Page 27 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

2.2 Exchanger types 23

weight per unit volume, (e) low volume of fluid hold up that is important for expensive fluids, (f) faster
transient response and (g) efficient process control. A smaller temperature difference between the
fluids allows good temperature control. In addition, the ease of cleaning makes the PHE useful for
sterilisation/pasteurisation in food and beverage processing and pharmaceutical industries. They are
also used in synthetic rubber industry, paper mills, process heaters/coolers and closed circuit cooling
system of large petrochemical and power plants. The material of gasket and use of plates often limits
the maximum pressure, maximum temperature and maximum pressure and temperature difference
between the fluids. They are usually operated below 1.0 MPa(g) and 150 C to avoid use of expensive

gasket material. Welded PHEs ensure almost leak proof operation but these cannot be dismantled for
cleaning and therefore can be used only with clean fluids. The overall pressure drop is comparable to
shell and tube exchanger. Some of the recent designs of plate-type exchangers include double wall
PHE, spiral PHE, lamella heat exchangers, printed circuit heat exchangers, etc.
Compared to shell and tube exchanger, compact heat exchangers are characterised by a large heat
transfer surface area per unit volume, resulting in lighter devices
with lower footprint and fluid inventory. A gas to fluid exchanger
is classified as ‘compact’ if it has a surface area density (b ¼ ratio
Compact Heat Exchanger 2 3
of heat transfer surface area to volume) > 700 m /m or a hy-
draulic diameter (D h ) 6 mm for operating in a gas stream and b
3
2
> 400 m /m for operating in a liquid/phase change stream. A
laminar flow exchanger (mesoscale heat exchanger) has a surface area density greater than about
2 3
3000 m /m (100 mm D h 1 mm) and micro (scale) heat exchanger has b greater than about
2 3
15,000 m /m (1 mm D h 100 mm). Examples of compact heat exchangers are plate fin, tube fin
and rotary regenerators for gas flow on one or both the fluid sides and gasketed, welded or brazed heat
exchangers and printed circuit exchangers for liquid flow. The basic flow arrangement in compact heat
exchangers are single-pass cross-flow, counterflow and multipass cross-counterflow.
3
2
The tubular and plate-type exchangers with b less than 700 m /m give a heat exchanger effec-
tiveness around 60% or less. Effectiveness (ε) of a heat exchanger is defined as the ratio of the actual
heat transfer rate to the maximum possible heat transfer rate thermodynamically permitted. This is
discussed in greater detail later in this chapter. For a much higher effectiveness (around 98%), a more
compact surface is required. Fins are usually added to increase surface area and exchanger
compactness for the same temperature difference. Depending on the design, they can increase the
surface area by 5e12 times the primary surface area and the resulting exchanger is referred to as an
extended surface exchanger.
Fins while increasing the heat transfer area may or may not increase the heat transfer coefficient.
Interrupted fins (strips, louvers, etc.) increase area as well as heat transfer coefficient. The increase can
be two- to four-fold. Usually an increase in fin density reduces the heat transfer coefficient associated
with fins. Plate fin and tube fin geometries are the two most common types of extended surface ex-
changers. Internal fins in tube are less common.
Mostly low finned tubes are used in shell and tube exchangers to increase the surface area on the
shell side when the shell-side heat transfer coefficient is low. Highly viscous liquids, gases or film-wise
condensing vapours on the shell side cause low heat transfer coefficient. Fins add to structural strength.
Fins/studs may also be used to aid thorough mixing of a highly viscous liquid. The low finned tubes
usually have helical or annular fins. Double-pipe exchangers usually employ longitudinal fins. Fins on
the inside of the tube are either integral fins or attached fins. The fin efficiency increases with