3 Air distribution
Clasification Society 2024 - Version 9.40
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 - 3 Air distribution

3 Air distribution

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

3.1 Visual study with visible smoke

 In order to assess air change rate the movement of air and the existence of poorly ventilated areas, visible smoke can be released into the space. With the ventilation system operating, the movement of air and the dissipation of smoke can be studied and the air change rate estimated.

3.2 Measurement with tracer gas

 By use of tracer gas it is possible to estimate air change rate and air distribution in chosen points in the ro-ro cargo space.

Measurement with tracer gas involves mixing a gaseous component with the air. The atmosphere in the space is examined to determine how dilution of the tracer gas is tracked at chosen points in the ro-ro cargo space whilst the ventilation system is operational.

This method should be carried out with and without vehicles.

3.2.1 Test procedures

 The placing of the measurement probes shall be chosen with regard to the purpose of the measurement. The probes are not to be placed near to the supply air terminal devices or at places where a so-called ventilation shadow can be expected, such as behind pillars, webs etc. As a rule the probes are placed at the head height and in the vicinity of persons working on the deck.

The tracer gas should be spread and mixed in the air as completely as possible. The mixing may be done by the ordinary ventilation plant or with help of external fans. In order to reach an adequate accuracy, the concentration of the tracer gas ought to reach at least 50 times the detection limit of the analytical instrumentation.

When the tracer gas concentration is adequate the ventilation plant as well as the measurement equipment should be started. Tracer gas concentration should be recorded until the detection level is reached.

3.2.2 Calculation

 With a dilution ventilation system the logarithm of the concentration of tracer gas will be linear with regard to time (see figure below).

The relation between the concentration of tracer gas and time ( the inclination of the graph) is a straight measure of the effect to the ventilation expressed in number of air changes according to the following formula:

where
N = number of changes
C 0 = the concentration at the beginning of the effective dilution
C 1 = the concentration at the end of the effective dilution
t 0 = the point of time at the beginning of the effective dilution
t 1 = the point of time at the end of the effective dilution

Figure 1 The Logarithm of the Concentration of Tracer Gas

3.3 Alternatives

 As an alternative to the tests in paragraph 3.1 and 3.2, air flow distribution in the ro-ro cargo space may be evaluated by use of an anemometer.

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