Hudson Products Corp.

 

Hudson Products Fin-Fan®  Air-Cooled Heat Exchanger
"Frequently Asked Questions"
:

 

How do I specify fouling factors for an air cooler?

 Usually a fouling factor is only included for the process side of the cooler. The airside is usually considered clean service, although some customers will specify an airside-fouling factor as a result of airborne contaminants. The fouling factor is actually a resistance to heat transfer and is actually added to the other resistances due to the heat transfer coefficients.  It is usually recommended that the resistance due to fouling factor be no more than 33% of the total resistance to design the heat exchanger, otherwise the heat exchanger ends up being designed for fouling rather than for heat transfer coefficients. Some customers specify fouling as a "not to exceed %" of the total area.

How do you select the design air temperature for 
your cooler?

The design air temperature is not exactly the maximum air temperature during the year. Many air-cooled heat exchanger designers recommend that the design air temperature is the one that is only exceeded 2% of the days of the year.

 What is less expensive, air coolers or water coolers?

Plants, in general, never have enough cooling water. Water users at the plant always exceed the supply. A water cooler will almost always be less expensive and smaller than an air-cooled heat exchanger. However, there may be cases when, because of the process temperature involved, the film temperature of the cooling water becomes excessive and water causes excessive fouling in the water system. In these cases, the use of an air cooler is recommended.

    • When are belts or gearboxes used in air coolers?

Belts are usually used in drives up to 50 HP motors. Gearboxes are recommended for motors in excess of 50HP.

 How do I specify the pressure drop for an air cooler 
process side?

For gas coolers or condensers, the pressure drop is usually limited to 2 psi. For liquid coolers, the pressure drop is usually limited to 10 psi for an economical design. If the process fluid has a high viscosity, then the allowable pressured drop may be specified up to 20 psi or greater depending on pumping capacity and power consumption. 

 What is a guideline for the maximum design pressure 
of an air cooler?

Air coolers can be designed for up to 15000psi or more.

    • What is the limit on the width of the air cooler?

There is no limit on the total size.  However, there is a limit on the width of one bay and height during transportation as restricted by roads and bridge heights.  Most manufacturers limit the width of one bay to 16 ft., which often coincides with the maximum 50HP belt drive system.

    • What is the limit on the length of an air cooler bay?

The maximum length of the tubes available limits its. Mills usually limit tube size to 60 ft.

What is the difference between seal weld and strength 
weld of tubes in an air cooler?

Seal weld is usually performed to avoid leaks of process fluids to the atmosphere. Strength weld is usually specified for high-pressure units and it is controlled by ASME. 

What is the maximum number of rows that air coolers 
can be designed with?

Air coolers can be designed with up to 16 tube rows finned and in bare tube cases designs have utilized over 20 rows.

    • How is the tube thickness specified for an air cooler?

The design of an air cooler is dictated by API 661, which specifies minimum thickness for tubes depending on the tube material. 

What is the limit on the outlet air temperature for an air cooler?

The fan blade material usually limits this air temperature, grease system or bearing.  220 F is a conservative guideline.

    • What is the maximum number of passes on an air cooler?

There is no limit. Air coolers can be designed with up to 10 or more passes. 

What tests can be specified to make sure that a new air cooler does not leak?

Submerged bundle air tests, halide testing, hydro testing, and soap testing.

    • When is the use of a split header recommended?

A split header is recommended when the difference between the inlet and outlet process temperature is such that the tubes in the tube bundle are subject to different thermal expansion that may cause a tube to loose contact with the tube sheet. 

When are extruded serrated fins recommended?

Extruded serrated fins are recommended to improve the heat transfer coefficient on the airside by causing turbulence. This is beneficial when a compact (small) unit is needed because of available plot plan. Extruded fins are an excellent option when alloys (tubes and headers) are specified, in order minimize the cost of the unit.

    • What standards govern the design of air-cooled heat exchangers?

API-661, ASME Section VIII notably appendix 13 for header design, ISO 13706

    • What constitutes the most economical cooler design?

Usually the longest narrowest design possible meeting API - 661requirements for fan coverage

What component contributes the most to the noise 
generated by a fin-fan cooler?

Fan speed.

    • Are multi-blade fan designs the solution to noise problems?

Not necessarily, since the most efficient fan system utilizes the least number of blades.  It is therefore preferable to have a minimum of wide cord airfoil blades versus a number of narrower blades.

    • For the average air-cooled exchanger, what is the largest practical fan diameter to be considered?

As a rule of thumb, 16' usually is the limit allowing the usage of economical drive systems under 50HP.

    • Are their designs of fin tube that will help reduce the overall size of the cooler plot layout?

Using extruded serrated fins on airside sensitive designs can greatly reduce cooler bank sizing.

    • What standards are available for guidelines for performance testing?

ASME PTC-30.

What are differences between induced and forced 
draft fans?

- cost about the same

Induced:
- better air distribution across the bundle
- less like to suffer from hot air recirculation

Forced:
- marginally less power consumption
- easier to maintain and to install over the side recirculation systems

    • Can I use the top louvers to control the process and the
      inlet and recirculation louvers to control the plenum temperature on a winterized unit?

This creates an unstable system.  It is better to let the louvers control the plenum temperature only with a VFD or auto-variable fan controlling the process temperature.

Is a one-pass design recommended for a low-pressure 
steam or process condensing service?

Single pass condensers are subject to vapor locking and corrosion in the bottom (coolest) row due to the temperature and hence pressure gradient created as the air passes through the bundle.  This causes condensate backup in the bottom row and should definitely not be used.

