Air-cooled exchangers generally operate for a
longer period of time between maintenance intervals than do shell and tube exchangers. However, performance problems
do arise.
longer period of time between maintenance intervals than do shell and tube exchangers. However, performance problems
do arise.
Below
are listed, in order of frequency, common causes of impairment, ways to detect
them, and their remedies:
are listed, in order of frequency, common causes of impairment, ways to detect
them, and their remedies:
I:
Fouling of Extended Surface:
Airborne contaminants accumulate on and between the fins of air-cooled
exchangers. Contaminants include, but are not limited to dirt, lint, insects,
and overspray from nearby painting / sandblasting operations.
Fouling of Extended Surface:
Airborne contaminants accumulate on and between the fins of air-cooled
exchangers. Contaminants include, but are not limited to dirt, lint, insects,
and overspray from nearby painting / sandblasting operations.
Impairment
occurs due to reduced airflow across the tube bundle.
occurs due to reduced airflow across the tube bundle.
Note:
The practice of spraying water on air-cooled exchanger extended surface to
increase capacity during maximum ambient conditions, in addition to
accelerating corrosion, generally leads to deposition of external foulants.
These foulants cannot be removed without damaging extended surface.
The practice of spraying water on air-cooled exchanger extended surface to
increase capacity during maximum ambient conditions, in addition to
accelerating corrosion, generally leads to deposition of external foulants.
These foulants cannot be removed without damaging extended surface.
DETECTION:
Inspect bundles from
underneath. Finned tubes of induced draft units are in plain view. View finned
tubes of forced draft units through the fan ring.
Inspect bundles from
underneath. Finned tubes of induced draft units are in plain view. View finned
tubes of forced draft units through the fan ring.
REMEDY:
Most foulants can be removed by water
wash of extended surface when unit is off line. Detergent may be required to
removed corrosion from aluminum fins. Notes of caution on washing:
Most foulants can be removed by water
wash of extended surface when unit is off line. Detergent may be required to
removed corrosion from aluminum fins. Notes of caution on washing:
Water
wash should generally be avoided while the unit is in service. Most Air-cooled
exchangers are hot enough that thermal shock, and consequent roll leaks could
result.
wash should generally be avoided while the unit is in service. Most Air-cooled
exchangers are hot enough that thermal shock, and consequent roll leaks could
result.
Water
blasting should not be used. Aluminum fins are easily bent.
blasting should not be used. Aluminum fins are easily bent.
Apply
water spray in a plane parallel to the fins only, to avoid bending. Exercise
extreme caution in washing serrated-type fins. Serrated fins are formed, and
thus it is not possible to maintain spray in a parallel plane.
water spray in a plane parallel to the fins only, to avoid bending. Exercise
extreme caution in washing serrated-type fins. Serrated fins are formed, and
thus it is not possible to maintain spray in a parallel plane.
II:
Drive-belt slippage: Most
air-cooled exchangers utilize belts as fan drives. Multiple “V”–belts are the
most common. Slippage of these belts will lower fan RPM, and severely limit
airflow across the tube bundle.
Drive-belt slippage: Most
air-cooled exchangers utilize belts as fan drives. Multiple “V”–belts are the
most common. Slippage of these belts will lower fan RPM, and severely limit
airflow across the tube bundle.
DETECTION
/ REMEDY:
/ REMEDY:
Most
slipping belts will emit a screeching sound, readily noticeable to personnel
walking under the fan. Slippage accompanied by screeching can usually be
remedied by tightening or replacing the belt.
slipping belts will emit a screeching sound, readily noticeable to personnel
walking under the fan. Slippage accompanied by screeching can usually be
remedied by tightening or replacing the belt.
Slippage
due to a severely worn sheave will frequently not be accompanied by screeching.
Extreme sheave wear allows the belt to ride against the bottom of the sheave.
Inspect for this condition by measuring fan RPM with a strobe tachometer.
Compare actual RPM to design RPM on Manufacturer’s Data Sheet.
due to a severely worn sheave will frequently not be accompanied by screeching.
