Heat and Mass Transfer
The most commonly used method for the design of duct size is the

Static regains method
Dual or double method
Velocity reduction method
Equal friction method

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Heat and Mass Transfer
In counter flow heat exchangers

One fluid is in hottest state and other in coldest state at inlet
Both the fluids at exit are in their hottest state
Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state
Both the fluids at inlet are in their hottest state

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