By Selele Mashilo
Although comfort air conditioning is the basic requirement in ships, there are several other requirements for providing air conditioning.
Mostly, the requirements will be to protect material conditions due to high air humidity and salt contamination from ocean water. Other requirements are to increase crew efficiency, to increase the reliability of electronic equipment and to safeguard against rapid deterioration of special weapons aboard naval ships.
Design criteria
Corrosion is one of the critical elements to be considered for selecting material of equipment construction due to effects of sea, air and water. The equipment should be able to operate under conditions of roll and pitch to which the ship is subjected to. Ships on a voyage are not easily serviced, therefore the equipment should be able to operate for long periods without interruptions. Noise from the equipment must be very low especially when in port to maintain good ship voyage assessment conditions. Equipment weight should be kept to a minimum. The ship is subjected to various weather conditions during a voyage within hours or days, therefore the air conditioning system must be easily adaptable to the changing conditions.
Outdoor and indoor conditions
It can be easy to maintain indoor conditions by using air conditioning systems. For comfort purposes the human metabolic conditions are easily maintained to human satisfaction. Indoor temperatures of 24⁰C dry bulb and 45% to 50% relative humidity can be easily maintained. The difficult situation can be the outdoor design conditions. As indicated above the ships are subjected to various weather conditions on their voyage. Outdoor summer design conditions for the North Atlantic are 35⁰C dry bulb and 25.6⁰C wet bulb temperatures. For winter outdoor heating conditions can go as low as -17⁰C. Now the indoor conditions must be maintained by the same equipment as used for various outdoor conditions which can make equipment selection work quite sensitive.
Ventilation requirements
Due to their environment of operation, ships need particular ventilation systems. Air quantities of 5.9ℓ/s to 7.1ℓ/s per person are required. Even higher quantities can be supplied, so air changes per hour must also be considered and the selection will be based on the comparison and whichever is higher must be considered. Sometimes the specification may require maximum air requirements irrespective of the size of the space, such as 37.8ℓ/s.
Load determination
The cooling load estimates should consists of the following:
- Solar radiation
- Heat transmission through decks and bulkheads
- Occupants
- Lights
- Ventilation Air
- Motors for fans and machinery which dissipates heat.
The heat load should consist of the following:
- Heat losses through decks and bulkheads
- Ventilation air
- Infiltration
Equipment selection
Factors to be considered in the selection of equipment are the following:
- Initial installation costs
- Space available for plant rooms and reticulation
- Operating costs including maintenance
- Noise levels and weight of the equipment
Typical systems may be single zone central systems, multizone central systems, terminal reheat systems and high velocity dual duct systems.
References:
ASHRAE
