2 Nominal air change
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC/Circular.729 – Design Guidelines and Operational Recommendations for Ventilation Systems in Ro-Ro Cargo Spaces – (Adopted on 4 July 1996) - Part 1 - Design Guidelines and Operational Recommendations for Ventilation Systems in Ro-Ro Cargo Spaces - Appendix 2 - Ventilation of ro-ro cargo spaces - Air flow testing procedures - 2 Nominal air change

2 Nominal air change

 In order to verify that the calculated quantity of air is supplied to the ro-ro cargo spaces, the air flow rate shall be measured in each supply air and, where appropriate, exhaust air terminal device.

Parent topic: Appendix 2 - Ventilation of ro-ro cargo spaces - Air flow testing procedures

2.1 Instruments for Measurement of Air Flow

 Although alternative techniques, such as the pilot traverse method are available, anemometers are generally employed for low velocity air flow measurements. There are two general types of anemometers:

  • (a) The direct-reading anemometer of the electronic type which registers the air velocity almost instantaneously. This has a distinct non-uniform airflow as any instability or random changes of velocity are immediately seen and the true mean of the velocity at a point can be judged. It is also very quick to use.

  • (b) The mechanical type of direct reading anemometer with a rotating vane. The movement is a rotary deflection against the action of a spring.

These types of anemometer are small and compact, easy to read and use, give reasonably steady readings and any fault or inconsistency developing is usually quite apparent. Where a correction chart is supplied with an anemometer the correction factors should be applied to the measured velocities before comparing them. With a good quality instrument in proper repair used by an experienced operator, the probable error on the comparative value obtained will range from a maximum of ±2% when comparing similar velocities to a maximum of ±5% when comparing widely differing velocities.

2.2 Air Flow Measurement Procedure footnote

 For supply or extract grilles the anemometer is used as follows:

The gross grille area is divided into 150-300 mm squares, depending upon the size of grille and variation in the velocity pattern.

The anemometer is held at the centre of each square with the back of the instrument touching the louvres which must be set without deflection. The instrument will give an immediate reading of the indicated velocity at each square and this reading should be recorded. When the indicated velocities at the centre of all squares have been recorded, the average value of these velocities should be calculated, this average value should be taken as the 'indicated velocity' for the whole grille.

This method will normally provide repeatable results. In practice the only inconsistency which is necessary to consider appears where the grille damper is well closed down, causing the air to strike the anemometer vanes in separate jets rather than with uniform velocity. In this case a hood may have to be used with the anemometer.

2.3 Calculations

 The air flow rate at each supply-extract grille is calculated as follows:

Air flow rate (m3/s) = 'indicated velocity' (m/s) x area of supply/extract grille (m2)

The global rate of air change per hour achieved by the vehicle deck system(s) is subsequently calculated as follows:

2.4 Report

 A report should be drawn up in accordance with paragraph 4 of this appendix.

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