Engine Room Ventilation

In addition to providing sufficient air for combustion purposes in the main engine, the auxiliary diesel engines, the fuel-fired boiler, etc., the ventilation system should be designed to remove the radiation/convection heat from the main engine, auxiliary engines, boilers, and other components.
A sufficient amount of air should be supplied and exhausted through suitably protected openings arranged in such a way that these openings can be used in all weather conditions, and care should be taken to ensure that no seawater can be drawn into the air intake.
Furthermore, the ventilation air inlet should be placed at an appropriate distance from the exhaust gas funnel in order to avoid the suction of exhaust gas into the engine room.

Air supply
In the case of a low-speed two-stroke diesel engine installed in a spacious engine room, the capacity of the ventilation system should be such that the volume of air is at least 1.5 times the total air consumption of the main engine, auxiliary engines, boiler, etc., all at maximum continuous rating (MCR).
As a rule of thumb, this corresponds to about 1.75 times the air consumption of the main engine at MCR, and often 2.0 times the air consumption of the main engine at MCR may be sufficient.
On the other hand, for a compact engine room with medium speed or small two-stroke diesel engines, the above factor of 1.5 is recommended to be higher, at least 2.0, because the radiation/ convection heat losses from these engines are relatively greater than from large two-stroke engines, and because it may be difficult to achieve an optimum air distribution in a small engine room.

To obtain a correct supply of air for the main engine’s combustion process, about 50% of the ventilation air should be blown in at the top of the main engine, near the air intake to the turbochargers.
At the same time, in order to ensure that all the heat is removed, care should be taken that a sufficient amount of air is supplied to areas with a high heat dissipation rate, for instance around auxiliary
engines/generators and boilers. Ventilation ducts for these areas, etc.
In wintertime, the amount of air needed to remove the radiation/convection heat from the engine room may, of course, be lower. The engine room temperature should not normally be lower than 5 °C, which is ensured by stopping one or more of the ventilating fans.
 The engine room temperature is normally 10.0-12.5 °C higher than the ambient air temperature and, as explained above, this temperature difference is even higher with winter ambient air temperatures. 

Air pressure
The air in the engine room should have a slightly positive pressure, but should not be more than about 5 mm WC above the outside pressure at the air outlets in the funnel. Accommodation quarters will normally have a somewhat higher over-pressure, so as to prevent oil fumes from the engine room penetrating through doors into the accommodation. The air can be supplied, for example,
by fans of the low pressure axial and high pressure centrifugal or axial types.
The required pressure head of the supply fans depends on the resistance in the air ducts.

Two examples of air supply fan systems are normal air ventilation plant, while an advanced plant which is recommended for reducing the total power requirement of the fans.

1) Low-pressure supply fans only
All air is delivered by low-pressure air supply fans which, to obtain sufficient air ventilation in all corners of the engine room, may require extensive ducting and therefore a relatively high pressure head as stated below.
Low-pressure fans,
Δp = 60-100 mm WC

2) Low and high-pressure supply fans
In order to reduce the power consumption of the air supply fans, high-pressure fans supplying only a minor part of the air can be used to vent the ‘engine room corners’. The remaining and major part of the air can then be supplied by low-pressure fans with a lower pressure head.

Low-pressure fans,

Δp = 35-40 mm WC

delivering about 80-85% of the air, short ducts and an air duct velocity of max. 10 m/s.

High-pressure fans,

Δp = 175-225 mm WC

delivering about 15-20% of the air, round ducts with smooth bends and T-pieces and an air duct velocity of max. 20 m/s.