6.5 Oil coolers
The function of the oil coolers is to ensure that the oil temperature leaving the bearings is below the permitted limit by removing the heat absorbed by the oil within the pumps and the bearings. Redundant coolers are provided in order to enable repairs and tube cleaning to be carried out whilst the unit is on-load. Normally, three 50% duty coolers are provided, but two 100% units are sometimes used.
The oil coolers are mounted vertically with a two-pass water flow through the tubes and a single-pass oil flow through a series of baffles, to ensure even cooling. The cooler shells are of mild steel and modern practice is for the tubes to be made of titanium. The tube nest has a fixed and a floating tubeplate to contain thermal expansion. The coolers are provided with re¬movable end plates so that cleaning can be carried out without dismantling the water pipes. The heat dissipation requirements are around 2 MW per cooler. On some designs, the oil coolers have the main lubricating oil filters integral with the cooler shell. A typical example of an oil cooler, which also shows the integral oil filters, is shown on Fig 2.60.
Arrangements are made for automatic by-pass of the oil coolers upon excessive pressure drop across the coolers. Automatic control of the oil outlet temperature is provided.