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In an Indirect-Contact Heat Exchangerr, the fluid streams remain separate and the heat transfers continuously through an impervious dividing wall or into and out of a wall in a transient manner. Thus, ideally, there is no direct contact between thermally interacting

fluids. This type of heat exchanger, also referred to as a surface heat exchanger, can be further classified into direct-transfer type, storage type, and fluidized-bed exchangers.

Types of Heat Exchangers

 According to the heat transfer process — the heat exchanger may use an indirect contact or direct contact heat transfer method.

In indirect contact heat transfer, the fluids in the system are separated by a thermally conductive boundary layer which allows heat energy to flow but prevents mixing or contamination.

• In direct contact heat exchangers, the fluids are immiscible (e.g. gas and liquid) and therefore do not require physical separation when transferring heat.

According to the number of fluids — the heat exchanger may incorporate two, three, or more than three heat transfer fluids in the system.

According to flow arrangements — the heat exchanger may be either single pass or multi-pass. It may incorporate cross-flow, counter-flow, or co-current flow. Typically heat exchangers utilize combinations of these flow patterns to maximize thermal efficiency.

• In single pass heat exchangers, fluids flow by each other only once in the system.
• Fluids in multi-pass heat exchangers are looped back to flow by each other multiple times.
• In counter flow heat exchangers, fluids flow from opposite directions towards each other.
• In cross flow heat exchangers, fluids flow perpendicular to each other.
• In co-current flow heat exchangers, fluids flow parallel to each other.

According to heat transfer mechanism – the heat exchanger uses single-phase convection, two-phase convection, and/or radiative heat transfer on each side of the exchanger.

According to construction – the heat exchanger may incorporate shell & tube, plated, or air-cooled construction. Users should keep these characteristics in mind when considering different heat exchanger design.