By Andrew Perks
Not too long ago it really was all doom and gloom, but suddenly we had no more power cuts and everything was surging ahead. But don’t get too comfortable…

The outcome of this year’s election will have a significant impact on the future of South Africa; let’s all work together to make a difference.

However, let’s not fool ourselves: power and water problems are here to stay.

I was at a South African Institute of Refrigeration and Air Conditioning (SAIRAC) meeting during March, presented by Bitzer, where some significant statistics were tabled. I have mentioned before that the cooling industry accounts for a large proportion of the power usage in the world – a figure of some 17% was spoken of. That is a large portion when you think that the requirement for cooling is increasing rapidly as we have an increase in the global population.

India is set to overtake China as the most populated country in the world but they have a largely ineffective cold chain with thousands of kilograms of food being regularly wasted and dumped due to a lack of cooling facilities. The estimated increase in power demand to keep pace with rising world population is between five and eight percent. We are seeing the same trends in South Africa. However, some projects are being put on hold due to the lack of services availability.

So, the industry needs to look at power savings and increasing efficiencies. To get back to the Bitzer presentation, it was all about how we can go about increasing efficiency and operating more effectively. It is in our interests that we reduce the power usage not only to conserve our natural resources, but also to save operating expenses. Eskom is currently looking at tariff increases of 9.41%, a bit more respectable than the 15% they wanted. Think about how that affects the cost of cooling, which will eventually be passed on to consumers.

So, now we need to look at what we as an industry do about it. I know you will all say, “There he goes again about his precious ammonia,” but it is a fact that you get more cooling for your operational costs using ammonia than the other current refrigerants. Ammonia as a common refrigerant is the cheapest around, but not the cheapest to install. That is a given.

Ammonia plants are expected to last conservatively for 30 years, and if properly designed and maintained, 50 years is attainable. Normally new trends and improved efficiencies are the current major reason why plants are upgraded or replaced. Not so with the “Freon” (synthetic refrigerant) industry. It is generally accepted that a 10-year period is the norm. Maintenance and proper operational consideration will improve on this but over the long term, where initial costs are not the prime concern, ammonia is by far the most cost-effective overall.

Better energy efficiency with ammonia is an accepted fact but even that can be improved. The Bitzer chat was all about a way to increase operating efficiencies by looking at cascading capacity control systems to give the most operational efficiency of the compressors.

The Bitzer argument revolves around the installation of three identically sized capacity compressors to suit the total designed cooling demand. Now we all know that we mostly over-design a system to cope with the 20% additional peak load. We have to, as it is not acceptable to lose temperatures during the peak period through from January to March – our warmest period. Consider that for the rest of the year the plant is oversized.

Have a look at the following load profile from a cooling system in the UK. They don’t have the higher ambient temperatures that we have to cope with in South Africa.

Load profile of cooling system in the UK.Load profile of cooling system in the UK.

It’s a given that screw compressors are high capacity low foot print compressor assemblies. However, while the compressors have a completely variable capacity control system, this is good because they steplessly match the system’s demand; they have really poor energy efficiency at low load capacities. At 20% capacity they could be pulling 50% of the connected power depending on the operational range. With the overcapacity installed for the lower ambient/ production periods this part load performance is significant on the effective energy consumption.

I really like the Bitzer solution where the leading compressor runs on a variable speed drive. So instead of unloading, the compressor drops its speed to around 50% before it starts to unload. The screw compressor’s efficiency is best maintained in the 100% to 80% capacity range. By specifically designing your capacity control system you can operate at peak efficiencies.

There are electronic systems out there that automatically optimise the running power and performance to give the most efficient plant operation. Sure, you need to spend some money on this, but with the continual increase in Eskom tariffs can you afford not to look at this solution? It’s a retrofit without any mechanical plant upgrades other than a variable speed drive or two depending on the system involved.

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