HEAT EXCHANGER

Heat exchanger

• Heat exchanger are thermal devices that transfer or exchange heat from one fluid stream to one or more others.
• Heat exchanger transfer heat by one of three ways

1. By recuperation, or recovery, of heat from hot stream to a cold stream
2. By regeneration, as the hot and cold stream alternatively flow through a matrix
3. By direct contact of one fluid stream with another

Classification of heat exchangers

• Heat exchanger are classified either by flow arrangement, by construction or by their degree of compactness

Classification by flow arrangement

• Heat exchanger can be classified according to flow arrangement as follows

Parallel flow heat exchanger

• In a parallel flow (also referred to as cocurrent or cocurrent parallel stream) exchanger, the fluid streams enter together at one end, flow parallel to each other in the same direction, and leave together at the other end

• In a parallel flow exchanger, a large temperature difference between inlet temperatures of hot and cold fluids exists at the inlet side, which may induce high thermal stresses in the exchanger wall at the inlet.

Counter flow heat exchanger

• In a counterflow or countercurrent exchanger, the two fluids flow parallel to each other but in opposite directions within the core.

• The counterflow arrangement is thermodynamically superior to any other flow arrangement.
• It is the most efficient flow arrangement, producing the highest temperature change in each fluid compared to any other two-fluid flow arrangements for a given overall thermal conductance (UA), fluid flow rates (actually, fluid heat capacity rates), and fluid inlet temperatures.

• The maximum temperature difference across the exchanger wall thickness (between the wall surfaces exposed on the hot and cold fluid sides) either at the hot- or cold-fluid end is the lowest, and produce minimum thermal stresses in the wall for an equivalent performance compared to any other flow arrangements.

Cross flow heat exchanger

• In this type of exchanger, the two fluids flow in directions normal to each other.
• In a crossflow arrangement, mixing of either fluid stream may or may not occur, depending on the design.

CLASSIFICATION OF HEAT EXCHANGER BY CONSTRUCTION