Controlled environments are traditionally big energy users and HVAC systems are the main culprits. Allen Chasteauneuf, managing director, Alkemy, outlines efficiency gains that can be realised in HVAC systems with a new coating.
Imagine a surface coating that improves significantly the efficiency of HVAC systems. Heat transfer coefficients improve by several orders of magnitude; self-cleaning surfaces inhibit surface fouling and corrosion is reduced. Running costs are lowered and maintenance service intervals lengthen. In addition, employee respiratory problems decline rapidly.
Seems unlikely? Alkemy believes that these illusive benefits can be realised with its recently developed family of coatings called Alkemy Gold. These coatings address the problem areas where performance is critical and Alkemy has gathered the evidence to support the claim of efficiency improvement.
One solution to the heat transfer problem where heat transfer coefficients are too low is drop-wise condensation. Condensation continues to be one of the most important heat transfer processes in many HVAC systems. There have been various enhancement techniques employed over the years to obtain drop-wise condensation. For example, the use of noble metals, the injection of fatty acids into the stream and a number of surface treatments. However, these attempts have either been unsustainable, too expensive or simply not appropriate.
The drive to obtain drop-wise condens-ation is understandable: heat transfer efficiency improvements of at least an order of magnitude, possibly more, can be obtained, and that is worth having.
Alkemy Gold has been tested by Dr Raya Al-Dadah, a leading UK authority on heat transfer, at the Birmingham University Mechanical Engineering Laboratory. It was tested on a rig containing two condensing copper elements; one was gold-plated, the other coated with Alkemy Gold.
Gold or gold plated condensers are the standard for demonstrating drop-wise condensation. The Alkemy Gold coated element produces a condensation heat transfer coefficient comparable to the gold plated one (see Graph 1). The heat transfer coefficients of both condensers are fitted by two coinciding curves. A similar result occurs when heat flux values are compared.
What does this mean relative to the normal condensation process for water-cooled condensers, i.e. film-wise condensation? Graph 2 depicts the enhancement factor of drop-wise condensation compared with film-wise condensation. The heat transfer coefficient of the Alkemy Gold coated condenser has been plotted against the wall subcooling. The enhancement ratio is the ratio of the Alkemy Gold-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.
Graph 2 shows that a heat transfer enhancement factor of up to four times can be obtained. Dropwise condensation achieves its optimum benefits with water-cooled condensers.
Antifouling
Fouling is a general term used to describe any deposit of extraneous material that appears upon the transfer surface of an 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 HVAC 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 HVAC units. They reduce efficiency, increase maintenance costs and can cause systems to fail at times of peak demand.
Alkemy Gold coatings change 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 in Figures 1 and 2, where an uncoated surface is compared with an Alkemy Gold coated one. Movement of air or water on the non-stick surface is sufficient to displace most contaminants.
On close examination, the dirt and dust 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 fulfils the nutrient requirements for fungal and bacterial growth and a wide range of fungi have been identified as forming colonies on these surfaces.
Water and moisture play an important role in fungal growth and colonisation. This combination of factors, together with others, such as chemical pollutants from cleaning fluids, vapour from office machinery and work stress are the classic ingredients for sick building syndrome. Though a poorly understood phenomenon, it occurs in certain types of buildings: typically open-plan offices with automated HVAC systems. The range of symptoms includes headaches, fatigue, skin rash, shortness of breath and loss of concentration. Symptoms disappear when the person leaves the building.
Alkemy Gold-coated surfaces can be modified with a functional ingredient to be actively antifungal and antibacterial. The modified Alkemy Gold 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 onto 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.
In an experiment to determine bacterial adhesion, a partially Alkemy Gold-coated surface was covered with E. coli. It showed a marked difference at the junction between the uncoated and the coated surfaces, with the accumulation of E. coli cells clearly seen on the uncoated portion.
The additive incorporated in Alkemy Gold has also been tested and proved efficacious against various fungi species including: Aspergillus, Penecillium and yeast, among many others.
The antibacterial/fungal efficacy has been tested for durability at an independent testing laboratory. A sample piece of aluminium was coated with Alkemy Gold and artificially aged, in this case by processing the aluminium pieces in a dishwasher for more than 100 wash cycles. Using the standard ISO 22196 to determine antibacterial effectiveness, the results showed that the number of dishwash cycles did not have any statistical impact on the antibacterial performance of Alkemy Gold. The bacterial populations were reduced by four orders of magnitude following six hours contact to below the limit of detection in 24 hours.
Corrosion
Most HVAC units suffer more or less from a corrosive environment; it is 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 downtime, 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. Alkemy Gold prevents corrosive/oxidising substances from contacting metal surfaces. A number of tests have been undertaken with Alkemy Gold-coated surfaces 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 Figures 3 and 4.
At their most fundamental, the Alkemy Gold family of coatings are very thin (less than 10µm thick), chemically bonded to the substrate surface, completely dense and non-porous – factors that contribute to anticorrosion performance and durability.
Three problem areas have been identified that contribute to inadequate HVAC system performance, resulting in increased costs and sometimes failure. Alkemy Gold addresses these problems.
Heat transfer is one of the most important industrial processes. In most industrial facilities, heat must be added or moved from one process stream to another. For water-cooled condensers, drop-wise condensation would achieve substantial heat transfer improvements. Indeed, compared with film-wise condensation, an enhancement factor of up to four has been achieved.
Surface fouling also contributes to the impairment of 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 benefit; adding proven antifungal and antibacterial functionality addresses the hygienic issues. This will inhibit biofilm build-up, bacterial and fungal growth; an added benefit regarding sick building syndrome.
Corrosion in systems is an ever-present issue in many applications. Due to its dense, non-porous structure, Alkemy Gold has excellent anticorrosion properties and has demonstrated its corrosion-resistant capability. These functionalities, taken together, indicate that Alkemy Gold has the potential to add real value and potential improvements in HVAC systems can be realised.