etc., can cause a significant decrease in
  performance.
• Presence of a fouling layer lowers the overall U
  value.
• Plugging between fins reduces the extended
  (finned) surface area.
• Increases the static pressure drop.
• May decrease the airflow, which in turn lowers
  the U value and results in a higher air exit
  temperature.
• Reduction in the MTD.
• Can be very difficult to clean/restore to new
  condition.
• Selection of an effective cleaning method for the
  type of fouling is important.

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High Pressure Water Cleaning
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Chemical/Foam Cleaning

• Recent advancements in bio-degradable foams allow
  a cleaning agent to be sprayed on both the top
  and underside of the tube bundle
• The agent reaches the inner tube surfaces and
  gels, dissolving most organic deposits
• The foam is then rinsed with a low pressure water
  spray

• Can be completed with air cooler in service, but
  the tube bundle must be within appropriate
  temperature limits before applying the foaming
  agent.

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Other Cleaning Methods

• High Pressure Air
• Dry Ice (CO2 pellets)
• Other?

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Suggested Roundtable Questions

• What are users doing to clean fins?
• What works and what doesnt?
• Online vs. offline cleaning experience?
• What are the temperature limits when on-line
  cleaning is performed?
• What are good design practices to minimize or
  prevent airside fouling?
• Use of airside fouling factors?

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Performance Enhancements
U A MTD
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Air Side

• Increase Fan RPM and Motor Size
• Increase the blade pitch angle
• Replace AV pitch with VFD and fixed blade pitch
• Upgrade to high efficiency fan blades
• Reduce fan-ring tip clearance
• Add an inlet bell to the fan ring
• Humidify the inlet air stream

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Tip Seals (Honeycomb)
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Spraying with Raw Water
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Tube Inserts
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Tube Inserts

• Available in many shapes and configurations
• Basic purpose for all tube inserts is the same
  to promote increased turbulence and wall shear
  stress.
• Best overall performance increase is realized
  when the tube side HT film coefficient dominates
  the overall thermal resistance.
• Largest benefit is in moderate, high viscosity
  services where the Re number is low and laminar
  flow is the predominant flow regime
• Promote mixing
• Adds significant pressure drop.

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Suggested Roundtable Questions

• What other types of airside enhancements are
  being used?
• What is the user experience with tube inserts in
  fouling services?
• What are some success stories?
• Are there any velocity limits to be aware of when
  using tube inserts ?

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Fans
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Basic Fan Laws

• Airflow ? (Fan RPM) 1
• SP, TP ? (Fan RPM) 2
• HP ? (Fan RPM) 3
• Noise ? (Fan Tip Speed) 5.6

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Effect of Blade Shape
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Fan Tip Clearance

• Minimize tip losses by complying with API 661 /
  ISO 13706
• Radial Fan Dia. (ft) Clearance (in)
• 3 9 1/4 1/2
• 10 11 1/4 5/8
• 12 1/4 3/4

• Use tip seals to further reduce losses.

Tip Vortex Leakage
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Fan Ring Geometry
Airflow with no inlet bell
Airflow with inlet bell
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Optimizing the Fan Pitch Angle
What are some common errors in adjusting the fan
pitch angle?
Max amps ? Max airflow
Typical Fan Curve
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Suggested Roundtable Discussion

• Open forum all subjects
• Q A