Extreme sheave wear allows the belt to ride against the bottom of the sheave.
Inspect for this condition by measuring fan RPM with a strobe tachometer.
Compare actual RPM to design RPM on Manufacturer’s Data Sheet.
Remedy
for this type of slippage generally requires replacement of the entire drive.
for this type of slippage generally requires replacement of the entire drive.
III:
Incorrect Fan Blade Pitch: Fans on
process Air-cooled exchangers of five feet or greater diameter generally are
specified to have adjustable-pitch blades. Most units are pre-assembled at the
vendor’s shop, with the pitch preset.
Incorrect Fan Blade Pitch: Fans on
process Air-cooled exchangers of five feet or greater diameter generally are
specified to have adjustable-pitch blades. Most units are pre-assembled at the
vendor’s shop, with the pitch preset.
Except
in the case of automatic variable pitch fans, blades must be set so that under
summer ambient conditions, motor horsepower draw is less than nameplate. If
motors were loaded to nameplate at summer ambient, they would be overloaded
during winter.
in the case of automatic variable pitch fans, blades must be set so that under
summer ambient conditions, motor horsepower draw is less than nameplate. If
motors were loaded to nameplate at summer ambient, they would be overloaded
during winter.
The
above is necessary since as a constant velocity machine, a fan will move the
same volume of air regardless of air temperature and, consequently, density. An
identical volume of cold air weighs more than hot, and its movement will result
in more work done, and more horsepower expanded. If fans have been pitched back
during winter, they may deliver insufficient airflow during the summer.
above is necessary since as a constant velocity machine, a fan will move the
same volume of air regardless of air temperature and, consequently, density. An
identical volume of cold air weighs more than hot, and its movement will result
in more work done, and more horsepower expanded. If fans have been pitched back
during winter, they may deliver insufficient airflow during the summer.
DETECTION:
Motor amperage draw during
design ambient conditions should correspond with fan horsepower stated on the
Manufacture’s Data Sheet. Design ambient is located on this document also.
Motor amperage draw during
design ambient conditions should correspond with fan horsepower stated on the
Manufacture’s Data Sheet. Design ambient is located on this document also.
Note:
Low amperage draw may be due to belt slippage, as well as too little pitch.
Low amperage draw may be due to belt slippage, as well as too little pitch.
REMEDY:
Use Fan Manufacturer’s Curve to
determine correct pitch. Prior to setting pitch, consult Manufacturer’s
instructions. Improper torquing of blade clamp bolts, etc. can cause the fan to
fail violently.
Use Fan Manufacturer’s Curve to
determine correct pitch. Prior to setting pitch, consult Manufacturer’s
instructions. Improper torquing of blade clamp bolts, etc. can cause the fan to
fail violently.
NOTE:
Avoid overpitching fans. When operated beyond maximum design pitch angle, the
fan will “stall”, and deliver a smaller volume of air.
Avoid overpitching fans. When operated beyond maximum design pitch angle, the
fan will “stall”, and deliver a smaller volume of air.
IV:
Cooled Air Bypass/ Hot-Air
Recirculation: Thermal design of air-cooled exchangers is performed on the
assumption that all the air delivered by the fan(s) will cross the extended
surface of the unit. A second assumption is that, once heated by the process,
air will be directed upward and away from the unit, and not pulled back into
it.
Cooled Air Bypass/ Hot-Air
Recirculation: Thermal design of air-cooled exchangers is performed on the
assumption that all the air delivered by the fan(s) will cross the extended
surface of the unit. A second assumption is that, once heated by the process,
air will be directed upward and away from the unit, and not pulled back into
it.
Units
are constructed with seals to keep air from bypassing the extended surface.
These seals may loosen over time due to vibration, corrode away, or be removed
during maintenance and misplaced. Corrosion of plenum bottom panels is another
common cause of cold-air bypass.
are constructed with seals to keep air from bypassing the extended surface.
These seals may loosen over time due to vibration, corrode away, or be removed
during maintenance and misplaced. Corrosion of plenum bottom panels is another
common cause of cold-air bypass.
