The following information is necessary before deciding whether to ventilate or not:
- Temperature and dewpoint of the outside air.
- Temperature and dewpoint of the air inside the hold.
- Temperature of the cargo surface.
- Temperature of the steel structure (Ships) inside the cargo compartment and
- The moisture content of hygroscopic cargoes if applicable.
Whether to Ventilate or not to ventilate?
The following basic Dew Point rule provides relevant guidance which is:
- If the dew point of the air within the hold is higher than the dew point of the OUTSIDE atmosphere, then ventilate.
- If the dew point of the air within the hold is lower than that of the OUTSIDE atmosphere, then do not ventilate.
ANOTHER WAY OF EXPRESSING THIS IS –
- If the Dew point of the outside air is lower or equal to that of the hold atmosphere……continue ventilation.
- If the Dew point of the outside air is higher than the hold atmosphere dew point ………do not ventilate with outside air.
A further extension of the basic rule must be examined depending upon the two basic types of cargo.
HYGROSCOPIC CARGO -Warm region to Cold region:
As the ship proceeds from a warm region to a cold region it will experience a gradual drop in the temperature and a drop in the dew point. The atmosphere within the hold will receive large quantities of moisture from the cargo. Hence in a situation such as this, ventilate vigorously during the early stages but eventually the outside Dewpoint will be too low.
NON-HYGROSCOPIC CARGO –Warm region to Cold region:
Ship sweat is inevitable but cargo unaffected unless, condensation drips back onto the cargo.
HYGROSCOPIC Cargo:Cold region to warm region:
Not critical, therefore ventilation not essential. In some cases on opening hatches at the discharge port immediate condensation may form on surface but will dry off as cargo is discharged.
Non-HYGROSCOPIC Cargo:Cold region to warm region:
No Ventilation. Cargo sweat would occur on the surface of the cargo if relatively warm moisture laden air was admitted.
If due to any reason it is not possible to take readings within the hold then ‘Three Degree Rule’ can be used;
- During loading regular cargo temperatures should be recorded. Whilst on passage if the atmosphere dry bulb temperature is at least 3ºC cooler than the average temperature of the cargo when loaded, then ventilate the hold, if the temperature of the atmosphere is less than 3ºC cooler than the average temperature of the cargo when loaded, the do not ventilate the hold.