By Allen Chasteauneuf, Managing Director, Alkemy Ltd


If the challenge for heat transfer and HVAC sectors is to lower running costs and lengthen maintenance service intervals, then a coating that achieves this by reducing surface fouling with virtually self-cleaning surfaces, that dramatically improves heat transfer coefficients, and reduces corrosion must be good.  It sounds unbelievable.  Can these illusive benefits actually be achieved?  The evidence presented here indicates that they can.

Reducing Surface Fouling
Fouling is a general term used to describe any deposit of extraneous material that appears upon the transfer surface of a heat exchanger or HVAC unit.  Whatever the cause or exact nature of the deposit, an additional resistance to heat transfer is introduced and the operational capability of the unit is correspondingly reduced. In some cases, the deposit is heavy enough to interfere with fluid flow and cause a pressure drop.   Here, we are more concerned with dirt and dust accumulation: a significant problem affecting the performance of heat exchangers and HVAC units.   They reduce efficiency, increase maintenance costs and can cause systems to fail at times of peak demand.  The coating changes the surface topography, making the surface non-stick to the point of being anti-fingerprint, resulting in the reduction of adhesion of a wide range of contaminant substances to the surface.   In the presence of any moisture, water or rain, the surface causes the formation of water droplets.  As water droplets roll over the surface, they pick up surface contaminants and remove them easily from the non-stick surface.  This is shown clearly in figure 1 which compares an uncoated and a coated surface.  Movement of air or water on the non-stick surface is sufficient to displace most contaminants.


The dirt and dust particles, found principally on fans, but also present on air ducts and coils, comprise a number of identifiable particles including; fungal spores, pollen particles, cellulose fibres, synthetic fibres, decayed leaves, insect parts and other organic matter.  The particles come from indoor and outdoor sources.  Some of the material fulfills the nutrient requirements for fungal and bacterial growth and there are a wide range of fungi which have been identified as forming colonies on these surfaces.  Water and moisture play an important role in fungal growth and colonization.  

The coating can be modified with a functional ingredient to be actively anti-fungal and anti-bacterial.  The modified coating actively inhibits the adhesion and growth of fungi, bacteria and biofilm.  In normal operating conditions, fungal spores, bacteria, organic and inorganic soils are deposited on to surfaces to form a conditioning film.  Once established the biofilm becomes increasingly difficult to remove.  Fouled surfaces become rougher, offering more surface area and therefore more shelter to fungal spores and bacteria, creating the potential for increased bacterial and fungal retention.  Figure 2 shows an electron microscopic scan of a coated surface in an experiment to determine bacterial adhesion, in this example E Coli.   The junction between the uncoated (left) and the coated surface (right) is quite marked; with the accumulation of E Coli cells clearly seen on the uncoated portion.

The additive incorporated into the coating has also been tested and proved efficacious against various fungi including several of the species Aspergillus, Penecillium and yeast, amongst many others; fungi which are frequent colonizers of air cooled and evaporative condensers.

Figure 2: Electron Microscopic Scan of Coated Surface with Antimicrobial Functionality

The anti-microbial / fungal efficacy has been tested for durability at an independent testing laboratory.  A sample piece of aluminium was coated and artificially aged; in this case by processing the aluminium pieces in a dishwasher for over a hundred wash cycles.  Using the standard ISO 22196 to determine anti-microbial effectiveness, the results showed that the number of dishwash cycles did not have any statistical impact on the anti-microbial performance on the coated surface.  The bacterial populations were reduced by four orders of magnitude following six hours contact to below the limit of detection in twenty four hours. 


Improving Heat Transfer Performance
Heat transfer coefficients are too low; one solution to the heat transfer problem is drop-wise condensation.  Condensation continues to be one of the most important heat transfer processes in many systems. The average heat transfer coefficient for drop-wise condensation is much higher than film-wise condensation.  Essentially, a surface which promotes drop-wise condensation is in an optimum state for further condensation.  There have been various enhancement techniques employed over the years to obtain drop-wise condensation, some ingenious to say the least.  For example, the use of noble metals, injection of fatty acids into the stream and a number of surface treatments.  However, these attempts have not been sustainable or are too expensive or simply not appropriate. The drive to obtain drop-wise condensation is understandable: heat transfer efficiency improvements of at least an order of magnitude, possibly more, can be obtained.  