Hot-air
recirculation (air being sucked back into the unit), occurs when multiple units
are fairly near to one another, but gaps are existing between units.
recirculation (air being sucked back into the unit), occurs when multiple units
are fairly near to one another, but gaps are existing between units.
Recirculation
also occurs when the wind is parallel to a bank of units. Exchangers are usually
placed so that the bank is not parallel to the prevailing wind.
also occurs when the wind is parallel to a bank of units. Exchangers are usually
placed so that the bank is not parallel to the prevailing wind.
A
third cause of recirculation is blockage of wind by structures immediately
downwind of the unit.
third cause of recirculation is blockage of wind by structures immediately
downwind of the unit.
DETECTION/REMEDY:
Conditions leading to cold air bypass
are readily discernable through visual inspection, either from beneath, from
header ends, or in the case of induced-draft units, down through the fan rings.
Tighten or replace structural components.
Conditions leading to cold air bypass
are readily discernable through visual inspection, either from beneath, from
header ends, or in the case of induced-draft units, down through the fan rings.
Tighten or replace structural components.
Inspect
for recirculation by walking under the unit on motor servicing walkways.
Significant recirculation will be evidenced by a noticeable increase in air
temperature.
for recirculation by walking under the unit on motor servicing walkways.
Significant recirculation will be evidenced by a noticeable increase in air
temperature.
Recirculation
can frequently be controlled by wind walls and air seals between units.
can frequently be controlled by wind walls and air seals between units.
V:
Deteriorated or Bent Extended
Surface: Extended surface of process air-cooled exchangers usually consists of
helically wound, aluminum fins, 5/8” high, 0.016” thick, and from eight to ten
fins per inch. This surface is easily bent, and prone to corrosion when
subjected to acid atmospheres (not uncommon when units are located downstream
of a stack, in a humid environment).
Deteriorated or Bent Extended
Surface: Extended surface of process air-cooled exchangers usually consists of
helically wound, aluminum fins, 5/8” high, 0.016” thick, and from eight to ten
fins per inch. This surface is easily bent, and prone to corrosion when
subjected to acid atmospheres (not uncommon when units are located downstream
of a stack, in a humid environment).
Additionally,
aluminum fins can lose contact with the process tube if upset conditions push
them beyond their temperature limit. This temperature
frequently is considerably less than the ASME Code design temperature. Thermal
blanketing occurs when the process tube corrodes under the fins. This condition
is generally not a problem, unless the unit has been out of service for a long
period, in a humid environment.
aluminum fins can lose contact with the process tube if upset conditions push
them beyond their temperature limit. This temperature
frequently is considerably less than the ASME Code design temperature. Thermal
blanketing occurs when the process tube corrodes under the fins. This condition
is generally not a problem, unless the unit has been out of service for a long
period, in a humid environment.
A
last but not uncommon problem is bent fins. This can be due to foot traffic on
the upper row; or incorrect washing technique, top or bottom row.
last but not uncommon problem is bent fins. This can be due to foot traffic on
the upper row; or incorrect washing technique, top or bottom row.
DETECTION:
With exception of thermal
blanketing, these conditions are discernable through visual inspection.
With exception of thermal
blanketing, these conditions are discernable through visual inspection.
CORROSION:
Bottom-row fins are most prone
to dewpoint corrosion, since they see the coolest air. Fins will have a notched
appearance, and frequently flake off when touched. It is possible for
bottom-row fins to be corroded away completely, and top-row fins to appear as
new.
Bottom-row fins are most prone
to dewpoint corrosion, since they see the coolest air. Fins will have a notched
appearance, and frequently flake off when touched. It is possible for
bottom-row fins to be corroded away completely, and top-row fins to appear as
new.
DAMAGE
DUE TO EXCESSIVE TUBEWALL TEMPERATURE: Top
row fins are most susceptible to this impairment. Fins will exhibit a cupped
and laid over appearance. The degree of cupping and bending will usually not be
uniform.
DUE TO EXCESSIVE TUBEWALL TEMPERATURE: Top
row fins are most susceptible to this impairment. Fins will exhibit a cupped
and laid over appearance. The degree of cupping and bending will usually not be
uniform.