    • What is the lowest delta T attainable with an air-cooled exchanger?

As the outlet process temperature approaches the inlet air temperature the unit size increases disproportionately.  The is dependent upon the thermal conditions but approaches less than 15 F should raise a flag.

The API-661 allowable piping loads are not adequate to 
meet
my piping design.  Can they be increased?

It is not uncommon to design coolers to support piping loads of 2 - 3 times the API allowables.

What governs the design of the structural steel 
supporting the
cooler?

This is commonly the most under designed portion of the cooler when it should be the most conservative.  Local building codes usually are specified to ensure member sizing meets local structural requirements along with the stipulation that the design be authorized by a certified registered structural engineer.

 What is the best system to clean an air-cooled 
exchanger on the fin side?

Please see our cleaning page and recommended solutions on this website.

    • Should I use a VFD for process control?

VFD pricing has come down significantly in the recent past and is an excellent system for process control and allowing full usage of all available motor power during hot summer ambients.

 What are the basic differences between API -661 and non-API coolers?

Sample differences required for API:
- tube diameter minimum 1"
- geometry with regards to fan proximity to bundle
- temperature limitations for considering split headers
- lateral movement allowance of bundle
- sloping on multi-pass condensers
- header velocity limitations
- minimum header material
thickness
- minimum nozzle dimensional tolerances, and maximum allowable nozzle loads
- tube gauge minimums
- restrictions on fan capacity, tip speeds, and motor minimum sizing
- natural frequency, vibration limitations
- minimum specifications on mechanical design of transmission components
- louver minimal construction specifications
- limitations on structural steel design, loads, and construction
- minimum restrictions on welding procedures
-     specified noise measurement criteria and guidelines for winterization

    • What are the most common problems in cooler non-performance?

- poor thermal design, insist on third party check rating, or onsite performance test in accordance with ASME-PTC-30

- L-Base wrap on fin separation creating gap resistance between fin and tube

- commonly overstated fan efficiencies, insist on air side performance tests

- hot air recirculation off the top to the inlet of same or neighboring cooler 

- request computational fluid dynamics analysis when laying out the plant, watch high intake velocities

- under estimating the air side fouling factor due to airborne contaminant

- excessive tip clearances between fan in fan ring

-  the absence of inlet bells to the fan ring for efficient air entry to the fan 

-  system freezing of process fluid 

-  insist on maximum side recirculation duct face velocities and adequate  instrumentation

    • How do I control sub-cooling in a condenser?

In order for an air cooled liquid cooler to function properly, the inlet and outlet piping must be arranged in a way that ensures the flooding of the entire volume of the tubes.  If the outlet piping is connected to a tank whose liquid level is below the level of the exchanger, and no loop seal is used to trap the liquid, the liquid will not fill the tubes, and will run out before the expected cooling takes place.  Also if no provision is made for venting gasses from a high point of the exchanger or piping, it will be difficult or impossible to purge gasses from the exchanger, and again the expected cooling will not be achieved.

Both of these problems are aggravated when one attempts to use a single tube bundle for both condensing and sub-cooling.  In theory, any tube pass consisting of only one row of tubes which is sloped in the direction of flow could be wholly or partially flooded with a properly designed liquid level control, but this may not be practical.  Airflow control is normally used to maintain a constant condensing pressure and temperature.  Therefore, as tubeside flow and ambient air temperature change, the liquid level and the extent of flooding would have to be adjustable in order to maintain a constant amount of sub-cooling.

Air cooler passes are seldom sloped more than 0.125" per foot.  For an average sized exchanger, say 10' wide by 30' long, if it were determined that an acceptable range of sub-cooling would be achieved by flooding between 10' and 14' of the bottom row, then the range of variation in the liquid level would have to be 0.5" or less.  This would be difficult to accomplish in an exchanger of this size.  Increasing the slope of the tubes would add to the cost because the tube bundle gets deeper, and supporting the tubes becomes more difficult.  If automatic variation of the flooding were required to compensate for changes in tubeside flow and ambient temperature, the cost of such a control system might be higher than the cost of simply using a second tube bundle for the sub-cooling, in which case separate air flow controls could be use for the condensing and for the sub-cooling tube bundles.

Problems with venting further complicate the design and operation of a condenser / subcooler tube bundle.  Even in highly pure, single component processes, some small amount of non-condensable gas is usually present.  And when this is not the case, the exchanger will still be filled with air or nitrogen at startup.  If a whole row is flooded, gas can be vented at the header entrance to that row, but the degree of sub-cooling will not be controllable.  And if over cooling of the condensate is acceptable, then increasing the vertical separation between the bottom row of tubes and the next row up would make the liquid level less critical.  However, in the above example, the last 16' of the bottom row would be wasted.  If only part of a row is flooded, it is impossible to vent gas from the middle of the tubes.

For the above reasons, Hudson Products recommends that separate tube bundles be used for condensing and for sub-cooling, and that their piping be arranged so that none the condensing bundle and the entire sub-cooling bundle is flooded.  The system can be vented at the condenser's outlet, at the sub-cooler's inlet, or at both places.  The bundles may be combined in a common bay, and share fans, plenum and substructure.  If a single bundle is used, Hudson cannot guarantee that the desired amount of sub-cooling will be achieved.

    • What are the rivets for on the Hudson Tuf-lite II blades?

They are only used to secure the blade while the epoxy cement is permanently bonding the blade to the hub section.  They are not necessary for any strength requirements.   However, they should not be intentionally removed as this would create a hole for moisture entrance.  In addition, it could affect the balance of the blade.  

 

 

 

 

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