Dr. Raya Al - Dadah, one of the leading UK authorities on heat transfer, has undertaken test work on a coating that promotes drop-wise condensation at the Birmingham University Mechanical Engineering Laboratory.  The test involved a rig containing two condensing copper elements; one was gold-plated, the other was coated with the test coating.  Gold or gold plated condensers are the standard for demonstrating drop-wise condensation.  The element with the test coating produced a condensation heat transfer coefficient comparable to the gold plated one.  This is shown in Figure 3.  The heat transfer coefficients of both condensers are fitted by two coinciding curves.  A similar exercise was conducted for heat flux values, with comparable results.  This is quite an achievement.

Figure 3.  Comparison of Heat Transfer Coefficients

What does this mean relative to the normal condensation process for water cooled condensers, i.e. film-wise condensation?  Figure 4, below, depicts the enhancement factor of drop-wise condensation compared to film-wise condensation. The heat transfer coefficient of the condenser with the test coating has been plotted against the wall subcooling.   The enhancement ratio is the ratio of the coated condenser’s heat transfer coefficient versus the film-wise condensation that would be obtained at the same steam saturation temperature and the wall subcooling. 


Figure 4: Drop-Wise Condensation versus Film-Wise Condensation Heat Transfer Enhancement Factor

Figure 4 shows that an enhancement factor of up to four times can be obtained. Dropwise condensation achieves its optimum benefits with water cooled condensers.

Reducing Corrosion
Most HVAC units suffer more or less from a corrosive environment; it’s just a matter of degree.  It may be due to construction, i.e. the combination of varying and dissimilar metals, polluting / fouling environment, cleaning frequency, or even fluid chemistry.  The range of operating environments is extreme.  The cost of corrosion is always more significant than just the replacement of the heat exchanger or condenser.  Process down time, change out cost, accessing the equipment and pressure testing are all additional costs.   If corrosion can be prevented, halted, or even delayed, the performance and lifetime of the unit will be improved.  The coating prevents corrosive / oxidizing substances from contacting metal surfaces.  A number of tests have been undertaken with the coating to demonstrate anti-corrosion capability, such as:

    heat exchanger after 18 months exposure without a filter;  no corrosion, no dirt adherence and no blockage of fins
    aluminium sample, no chromating; no corrosion after 1400 hour salt spray test
    500 hour salt spray fog test on heat exchangers, see figure 5.

At its most fundamental, the coating is very thin, less than 10 microns thick, completely dense and non-porous and is chemically bonded to the substrate surface.  These factors contribute to the anti-corrosion performance and durability.

Figure 5. Comparison of Heat Exchangers after 500 hours salt spray test

Conclusions
Three areas have been identified as the main contributors to poor performance for the heat transfer and HVAC sectors, resulting in increased costs and sometimes equipment failure.  Surface fouling exacerbates heat transfer performance.  Making surfaces non-stick, enabling a virtually self-cleaning functionality facilitates efficiency improvements.  Inorganic and organic contaminants are inhibited from adhering to the surface.  There is an additional hygiene benefit; adding proven anti-fungal and anti-bacterial functionality inhibits biofilm build-up, microbial and fungal growth.

Heat transfer is one of the most important industrial processes.  Throughout most industrial facilities, heat must be added or moved, from one process stream to another.  For water cooled condensers, drop-wise condensation would add considerable benefits by achieving substantial heat transfer improvements. Indeed, compared to film-wise condensation, an enhancement factor of up to four has been achieved.  Corrosion in systems is an ever present issue in many applications and the coating presented here has demonstrated its corrosion resistant capability.  These functionalities, taken together, indicate that the challenge of improved efficiencies can be achieved.


Alkemy Ltd
Stratford on Avon
Warks.

Can be contacted on
Tel: 0844 588 3103