By Andrew Perks
As I mentioned in my last article, I was so looking forward to my overseas visit to my first IIAR conference in Orlando US. I have been to many exhibitions and conferences in the past but the thought of a real live Ammonia one really captured my imagination.
Well we all know what happened – the whole world went crazy and we are still a long way from the end of it. I was chatting to someone about the last pandemic, the Spanish Flu of 1918. The scary bit is that it lasted for 2 years and the second wave was more deadly than the first as by then most people were just fed up with the restrictions placed on their lives. What’s that old adage? “If we don’t learn from the past, we are destined to repeat it”. So, everyone out there just stay safe and take care.
So, what is this about water? We all know H₂O is pretty good stuff, 71% of the earth’s surface is covered with it. Our adult bodies are between 57 to 60% of it, and if you read the world statistics, our total consumption as a country is 791ℓ per person. I just can’t imagine my whiskey without its ice and the good news is water has no calories. All in all, it’s pretty good stuff in the right place.
However, (there is always a however isn’t there?), it’s not good stuff when we get it into our refrigeration system. I attended a recent webinar from IIAR on “Water Contamination with Ammonia refrigeration systems” and found it quite enlightening.
Anhydrous Ammonia as we know is a robust industrial refrigerant that due to its affinity with water can, and does, absorb water easily, resulting in it becoming what is known as an Aqueous (wet) Ammonia. Unlike with the Hydrocarbons, water mixes readily with Ammonia and does not separate thus freezing up in the system. Whilst with Ammonia this may be seen as a good thing that doesn’t totally wreck the refrigeration process, it’s not supposed to be there and does have an effect on the system’s operation and efficiency.
IIAR have published various standard publications on the effect of water in the system initially with Bulletin 108 which has been updated with ANSI/IIAR 6-2019 and the later ANSI/IIAR 2 update revision and I am going to refer to these as we go.
When water and Ammonia mix, Ammonium Hydroxide is formed which can be extremely corrosive as concentrations rise unnoticed over a period of time. There are also other detrimental effects on the system with high levels of water in the Ammonia. Whilst it is still a functioning refrigerant its saturation pressures need to be depressed to achieve the correct evaporation temperature required in the system.
Some of the symptoms of water in the system are as follows:
- Pressure/temperature relationship is affected
- Chemical changes occur in the compressor oil
- Pressure drops increase in piping
- There is an overall decrease in the plant’s performance
- To maintain cold room temperatures compressors must operate more
Commercial grade Ammonia is not really dry but is classed as anhydrous Ammonia with a purity of 99.995% Ammonia. Our systems are permitted to have some water content in the region of between 50 to 5 000 ppm, whilst the limit for oil is 50 ppm, without affecting overall plant performance.
The net effect of water in the system results in us having to push down suction pressures to achieve the correct temperature of refrigerant at the evaporator. As the water content increases this effect increases resulting in lower and lower suction pressures in the system. It should be remembered that as the suction pressure decreases so does the gas density, meaning that there is less refrigerant per cubic meter compressed by the compressor. Less refrigerant flowing around the system means less of a cooling-effect. The system tries to balance this out by increasing compressor operating with an overall impact on the wear and tear on compressors and energy efficiency, resulting in a lose-lose situation.
As a yard stick, for every 1º C the system pressure or saturation temperature drops the compressor capacity drops by approximately 4.5% due to the change in the gas density.
This is quite a lengthy subject that needs some serious attention and I will be continuing on this topic in the next issue.