How can i optimize and increase the heat rejection and decrease the coil surface of air cooled condenser?

what should i do to decrease the coil surface of the condenser coil in air cooled condenser unit ? what parameter should i change? material of tube? diameter of tube? material fin? arrangement of coils? what is the best arrangement of condenser coil in air cooled condenser unit? flat or v-type ?How can i optimize and increase the heat rejection and decrease the coil surface of air cooled condenser?
What is this condenser's service? Refrigerant, steam, other? Do you need any sub-cooling.



Materials for the tubes or fins have very little to do with the over all heat transfer rate. Air flow rate and velocity inside the tube have a bigger impact.



If you do not need any sub-cooling, then having a configuration that will get the condensed liquid out of the condenser so more surface is exposed to the condensing process is important.



One important thing that is overlooked many times is air recirculation. Having a forced draft arrangement where the fans push the air across the condenser seems at first to be the most economical arrangement but in fact an induced draft arrangement where the air is pulled across the tubes and then discharged by the fans minimizes an air recirculation and actually results in better performance. You should note that most air conditioning condensers use this principle.How can i optimize and increase the heat rejection and decrease the coil surface of air cooled condenser?
Well ,you can increase heat rejection by increasing the resistsnce which inturn can be done by increasing the length.
Introduction



Refrigeration is technology which makes a major contribution to humanity in many ways including food preservation, control of indoor air quality, gas liquefaction, industrial process control, production of food and drink and computer cooling. Without refrigeration, modern life would be impossible. About 15% of the world’s electricity is used to drive refrigerating and air-conditioning systems. Inefficient use of energy is a waste of valuable resource and contributes to global warming. Most of the global warming effect of refrigerating systems comes from generating energy to drive them. Only a small proportion comes from the release of certain refrigerants. This informatory note describes how the efficiency of refrigerating systems can be maximized thereby minimizing their global warming impact.

Fundamentals



Refrigeration is the science of making heat flow “uphill” from low to high temperatures. A refrigeration system extracts heat from the substance being refrigerated (cold reservoir) and rejects it to the ambient at a higher temperature (hot reservoir) as indicated in Figure 1. This is analogous to the pumping of water to an elevated storage tank. The energy consumption of a refrigerator is roughly proportional to rate of heat extraction (amount of water pumped) and to the temperature lift through which the heat is raised (height water is pumped).



The energy efficiency of a refrigeration system is usually expressed as a Coefficient of Performance (COP) which is the ratio of the heat extraction rate to the rate of energy use.



Whatever type of refrigerating system is being used, it is fundamental to minimize the required heat extraction and to keep the difference between TC (condensing temperature) and T0 (evaporating temperature) as small as possible. Minimizing heat extraction is done by insulating the refrigerated room and low-temperature parts of the refrigeration system, minimizing ambient air infiltration (e.g. door openings and leakage) and reducing energy use in refrigerated applications (e.g. fans and forklifts). Reducing (TC – T0) is done by maximizing condenser and evaporator heat transfer performance and minimizing refrigerant pressure drops in suction and discharge pipelines.

Description of a Vapour Compression Refrigeration System



The standard vapour compression refrigeration system consists of a refrigerant in a closed circuit comprising a compressor, a condenser, an expansion device, an evaporator and interconnecting piping (Figure 2). In the condenser, compressed refrigerant vapour at high pressure is condensed at high temperature by heat transfer to the surroundings. The high-pressure refrigerant liquid is reduced to a low pressure at the expansion valve. At low pressure, the refrigerant will evaporate at a low temperature enabling it to extract heat from the substance to be cooled. To complete the cycle, the low pressure refrigerant vapour exiting the evaporator is compressed to high pressure by the compressor. The total heat rejected in the condenser is the sum of the heat extracted plus the compressor energy use.







Figure 2. Schematic of a Simple Vapour Compression Refrigeration System



Loss of refrigerant from the circuit would have a very detrimental effect on the reliability of the system, so great care is taken to make refrigerating systems as leak-tight as possible. Individual domestic refrigerators, of which there are more than one billion, each contains a very small amount of refrigerant. Such systems are expected to run for more than 20 years without addition of refrigerant. The global warming effect of such refrigerators is significant but nearly all of it is caused by carbon dioxide produced when the electricity to run the refrigerator is generated.

