What is the difference between TEMA and API 660? TEMA is the mechanical design and fabrication standard a shell-and-tube heat exchanger is built to, published by the Tubular Exchanger Manufacturers Association and used across industries. API 660 is a refinery-industry purchase specification, published by the American Petroleum Institute, that invokes TEMA underneath and adds the requirements a refinery or petrochemical owner needs on top of it. One is the mechanical base. The other is the purchaser's layer built on that base, so on a refinery job an exchanger is read against both at once.
This is a scope reference for the engineer who has to know which document governs what. It covers what each standard covers, how they stack on a refinery job, and what the exchanger carries onto the P&ID and the tag register once the mechanical standard is settled elsewhere. It is not a thermal or mechanical design guide.
Key takeaways
- TEMA is the general mechanical design and fabrication standard for shell-and-tube exchangers, used across industries.
- API 660 is a refinery-industry purchase specification for shell-and-tube exchangers that invokes TEMA rather than replacing it.
- On a refinery job the two are read together, with API 660 as the purchase spec and TEMA (commonly Class R) as the mechanical standard underneath.
- TEMA's three-letter code names the front head, shell, and rear head configuration, and it is a naming convention, not a calculation.
- The instrument engineer's interest is downstream of both, namely the exchanger tag, its type designation, and the instrumentation on its inlet and outlet.
What is the difference between TEMA and API 660?
The two documents sit at different layers of the same job.
| Dimension | TEMA | API 660 |
|---|---|---|
| Publisher | Tubular Exchanger Manufacturers Association | American Petroleum Institute |
| What it governs | Mechanical design and fabrication of shell-and-tube heat exchangers | Purchaser requirements for shell-and-tube exchangers, layered on top of the mechanical standard |
| Service scope | General industry, any shell-and-tube application | Refinery and petrochemical general service specifically |
| When it applies | Whenever a shell-and-tube exchanger is designed or fabricated to a recognized mechanical standard | When a refinery or petrochemical owner's specification calls for it, on top of TEMA |
| Relationship to the other | The mechanical-design base | The purchaser-facing layer that invokes TEMA and adds refinery-specific requirements |
Neither one is optional where it applies. A refinery exchanger is built to a TEMA class, and API 660 is the purchase specification that names which class and wraps its own requirements around it.
What TEMA governs
TEMA is the mechanical standard, covering how the exchanger is designed and fabricated as a piece of equipment, independent of which industry is buying it. Two parts of it are worth knowing at a naming level, without going into the mechanical allowances behind them.
The first is the three-letter type designation, which names the exchanger's construction by position.
| Position on the exchanger | Letter position | What it names |
|---|---|---|
| Front head | First letter | The stationary head design at the tube-side inlet |
| Shell | Second letter | The shell configuration |
| Rear head | Third letter | The rear head design, fixed or removable |
The second is TEMA's three classes, R, C, and B, each built for a different severity of service. Which class a given exchanger is designed to is a mechanical engineering decision made off the process conditions, and it is not something to infer from the P&ID.
What API 660 governs
API 660 covers shell-and-tube heat exchangers in general refinery service. It invokes TEMA, commonly TEMA Class R, the class built for severe, continuous petroleum service, as its mechanical basis, and adds the requirements a refinery owner expects on top, including a defined data-sheet format, inspection and testing expectations, and purchasing-specific requirements beyond a generic TEMA-only order. An exchanger ordered to API 660 is still a TEMA-class exchanger underneath, with an additional layer of refinery-specific purchaser requirements wrapped around it.
How they stack on a project
On a refinery or petrochemical job, both standards show up on the same exchanger data sheet, with API 660 as the purchase specification named in the project's engineering specs and the TEMA class it invokes as the mechanical basis. Neither stands alone. This is the same pattern used elsewhere on the same unit, and API 682 layers a refinery purchaser's requirements onto a pump's mechanical seal design the same way API 660 layers onto TEMA. The exchanger's process connections still tie into piping governed separately, under ASME B31.3, which is a different document again and not something API 660 or TEMA replaces.
Outside petroleum and petrochemical service, a project may specify TEMA directly, choosing the class for the service, with no API 660 in the picture at all. The decision of which document set applies belongs to the owner's engineering specification, not to the exchanger itself.
What the exchanger contributes to the drawing and the register
None of the above changes how the exchanger reads on the P&ID or feeds the tag register. What shows up there is the exchanger's own tag, its type designation, and its instrumentation.
- The exchanger tag itself, for example
E-401, carrying the TEMA type designation as a data-sheet reference rather than a field on the P&ID. - Temperature and pressure indication on the inlet and outlet of both the shell side and the tube side, typically
TIandPItags at each nozzle. - A differential-pressure point across the bundle where the process design calls for one, commonly a
PDTtag. - A relief point sized to the exchanger's pressure design, tagged and routed per the unit's own convention rather than dictated by TEMA or API 660.
Once the exchanger is on the P&ID, its tag and instrumentation take their place on the instrument index, the I/O list, and the line list alongside everything else on the unit, with the signal class already assigned. How those three documents relate to each other is covered separately in instrument index vs. I/O list vs. line list.