It is a well-established fact that an HVAC system is comprised of so many parts and components. Some of these parts and components are striking in terms of appearance. But there are also others that are easily forgotten or overlooked — though they are just as important as the others. A perfect example of this kind of HVAC component is the fan guard.
What Is A Fan Guard?
Generally speaking, a fan guard, also known as a finger guard, fan grill, or fan cover, is the HVAC component that provides a physical barrier around spinning fan blades to prevent accidental contact with fingers or other objects. A fan guard is typically made of a metal wire or plastic, and it is designed in a way that would maximize airflow while minimizing noise. Because of this particular purpose, it’s not surprising that a fan guard is required for a lot of HVAC systems.
Because of the importance of fan guards, HVAC professionals like you should make sure that the HVAC systems that you’re offering are equipped with high-quality fan guards. And one thing that you can do is to make sure that the fan guards are coated with the right finish.
Why Is Finish Coating Important for Fan Guards?
The simple answer to the question of “why is finish coating important for fan guards?” is that finish coating helps prevent fan corrosion. The rate of corrosion for any application depends to a large extent on the concentrations of fumes, their temperature, and the amount of moisture associated with them. These parameters make it incredibly difficult to define the corrosion resistance of any coating by a single rating.
There are seven most common reasons to apply a protective coating to a fan, especially to a fan guard. These reasons are:
1) Protection from corrosion by acids and alkalies
2) Protection from weather elements (rain and UV)
3) Protection from abrasion and physical wear
4) Protection against process contamination
5) Protective coating provides easily cleanable surface or decontamination
6) Good aesthetics
7) Safety (fireproofing, low glare, and visibility)
Even though the coating is incredibly for fans in general, it is still worth noting that there are some restrictions on fans to be coated. These restrictions are as follows:
- Bearings cannot be placed in a corrosive airstream.
- The coating of variable intake vanes and outlet dampers is not recommended since it is almost impossible to properly protect some of their component parts like linkages, bearings, etc.
- Shaft seals of a variety of types are available and should be used. Special types of seals may be required in some instances.
- Belt and shaft guards are usually made from light gauge material. Steel blasting would distort these parts. As a result, steel blasting (and special coatings requiring steel blasting) is not recommended for these parts.
- Belt and shaft guards are usually painted OSHA yellow.
- Drains, especially in handling moist atmospheres, are a necessity.
With all these points in mind, the corrosion resisting paint or finish can be applied either to the airstream of the fan or to the entire fan, both inside and out. That said, bearings, motors, and drivers are items that are usually not coated by the fan manufacturer. Shafts are typically coated with a very thin coating of an asphaltum-based paint to prevent corrosion. A light asphaltum coasting has been specified for fans, but many field problems, such as rusting, were reported. As a result, a heavy coating of asphaltum must be specified so as to prevent these field problems.
Protective Coating on Fans and Fan Components
For all paints on steel surfaces, fans are phosphatized and washed, followed by one prime coat and one or more finish coats, depending upon the application and its requirement. Phosphating is the formation of a layer of zinc, iron, or manganese phosphate crystals on the surface of the part to be painted. It is used to increase corrosion resistance and improve paint adhesion.
The phosphate coating is a transition layer. It is less dense than most metals but denser than paint. It has an intermediate thermal expansion between paint and metal. As such, the effect is that the phosphate layer can smooth out the thermal expansion differences that would otherwise exist between the metal and the paint.
Generally speaking, the parts to be coated are treated with a phosphating solution, which is sprayed on. Upon curing, the paint solidifies and is locked into the phosphate pores. As a result, adhesion is greatly enhanced.
The method by which the phosphatizing process works is as follows. First, the acid attacks the metal at the crystalline grain boundaries. Then, the phosphate crystals begin to grow along the grain boundary lines. After the grain boundaries are attached, the acid begins to etch the grain surfaces, and phosphate crystals appear at these sites. The resulting mass of phosphate crystals spread over the surface and grow into one another. This process is controlled by regulating the temperature, chemical concentrations, and the rinsing pressure. As a result, the final surface appears smooth to the naked eye.