Effect of System Components on Efficiency

Refrigerant



Very few substances have properties appropriate for a refrigerant and, of these, few have stood the test of time and continue to be used as refrigerants. Figure 3 shows some of the substances that have been used as refrigerants and how their use has varied over time.







Figure 3. Typical Refrigerants and Their Historical Use







There is no ideal refrigerant. Selection of a refrigerant is a compromise between many factors including ease of manufacture, cost, toxicity, flammability, environmental impact, corrosiveness and thermodynamic properties as well as energy efficiency. A key characteristic is the pressure/temperature relationship. In general, for energy efficiency it is desirable for the refrigerant critical point (temperature above which the refrigerant cannot condense) to be high compared with the heat extraction and rejection temperatures.



Good transport and heat transfer properties are also important for energy efficiency as they reduce running costs and allow smaller temperature differences to be employed in evaporators and condensers and hence smaller overall temperature lifts. In general, refrigerants of low molecular weight and low viscosity will have the best properties.

Compressor



Compressors will lose efficiency if the temperature lift is higher than necessary and they will also lose efficiency if droplets of refrigerant liquid are present in the suction vapour or if the suction vapour becomes too hot. Compressor maintenance, where possible, and the preservation of lubricant quality are important to retain energy efficiency. For some compressor types (particularly screw and centrifugal), their part-load energy efficiency performance is poor compared with at full load, so sustained part-loaded operation should be avoided. Variable speed drive technology and improved control systems can minimize the energy penalty but increase capital costs.

Condenser



To keep refrigerant heat rejection temperatures as low as possible, condenser heat transfer rates should be maximized and the cooling medium temperature minimized. Evaporative condensers are often the most efficient because they reject heat to the wet-bulb temperature of the ambient air. For instance, humid air at 25°C and 60% relative humidity has a wet-bulb temperature of 16°C. However, they require careful maintenance to avoid Legionella contamination. Water-cooled condensers combined with cooling towers also approach ambient wet-bulb temperature but there is an additional temperature difference to drive heat from the refrigerant into the water, so refrigerant heat rejection temperature is generally higher. Water use can be excessive if a cooling tower is not used. Air-cooled condensers are usually the least efficient method as they reject heat to the air dry-bulb temperature, which is generally significantly higher than wet-bulb or water temperature. However, for small systems they are commonly used because they are cheap, simple and require little maintenance.



It is important to keep all types of condenser clean and free from fouling. Condensers rejecting heat to atmosphere must be allowed plenty of fresh air and protected against any tendency for the air to re-circulate back to the condenser inlet. Systems that operate with refrigerant suction pressures less than atmospheric (e.g. low temperature ammonia or air-conditioning with HCFC-123) should use purgers to remove non-condensables from the refrigerant.

Expansion devices



Many expansion devices require significant pressure difference to allow proper operation. Therefore condensing pressure is often maintained at artificially high levels, even at low ambient temperatures. The biggest culprit in this respect is the conventional thermostatic expansion valve which is often selected because of its very low cost. One solution is to use electronically controlled expansion valves.

Evaporators



As for condensers, evaporators should be designed to operate at minimum economic temperature difference so that the refrigerant heat extraction temperature can be as high as possible for a given substance temperature. Increasing heat extraction temperature also reduces the size of the compressor required.



As well as evaporator size, aspects such as refrigerant distribution, circuiting and velocity, use of enhanced surfaces, air speeds (for air coolers) can all significantly affect energy efficiency. Air coolers that operate at temperatures below freezing must be defrosted regularly to restore performance. Electric defrost is simple but is least efficient and therefore only suitable for small systems. Electric defrost has to be paid for at least twice, to put the electric heat into the cooler and to take it out again. Water defrost, hot gas defrost, and defrost by the circulation of warm fluid through the cooler, are all potentially more efficient. However, whatever the system, it is important to optimize the frequency and duration of defrost to avoid unnecessary defrosting.



Interconnecting piping



Efficiency can be reduced if interconnecting piping is of the wrong size or is arranged in ways that cause unnecessary pressure drop or inhibit oil return (e.g. excessive bends and fitting).

Importance of controls



A refrigeration system with well-designed components will not operate efficiently unless the components are correctly matched and controlled. Energy efficiency has not always been the prime consideration when selecting effective controls. If possible, the following control options should be avoided to maximize energy efficiency:



– slide valve unloading of over-sized screw compressors;



– hot gas bypass of compressors;



– throttling valves between evaporators and compressors;



– evaporator control by starving refrigerant supply;



– too frequent defrosts;



– condenser head pressure controls except when necessary.

