Wireforms for HVAC and refrigeration: vibration and corrosion

Where wireforms sit in an air path

Wire shows up all over an HVAC or refrigeration unit because it is cheap, stiff and open. Fan guards, coil supports, capillary clips, compressor mount springs, damper linkages, filter frames, drain pan clips and thermostat springs are all formed wire. Most of them are secondary parts in the bill of materials and primary parts in the failure report.

The duty is unusual. The part sits in moving air, at a temperature that cycles, in a stream that may carry dust, and in a cabinet that collects condensate. It is also expected to be quiet. A wire guard that rattles at low fan speed is a warranty call even if the metal is sound.

Corrosion starts where water sits or where two metals touch. Fatigue starts where a vibrating part is restrained. Both are designed in or out at the drawing stage.

Condensate, chlorides and galvanic couples

An evaporator coil runs wet for much of its life. Water collects on the fins, runs to the drain pan and splashes onto the supports. That water is not pure. It picks up dust, biological growth and, in coastal or industrial sites, chloride. A wire support that looked adequate on a dry bench is now in a shallow electrolytic cell.

The most common mistake is a zinc plated steel clip touching an aluminium fin pack or a copper tube. Zinc and aluminium sit at different positions in the galvanic series, and water bridges the joint. The aluminium becomes the anode and corrodes around the contact, while the steel clip keeps its shape and hides the damage. A coated clip, an isolating grommet or an aluminium support removes the couple.

Stainless is not automatically the answer. A 304 grade can pit in a chloride bearing condensate, especially in a crevice under a clip or at a tight bend where the stress is high. A 316 grade or a polymer isolated contact is the usual response where chlorides are expected.

Crevices deserve their own line in the review. Two wires twisted together, a clip closed over a tube and a coating that has lifted at an edge all create a gap where water sits and oxygen cannot refresh. That is where pitting begins, even on a good alloy.

Vibration: the failure that starts at a contact

A fan guard, a coil support or a capillary clip is a spring whether or not the drawing calls it one. It has a natural frequency, and if that frequency sits near the running speed of a fan or a compressor, the part will amplify the excitation. An illustrative fan at 900 rpm gives 15 Hz. A bracket with a natural frequency close to that will move far more than its static load suggests.

Fatigue cracks usually start where relative movement is restrained. A wire that passes through a hole, a clip that pinches a tube, a hook that rests on a bracket: each of those is a fretting point. Small oscillating movement wears the coating, exposes bare metal, creates a corrosion pit and then a crack. The two failure modes feed each other.

The first fix is stiffness, not material. Shorten the unsupported span, add a leg or a gusset, change the routing so the mass sits closer to the mounting, or add a compliant clip that lets the part move as a whole instead of flexing at a joint. The second fix is isolation: a rubber or polymer sleeve at the contact spreads the load, damps the movement and separates the metals.

Only after those changes should the alloy be upgraded. A stronger wire in a flexible layout will still fatigue, and it will cost more while it does so.

Match the material to the zone

An HVAC unit has several microclimates, and one finish rarely suits all of them. The table below is a starting point for the conversation with the designer and the coating supplier, not a substitute for a test in the actual unit.

What each coating does under a wet, vibrating duty

Zinc plating is the cheapest route and the weakest in a wet zone. It protects steel by sacrificing itself, so it works well until the layer is consumed, scratched or blocked by a tight contact. In a condensate stream it is a short term answer.

Powder coating gives a thicker barrier and a colour, and it hides the wire. Its weakness is the edge and the bend. The film thins on an outside radius, and a chip becomes an undercut where water creeps along the interface. Once that starts, the coating can hide a corrosion cell that is larger than the visible damage.

Zinc flake coatings sit between the two. They are thin, they tolerate tight tolerances and they protect at cut edges better than paint, which makes them useful on clips and tines where a thick film would change the fit. Abrasion at a contact point is still a limit.

Hot dip galvanising gives a thick, sacrificial layer that suits outdoor frames and guards. It is not a precision finish. Threads need clearing, small holes can bridge and the coating adds thickness that the drawing has to allow for.

Stainless removes the coating question and introduces others: cost, galling at threads, and chloride pitting if the grade is wrong. Use it where the part is difficult to recoat and the environment is known.

The design details that decide life

A few small calls do more for life than a change of alloy.

• Put the low point where water can leave. A U bend that holds a cup of condensate will corrode from the inside.
• Break the metal to metal contact at every clip with a sleeve, a coating or a change of material.
• Keep the wire away from sharp fin edges and from the tube wall, so vibration does not wear through either part.
• Increase the bend radius at any point that flexes. A tight radius concentrates strain and is the first place a crack appears.
• Route the part so the mass sits near a mounting point, and avoid long unsupported legs in the air stream.
• Leave room for the coating. A powder film on a tight fit can turn a clearance into an interference.

What to test and what to put on the drawing

A salt spray test to ISO 9227 is a useful comparison between finishes, but it is not a prediction of life in a condensate stream. The electrolyte, the wet and dry cycle and the galvanic couple in the real unit are different. Use the test to qualify a coating process, then confirm the part in the unit.

Vibration testing should reproduce the mounting and the running speed. A sine sweep finds the natural frequencies, and a dwell at the running speed shows whether the part will survive. Add a check of the contact points after the test, because fretting damage can be present before a crack is visible.

On the drawing, state the wire diameter and grade, the finish and its thickness, the masked areas, the critical fit dimensions and the coating standard. Add the zone and the cleaning method the unit will see. If the part is powder coated, say whether the coating must cover the cut ends, because that call changes the process.

Ask for a first article that records coating thickness on a bend and the bend radii. Those two records catch most field problems.

Sending the part out for quote

Send the drawing, the wire diameter and grade, the finish, the annual volume, the unit zone and the cleaning regime. Say whether the part is indoors, outdoors, coastal or wash down. Add the vibration source and its running speed if the part is close to a fan or a compressor.

A supplier can then propose the forming route and the finish together, which is the point. A part that is cheap to form and expensive to coat is not cheap, and a part that is over-specified in stainless may be beaten by a coated steel design with a better drain path.

The best HVAC wireform is usually the one that is stiff enough, drains well and separates its metals. Those three calls cost less than an upgrade to a higher alloy.