By Andrew Perks
I can’t imagine my whiskey without some ice, but it has some pretty negative effects when we add it to a refrigerant.
As mentioned in my previous article, water is very important to us as is evidenced in the Western Cape from the summer of 2018/19 with the droughts and ‘Day Zero’.
That given, ammonia can cope with water much better, but don’t fool yourself because there is a penalty to be paid. The International Institute of Ammonia Refrigeration (IIAR) bulletins No 108. ANS/IIAR 6-2019 and ANSI/IIAR 2 refer to the comments we are discussing further in this article on the subject.
Water contamination in the system occurs over a period of time. Due to this gradual process the effects slowly build up and go unnoticed and are just seen as a gradual decrease in efficiency, usually felt to be related to general wear and tear in the plant.
I can remember being called out to a plant at a winery where the client complained of rust in his compressor – it was the white wine cellar and that section of his cooling system only operated 2 months maximum a year. The client was doing a top end inspection and noticed the rust.
Being a first time for me, we stripped the compressor and found evidence of corrosion throughout the machine, even on the crankshaft which cleaned up nicely with some very fine emery paper. The winery’s ammonia was super saturated with water which on a normal 24/7 plant would never have caused this corrosion, but since the plant stood for 10 months of the year, the corrosive effect of the resultant Ammonium Hydroxide was noticeable. We purged the ammonia and drew a very deep vacuum (at least -95 kPa) which we broke 3 times with dry Nitrogen.
There are some graphs in the IIAR bulletins mentioned which highlight the effect of water dilution on a system running at –18 ºC room temperature and a nominal suction pressure of 106.9 kPa. With 10% of water dilution, due to the impact on the ammonia saturation conditions, the system now needs to run at 89.63 kPa to hold the same conditions of –18 ºC in the cold room which means the system now needs to run in a vacuum.
The effect of water dilution is much more pronounced in low pressure systems, resulting in overall compressor capacity loss, and therefore the requirement for more compressors to run. Which of course results in more power being used to achieve the same results. A lose-lose situation.
In the United States of America (US) they do not enforce stress relieving of ammonia vessels after construction like we do in South Africa. During construction of pressure vessels there is a lot of stress set up in the vessel which needs to be relieved.
Ammonia is known to create serious issues where there is localised stress but it has been found that a small concentration of 0.2% (2000 parts per million) of water can reduce the stress corrosion cracking effect as it scourges the loose oxygen particles which contribute to the problem.
So, a little bit of water has an advantage where there is the possibility of stress corrosion cracking taking place in the system. Water content above 2% or higher results in energy penalties. Sometimes you can get too much of a good thing.
So, where does this water come from? Usually bad service procedures such as poor initial vacuum being drawn on the plant at construction stage (should be at least -95 kPa.) to remove any air and moisture.
It can also happen when systems are being blown down during maintenance into water drums to absorb the smell of the ammonia being released. What happens is that as the last of the vapour is vented from the system the remaining vapour condenses due to the reduced pressure/temperature relationship in the plant, drawing whatever water is in the hose back into the plant which is now under a vacuum. This can easily result in 200ℓ being drawn into the system if the technician is not vigilant.
Before we pull a full vacuum (-95kPa) the plant needs to be properly pressure tested to ensure it is tight. As mentioned, there are two reasons why we pull a vacuum on a plant: firstly, to remove the air – this is normally quite quick, and the second is to flash off (evaporate) and remove any moisture that is in the system.
When you reach the correct vacuum pressure, the system pressure must not rise when you switch the vacuum pump off. If it does with a tight system, then it can only be water that is evaporating and raising the pressure. It is very difficult if the system is already cold to get this water out as the driving force to create the evaporation of any moisture through temperature difference of components being vacuumed is greatly reduced.
Other areas where moisture enters the system can be on plants running in a vacuum, improper oil draining procedures, a failed heat exchanger where the fluid on the secondary side of the system is forced into the plant, and general poor service procedures similar to the pump down procedure mentioned above. The highest quantity of water is normally found on the low side of the system, and that’s where we need to look if we want to remove it. Water also gravitates to oil pots in which, due to relative densities, oil floats on water and ammonia floats on oil.
IIAR recommends that at least every three years, all ammonia systems should have a sample removed and inspected for water concentration. This might seem excessive to some, but when we start to look at the performance penalties, we find with water in the system it makes financial sense.
So, how do we remove it? Well if we have water at 0kPa which is sea level pressure, it will boil at 100ºC and with ammonia at 0kPa it boils at – 33.35ºC. This tells us that we can flash off ammonia vapour from a solution and leave the water behind. A proficient SAQCC Gas artisan should be able to do this.
There are automatic air-purgers out there that also de-water the ammonia on the systems, working on these same principles. However, it is much more efficient if we do this on the low-pressure side of the system.
Some of the larger contractors have dewatering machines that they hire out which makes dewatering really worth doing. As we know today it is all about efficiency, so let’s get the system back to pure ammonia. It’s what it is designed for.
I trust that this was interesting. If you really need to find out more, get yourself a copy of the IIAR bulletins mentioned above – they are worth every dollar.
Till next time, stay safe out there, and I don’t need to tell ammonia people – wear your mask.