Conclusion



Improving the energy efficiency of refrigeration systems is not difficult and should be encouraged because of the environmental benefits. It often involves a trade-off between initial costs and on-going operating costs. There are many situations where economics motivate the equipment supplier to provide the cheapest solution, especially if the supplier does not have to pay for the running costs of the system. Standards should be set for energy efficiency for all types of refrigerating system. Governments should legislate to ensure that suppliers are penalized for supplying systems that do not reach acceptable standards of efficiency and to ensure that users of efficient systems benefit by more than the resulting reduction in running costs. If this were done, it is reasonable to suppose that the energy consumption of refrigerating systems could be reduced by at least 20% in the short term. An objective of 30-50% reduction — depending on applications — by 2020 is a goal which could be achieved.







This Informatory Note was prepared by S. Forbes Pearson, winner of the IIR Gustav Lorentzen Medal awarded at the 21st IIR International Congress of Refrigeration in Washington DC in August 2003. It was reviewed by 24 experts worldwide.











The International Institute of Refrigeration (IIR) is an intergovernmental organization comprising 61 Member Countries representing over 80% of global population.



The IIR's mission is to promote knowledge and disseminate information on refrigeration technology and all its applications in order to address today's major issues, including food safety, protection of the environment and development of the least developed countries.



The IIR provides a wide range of services: organization of conferences, congresses, workshops and training courses, a database (Fridoc) containing 70 000 references, several publications (journals, manuals, technical books, conference proceedings, informatory notes), and a Web site providing a wide range of information (www.iifiir.org).











Technical Note on Refrigerating Technologies



Ice Slurry: a Promising Technology



For centuries, ice has been considered as effective storage material for temperatures around 0°C. Using ice can reduce the size of a water storage tank by a factor of two to ten, depending on the temperature range used for operating the system. The reason for the high energy density is the latent heat of phase change. For a pure substance, under constant pressure, at the freezing temperature, a large amount of energy is required to build up a regular crystalline structure, which leads to the solid phase. In the opposite process of melting, the crystal is destroyed and energy released at the same temperature, now called the melting temperature. For temperatures other than 0°C, other materials that exhibit a change of phase, phase-change materials (PCM), can be used. In technical applications, mixtures are the most used. They show a temperature glide (continuous transition) in the enthalpy function during the phase change.



If a PCM is finely dispersed in a carrier fluid, a phase-change slurry is obtained. The particles need to be stable and should not lead to high stratification effects in the system, caused by the buoyancy force. Phase-change slurries may be micro-emulsions, shape-stabilized paraffins, clathrates, microencapsulated phase-change slurries, etc.1 In April 2003, in Switzerland, an international conference and business forum on the new fields of PCM and energy storage based on these materials was organized.2 In this type of energy storage, PCMs – which have a high thermal energy density and stable temperature due to the phase change – are also used for the transport of cold or heat. Ice slurry is the oldest and most commonly used substance in the phase-change slurries group.



This technical note briefly highlights the state of the art of this promising technology.



Definitions



In early times, to cool their food, the Romans used naturally occurring ice slurries, e.g. snow-water mixtures, crushed ice, flake ice, etc. Last century, ice slurries were created artificially. Initially, these were basically water with large ice particles with a characteristic diameter of one to several centimetres and were mainly used to cool coal and silver or gold mines. The production of fine-crystalline ice slurries then allowed the technique to be applied in small-scale systems, e.g. for the cooling of display cabinets in supermarkets (see Figure 1).



It is difficult to define “ice slurries”. Ice slurries can be classified using the following definitions3:



Definition 1: An ice slurry consists of solid ice particles in a fluid forming a suspension with two phases.



Definition 2: A fine-crystalline ice slurry is a substance comprising ice particles with an average (characteristic) diameter which is equal to or smaller than 1 mm.



Definition 2 is a little arbitrary, but still very useful. This technical note only addresses fine-crystalline ice slurries, produced for example with mechanical scraper-type ice-slurry generators. With this method, the ice particles created have a characteristic diameter of approximately 200 mm.

