Why condenser back-pressure must be low? How it is achieved?

Why condenser back-pressure must be low? How it is achieved? 
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Condenser back- pressure must be low, because steam should be dumped into the condenser so as to recycle it to boiler through the recycle process. It improves efficiency of the turbine, as the heat rejection is less. It is achieved by the help of ejectors and also passing cold water in the condenser through the tubes of the condenser so that maximum vacuum can be obtained.
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