Flexible cooling with shell-and-tube heat exchangers

Paul Boughton

Shell-and-tube heat exchangers are the cooler of choice for many applications. In addition to one-off and custom designs, numerous standard types have been developed to meet the specific requirements of common applications. Among these is oil-water cooler BNZ from R+L Hydraulics, designed for cooling oil hydraulics systems, gearboxes and compressors. Its use in an offshore screw-type compressor demonstrates its high flexibility and suitability for use with sea water.

With its aluminium fins and a fixed tube bundle the NewCool BNZ oil-water cooler from R+L Hydraulics has a much larger cooling surface than other designs (Fig. 1). Compact and rugged, with built-in measuring points and low pressure loss the BNZ oil-water coolers achieve a cooling efficiency of 500 kW 670 HP)at a volume flow rate of up to 850 l/min (225 GPM).

The special feature of the BNZ is its tube bundle (Fig. 2), which is made from a set of copper-nickel tubes onto which thin aluminium fins are threaded. The tubes are then expanded so that they form a firm interference fit with the fins. This results in a much larger contact area for heat transfer than that of conventional tube bundle. In the gaps between the aluminium fins strong turbulence occurs, which further increases heat transfer. The open tube ends are rolled into the tube sheets of the cooling package to create a firm form-fit connection also at this point. Fitted with suitable inlet and outlet covers, the cooling packages can be used to produce both one-pass and multi-pass coolers.

With a choice of suitable materials BNZ coolers are also suitable for use with sea water. The cooler’s flexibility is demonstrated by its use in a screw-type compressor from Aerzen Nederland B.V. for use in the offshore industry (Fig. 5). Aerzen Nederland is a subsidiary of Aerzener Maschinenfabrik GmbH, which manufactures fans, compressors and gas meters and is a world-leader in gas conveying and compression applications. Aerzen’s screw-type compressor is used on board oil rig supply ships for conveying cement and concrete.
The process of compressing air generates heat in the compressor, some of which is dissipated through the compressed air aftercooler. The remaining heat of compression generated in the compressor heats up the compressor oil. In its screw-type compressors Aerzen used the BNZ heat exchangers both as an aftercooler and for -cooling the compressor oil (Fig. 4). Because of the high volume flow rates and the required cooling rate, two BNZ 175 coolers with a jacket tube diameter of 175 mm and SAE flanges at both the oil-air end and the cooling water unions are used. The air cooler is much shorter than the oil cooler since the friction losses are much higher than the heat generated through compression. The coolers are connected in series at the water end, so that the cooling water flows through the air cooler before passing through the oil cooler.
As well as tank-mounted coolers BNZ for marine and industrial applications R+L Hydraulics offers tank-mounted coolers BU and in-tank coolers BTU, all with removable tube bundle. For special applications R+L Hydraulics also supplies custom BNZ shell-and-tube heat exchangers in non-standard sizes. The standard BNZ range comprises units with four different diameters and standardized lengths of up to 2100mm (Fig. 3).

For more information, visit www.rl-hydraulics.com

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