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A coil is the simplest stainless exchanger we build, and the easiest to get wrong
A heat exchanger coil is a length of stainless tube bent into a helix or a serpentine grid, with a utility inside and the tank contents or an air stream outside. There is no shell, no gasket and no plate pack to inspect; the whole exchanger is the tube wall. That makes a coil the lowest-cost way to heat or cool a tank that already exists, and it is the only practical answer for a duct, a flue or a vessel with an agitator in the middle. HeatX sizes and builds coils in 304L and 316L tube from 1/2 in. to 2 in. OD, with 3/4 in. and 1 in. the sizes we use most.
Three things decide the coil: the tank or duct it goes into, the utility that runs inside it, and the grade of the tube. The tank sets the shape, the mounting and how much surface fits. The utility sets the pressure rating and the drainage layout; steam coils to 150 psig are the common build, and steam at that pressure condenses at about 366°F, so a steam coil is a hot, moving piece of metal that must be free to grow. Hot water, glycol and chilled water coils run cooler and flooded, and the design question becomes velocity and venting rather than condensate. Ammonia coils are all stainless with no copper anywhere in the circuit.
The tube grade is chosen for the outside of the coil, not the inside. The utility is almost always benign to stainless; the tank contents are what pit it. A 304L coil is right for glycol, condensate and clean process water. The moment the tank holds brine, a chloride sanitizer, an acid pickling bath or seawater-derived water, the coil goes to 316L, and above about 1,000 ppm chloride or at temperatures where chloride stress corrosion cracking becomes a risk, roughly 140°F, the conversation moves to duplex or a higher alloy. The grade comparison covers the limits in detail.
Immersion coils run 304L on glycol, condensate and clean process water, 316L where the tank holds brine, sanitizer, acid or any chloride-bearing liquid. The Material & Construction page compares the grades.
Which coil for which duty
The four coil pages cover the builds that come up most. Most tank and duct coils land on the custom or welded page; ammonia and transformer oil have their own rules.
| DUTY | USUAL COIL | WHY |
| Heating a process tank with steam | Helical 304L or 316L coil, flanged through-wall nozzles | Steam needs a sloped coil that drains condensate to a trap below the tank; the helix does that naturally. |
| Cooling a brine or glycol tank | Serpentine 316L grid against the tank wall | A flooded grid keeps velocity up on the glycol side and leaves the center of the tank free for an agitator. |
| Ammonia-cooled process or brine tank | All-stainless coil, 304L on glycol, 316L or duplex on brine | Ammonia attacks copper and brass; every wetted part, down to the fittings, has to be stainless or steel. |
| Cooling transformer or rectifier oil | 316L tube on the water side; 304L or 316L for oil-to-air | Oil is laminar and viscous when cold; the cooler is sized on the oil film, and the water side must not leak inward. |
| Heating or cooling air in a duct or flue | Finned or bare 304L tube bank with welded headers | Gas-side coefficients are low, so the bank needs fins or a deep tube count and a casing that handles expansion. |
| Replacing a coil that pitted or cracked | Coil to your drawing in a higher grade | Send the old coil's tube size, pitch and nozzle spacing; we match the envelope and move the grade up. |
Before you ask for a coil quote
Immersion or duct mounting
- Immersion coils see a liquid outside: coefficients are high, so surface is modest but every tube must stay submerged
- Duct and flue coils see gas outside: coefficients are low, so expect fins or many more tube rows for the same duty
- Agitated tanks roughly double the outside coefficient of a still tank; tell us whether the mixer runs during heat-up
- A coil that hangs from a manway needs its own support frame; a through-wall coil needs nozzle pads on the tank
Expansion, draining and venting
- A 20 ft run of stainless tube grows about a quarter inch for every 100°F of temperature rise; fix one end and let the other slide
- Steam coils slope toward the outlet so condensate leaves; a level coil hammers and the first weld to fail is at the nozzle
- Water and glycol coils need a high-point vent, or the top turn of the helix sits in air and does nothing
- Put the drain at the true low point of the coil, not at the nozzle, so the coil empties for freeze protection
The mistake we see most
- Buying the coil by the tank dimensions instead of by the duty: it fits beautifully and has half the surface the heat-up time needs
- Choosing copper tube for the price in a tank that holds brine, sanitizer or acid, then replacing it in stainless a year later
- Specifying 304L on a chloride tank because the utility inside is clean water; the outside of the tube is what corrodes
Related
Heating & Cooling Coils
Welded Coils
Fully welded coil assemblies in 304L or 316L, with welded headers, support frames and sloped runs that drain, for steam, water and glycol in tanks, ducts and flues.
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Heating & Cooling Coils
Custom Coils
Helical and serpentine coils made to your tank drawing in 304L or 316L, with the tube size, pitch and nozzle layout chosen for the duty and the agitator.
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Heating & Cooling Coils
Ammonia Coils
Evaporator coils for ammonia in brine, glycol, ice-bank and process tanks, built entirely in stainless with no copper-bearing part on the circuit.
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