In some cases, steel, sand, or glass bead blasting of the parts to be coated is needed. For the paint to adhere, the profile generated by steel blasting is required. The surface profile for most fans coated with special paint is 1–2 mils peak to peak.
The two basic standards to describe surface preparation are the National Association of Corrosion Engineers Standards (NACE) and the Steel Structures Painting Council (SSPC). The following are some of the SSPC specifications for surface preparation.
- Solvent Cleaning. It is the removal of oil, grease, dirt, soil, and contaminants by cleaning with a solvent, vapor, alkali, emulsion, or steam.
- Hand Tool Cleaning. It is the removal of loose rust, loose mill scale, and loose paint by hand chipping, scraping, sanding, and wire-brushing.
- Power Tool Cleaning. It is the removal of loose rust, loose mill scale, and loose paint by power tool chipping, descaling, sanding, wire-brushing, and grinding.
- White Metal Blast Cleaning. It is the removal of all visible rust, mill scale and paint, and foreign matter by blast cleaning.
- Commercial Blast Cleaning. It is blast cleaning until at least two-thirds of each square inch is free of all visible residues.
- Pickling. It is the complete removal of rust and mill scale by acid pickling, duplex pickling, or electrolytic pickling.
- Near White Blast Cleaning. It is blast cleaning until at least 95% of each square inch is free of all visible rust, mill scale, paint, and foreign matter.
Selection of Coatings
Paints can be grouped based on their physical type, such as waterborne, high-solids, or powder metal paints. There is no paint that can meet the requirements of every application. The selection process must answer the following questions: in what environment will it be used? How long must it last? How much cost can be tolerated?
The following are some of the most common coatings used for fans and their components.
Waterborne coatings are those in which water is the major solvent or dispersant. Numerous resins are used in waterborne systems; some of the available ones include modified epoxies, polyesters, acrylics, alkyds, vinyl acetate-acrylics, and styrene-acrylics. Both thermoplastic and thermosetting formulations are used. Some resin systems cure when heated while others require the addition of a crosslinking agent.
The major advantages of waterborne coatings are 1) reduced fire hazard, 2) reduced solvent emission, 3) lower toxicity, and 4) they use equipment similar to solvent spray. On the flip side, the primary limitations of waterborne coatings include 1) surfaces must be very clean, 2) stainless steel or plastic pipe and fittings are recommended (due to corrosion in application equipment), 3) some formulations must be protected from freezing, 4) better control of booth temperatures and humidity may be required, and 5) longer flash-off times may be needed.
High Solid Coatings
High solid coatings are usually solvent-based and contain greater than normal amounts of pigment and binder — usually at least 40% on a weight basis. Some coatings are 65% solids or higher.
The primary advantages of high solid coatings include 1) less paint must be shipped, stored, pumped, and sprayed, 2) lower oven air volumes are required, 3) less paint must be sprayed to provide a given film build, 4) formulations may be less expensive to produce, 5) less energy is needed for solvent evaporation, and 6) less solvent is emitted to the atmosphere.
On the other hand, the main disadvantages of high solid coatings are 1) high viscosity, 2) difficulty in pumping and atomizing, especially when cold, 3) cleaning and phosphating quality is usually more important than for conventional paints because there is less solvent present to clean as it coats, and 4) sticky overspray, which is messy to clean up because it never dries. All these disadvantages may actually be beneficial since it means that pretreatment must be better than usual. As a result, corrosion resistance may be improved.
Powder coating is the application of paint in the form of a finely ground powder. The powder adheres weakly by means of electrical attraction. After application, the coated part is heated to melt the powder then cooled so the melted powder forms a solid film.
Powder coating has a number of advantages. The primary advantages include 1) little waste since overspray can be used, 2) no solvent cost or handling problems, 3) less fire hazard (higher ignition energy than solvent air mixtures), 4) less toxicity, 5) no air or water pollution, 6) no liquid mixing or pumping is required, 7) no flash-off space is needed, 8) less tendency to trap airborne dirt, and 9) fewer shrinkage stresses developed during cure.
However, powder coatings also have their own share of disadvantages. These are 1) powder films have more appearance problems, 2) the powder must be kept dry and this demands constant attention to equipment and technique, 3) changing colors is difficult, 4) cleaning before the application is very important, 5) powder coatings require a baking oven, 6) they’re hard to apply in tight areas (i.e. corners, behind angles, and hidden areas).
