Thermosyphon Oil Coolers

Thermosyphon oil coolers for compressors have been designed in several ways. It has been a common design for the oil to be circulated through the “shell side” of a shell and tube heat exchanger, while ammonia is circulated through the tube side. These oil coolers are typically ASME pressure vessels and bear an ASME stamp. When shut-off valves for the oil cooler are fi tted, thus enabling isolation from the compressor package oil separator, they must also be fitted with pressure relief valves.

It might be presumed that a hydrostatic relief valve would serve as adequate protection. However, the shell might be full of oil, or it might be full of ammonia vapor or it could be a combination of both. In some cases it could even contain some liquid ammonia. But because it is an ASME vessel, the ASME Boiler and Pressure Vessel Code, IIAR 2 and ASHRAE 15 all require a vapor relief valve in the event of a fire. So the issue of concern is how to handle both the compliance for vapor relief and the more likely scenario of hydrostatic relief of fluid. The following outlines a few ways to address this situation.

When this design was prevalent, one compressor manufacturer would generally use a “dual stamped” relief valve, rated for vapor and liquid relief. This manufacturer coordinated with a relief valve manufacturer to have a typical relief valve tested and rated for both liquid and vapor service. An internet search reveals that there are other relief valve manufacturers, who seem to serve the petroleum industry, that provide vapor relief valves with “liquid trim” and vice versa which could also be used.

If the oil side of the shell and tube cooler is stamped for 400 psi it can be relieved to the oil separator vessel if the vessel is rated for 300 psi. A 75psi relief valve would be installed between the two vessels. The oil separator relief valve must include the capacity of the relief valve from the cooler.

If the design pressure of the oil cooler is the same as the oil separator you can relieve the cooler to the oil separator with a bellows or balanced piston type relief valve. These types of valves are not appreciably affected by backpressure. However, this is generally expensive.

Another method to address this situation is to remove the stop valves on one of the oil lines back to the separator to provide an un-valved relief path. This can be done if the design pressure of the oil cooler is the same as the oil separator. Again, the oil separator relief valve must be sized to handle the volume of both vessels. However, this generally means someone would have to depressurize the oil separator to replace an oil filter or service the oil cooler, an obvious inconvenience.

If none of the above is acceptable, the oil side of the vessel can be piped with an atmospheric (vapor) relief valve at the design pressure of the oil side of the cooler, generally through a dedicated relief line, because it is unacceptable to introduce anything but ammonia vapor into a common relief header for vapor relief valves. However, the risk of opening and blowing oil to the roof is not desirable. One means of avoiding this is to combine an atmospheric relief valve with a small relief regulator in parallel, set roughly 40 psi below the atmospheric relief valve and piped back to the oil separator. If the pressure goes up in the oil side of the cooler the relief regulator would open and relieve the pressure. This will relieve an oil pressure hydraulic spike and avoid opening of the atmospheric relief valve. And the atmospheric relief valve provides the code compliance.

The more common current design of thermosyphon oil coolers is to use a “plate and shell” heat exchanger. In this arrangement, oil is circulated through the plate, and ammonia is circulated through the shell. The shell is an ASME stamped vessel and will require typical relief protection, which can be back to the oil separator or to the atmosphere, in the same manner as described above. The plate side is not an ASME vessel, and if fitted with shut-off valves, these can be protected with a common hydrostatic relief valve. Alternatively, if there is a procedure in place such that trained technicians isolate the plate only during maintenance, the hydrostatic relief valve can be omitted. This arrangement would also work for shell and tube heat exchangers with ammonia in the shell and oil in the tubes.