Page 140 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Construction of experimental liquid-metal facilities 113

Fig. 3.3.3 LBE-to-air shell-and-tube heat exchanger in the THEADES loop.

In addition to the thermal process specifications, the selection of heat exchanger
geometry is influenced by mechanical and material considerations; see, for example,
Shah and Sekuli c (2003). Tubular heat exchangers are generally most versatile and
can be constructed in a wide range of sizes and operated at elevated temperature
and pressures. Mostly shell-and-tube heat exchangers are constructed at KIT, while
a double-tube construction is suitable for laboratory applications at low power densi-
ties, or in reactor setups considering, for example, mitigation of steam generator tube
rupture (SGTR) event.
Regarding the sizing of a liquid-metal shell-and-tube heat exchanger, it should be
noted that due to their high thermal conductivity, the heat-transfer resistance is for
most practical purposes dominated by the other media side, particularly if it is a
gas such as air. In such unbalanced scenarios, extensions of the heat-transfer area such
as fins can be adequate, but additional challenges related to the mechanical stability at
high temperatures, pressure drop, and fouling can limit the application of fins to small
components. Alternatively, placing the gas flow on the tube side can lead to higher
velocities and consequently higher heat-transfer coefficients and a more balanced
and compact design. This approach is followed in most facilities at KIT, for example,
in the main heat exchanger of THEADES LBE loop, shown in Fig. 3.3.3.
Furthermore, heat exchangers are the components with the largest temperature dif-
ferences in the facility. Thus, the construction must account for thermal expansion and
mechanical stresses during transients, for example, preheating to standby conditions.

3.3.2.2 Instrumentation
For the operation and control of an experimental facility, several integral quantities,
such as flow rate and differential pressure, must be measured as accurately as possible.
In principle, measurement techniques well proved in air and water can be transferred