Page 6 - Industrial Technology EXTRA 8th June 2020

range are able to use these cooling systems, but system an air-to-water heat exchanger should be
careful planning is required prior to installation. added to transfer the brake energy to the ship’s
Most ships have a chilled water system, which chilled water system.
circulates cool water throughout the vessel to be
used in both air conditioning and equipment Resistance and power rating
cooling. By using electric drives and dynamic braking
However, there is also a sea water variant with resistors (DBRs), ships can benefit from increased
construction and ratings similar to fresh water control, mechanical simplicity and significant
designs, but with titanium-sheathed elements in weight savings. However, for many marine
higher-grade stainless steel vessels. Although this applications, the range of power required for DBRs
alternative is suitable for continuous duty in heated can vary from a few kilowatts (kW) to hundreds of
sea water, its elements cannot be used at an megawatts (MW), and being aware of the unique
operating voltage of much above 1.5kV – power requirements for each application is
significantly lower than the voltage use of other important.
drives. The resistance, measured in ohms (Ω),
For braking resistors operating at more than 2kV, determines how effectively the resistor transforms
sheathed elements are unsuitable and it is the electricity into heat energy, whereas the power
necessary to employ live elements. Normally, rating, measured in watts (W), is the amount of
cooling is by simple natural convection. However, if power which can safely pass through the resistor
the heat losses after braking would impose without risking damage. The properties of these two
unacceptable loads on the ship’s air conditioning parameters are intrinsically linked as power is equal

June 2020 • INDUSTRIAL TECHNOLOGY EXTRA! • p6