From the Technical Department: 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 fitted, 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. 

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Avoiding That “Oh No” Moment

Having well thought out and properly used operating procedures is important, but having good plans and people in place in case something goes wrong can make a world of difference.

Think back to your own past. Have you ever been doing something and all of a sudden you get this cold queasy feeling in the pit of your stomach and breakout in a cold sweat? Well here is one of those “oh no” moments and a great lesson learned.

This particular company put a lot of time and effort into developing good standard operating procedures, or SOP’s, and in training and documenting training of all of their refrigeration operators.

One of their SOP’s addressed oil draining. In order to do the oil draining procedure, the person had to not only understand the procedure, but be trained under supervision then finally authorized to do the procedure. In this particular procedure, the direction was that “two refrigeration operators” would be involved in any oil draining procedure.

It also directed that the appropriate personal protective equipment would be worn. The PPE in this case includes a full face ammonia cartridge mask, chemical resistant gloves, and coveralls. 

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Safety and Refrigeration Manifold Sight-Glasses

The guidance is clear, if you are a refrigeration engineer and need to leak detect or undertake strength testing on a refrigeration system, then you should not be using a refrigeration manifold with a sight-glass. This is the advice from the HSE – (A Study of Current Working Practices for Refrigeration Field Service Engineers 2011) and the IOR (Institute of Refrigeration – Good Practice Guide 24).

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Reducing Risks Associated with Hot Gas Defrost and Avoiding the Liquid Hammer Effect

In a paper titled “Research on risk of hot gas defrosting and liquid hammer in ammonia refrigeration,” Yitai Ma and Haoxiang Gu from the Key Laboratory of Efficient Utilization of Low and Medium Grade Energy in China, gave an overview of the safety requirements for ammonia refrigeration. The paper was presented at the Gustav Lorentzen 2014 Conference on Natural Refrigerants, which also included a paper focused on hot gas defrosting and the risk of liquid hammer phenomenon.

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