Drought: How a large international cold store survived
As the Western Cape drought reaches critical levels, Gabor Hilton, Oxford Cold Store’s Engineering Manager describes some of the measures they took in Melbourne Australia to keep their cold store operating in a recent drought.
We had our share of drought 10 years ago and our dams were down to very low capacity for a couple of years. In 2008, the Victorian government enacted a couple of very useful initiatives along with some very costly and panic driven infrastructure projects. A huge desalination plant (mothballed for 10 years and now producing water not required but contracted to take and using lots of power now adding to our energy crisis) and a new pipeline linking Melbourne catchments to the drought affected North of the state, also never used.
Some of the initiatives were:
Required industry to participate in a Water Map project that had to be submitted to the water authorities with annual reports on its progress. The Water Map required large industrial water users to fit water meters (supplied free of charge) to equipment and drains and to measure where water was being used and to come up with initiatives to reduce water use and waste. The threat was that unless industry complies, there was a chance that the potable mains water supply pressure will be lowered and the flow of water into our condensers will be reduced. The lack of water from the street mains was already a problem during times of very hot weather and we often needed to resort to filling up the condenser sumps with additional water from the fire mains.
We estimated that each millimetre of rain will yield 1 litre of water per square meter of roof space and calculated that retrofitting rain water harvesting to our large roof areas will provide us with about 4 days of water supply when the tanks are full. This usually happens on average once a month but rain water collected at other times still reduced our annual water use by some 30%. Our annual rain fall in Laverton North is around 450mm. We usually maintain our rain water collection tanks at 15-20% to cover 1 day’s requirement and to maintain full pressure to our condenser float valves at time of high water usage in hot weather or during times when no potable water is available.
The additional benefit is not paying charges for the bleed water discharged to sewers as these are based on the water passed through the authority’s water meters. Under Water Map, the cost of the retrofit was subsidised by 40%. The cost of water since we completed the retrofit has jumped from 85cent/lt to $2.75/lt with a corresponding increase in sewage charges.
- Having the water meters fitted, we now measure and monitor the monthly water used and have taken the following steps to reduce water consumption
- Checked all condenser coils for scale and revisited the chemicals used in water treatment to eliminate the scale. Scale on tubes reduces the rate of heat transfer and consequently the efficiency of the condensers, which results in higher discharge pressure and the plant and condenser fans running for longer periods to achieve the required chamber temperatures
- Checked and replaced condenser drift eliminators to reduce or eliminate carry-over
- Checked the water level in condenser sumps to ensure they are not overfilled
- We retrofitted VFDs to all condenser fans and as the fans operate at reduced speeds most of the time, evaporation and water carry over through the eliminators is reduced. (see the attached paper on the energy efficiency benefits of reducing fan speeds)
- Ensure that the condensers are clean and free of mud to reduce the flow of water through automatic bleeds.
- Check that all condenser sprays operate at full flow and cover the whole coil surface and there are no dry spots.
- Where manual bleeds were used, replaced them with an automated system that works on the level of TDS (total dissolved solids) in the sump water.
- Check the size of the condensers, undersized condensers will decrease energy efficiency, will increase discharge pressure and operate flat out for most of the year.
I attached a couple of papers I have written on the subject, one was the case against hybrid condensers which was published 10 years ago in the AIRAH journal Ecolibrium and a case study that was published in the AIRAH Ammonia Plant Operation training manual.
There are a number of very large cold stores, including Americold who operate with air cooled condensers with condenser fans on VFDs and using water sprays over the condenser. They believe that the savings in water costs and treatment offsets the increased running costs, but I don’t agree.