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Quotes - Engineering - SalesWhat goes on the drawing
A custom coil starts with a tank, not a catalog. Someone has a 3,000 gallon jacketless vessel that needs to go from 60°F to 180°F in two hours, or a dip tank that must hold 40°F against a line of hot parts, and the coil has to fit through the manway, clear the agitator sweep and still carry enough surface to make the time. HeatX sizes that coil from the duty first, then shapes it to the tank. The drawing carries the tube OD and wall, the coil diameter or grid width, the pitch between turns, the number of turns or passes, where the headers break the tank wall and how the assembly is held up.
The surface comes from the duty. One gpm of water raised 1°F takes about 500 Btu/h, and a tank heat-up is a batch calculation: liquid mass, temperature rise, time allowed and wall losses set the average duty, and the coil must do that duty at the smallest temperature difference it will see, near the end of the heat-up when the tank is already hot.
A custom coil is 304L on glycol, hot water and condensate duties and 316L once the tank contents carry chlorides, acid or sanitizer, with duplex or titanium tube quoted for strong brine. The Material & Construction page compares the grades.
Tube and pitch
- Tube OD from 1/2 in. to 2 in.; 3/4 in. and 1 in. OD in 0.065 in. wall is the usual immersion coil tube
- Pitch of 1.5 to 2 tube diameters center to center; tighter than that and the liquid between turns stagnates
- Helix diameter around 70–80% of the tank diameter, leaving the agitator its sweep and the wall its clearance
- Long-radius bends on 316L and duplex, which work-harden; tight bends on thin wall flatten and lose flow area
Headers and nozzles
- Through-wall with flanged or tri-clamp stubs on a nozzle pad, or over-the-top through a manway with a support frame
- Steam in at the top of the coil, condensate out at the bottom, every turn sloped toward the outlet
- Water and glycol in at the bottom, out at the top, with a vent at the high point
- Multiple parallel passes on a large grid, fed from a common header, with pass lengths matched within a few percent
Mounting and clearance
- Stainless legs or brackets to the tank floor, or hangers from a ring on the wall, never from the nozzle alone
- Agitator clearance drawn, including the blade tip radius and the shaft wobble
- Manway entry dimension checked against the coil envelope, or the coil is built in bolted sections
Talk to an Application SpecialistSize the coil to the heat-up time and end temperature, then make it fit; a coil shaped to the tank first is short on surface.
Duty first, then tube size, pitch and the drawing.
Getting a custom coil right
Helical or serpentine
A helical coil suits a round tank with a center agitator: the helix hugs the wall, the flow in the tank sweeps across every turn and the coil drains by its own slope. A serpentine grid suits a rectangular tank, a side-entry mixer or a wall where the coil must lie flat; it is also what fits through a small manway, since the grid can be built in panels. The grid needs more care with slope, because each horizontal run must still drain, and more support, because a flat panel of tube sags when hot.
In an unagitated tank the outside coefficient is natural convection and the coil needs two to three times the surface of the same duty in a stirred tank. Tell us whether the mixer runs during heat-up or cool-down; it changes the coil size more than any other input.
Thermal growth of a long coil
Stainless grows about 0.23 in. per 20 ft of tube for every 100°F it is heated, so a 60 ft helical coil brought from a 70°F shop to 150 psig steam at about 366°F grows close to 2 in. over its length. We anchor the coil at one header, guide it at the other and let the helix breathe on slide-type supports; a serpentine grid gets a loop in the header run. A coil clamped rigid at both nozzles cracks at the nozzle weld on the cold side, usually within the first few dozen heat-up cycles.
Grade for the outside of the tube
The utility inside a custom coil is nearly always steam, condensate, hot water, glycol or chilled water, none of which trouble 304L. The grade is chosen for the liquid in the tank. Plating rinse water, pickling acid, caustic cleaning solutions, chloride-dosed process water and any brine push the coil to 316L, whose 2–3% molybdenum resists chloride pitting to roughly 1,000 ppm at ambient. Hot chloride tanks are the trap: above about 140°F, chloride stress corrosion cracking can take 304 and 316 alike, and a heating coil is by definition hotter than the tank. For a hot chloride tank we quote 2205 duplex, and for seawater or a strong brine, titanium tube (Grade 7 or 12 on a hot wall above about 160°F), which conducts slightly better than 316L.
The mistake: pitch too tight
The most common fault on a coil someone else drew is turns packed so close that the tubes nearly touch. On paper the surface is all there; in the tank the liquid between the turns stalls, the outside coefficient collapses and any solids pack into the gap. The coil reads as fouled within weeks and the heat-up time doubles. We hold the pitch at 1.5 to 2 tube diameters and, if the tank cannot fit the surface at that spacing, go to a larger tube, a second bank or finned tube rather than crowding the turns.
Custom coil questions
Yes, and that is usualy the better start. Send the tank inside diameter and straight side, the manway size, the agitator type and swept diameter, the nozzle positions you can use and the liquid level range. Add the duty: the batch volume, the start and end temperatures, the time allowed and the utility available. We draw the coil to that, and the drawing you approve becomes the build drawing.
From the tank contents, the temperature rise and the time allowed, plus the tank's heat loss while it heats. That gives an average duty, but the coil is sized for the end of the batch, when the tank is near its final temperature and the driving temperature difference between the utility and the liquid is smallest. A coil sized on the average is fine for the first hour and crawls through the last one.
A helical coil with a 70–80% tank-diameter helix will not, and we do not try to spring it. For a tank with no top opening large enough we build the coil in bolted or flanged sections that pass the manway and are joined inside, or as serpentine panels that mount flat against the wall. Tell us the manway dimension and whether a person can work inside the tank.
Often, if the utilities are compatible and the plant will purge between them. A coil that runs 150 psig steam, then chilled water, cycles through about 320°F, and that swing drives the expansion and the fatigue at the nozzle welds. We design the supports for it, slope the coil to drain condensate before the water goes in and size the surface for whichever duty is harder, usually cooling.
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