Thermoplastic is a coating that will dissolve in its own solvent system. The term “thermoplastic” means that if the coating is exposed to heat (thermo-), then it will become more plastic (i.e., soft and pliable). It is typically a single package material that dries only by solvent evaporation. Thermoplastic coatings are typified by vinyl and phenoxy. They cure by solvent evaporation, and there is no crosslinking of the resin. The coating will re-dissolve readily in the original solvent system.
There are three chemical compositions of generic coatings for thermoplastic. They are as follows:
- Acrylics. Acrylic acid, made from acrylonitrile, is converted to various acrylates. These and methyl methacrylate are the raw materials that are polymerized into acrylic resins. The advantages of acrylics include 1) excellent gloss retention, 2) weathering resistance, 3) moderate flexibility, and 4) high-temperature resistance. Meanwhile, the disadvantages are 1) poor chemical resistance and 2) poor solvent resistance.
- Vinyl. Resins are usually made by linking together (polymerization) of vinyl chloride, a very toxic gas, vinyl acetate, and vinyl alcohol. These resins are formed by a reaction between acetylene and an acid.
The advantages of vinyl are 1) fast dry in warm weather, 2) excellent water and acid resistance, 3) good adhesion, and 4) flexibility. On the other hand, the disadvantages are 1) usually low film builds, 2) the possibility to lift other coatings, and 3) poor solvent resistance.
- Alkyds. Commonly referred to as oil-based paints, alkyds are chemically modified polyesters. The modification is the natural oil, used along with the alcohol, to reach with a dibasic acid to form the polymer. The curing mechanism is a reaction between oxygen in the air and the oil of the polymer. This is a complicated reaction and not completely understood.
The primary advantages of alkyds include 1) inexpensiveness, 2) good adhesion, 3) flexibility, 4) good gloss, and 5) the fact that it can be applied over most other alkyds. However, alkyds are also known for their poor chemical resistance, slow drying, and low film build.
The opposite of thermoplastic, thermosetting is a coating that doesn’t dissolve in its own solvent system. It is normally a two-component coating, which cures by solvent evaporation followed by a chemical reaction. This chemical reaction of crosslinking enables a low molecular weight material to become a large molecule or network of large molecules.
Thermosetting also has its own share of generic types. The chemical compositions of the generic coatings of thermosetting include:
- Epoxy. Epoxy requires a catalyst to bring about the crosslinking that gives the coating its many desirable properties. The most commonly used catalysts are reactive amines and polyamide resins.
- Polyesters. These are coatings that are based not on epoxy resins but on modified alkyd chemistry, formed by the polycondensation of dicarboxylic acids and dihydroxy alcohols. The advantages of polyesters include 1) high solids, 2) excellent mineral acid resistance, and 3) strong and hard films. However, polyesters also have disadvantages, namely poor solvent resistance and marginal adhesion.
- Phenolics. Phenolic resins are made by the reaction of phenol and formaldehyde. The reaction is driven by heat.
- Inorganic Zincs. Zinc dust is added to inorganic binders. The zinc metal acts as the cathode and corrodes in preferences to the steel substrate. The main advantages of inorganic zincs are 1) they protect steel with a single coat and 2) they are also not subject to ultraviolet degradation. That said, inorganic zincs are also known for dry spray in hot windy weather, porous film to topcoat, and limited pH range.
- Epoxy Phenolics. Coatings are modified phenolic coatings that are created by blending phenolic resins with epoxy resins. Epoxy phenolics are known for their heavy build and excellent acid and solvent resistance. However, they also require shot blasting, which is a disadvantage.
- Polyurethane. These are coatings that are derived from prepolymers containing isocyanate groups and hydroxyl-containing materials, such as polyols and drying oils. Polyurethane has the following advantages: 1) excellent weathering, 2) enamel-like finish, 3) flexibility, and 4) damage resistance. On the flip side, it also has disadvantages, namely low film build, costliness, moisture intolerance, chemical cure, and temperature sensitivity.
