Calculations Prove Copper Shelled Chill Rolls Offer Massive Energy Savings
- 27 March 2018
- By BEP
- Chill Rolls
Manufacturers and laminators in the packaging industry could cut water cooling costs by up to 50% with the latest 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 1°C increase in cooling water temperature delivers a 5% saving in energy. In practice this means that if a steel shell chill roll operates at 10°C and copper shelled chill roll at 20°C the potential exists to cut refrigeration costs by up to 50%. Another significant benefit of operating rolls at 20°C rather than 10°C 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 reported to be enhanced.
Calculations are based on water, entering and flowing through a chill roll, being cooled by a heat exchanger which 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(s) in order to obtain a constant pressure and thus temperature. If the evaporating temperature for a particular application is +10°C, 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 11°C the pressure will also rise but the specific gas volume decreases to 0.1987m3/kg. This alone means that at constant load the compressor dynamic displacement (swept volume [m3/s]) can be 3.4% 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 p1 and its evaporating pressure p0) at which the compressor is working. A conventional ‘industrial’ compressor would adequately account for the remaining 1-1.5 % energy saving. At lower evaporating temperatures and pressures the effect will increase and vice versa. There are, of course, 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, using less energy and can solve historical condensate problems.
Historically, laminating rolls used in the packaging industry have been manufactured in steel. Aluminium has been used for the outer shell, but although it has a higher thermal efficiency than steel it is subject to internal corrosion, particularly on the welded seam. Also roll surfaces are difficult to re-process when they are damaged.
Standard steel rolls are normally doubled shelled and constructed with an inner steel core and an outer steel shell between which there is a gap of approximately 12 mm. Steel or rubber spirals are wound around the core between the two shells to guide the chilled cooling water evenly around the roll.
BEP’s unique proposition is to replace the outer steel shell with a fine grain electroformed copper shell 16mm thick which is shrink fitted over stainless steel guiding spirals welded to the inner core. This provides great strength and resistance to deformation.
While copper shelled chill rolls cost around 50% more than conventional steel rolls, depending on size and design, their performance is estimated to deliver an ROI (return on investment) within 12 months. However, copper shelled rolls should have a considerably longer, less troubled, life when because they are far more resistant to internal corrosion thus eliminating local hot spots associated with older (steel) rolls where internal rusting occurs. Also if a copper shelled roll is damaged externally it can invariably be machined to remove the damaged and then restored to the original diameter by electroplating with copper.