Page 72 - Offshore Electrical Engineering Manual

Cooling Systems 59

Table 2.4.1 Heat Balance Values
Electrical
Energy Gain Heat Loss Output

Energy provided by fuel – 1500 kW
Heat lost to engine water jacket 450 kW
Heat radiated from engine 50 kW
Net mechanical output from engine – 500 kW
Losses from alternator (eff. = 90%) 50 kW 450 kW
Heat lost to turbo intercooler 50 kW
Heat lost to engine exhaust 450 kW
Total 1050 kW 450 kW

COOLING SYSTEMS

Although this subject and most of the following ones in this section are definitely
in the realms of mechanical and other engineering disciplines, the electrical engi-
neer needs to be vigilant for the proceedings or risks that occur when problems
such as insufficient cooling arise during commissioning of the generator module.
At some stage during purchase and manufacture of the generator set, the manu-
facturer will provide heat balance figures. Typical heat balance figures are listed
in Table 2.4.1.
As with an accountant balancing his books, all the waste heat from the engine
must be accounted for in the design of cooling and ventilation systems. The ratings
of the engine and alternator are based on designed operating temperature bands,
and if these are exceeded when the engine is running at its rated power output
because of poor cooling and ventilation, the generator manufacturer will have to
derate the equipment accordingly. The above-mentioned heat balance example is
for a reciprocating engine, but the same principle may be applied to turbines. In
generator modules where all ventilation is provided by the engine radiator fan, an
allowance must be made for the temperature rise caused by heat dissipated within
the module before the airflow reaches the engine radiator. Wind speed and direc-
tion will also affect the airflow through the module and when the wind is strong and
blowing directly against the fan, it may stall the airflow completely, causing a rapid
temperature trip. If the radiator fan is electrical, ‘windmilling’ of the fan should be
prevented when the engine is not running, otherwise when the engine is started the
fan motor may trip on overload due to the excessive acceleration time from some
negative to full forward speed. If power is available from another source after the
generator has stopped, it is advisable to have another smaller fan running to pre-
vent a build up of heat in the compartment while the generator set is cooling down.
Without this, the temperature in the compartment may exceed maximum allowable
values for electrical equipment or insulation.