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Home » Heat exchangers – Part 2

Heat exchangers – Part 2

By Marius La Grange, general manager, Thermocoil

A basic introduction to fin and tube designs.

“Heat exchangers (HEs) transfer heat from one fluid to another without the fluids coming in direct contact” – (ASHRAE, 2008, Chapter47). An exchange of heat energy taking place between at least two fluids. A fluid of course being a vapour or a liquid or a combination of both.

Types of fin and tube heat exchangers commonly used

Due to the relative ease of manufacturing fin and tube designs, these are used in a variety of HVAC&R applications. Not limited to these of course, but these would be the most commonly found in operation.

  • Dry coolers
    Similarly, to an air-cooled condenser, a dry cooler is an air-cooled device making use of ambient air to remove heat from the enclosed fluid. The cooling of a secondary coolant circulated through the internal portion of the coil with the heat transfer towards the ambient air drawn across the surface area. The internal fluid could of course be in a superheated gaseous state of which the temperature is reduced.
  • Gas-coolers (CO₂ systems)
    Systems operating with CO₂ (R744) as refrigerant offers substantial direct and indirect benefits to the environment hence these applications have grown exponentially, and the technology is now well established.

    R744 is a natural refrigerant like R717 but it typically operates and higher pressures when compared to R717 or most synthetic refrigerants. In trans-critical applications a gas-cooler is used to lower the discharge temperature from a compressor with ambient air. In warm ambient conditions the discharge gas simply experiences a reduction in heat energy. A distinct property of CO₂ is its relatively high critical temperature of 31°C.

    Ambient conditions above 31°C are common in many parts of the world so condensing with ambient air is not possible but the discharge gas from the compressors could be significantly reduced. Operating pressures of up to 120bar is common for gas-coolers. In low ambient conditions a gas-cooler would function very similarly to an air cooled condenser with high pressure condenser liquid draining from the outlet of the coil.

Materials commonly used

Heat exchangers could be made with tubes only with no fin material in specific  applications like ‘tank coils’ where an evaporator coil might be used to cool a fluid.

  • Tube materials
    In the case of the fin and tube coil the tubes are most-commonly copper with a few tube diameters options on offer. There are many reasons for this, but the major ones would be that copper is a very good conductor of heat (second best behind gold) and it is also rather easy to work with and weld.

    The operating pressures of any HCFC or HFC type system is well within the safe pressure limits of copper tubing options. As is the case with refrigeration tubing the imperial designation is still commonly used. 3/8”, 12mm and 5/8” is very common globally (don’t ask me about the logic). (If you have not reached your thirtieth birthday yet 1 inch is 25.4mm).

    Depending on the fluid circulated within the coil, the type of copper tube is selected. In the case of water tubing with smooth inner sidewall would be selected. Applying fluid dynamic laws one can understand that laminar fluid flow would reduce the pressure drop from inlet to outlet.

    In the case of DX applications tubing with an ‘inner groove’ is on offer as well. This could be used in evaporator or condenser applications. Various inner groove patterns are on offer, but they all aim to increase the inner surface area. The inner surface area could be up to 68% bigger with an increase in heat transfer of as much as 30% compared to a similar application with smooth bore tubbing used (Onan, Ozkan, and Ceran 2013).

    Copper tubes and return bends are gas welded with a suitable filler rod after the materials are correctly prepared.

    Stainless Steel (SS) is also very commonly used to make tube and fin type heat exchangers. The main reason in such cases would be the required operation pressures and the type of refrigerant used. In the case of a trans-critical CO₂ (R744) systems the operating limit of a gas cooler might be 120 Bar, as mentioned previously, making the SS tubing an obvious choice. Grade 304SS (A2) is most common.

    In the cases of systems operating with Ammonia (R717) as refrigerant SS is also a good choice since the Ammonia passing through the systems constantly would corrode any copper items. Steel tubing is also an option in the case of ammonia systems and historically it was the norm but has made way for SS as tube material for the greater part.

    The designed operating pressures of the coil influence the material thickness. Larger diameter tubes inherently have a lower operating pressure limit from the same material. Hence smaller diameter tubes are used when a higher operating limit is need. Should copper tubing not offer a safe operating pressure SS is commonly used. SS tubing is commonly tungsten inert gas (TIG) welded or fused.

