By Andrew Perks
This month I want to chat about some interesting experiences I have had over the years, especially an interesting one involving this topic.
Well, as it’s that time of month again when I sit down and write my article – for any of you that has done this, you will understand the process, firstly think of a topic then write the article around it.
I have worked on a few Ammonia installations on vessels but the most have been “Freon”. Well as you all know, there is no such thing as Freon. Freon is an old Dupont name for a range of their refrigerants which includes CFCs and HCFCs. It does not include the natural refrigerants such as Ammonia, CO2 or the other HCs.
Being simple people, for us refrigeration folk its either Freon or Ammonia. Anyway, I digress – my story revolves around an issue on a trawler in Cape Town harbour. As any refrigeration tech will know our life is really not our own, we are batted around from pillar to post in the service of our clients.
This story starts with what was supposed to be a weekend off where I was going to my son’s school concert in the late afternoon. However, one of my client’s trawlers had a problem which the on-board chief could not solve.
After lots of communications I was on a boat on my way to the vessel in Cape Town harbour around 2 pm, I thought I might still make the concert but sadly that was not to be. On arriving on the vessel, I was confronted by a hold cooling system that kept tripping on high discharge pressure. This vessel had an R-22 system with deck head coils in the hold and one Sabroe compressor condenser receiver refrigeration package.
Obviously the first thing I did was close the suction and start the compressor, no problems till I tried to open the valve. This is where it all went pear shaped.
The discharge pressure rocketed and the internal relief valve short-circuited back into the suction line. No, the discharge valve was not closed. On checking the HP switch it was wound right off the scale. Next thing to do was check water flow and liquid level in the receiver. I couldn’t see a level in the receiver and there was plenty of water. Now for some investigation but the chief was indisposed so I got a hold of the second and asked him how they normally started the plant, only to be told they blow off about a bottle of freon and then it starts but there is no spare freon on board.
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Well, that was not going to happen. Eventually it came out that when they come into port, they pump back the system into the receiver which on further investigation was found to be undersized for the liquid volume resulting in the condenser filling up as well. Net result – no space for condensing on start up. Ok so you are thinking no problem jack open the solenoid and get the liquid into the system. Tried that, that didn’t work even with the high-pressure differential the liquid refused to flow.
One of those back to basic problems, when something just doesn’t gel it’s always good to go back to basics. Due to the compressor internal relief valve the suction was sitting at around 5 bar and the hold was at -2°C (saturated pressure of around 3 bar) this got me thinking the problem was the expansion valve not opening. Fortunately, on fishing vessels there are plenty of isolating valves so, I thought take the orifice out of expansion valve and get the liquid to flow out of the receiver into the system and once the level was down in the receiver get it all back together and that should be that.
Firstly, I disconnected the external equaliser – and wow the valve opened and started to feed. After getting everything up and running I put the connection back again and with the hold starting to cool I left the vessel about 2 am – way too late for the concert but happy to be going home.
So, what was the issue? The problem was that the system suction pressure was way above the saturated pressure on top of the expansion valve holding it closed. Ok so you are thinking why did I not immerse the bulb in hot water? Easy, there was no way to get to the bulb. By releasing the external equaliser pressure, I removed the pressure from under the diaphragm.
Well, you should have seen the second’s face when he saw the pressure drop and the plant start without having to blow off Freon. He got a lesson on refrigeration that day and it taught me a valuable one as well, never assume anything is what you first think it is and always go back to basics. At the end of the day the problem was an undersized receiver.
I hope you have enjoyed this, just remember that things are not always what you think. Sometimes you need to make a problem happen to understand the issue.
Till next month…
About Andrew Perks
Andrew Perks is a subject expert in ammonia refrigeration. Since undertaking his apprenticeship in Glasgow in the 1960s he has held positions of contracts engineer, project engineer, refrigeration design engineer, company director for a refrigeration contracting company and eventually owning his own contracting company and low temperature cold store. He is now involved in adding skills to the ammonia industry, is merSETA accredited and has written a variety of unit standards for SAQA that define the levels to be achieved in training in our industry.