Why the jacket design matters

A heating or cooling jacket transfers heat between a service fluid — steam, hot oil, chilled water, glycol — and the product inside the tank, through the tank wall. Two things govern how well it works: the heat-transfer area and the velocity and turbulence of the service fluid against the wall. The three common jacket designs make very different trade-offs on both, plus on how much pressure the jacket can take and how much it costs to build.

1. Conventional (annular) jacket

The simplest jacket: a second shell spaced off the tank wall, creating an annular gap the service fluid flows through. It is economical, robust, and easy to build over large areas. Its weakness is flow — in a plain annular space the fluid moves slowly and can short-circuit or stratify, giving a lower heat-transfer coefficient. Agitation nozzles or spiral baffles inside the jacket fix much of this by forcing a defined, faster flow path. Conventional jackets are limited in pressure rating because the flat outer shell must be reinforced to resist the jacket pressure.

2. Dimple jacket

A thin outer sheet is plug-welded to the tank wall in a regular pattern of dimples, creating a shallow, channeled space. The dimples do two jobs: they tie the sheet to the wall so it resists pressure without heavy reinforcement, and they trip the flow into turbulence, which sharply raises the heat-transfer coefficient. Dimple jackets have low hold-up volume (less service fluid to heat or cool, so faster response), good heat transfer per dollar, and work well for both heating and cooling. They are a common default for sanitary and process tanks.

3. Half-pipe coil jacket

A pipe split lengthwise is welded in a spiral or in zones around the tank shell, forming high-velocity channels. Half-pipe jackets deliver high, predictable heat transfer and the highest pressure rating of the three, which makes them the choice for steam and high-temperature hot-oil service and for large reactors. They can also be split into independent zones for staged heating along the tank height. The trade-off is cost — more welding and material than a dimple jacket.

Quick comparison

Insulation and the service media

A jacket only pays off if the heat stays where you put it, so jacketed tanks are almost always insulated and clad — mineral wool or polyiso under a stainless or aluminum jacket sheet — to cut losses, hold temperature, and protect personnel from hot surfaces. The service media (saturated steam, thermal oil, hot or chilled water, glycol) is chosen for the temperature range and sets the jacket's design pressure and material. On code work, remember the jacket is its own pressure boundary and is designed and tested to ASME Section VIII just like the vessel it wraps.

Frequently asked questions

Which tank jacket has the best heat transfer?

Dimple and half-pipe jackets both transfer heat well because they force turbulent, high-velocity flow against the wall. Half-pipe gives the most predictable performance and the highest pressure rating, which is why it's preferred for steam and large reactors. A plain conventional jacket transfers less heat unless it's baffled.

What is a dimple jacket?

A dimple jacket is a thin outer sheet plug-welded to the tank wall in a pattern of dimples. The dimples anchor the sheet against pressure and trip the service fluid into turbulence, giving high heat transfer with low hold-up volume and fast temperature response — a common choice for sanitary and process tanks.

Do jacketed tanks need to be insulated?

Almost always. A jacket transfers heat efficiently into the tank, but without insulation and cladding that heat bleeds straight back out to the room — wasting energy, making temperature hard to hold, and creating a hot-surface hazard. Insulation is what lets the jacket actually do its job.

Is the jacket part of the ASME code stamp?

Yes, when the vessel is a code vessel. The jacket is a separate pressure boundary with its own design pressure, so it's calculated, fabricated, and pressure-tested to ASME Section VIII alongside the inner vessel and documented on the same data report.