  • Fin materials
    The greater majority of fin and tube coils are made with aluminium alloy fins. Aluminium is light and easy to process in big volumes and offers the best heat transfer value for money. A variety of thicknesses and alloy variations are generally on offer depending on the applications. The greatest majority of all fin and tube type heat exchangers would be making use of regular aluminium fin material.

    With a constant mass of air drawn across an air-cooled condenser the fin material is exposed to potentially corrosive elements constantly. Corrosion to the fin material lowers the unit’s potential heat transfer capability of the coil. In acid environments the outer surface other coil could be coated with a suitable epoxy coating to reduce the corrosion rate.

     “How good are these coatings” is a question often asked. Testing done in accordance with ASTM B117 norms (5% NaCl, 35°C) showed epoxy coated fin materials to last 4-7 times longer than bare aluminium fins. Epoxy coating is therefore a good option to consider in acidic or salty environments. An epoxy coating would be applied to the coils outer surface area after the coil is completed.

    In some cases where a large amount of condensate would be present on the aluminium fin area for prolonged periods corrosion is also accelerated. A hydrophilic coating on the fin’s surface materials offer a lower surface tension and allows the water droplets to flow off the aluminium easier. In this case the hydrophilic coating is applied to the aluminium fin material during production of the fin material, so it is not added at a later stage during manufacturing as it the case with epoxy coatings.

    This solution also offers good resistance to corrosion in salty or acid environments. Comparative tests according to ASTM B117 norms (5% NaCl, 35°C) have shown hydrophilic coated fins to last up to 3 times longer than unprotected aluminium fins (Friterm 2004).

    The closer the fins are together the greater the surface area available for the heat transfer to take place. The fins are cut and shaped from the raw material into the required sizes. A collar is shaped to be the contact area between the fins and tubes and serves as suitable fin spacing.

    Another alternative material is copper fins. Copper is an alternative to aluminium in more acid or salty environments. Copper is of course significantly heavier and offer a slightly better potential heat rejection that will in return effect the solution’s specific U-value. The copper fin materials could also be epoxy coated to improve resistance to corrosion.

Planned maintenance

A factor that affects the fin material life span like with most things is the specific preventative maintenance required. The huge volumes of ambient air drawn across an air-cooled condenser application contains dust particles and other airborne particles. These enter the fin face area and restricts the air flow. Less air flow means less heat rejection is now taking place every hour in operation.

The inlet of the coil face needs to be cleaned on a regular basis to remove restrictive matter. Any mould that might have accumulated also needs to be cleaned since it might pose health and safety risks. Cleaning condenser coils thoroughly is crucial but often quite challenging task. A suitable coil cleaning solution needs to be applied to removed greasiness and dust. A thorough rinse is needed to remove the dust and cleaning solution.

A slightly acidic solution allowed to dry on the finned area is likely to result in corrosion. Ideally the system would need to be switched off in order for the condenser to cool down to close to ambient conditions (the discharge gas/vapour might be close to 100°C in operation) as part of any Standard Operating Procedure (SOP). It is obviously a challenge in many applications to switch a system off for an hour or two with goods in the freezer room gaining temperature.

Many cleaning solutions applied and not thoroughly rinsed prior to the condenser coil placed in operation again might well adversely affect the fin material and reduce the unit’s ability to reject heat energy. Follow the required procedures as specified by the developer to the cleaning agent is a crucial part of the servicing SOP. 

Resources:

  • ASHRAE. 2008. “ASHRAE Handbook – HVAC Systems and Equipment.”
  • Friterm. 2004. “Epoxy and Hydrophilic Coatings Used in Aluminium Finned Heat Exchangers.” 2–3.
  • Onan, C., D. Ozkan, and L. Ceran. 2013. Heat Transfer Analysis of Internally Grooved Copper Tube R404-A Evaporators.
  • Stoeker, W. 1976. “Ch6- EVAPORATORS – AIR COILS AND LIQUID CHILLERS.” Pp. 169–250 in Industrial refrigeration handbook.