THERMAL
BLANKETING: This condition is
not readily discernible through visual inspection. Should no other impairments
be obvious, and considerable under-fin corrosion is evident, blanketing may be
suspected.
BLANKETING: This condition is
not readily discernible through visual inspection. Should no other impairments
be obvious, and considerable under-fin corrosion is evident, blanketing may be
suspected.
A
full-scale performance test, coupled with computer simulation of actual unit
operation is required to verify it.
full-scale performance test, coupled with computer simulation of actual unit
operation is required to verify it.
BENT
FINS: Fins will be laid over rather
uniformly in certain areas of the bundle.
FINS: Fins will be laid over rather
uniformly in certain areas of the bundle.
REMEDY:
Bending is the only condition that
can be remedied without at least partially retubing the exchanger.
Bending is the only condition that
can be remedied without at least partially retubing the exchanger.
VI:
Internal Fouling/Excessive Plugging:
Product or foreign matter can accumulate in any tubular exchanger. Likewise, as
individual tubes are sealed off due to leakage, heat transfer surface is reduced.
Internal Fouling/Excessive Plugging:
Product or foreign matter can accumulate in any tubular exchanger. Likewise, as
individual tubes are sealed off due to leakage, heat transfer surface is reduced.
Fouling
and plugging is not always uniform, and depending on the pass arrangement, the
effect on heat transfer may be greater than the percentile reduction in
surface.
and plugging is not always uniform, and depending on the pass arrangement, the
effect on heat transfer may be greater than the percentile reduction in
surface.
DETECTION:
Except in the case of a
material with an extremely low heat transfer coefficient, where a very thin
solid layer on the tube wall would impede performance, fouling will be
accompanied by an increase in pressure drop. Plugged tubes will always cause
increased pressure drop.
Except in the case of a
material with an extremely low heat transfer coefficient, where a very thin
solid layer on the tube wall would impede performance, fouling will be
accompanied by an increase in pressure drop. Plugged tubes will always cause
increased pressure drop.
REMEDY:
Most air cooler bundles have plug-type or cover plate headers, with
straight tubes; and thus can be readily freed of internal foulants.
Most air cooler bundles have plug-type or cover plate headers, with
straight tubes; and thus can be readily freed of internal foulants.
In
the case of sealed-off tubes, if enough are out of service to cause the unit to
be short, at least a partial retube is needed.
the case of sealed-off tubes, if enough are out of service to cause the unit to
be short, at least a partial retube is needed.
VII:
Change in Process Conditions: Over
time, feed rates, process fluid properties, inlet temperature, or inlet
pressure may be changed. Since operating personnel are periodically reassigned,
retire, or change jobs, employees who observed the unit when it was new may be
long gone.
Change in Process Conditions: Over
time, feed rates, process fluid properties, inlet temperature, or inlet
pressure may be changed. Since operating personnel are periodically reassigned,
retire, or change jobs, employees who observed the unit when it was new may be
long gone.
When
confronted with the statement “This exchanger never worked”, check to see that
existing conditions are similar to design conditions prior to embarking on
extensive research.
confronted with the statement “This exchanger never worked”, check to see that
existing conditions are similar to design conditions prior to embarking on
extensive research.
About Us: DtiCorp.Com
carries more than 35,000 HVAC products, including
industrial, commercial and residential parts and equipment from Honeywell,
Johnson Controls, Robertshaw, Jandy, Armstrong and more. Our online catalog is easy
to navigate and search, and all products have a picture and a description. If a
customer has any questions about a product, they can call and speak with one of
our product experts. Our mission is to offer the best prices anywhere to our
customers.
carries more than 35,000 HVAC products, including
industrial, commercial and residential parts and equipment from Honeywell,
Johnson Controls, Robertshaw, Jandy, Armstrong and more. Our online catalog is easy
to navigate and search, and all products have a picture and a description. If a
customer has any questions about a product, they can call and speak with one of
our product experts. Our mission is to offer the best prices anywhere to our
customers.
