The power rating of the ship electrical equipment is limited by the temperature rise above the ambient air that cools the equipment.
The industry-standard ambient air temperature is 40°C. It can be higher, up to 65°C, in special places such as the ship boiler room uptake. The maximum permissible operating temperature depends on the class of insulation used in the equipment.
For example, a temperature rise of 80°C is permitted in equipment’s with class B insulation. In 40°C standard ambient air, it would have an operating temperature of 80°C + 40°C = 120°C. For some insulation’s, such as NomexTM, the allowable operating temperature can be up to 200°C.
The temperature rise is determined by the cooling medium and the surface area available for dissipating the internal power loss on ship system that heats the equipment body.
Common Aspects of Power Equipment
Air-cooled ship equipment dissipates the internal heat mostly by convection and radiation, and negligibly by conduction. For electrical equipment on ship normally operating around 100°C, the temperature rise above the ambient air is approximately given by:
ΔT˚C = K × (watts)0.8 (3.23)
where watts = power loss to be dissipated and K = constant for a given body.