Application in refrigeration systems and environmental benefits



Experience has demonstrated that conventional direct evaporation systems are usually low-cost and are technically very reliable. However, they use the same fluid – the refrigerant – for “production” and “transport” of cold from the central refrigeration unit to the end users (e.g. display cabinets). As a result, these systems contain large masses of refrigerant and, in the case of permanent or accidental leakage, may lead to high losses with drastic consequences for the environment. Furthermore, high system charges of refrigerants lead to higher costs, because new replacements of CFCs and HCFCs are several times more expensive.



In indirect systems, the production of cold and its transport are separated. Cold is transferred in a heat exchanger from the primary to the secondary circuit. Indirect systems facilitate the use of refrigerants such as ammonia (R-717) or propane (R-290). A large number of fluids are available and used as “secondary liquid refrigerants”. The use of ice slurries is a development in such systems where the phase change is used in order to reduce the required mass flow for a given capacity compared with that obtained using a secondary liquid refrigerant.



The difficult search for alternative refrigerants, due to the phasing out of CFCs and HCFCs, has led to the envisaging of the development of ice-slurry technology.



Production methods



Currently, the most commonly applied techniques are mechanical ones. Usually, the refrigerant is evaporated in a cylindrical double-wall evaporator. In the inner domain, a water-additive mixture leads to the creation of ice crystals on the wall; these are then mechanically removed. As the crystals drop into the fluid, the number of ice particles per volume and, therefore also the ice concentration, increases. Mechanical-scraper type ice-slurry generators with:

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rotating knifes

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rotating cylindrical slabs (see Figure 2)

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rotating brushes

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screws in cylinders



have appeared on the market and are now widely used for experiments and in some installations.







Condenser

Textfeld: Condenser







Figure 1. A schematic drawing of an ice-slurry system, e.g. of a supermarket, is shown. The left part shows the ice-slurry generator









Figure 2. The principle of an ice slurry generator with rotating slabs allows a high ice production rate per volume



Other ice-slurry generators under investigation are:

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vortex-flow type (turbulent fluid eddies remove the ice particles from special treated surfaces with little adhesion)

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direct-injection or direct-heat exchange type (the refrigerant is directly injected into the water)

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fluidized bed ice generator (the flow enables steel or glass spheres to hit the ice crystals and remove them from the wall)

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ice generators using supercooled water with different types of nucleation initiation:

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by a momentum decrease (flow perpendicular to a cold wall)

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by an ultra-sound field

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by bubble nucleation

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vacuum ice generators (the pressure is lowered to the triple point of water)

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hydro scraped generator.



Advantages of ice-slurry technology



The potential advantages of ice-slurry systems, which are listed in this section, are valid in comparison with direct evaporation systems or/and indirect refrigeration systems containing brines as a secondary refrigerant:

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high cooling capacity given by the latent heat

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smaller tube diameters for the piping system (A)

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lower energy demand for the pumps (B)

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practical observations tend to prove that the “quality” of cold produced by ice-slurry systems is improved: better temperature stability, easier moisture and frosting/defrosting management…

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combined with energy storage, the high thermal capacity of the system may bridge small electricity supply cuts

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cheaper electricity and the low nocturnal condensing pressure can be taken advantage of

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increased safety by storing cold in storage tanks

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smaller filling mass of primary circuits

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if the cooling demand of an existing system has to be extended, then the electrical supply does not have to be increased, because cold production can be extended to 24 h

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the peak supply power of cold is many times larger in an ice-slurry system than in a conventional storage system (e.g. an ice-on-coil system).



The system design engineer may choose between (A) and (B) or take partial advantage of both. High heat transfer rates are possible because the ice particles are very finely dispersed in the fluid. Figure 3 shows numerous ice particles in an ice slurry, and Figure 4 the related surface of the total amount of ice particles in one kilogram of ice slurry.



Drawbacks and limits



Ice-slurry systems also show some significant disadvantages, which are listed below:

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additional heat exchanger between the primary refrigerant system and the secondary transportation system for cold

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additional pump

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additional energy demand for the pump to charge the storage tank and for the operation of the mixing element

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additional systems for controlling and monitoring the ice-slurry quality

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not adapted for use in air-conditioning and chiller systems, except where savings offered by this technology offset the thermodynamic penalty due to cooling below 0°C to fulfil a cooling task at only 12-14°C

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ice-water systems are most advantageous at temperatures close to the freezing point of water.



The latter disadvantages have recently led to the development of other PCMs, e.g. use of substances such as paraffins. With such substances the melting point can be continuously adjusted to the requirements dictated by the particular application.

























