TRUNKING IN SHIP

The method of ventilation and output are decided as discussed above, but to decide the size of fan to handle the total amount of air required it is necessary to investigate by calculation the resistance which will have to be overcome by the air in passing through the trunks and out through the louvres or terminals. Considering any one system, losses of head occur through friction of the air passing through the trunk, changes of section, bends in trunking, fittings such as coolers, heaters, valves, gas flaps, and discharges. Each of these losses is proportional to the square of the velocity of flow. The loss of head due to friction in straight trunking is proportional to the length of trunking and inversely proportional to the linear dimension of the trunk section. Apart from the obvious desirability of minimizing the length of all trunks, the principal points requiring consideration from the practical point of view are as follows:

·         ADOPTION OF EFFICIENT SHAPE OF SECTION

The rate of air supply to, or exhaust from, a compartment having been decided from consideration of its nature and size, comfort of the personnel there, or technical reasons, the sectional area of trunking will depend on the available fan pressure after deduction of 'fixed' resistance items such as coolers, heaters, etc. To simplify calculations the 'Equal Friction per Foot' method is used, the basis of which is to convert all bends and changes of section to an equivalent length of straight trunking to which is added the actual straight length of trunking in feet to give the total equivalent length of straight trunking. Knowing the total equivalent length of straight trunking, the volume of air passing through the trunking and the pressure available, the diameter of circular trunking for each section of the system can be obtained from standard trunk friction charts. Having determined the diameter of circular trunking it remains to decide the best practicable shape of section. Circular trunking is more efficient than rectangular but saving in time, space and cost of manufacture results in the adoption of the latter in the general case. Small trunks are more easily constructed circular. A rectangular trunk can be made in a variety of ratios of sides for the same equivalent diameter of section. Undue 'flattening' of the section for such reasons as increased head room or clearance of unimportant obstructions must be avoided. In warships the ratio of the sides of any trunk must not exceed 4 to 1.

·         Minimization of the number of bends and changes of section. Complete avoidance of change of direction is impossible, but by adopting easy bends with large radius, a material improvement is effected. Sudden changes of section must be avoided and in cases where a local enlargement is necessary, the change in sectional area must be carried out gradually to minimize loss of head. In cases where these preventive measures cannot be taken a splitter must be fitted inside bends or changes of section to reduce the resistance as much as possible by preventing the formation of eddies.

·         Layout of trunking 

 The trunking must be led in such direction and be of such length as to ensure a good distribution of air within the compartment. The layout should be such that the air leaving supply louvres does not pass directly to the exhaust trunk or other opening acting as exhaust in the compartment ('short circuiting'). Similarly in compartments exhausted by fan, the exhaust inlet in the compartment must be situated remote from doorways or hatches so that the fresh air travels some distance before being drawn into the exhaust inlet. The vertical position of the exhaust inlet will depend upon the nature of the noxious gases in the compartment. If these are lighter than cold air, the inlet is placed near the crown of the compartment, and if heavier than cold air, near the bottom.