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Energy efficiency with ammonia systems (continued)

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By Andrew Perks

In the last issue I spoke of energy efficiency, I would like to continue the same topic but this month I am going to refer to an article in the IIAR monthly magazine – Condenser.

The article in question is ‘Energy Performance Benchmarking: Considering the efficiency of an Ammonia Refrigeration Installation’. When we talk about the Montreal Protocol, we all think about limiting the release of chlorinated refrigerants and their impact on the ozone layer depletion, but in fact we are talking about climate change.

Following the Montreal Protocol in 1989 we have the Kigali Amendment of 2016 which entered into force in November of that year. The goal of the agreement is to limit global warming to well below 2ºC, preferably 1.5ºC compared to pre-industrial levels.

Also read: Energy efficiency with ammonia systems

I must say, I am encouraged with the world’s overall response to the Montreal Protocol. Who would have thought that nearly the whole world would have gotten together and are busy implementing a procedure that will impact our children’s future?

I don’t know if any of you saw the Al Gore video ‘An Inconvenient Truth’ where they picked up the problem with the Ozone layer depletion in the fifties. Climate change is real and has been with us for some time. We are now seeing its impact with all the weird temperatures, floods, and forest fires. It’s scary out there!

But again, we all feel that climate change is a ‘Freon’ problem – well it’s not. Carbon dioxide that is released from burning fossil fuels to generate electricity is a major contributor. The more energy we need the more fossil fuels we will consume, unless we develop a clean energy source.

We are now seeing and experiencing climate change’ impact with abnormal temperatures, floods and forest fires seen around the globe. Iamge credit: Malcolm Lightbody | Unsplash
We are now seeing and experiencing climate change’ impact with abnormal temperatures, floods and forest fires seen around the globe. Iamge credit: Malcolm Lightbody | Unsplash

We are now looking at battery-driven motorcars. How environmentally friendly is that? So, we are not burning petrol but what about the energy to make the batteries, to charge the batteries and what about disposing of them when their effective life span is reached. We really are travelling in uncharted territory.

An interesting development is the use of ammonia as a fuel source, whatever next?

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But I digress, how do we make our ammonia plants more energy efficient. As I mentioned in the last issue, we need to be aware of the overall application and facility operation when we design a plant. A rule of thumb for most production plants is that approximately 2/3 of the duty is processing and the remaining 1/3 is the base load of the system that is always there. Ideally the capacity control philosophy should be based around the set of parameters evident in the process being undertaken.

One of the conclusions in the IIAR article about high energy loads on systems is directly related to lack of attention to overall system energy performance at partial load conditions. Whilst the cold room temperatures are constant, ambient temperatures fluctuate throughout the year resulting in higher energy usage in summer.

In the design stage it is a simple exercise to quantify protracted cost savings with energy consumption relative to the type of cooling systems available. However, sometimes the client is only interested in the bottom line and not ongoing operational expenses.

An example of this is the replacement of a small 5 250 m³ refrigerated facility from air cooled single stage R 404A based condensing units with electric defrost to a dual stage centralised low charge ammonia plant with hot gas defrost and evaporative condensers. Sure, the R 404 system would have been the cheapest solution, but it is a recorded fact that a “Freon” plant has an effective life span of 10 years whilst Ammonia is around 30 years.

The recorded reduction in Specific Energy Consumption (SEC) was between 57 and 71% depending on the time of year and the plant control settings. I have heard it said that based on this the difference in the initial project cost is made up in as little as 3 to 5 years on the energy running cost savings and from then on, it’s money in the bank.

Another example was an existing ammonia plant where screw compressors were replaced with a low charge ammonia gravity feed system utilising reciprocating compressors and hot gas defrost. Annually after all running costs, the new plant had a saving of around USD190 000, resulting in a simple payback period of around 5 years and a plant with at least 25 years effective life remaining.

The final example was a 60 000m³ distribution facility with a 300 ton/week blast freezing capacity. When the new facility was commissioned, there was a projected 57% reduction in annual energy costs equating to nearly USD500 000 saving compared to an almost identical facility in the neighbourhood.

So, it is evident that with the correct approach and good engineering practices, energy consumption can be reduced. In some instances, the plant may need to be replaced, but from projected savings these costs can be recovered as power consumption and energy cost savings, and this starts to impact on the facilities operating expenses.

Stay safe.

About Andrew Perks

Image credit: Andrew Perks
Image credit: 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.

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