PARALLEL FLOW AND COUNTER FLOW

In counter flow the temperature range possible is greater, since, in theory, the fluid being heated can be raised to a higher temperature, than that of the heating fluid at exit.

In parallel flow, the final temperature of the fluids must be somewhere between the intial values of each liquid.

In Cross - flow Recuperators, for a given mass flow and given temperature changes, the required surface area is between that for parallel - flow and Counter - flow. As with counter - flow, the outlet temperature of the heated fluid in cross - flow can be raised to a higher temperature than the outlet temperature of the cooled fluid (i.e. t_A2 can be higher than t_B2), this is not possible in parallel - flow.

In order to make the heat exchanger more compact for space consideration and so to reduce heat loss from the outside surface, it is necessary to have a mixture of parallel - flow, counter - flow and cross - flow which is called mixed - flow. The analysis of a mixed flow heat exchanger is complex. The logarithmic mean temperature difference in counter - flow can be evaluated and then multiplied by the correction factor.

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