21-05-2012, 01:18 PM
Effectively Design Shell-and-Tube Heat Exchangers
Design of Heat exchanger.pdf (Size: 168.72 KB / Downloads: 55)
Components of STHEs
It is essential for the designer to have a
good working knowledge of the mechanical
features of STHEs and how they influence
thermal design. The principal
components of an STHE are:
Classification based on construction
Fixed tubesheet. A fixed-tubesheet
heat exchanger (Figure 2) has straight
tubes that are secured at both ends to
tubesheets welded to the shell. The construction
may have removable channel
covers (e.g., AEL), bonnet-type channel
covers (e.g., BEM), or integral tubesheets
(e.g., NEN).
The principal advantage of the fixedtubesheet
construction is its low cost because
of its simple construction. In fact,
the fixed tubesheet is the least expensive
construction type, as long as no expansion
joint is required.
Design data
Before discussing actual thermal
design, let us look at the data that
must be furnished by the process licensor
before design can begin:
1. flow rates of both streams.
2. inlet and outlet temperatures of
both streams.
3. operating pressure of both
streams. This is required for gases,
especially if the gas density is not
furnished; it is not really necessary
for liquids, as their properties do not
vary with pressure.
Classification based on service
Basically, a service may be singlephase
(such as the cooling or heating
of a liquid or gas) or two-phase (such
as condensing or vaporizing). Since
there are two sides to an STHE, this
can lead to several combinations of
services.
Broadly, services can be classified
as follows:
• single-phase (both shellside and
tubeside);
• condensing (one side condensing
and the other single-phase);
• vaporizing (one side vaporizing
and the other side single-phase); and
• condensing/vaporizing (one side
condensing and the other side
vaporizing).
Heat-transfer coefficient
The tubeside heat-transfer coefficient
is a function of the Reynolds
number, the Prandtl number, and
the tube diameter. These can be broken
down into the following fundamental
parameters: physical
properties (namely viscosity, thermal
conductivity, and specific heat);
tube diameter; and, very importantly,
mass velocity.