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Manufacturers and laminators in the packaging industry could cut water-cooling costs by up to 50 per cent by using copper-shelled chill-roll technology, according to BEP Surface Technologies.

The claim is based on cooling energy calculations undertaken in Sweden by refrigeration engineering consultancy Sollie AB and verified by engineers in the UK.

Refrigeration engineers have calculated that a 1C increase in cooling-water temperature delivers a five per cent energy saving.

In practice this means that if a steel-shell chill roll operates at 10C and copper-shelled chill roll at 20C, the potential exists to cut refrigeration costs by up to 50 per cent.

Another significant benefit of operating rolls at 20C rather than 10C is that in warmer climates condensation does not build up on the cold ends of the roll, so waste is minimised.

Laminating line speeds of up to 850 metres per minute plus are also achievable with copper-shelled rolls, whereas a steel roll runs at circa 400 metres per minute because this is the limit at which it can extract sufficient heat from the PE to make it set.

The dramatic increase in efficiency is due to the thermal coefficient of heat transfer of copper, which is 10 times that of steel.

In effect this means that during the time the copper shell is in contact with the PE, it can absorb up to 10 times more heat than a steel roll, thus reducing the contact time.

Quality is also enhanced.

Calculations are based on water entering and flowing through a chill roll and being cooled by a heat exchanger that uses an evaporating refrigerant in the refrigeration plant.

As evaporation is taking place there is a strict relationship between temperature and saturated vapour pressure.

Evaporated gas is drawn off by the refrigeration plant compressor to obtain a constant pressure and thus temperature.

If the evaporating temperature for a particular application is 10C, it corresponds to an evaporating pressure at 6.15 bar.

At this point the specific volume of the suction gas is 0.2053m3/kg (using R717 ammonia refrigerant).

If the evaporating temperature is increased to 11C the pressure will also rise but the specific gas volume decreases to 0.1987m3/kg.

This means that at constant load the compressor dynamic displacement (swept volume [m3/s]) can be 3.4 per cent smaller and yet will still transport the same mass flow (kg/s).

In addition savings will accrue from the compressor volumetric efficiency – the amount of theoretical swept volume (m3/s) that is active.

This factor is determined by the pressure ratio (the ratio between condensing pressure and its evaporating pressure) at which the compressor is working.

A conventional ‘industrial’ compressor would adequately account for the remaining 1-1.5 per cent energy saving.

At lower evaporating temperatures and pressures the effect will increase and vice versa.

There are local circumstances that will impact on these calculations, including the type of refrigerant used, the type and size of compressor being used – screw or reciprocating – and the type of evaporator being used – flooded or dry-expansion.

Based on these figures, BEP says its copper-shelled chill rolls have the capability to produce more product for the same floor space and using less energy.

BEP Surface Technologies

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