Figure 3. A microscopic photograph of an ice slurry is shown in this figure.3 After their creation, the ice particles grow slightly as a function of time, leading to time-related behaviour of physical properties





Figure 4. The total surface areas A of all the particles in one kg ice slurry, for different ice mass fractions, are presented. These results are valid for spherical particles with a diameter dp







Current applications



Ice-slurry systems can be applied in the following domains:

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refrigeration in supermarkets

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cooling in dairies and cheese production facilities

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cooling in breweries

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fast food cooling

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cooling of planes in airports (transport of cold over long distances to the docks)

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cooling of pharma parks (analogous to technoparks, but for pharmaceutical research)

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direct immersion of food (e.g. shrimps).

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cold storage in the food industry, in air conditioning and in district cooling.



These are some examples that can be spread to many other domains.



Possible future applications



Future applications are expected in the following domains:

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plastics production: temperature stabilization leads to more homogeneous temperature profiles in the plastic extruders. This will increase the product quality

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cooling of chemical processes

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immediate stopping of a chemical process by direct injection of ice slurry into the reactor to absorb as much heat as possible, for safety reasons

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mixing of concrete with ice slurry in a quantity so that after the melting of the ice particles exactly the right mass of water is added to the concrete. The latent heat will be used to absorb the reaction heat. Particularly in the construction of road and train tunnels, the technical cooling system may be reduced in size or even economized.



The IIR Working Party on Ice Slurries, which was set up in 1998, has organized five workshops and published proceedings of the papers presented. The contributions cover all important topics that still need to be investigated in order to favour the development of this technology: from physical properties and their time behaviour to fluid dynamics (e.g. pressure drop calculations in piping systems), heat transfer (Nusselt functions) for laminar and turbulent flows, ice generation, storage, mixing, piping, etc. Special contributions will be published in a Special Issue on Ice Slurries of the International Journal of Refrigeration in 2004. All useful and available practical knowledge will be brought together in an IIR Handbook on Ice Slurries, which will be published in 2005.



Conclusion



Ice slurry is undoubtedly a promising technology that should be encouraged because of its numerous advantages, in particular energy saving and environmental benefits.



Further research and development work needs to be carried out, particularly on how to generate ice slurry in an efficient, reliable and economical way, and on fluid properties and measuring techniques in order to open up this technology for use in a broader range of applications.5



References



1. Inaba H. et al. New Challenge in Advanced Thermal Energy Transportation Using Functionally, Thermal Fluids, Int. J. Therm. Sci., 39, 991-1003, 2000.



2. Proceedings of the Phase Change Material and Slurry Scientific Conference and Business Forum, Editors Egolf P.W., Sari O. Yverdon-les-Bains, Switzerland, April 23-26, 2003.



3. Egolf P.W., Sari O. A Review from Physical Properties of Ice Slurries to Industrial Ice Slurry Applications. Proceedings of the Phase Change Material and Slurry Scientific Conference and Business Forum, 15-25, Yverdon-les-Bains, Switzerland, April 23-26, 2003.



4. Brühlmeier J, Egolf PW. Flüssigeis - ein neuer K?ltetr?ger. Booklet printed in the framework of a special prize donated by the Swiss Bank Association (UBS), 1996.



5. Granryd E. Perspectives on ice slurries, Proceedings of the Fifth IIR Workshop on Ice Slurries, Stockholm, Sweden, 2002.
First, it will depend on how air will be flowing through the condenser. If it is a passive air flow, you might need a large condenser. If it has a fan pushing (or pulling) air through it, it can be smaller. Second, it depends on the K factor of the tubing and fin material. Metals like aluminum and copper transfer heat fairly rapidly, and using these metals will allow a smaller coil. Thirdly, what type of fluid is inside of the coil? If it is a liquid, more heat can be transferred in a smaller area, compared to a gas fluid. Lastly, the shape of the coil will only depend on the way that air will be flowing through it. Look at an air conditioner condenser coil outside of your house, and it is somewhat circular in shape, as it has a fan that pulls air through it. It is designed to maximize area and minimize space occupied. The air conditioner coil on an automobile, however, is flat, as it is designed to take advantage of the forward movement of the car in addition to the fan on the engine. Diameter of tubing and spacing of fins would need to be taken into account, but these parameters would depend on material of the tubes and fins, as well as fluid flowing through.
It might help if you could add a water spray to the cooling air. The unused water could be recirculated so that all the water consumed went to evaporative cooling.