API 661 Roundtable AirCooled Heat Exchanger Performance - PowerPoint PPT Presentation

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API 661 Roundtable AirCooled Heat Exchanger Performance

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Title: 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
2
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.

3
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

4
Roundtable Panel
  • Buddy Kluppel (Hudson Products)
  • Paul Harte (Shell Canada)
  • John Nesta (Fluor Canada)

5
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).

6
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.

7
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.

8
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 !
9
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.
10
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.
11
Incorrect and Correct Layouts
?
?
Arrange coolers from lowest to highest approach
?T considering direction of prevailing wind
12
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).

13
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
14
Bundle w/o Adequate Thermal Restraint Relief
15
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.

16
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

18
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.

19
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

20
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.

21
Airflow Measurement
Fan ring is broken down into a set of concentric
(equal area) segments. Measurements averaged to
estimate actual airflow.
22
Fan Ring Air Flow Measurement Points
23
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!

24
Measurements Points Ta, Te, Sp(From AIChe
Performance Test Guideline)
25
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.

26
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
27
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?

28
Air Side Fouling Cleaning
29
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.

30
High Pressure Water Cleaning
31
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.

32
Other Cleaning Methods
  • High Pressure Air
  • Dry Ice (CO2 pellets)
  • Other?

33
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?

34
Performance Enhancements
U A MTD
35
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

36
Tip Seals (Honeycomb)
37
Spraying with Raw Water
38
Tube Inserts
39
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.

40
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 ?

41
Fans
42
Basic Fan Laws
  • Airflow ? (Fan RPM) 1
  • SP, TP ? (Fan RPM) 2
  • HP ? (Fan RPM) 3
  • Noise ? (Fan Tip Speed) 5.6

43
Effect of Blade Shape
44
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
45
Fan Ring Geometry
Airflow with no inlet bell
Airflow with inlet bell
46
Optimizing the Fan Pitch Angle
What are some common errors in adjusting the fan
pitch angle?
Max amps ? Max airflow
Typical Fan Curve
47
Suggested Roundtable Discussion
  • Open forum all subjects
  • Q A
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