Three jackets on the same 3,000 L reactor give three different prices, three different heat-transfer rates and three different plant layouts. None of the three is wrong. Each one is wrong in the wrong place.
1. Start with the duty, not the jacket
A jacket is a pressure vessel wrapped around your process vessel. It has one job: move heat in or out at the rate the reaction needs, using the utility you actually have, without contaminating the product and without making the vessel impossible to install or clean.
Four numbers settle the choice before anyone talks about price: the required heat load (kW), the utility and its supply temperature and pressure (MPa), the ramp rate the process needs (°C/min), and the free space between adjacent vessels on your plot (mm). Write those four down and send them with the enquiry. A jacket sized against a guess is a jacket sized wrong.
2. The plain (conventional) jacket
Two concentric shells welded at top and bottom, with an annular gap around the full circumference of the cylindrical shell. The gap is commonly 25 to 100 mm, and the jacket carries a low-point drain nozzle and a high-point vent.
What it does well: it gives complete, even coverage of the shell, it is the cheapest jacket to fabricate per square metre, it drains fully by gravity, and its smooth outer surface is the easiest of the three to insulate. On a large vessel where surface area is plentiful and the duty is moderate, it is often the correct answer.
Where it fails: the annular gap is a large flow area, so at a given volumetric flow the medium velocity is low, and the film coefficient on the jacket side becomes the weakest link in the heat-transfer chain. The flat outer shell also has limited support, so the wall thickness climbs quickly as the utility pressure rises. A plain jacket is a poor place to put high-pressure steam or heat-transfer oil.
3. The half-pipe coil jacket
A pipe — typically DN25 to DN80 — is split lengthwise, and the half-round channel is wound in a spiral and welded to the outer shell. The medium flows through that half-round channel, not through an annulus.
The consequence is the whole point: the flow cross-section is far smaller than an annulus, so for the same volumetric flow the velocity is much higher and the jacket-side film coefficient rises with it. A pipe is also a natural pressure vessel, so the half-pipe form carries higher pressure with less metal than a plain jacket. The trade is coverage. There is a land between adjacent turns, and the dished ends and cones normally need a separate arrangement, so the coil never covers 100 % of the wetted surface. Ask for the actual heat-transfer area in m², not a percentage claim.
Cost sits above a plain jacket: more welding metres, more fit-up, more non-destructive testing, and therefore a larger share of the vessel price tied up in the jacket. The winding also adds to the outside diameter and leaves a ribbed surface, which makes insulation more work and widens the vessel envelope on the plot.
4. The dimple jacket
Also called a pillow-plate or spot-welded jacket. Two relatively thin sheets are welded together on a regular pattern of points, and the space between them is inflated to form the flow channel. The dimples act as stays, so the jacket resists pressure without a thick outer shell.
Heat transfer per square metre is good, because the labyrinth path keeps the medium turbulent. The real advantage is geometry: a dimple jacket can be formed onto dished ends, cones and other shapes where a half-pipe coil cannot easily follow. That makes it the option for a compact vessel that needs a lot of jacket area, or for a high-pressure duty on a non-cylindrical surface.
The trade-offs are practical. The flow path is a labyrinth, so it does not drain as cleanly as a plain jacket unless the drain route is designed in — confirm the drain point orientation on the drawing. The weld count is high, so weld quality and inspection matter more than on the other two types. And the outer surface is uneven, which affects insulation and appearance.
5. Side-by-side, on the four criteria
- Heat transfer per m²: dimple and half-pipe are both well above a plain jacket, because both force higher velocity through a smaller channel. Total area still decides the outcome — a large plain jacket can out-perform a small half-pipe coil. Work from Q = U·A·ΔT with your own U value, and make the supplier give you A in m².
- Pressure: a plain jacket is the lowest-pressure option and gets expensive fast as pressure rises. Half-pipe and dimple jackets both handle higher utility pressure. In every case, the jacket design pressure in MPa is stated separately from the inner vessel design pressure, and both belong on the drawing.
- Cost: plain is cheapest at the jacket level, half-pipe and dimple cost more in fabrication and testing. Be honest about the arithmetic — an undersized jacket that cannot hold the reaction temperature has not saved anything.
- Installation spacing: a plain jacket presents a smooth cylinder. Half-pipe and dimple jackets are ribbed, add to the outside diameter and make insulation bulkier. If your vessels sit close together, or the vessel has to pass through an existing opening, that envelope matters. Check the outside diameter on the drawing, not the nominal vessel diameter.
Two further points that buyers forget. First, draining and venting: a plain jacket drains by gravity to a low point; a half-pipe coil drains if the spiral is pitched correctly; a dimple jacket needs a designed path. Second, an internal coil or immersion tube gives you more area inside the vessel, but it interferes with the agitator and complicates cleaning. Both are listed alongside the jacket in the standard build options — decide which side of the wall the area should sit on.
6. The lining changes the answer
On glass-lined and PTFE-lined reactors, the jacket choice is not only a heat-transfer question. Glass lining is rated for cold shock up to 110 °C and heat shock up to 120 °C, and a jacket that puts high flux on one patch of the wall creates a steeper local gradient than a uniform one. If the duty is aggressive, state the utility temperature and the ramp rate, and let the lining limit — not the utility — set the maximum.
Glass lining is not suitable for hydrofluoric acid or fluoride-bearing media, phosphoric acid at 30 % and above above 180 °C, or strong alkali above pH 12 at 100 °C. The jacket type does not change that list, but it does change how hard the lining works for its living.
7. What to put in the enquiry
- Heat load in kW, and the U value you are assuming.
- Utility, its inlet and outlet temperature, and its pressure in MPa.
- Required ramp rate in °C/min, and the maximum utility-to-product temperature difference you will allow.
- Vessel volume in L, geometry, and whether the dished ends and cone need jacket area.
- Available clearance between vessels in mm, and any access restriction on the way in.
- Whether the jacket must drain completely between batches.
Then ask for the jacket design pressure in MPa, heat-transfer area in m², material of construction, the pressure vessel standard followed (GB/T, ASTM or ASME), the welding and NDT scope, the high-voltage test record in kV where the vessel is lined, and the hydrostatic test record. Testing described as available on request is testing that will not happen.
What to do next
Fix your heat load, utility pressure and plot clearance first. Then ask each supplier to justify the jacket type against those three numbers, in writing, with the area in m² and the design pressure in MPa. If two suppliers give you different jacket types for the same duty, that disagreement is the most useful document you will receive — read it before you read the price.