Page 153 - Introduction to Marine Engineering

Auxiliaries 139
provision is made at one end for expansion. The tubes are sealed into
the tube plate at either end and provide a passageway for the cooling
liquid. Headers or water boxes surround the tube plates and enclose the
shell. They are arranged for either a single pass or, as in Figure 7.4, for a
double pass of cooling liquid. The tube bundle has baffles fitted which
serve to direct the liquid to be cooled up and down over the tubes as it
passes along the cooler. The joint arrangements at the tube plate ends
are different. At the fixed end, gaskets are fitted between either side of
the tube plate and the shell and end cover. At the other end, the tube
plate is free to move with seals fitted either side of a safety expansion
ring. Should either liquid leak past the seal it will pass out of the cooler
and be visible. There will be no intermixing or contamination.

Plate type
The plate-type heat exchanger is made up of a number of pressed plates
surrounded by seals and held together in a frame (Figure 7.5(a)). The
inlet and outlet branches for each liquid are attached to one end plate.
The arrangement of seals between the plates provides passageways
between adjacent plates for the cooling liquid and the hot liquid (Figure
7.5(b)). The plates have various designs of corrugations to aid heat
transfer and provide support for the large, flat surface. A double seal
arrangement is provided at each branch point with a drain hole to detect
leakage and prevent intermixing or contamination.

Operation
Temperature control of coolers is usually achieved by adjusting the
cooling liquid outlet valve. The inlet valve is left open and this ensures a
constant pressure within the cooler. This is particularly important with
sea water cooling where reducing pressure could lead to aeration or the
collecting of air within the cooler. Air remaining in a cooler will
considerably reduce the cooling effect. Vents are provided in the highest
points of coolers which should be opened on first filling and occasionally
afterwards. Vertical mounting of single pass coolers will ensure
automatic venting. Positioning the inlet cooling water branch facing
downwards and the outlet branch upwards will achieve automatic
venting with horizontally mounted coolers. Drain plugs are also fitted at
the lowest point in coolers.

Maintenance
Clean heat transfer surfaces are the main requirements for satisfactory
operation. With sea water cooling the main problem is fouling of the
surfaces, i.e. the presence of marine plant and animal growth.