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Energy efficiency with ammonia systems

  • marimac 

By Andrew Perks

We should all be aware that energy costs in South Africa are escalating. Not only that, but with countrywide load shedding the unpredictability of electrical supply has led to the event of most cold stores and key industries now having generators fitted to ensure that services are maintained.

Properly considering the system demand characteristics will have a major impact on energy efficiencies and equipment performance. Image credit: Creative Commons | MVOPro | Pixabay
Properly considering the system demand characteristics will have a major impact on energy efficiencies and equipment performance. Image credit: Creative Commons | MVOPro | Pixabay

Sadly, this is a fact of life, and the lack of supply is throttling the expansion of the economy while another consideration is the impact of the energy efficiency of our cooling systems and their impact on power usage.

Energy efficiency is critical to the ongoing issues with climate change. We all know that ammonia is a climatically friendly refrigerant, but what about the carbon footprint from the energy consumption? It makes sense to take a look at the installed systems to see where we can improve efficiencies. This may mean a radical look at the plant and, we will discuss this next month, even the replacement of the existing operational plant with a more modern energy efficient solution.

Some time back, around the end of the 90s, Eskom started realising it had a problem balancing the power availability on the grid to the country’s expanding demand for power, and the industry was informed it should reduce power consumption by 10%. Up till this point, power was plentiful and cheap so we put up with the inefficiencies, but those days are now gone.

I was approached by a large fruit processing company in Ceres with a view to reducing their power usage. The plant had both a chilling application and a low temperature cold store. Each of these systems had their own design considerations. Being out in the country, the site had three different electrical tariff rates depending on when the demand for electricity was required.

After careful consideration it was decided that it was beneficial to switch off the freezer plant during peak demand tariff periods in the week and run full capacity over the weekends to catch up. This solution is not viable with a chilling plant. On the chilling plant we modified the condenser’s piping and replaced an old condenser with a larger unit. This resulted in a reduction of the condensing pressure from 37ºC to 33ºC. At that time, due to cost considerations, variable speed drives were not considered, so the brief was to go ahead with the other modifications. It was an interesting and successful project where we met all the requirements.

When people look at plant performance, it tends to always be at peak performance but, that is only two months in a year, and at that, only about 8 hours per day during the period. So, we need to consider part load performance of the system. Screw compressors are large capacity compressors with a very small footprint. The beauty of these machines is that they have an infinitely variable capacity control system allowing them to adjust their capacity to match plant requirements.

Sounds too good to be true! Well, there is a specific energy consumption (SEC) consideration depending on the application where at full speed the part load energy efficiency is drastically reduced. These days we are looking at variable speed drives to keep the compressors running at maximum capacity whilst adjusting the drive speed to match the system capacity demand. The variable speed drive uses between 5 to 10% power to drive it, and does improve the SEC, but only up to a point.

Each compressor, whether screw or reciprocating, has a minimum operational speed at which point we either switch off or we start to unload the compressor.

Properly considering the system demand characteristics will have a major impact on energy efficiencies and equipment performance. A well-designed capacity control system is paramount. I recently was at a plant where there was a glycol chiller that was being controlled by the glycol temperature instead of the system suction back pressure. The load was all over the place with dramatic surges in suction pressures resulting in excessive loads and liquid slug back on the compressors.

The net result was high power usage, massive wear and tear, and maintenance costs. There was a total of four compressors that regularly cycled in and out. Now that we have sorted out the control system, the plant has a stable suction, and only two compressors are operating. To top that there has been additional condensing capacity installed resulting in low head pressures and reducing water and electrical usage on the plant.

Next month I will chat about a report from the International Institute of Ammonia Refrigeration (IIAR) on this very subject, I am sure you will find it enlightening.