Page 193 - Introduction to Marine Engineering

Refrigeration, air conditioning and ventilation 179

Ventilation

Ventilation is the provision of a supply of fresh untreated air through a
space. Natural ventilation occurs when changes in temperature or air
density cause circulation in the space. Mechanical or forced ventilation
uses fans for a positive movement of large quantities of air.
Natural ventilation is used for some small workshops and stores but is
impractical for working areas where machinery is present or a number
of people are employed.
Forced ventilation may be used in cargo spaces where the movement
of air removes moisture or avoids condensation, removes odours or
gases, etc.
The machinery space presents another area which requires ventila-
tion. As a result of its large size and the fact that large volumes of air are
consumed a treatment plant would be extremely costly to run.
Ventilation is therefore provided in sufficient quantities for machinery
air consumption and also to effect cooling. The usual distribution
arrangement is shown in Figure 9.13.

Fan room

Machinery
>space
' casing

Purifier
extraction
Flat fan
Floor plates
Tank top

Figure 9.13 Machinery space ventilation—diagrammatic

Several axial-flow fans provide air through ducting to the various
working platforms. The hot air rises in the centre and leaves through
louvres or openings, usually in the funnel. The machinery control room,
as a separate space, may well be arranged for air conditioning with an
individual unit which draws air through trunking from the outside and
exhausts back to the atmosphere.