Page 24 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

20 Chapter 2 Heat transfer processes in industrial scale

elements such as headers, manifolds, tanks, inlet and outlet nozzles, pipes and seals. Primary surface is
the part of the heat transfer surface in direct contact with both hot and cold fluids. Transfer of heat
across the primary surface is by conduction. Fin appendages are added to the primary surface in finned
tube exchangers to increase the rate of heat transfer. Heat conducted through the fin is transferred to
surrounding fluid by convection and also by radiation. Fins are not primary heat transfer surface as they
do not separate the two fluids of the exchanger. In a heat pipe heat exchanger, heat is transported by
condensation and evaporation of the working fluid inside it. In cooling towers, cooling of process water
is predominantly by latent heat transfer due to evaporation of a small amount of water into the air in
contact with it. Except rotary regenerative exchanger (in which the matrix is mechanically driven to
rotate at some design speed) or a scraped surface heat exchanger, other exchangers do not have any
moving part.

2.2 Exchanger types
Due to the large number of heat exchanger configurations, several classification systems are devised
based on the basic operation, construction, heat transfer mechanism and flow arrangements.
• Mode of heat transfer e direct contact (between immiscible fluids, gaseliquid or vapoureliquid)
and indirect contact type. Indirect contact type may further be classified as direct transfer type,
storage type and fluidised bed exchangers.
Direct transfer exchangers are recuperators. Depending on the geometry, these may be
tubular, plate type and extended surface. Tubular exchangers are classified as double pipe, shell
and tube, spiral flow and pipe coils. Extended surface can be plate fin or tube fin which can be
fitted on either ordinary separating wall or pipe wall. Storage-type exchangers are re-
generators. Process industry exchangers are mostly recuperators. Regenerators are less common
but are used in some specific cases, particularly when heat transfer involves a dust laden gas like
the blast furnace gas in steel plants.
• Flow arrangement e single pass or multipass.

In single pass, the flow of the two fluids can be co-current, countercurrent, cross-flow, split flow or
divided flow. In multipass, the flow can be (a) parallel and counterflow (‘m’ shell passes and ‘n’
tube passes), split flow and divided flow in shell and tube exchangers, (b) cross-counterflow, cross-
parallel flow and compound flow in extended surface and (c) fluid 1em passes and fluid 2en passes
in plate-type exchangers.
• Heat transfer mechanism e (a) single-phase convection on both sides, (b) single-phase
convection on one side and two-phase convection on the other side, (c) two-phase convection on
both sides and (d) combined convection and radiation heat transfer.

2.2.1 Recuperator

In direct transfer heat exchangers/recuperators, heat transfer occurs continuously from the hot to
the cold fluid through a dividing wall. The fluids flow simultaneously in separate fluid passages