Title: API 661 Roundtable AirCooled Heat Exchanger Performance
1API 661 Roundtable Air-Cooled Heat Exchanger
Performance
2005 API SpringRefining Equipment Standards
Meeting- SCHTE -April 19, 2005
2Roundtable 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.
3Agenda
- 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
4Roundtable Panel
- Buddy Kluppel (Hudson Products)
- Paul Harte (Shell Canada)
- John Nesta (Fluor Canada)
5Equipment 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).
6Common 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.
7Hot 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.
8Hot 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 !
9Hot 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.
10Incorrect 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.
11Incorrect and Correct Layouts
?
?
Arrange coolers from lowest to highest approach
?T considering direction of prevailing wind
12Header 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).
13Split 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
14Bundle w/o Adequate Thermal Restraint Relief
15Fin 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.
16Suggested 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?
17Performance 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
18Test 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.
19Measurements
- 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
20Airflow 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.
21Airflow Measurement
Fan ring is broken down into a set of concentric
(equal area) segments. Measurements averaged to
estimate actual airflow.
22Fan Ring Air Flow Measurement Points
23Air 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!
24Measurements Points Ta, Te, Sp(From AIChe
Performance Test Guideline)
25Additional 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.
26Calculations
- 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
27Suggested 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?
28Air Side Fouling Cleaning
29Air 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.
30High Pressure Water Cleaning
31Chemical/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.
32Other Cleaning Methods
- High Pressure Air
- Dry Ice (CO2 pellets)
- Other?
33Suggested 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?
34Performance Enhancements
U A MTD
35Air 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
36Tip Seals (Honeycomb)
37Spraying with Raw Water
38Tube Inserts
39Tube 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.
40Suggested 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 ?
41Fans
42Basic Fan Laws
- Airflow ? (Fan RPM) 1
- SP, TP ? (Fan RPM) 2
- HP ? (Fan RPM) 3
- Noise ? (Fan Tip Speed) 5.6
43Effect of Blade Shape
44Fan 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
45Fan Ring Geometry
Airflow with no inlet bell
Airflow with inlet bell
46Optimizing the Fan Pitch Angle
What are some common errors in adjusting the fan
pitch angle?
Max amps ? Max airflow
Typical Fan Curve
47Suggested Roundtable Discussion
- Open forum all subjects
- Q A