- Epoxy Polyamides. These coatings have a slower crosslinking, which leads to a more flexible finish. The advantages of epoxy polyamides include 1) moisture tolerance during cure, 2) long pot life, and 3) excellent adhesion. Meanwhile, the disadvantages are 1) less solvent resistance, 2) costliness, 3) and less chemical resistance.
When choosing the coating for fans and their components, it is incredibly important to test them first. This must be done so as to evaluate the products, control the quality, and ensure process control. The following are the primary characteristics of paints that must be checked prior to the coating.
Paint viscosity is an incredibly important property to control since it affects the quality of the applied paint. Viscosity is the resistance of a liquid to flow. As such, paint viscosity must be such that the proper atomization and flow-out can be achieved.
There are a lot of instruments that are used to measure viscosity. A common method is to use a Zahn cup. Paint viscosity is determined by measuring the time required for a given quantity of paint to flow through a hole in the bottom of a metal cup.
The film thickness of dry paint is crucial as well. Thin films will not be able to provide corrosion resistance. Meanwhile, thick films are likely to crack. There is a direct correlation between wet film and dry film. A dry film is usually controlled by measuring the wet film with a gauge that has various “V” notches. There are many commercial gauges to measure dry film thickness.
Tape adhesion is a test to measure the adhesion of a paint film to its substrate. It is often measured by pulling the paint away from an inscribed “X” or grid with a strip of tape. The tape is pulled back upon itself as nearly in the plane of the painted surface as possible. A numerical rating system is used to evaluate the tape adhesion test results. If an inscribed grid has been used, the failure of adhesion may be expressed as the percentage of squares that have experienced some loss of paint.
Salt Spray Testing
Salt spray testing is the use of a salt spray system in an attempt to accelerate the corrosion process and cause early paint failure. Panels are usually exposed up to 14 days to a mist of 5% (weight to volume) sodium chloride solution at 92°F–97°F. The mist is produced by blowing hot, saturated air through a 5% salt solution.
The panels are then evaluated for two types of rusting. The first is face rusting, which is the percentage of the surface that has rust visible through the paint. And the second is the scratch-creep back rusting, which evaluates the distance in 32nds of an inch that has rusted away from a line scratched through the film to the metal.
Sometimes, acetic acid is added to the salt spray solution to accelerate the corrosion. A common problem has been that customers have asked if the paint is salt spray tested to an ASTM standard. The problem with this is that the ASTM standard does not have acceptance/rejection limits. For example, the specification should read that the paint should be “salt spray tested to ASTM B117 for 200 hours and have no scribe blisters. Maximum creep from the scribe should be 1 mm creep.”
Color matching is both important and difficult. This is important because of the customer reaction to mismatched colors and difficult because of human variability in color perception. Two panels that appear to be the same color to one person may seem different to someone else. Additionally, a color may also change when viewed from different lighting, i.e. fluorescent versus outdoor light. Color chips are used as a general practice in the fan industry to offer matched colors.
In the HVAC industry, there are so many kinds of HVAC systems, and there are also so many kinds of HVAC components that comprise these systems. A popular kind of HVAC system is the fan, and for this system, the fan guard is an important component.
Simply put, a fan guard, also known as a finger guard, fan grill, or fan cover, is the HVAC component that provides a physical barrier around spinning fan blades to prevent accidental contact with fingers or other objects. A fan guard is usually made of a metal wire or plastic, and it is designed in a way that would maximize airflow while minimizing noise. With that said, fan guards are undeniably crucial for a lot of HVAC systems.
Because of the importance of fan guards, it is your responsibility as an HVAC professional to make sure that the HVAC systems that you’re offering are equipped with high-quality fan guards. And one thing that you can do is to make sure that the fan guards are coated with the right finish.
Applying finish coating to the fan guards is a necessary process because doing so helps prevent corrosion to the fan guards. However, it can also be an incredibly tedious process because there are so many things to consider. First, you have to do a proper surface preparation to the fan guards, and then you have to go through a long list of possible paints. And of course, before you decide on which paint to use, you have to test a variety of them first to test their quality. Basically, it takes a lot of time and effort to choose the right paint for your fan guards. But the process has to be done so that you can provide high-quality fan guards to your customers.
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