A plate heat exchanger,
configured in the form of a spiral, is capable of being considered. The
channels, formed between the plates, are traversed by the fluids. Constructed
from extended sheets, ranging from 150 to 1800 mm in width, they are shaped
into a pair of concentric spiral channels. Gasketed end-plates, bolted to
an outer case, enclose these channels. Inlet and outlet nozzles are affixed to
the case, connecting to the channels. The spacing between the sheets varies
from 4 to 20 mm, contingent upon the exchanger's size and application. They can
be manufactured from any material amenable to cold-working and welding.
Compact in nature, spiral
heat exchangers occupy a volume of about 10 m3 for a unit
with an area of approximately 250 m2. The maximum operating
pressure is restricted to 20 bar, with a temperature limit of 400 oC.
The pressure drop across a spiral heat exchanger is typically lower than
that experienced in an equivalent shell-and-tube exchanger for a given duty.
True counter-current flow is
achieved by spiral heat exchangers, suitable for situations where the
temperature correction factor Ft in a shell-and-tube exchanger would be
inadequately low. While the heat transfer coefficients are not as elevated as
those in a plate exchanger without corrugations, they surpass those in a
shell-and-tube exchanger due to the presence of curved rectangular passages.
Consequently, the surface area requirement is approximately 20% lower
compared to a shell-and-tube unit for the same heat duty.
Due to their high turbulence in
the channels and easy cleanability, spiral heat exchangers find application in processes
involving very dirty fluids and slurries. The accessibility for cleaning
via high-velocity water jets is facilitated by the easy opening of spiral-plate
exchangers.
The heat transfer coefficient and pressure drop in the channels can be estimated using correlations for flow in conduits, employing the hydraulic mean diameter as the characteristic dimension.
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