Structured Evaporators

The optimization of the heat transfer during evaporating a liquid is of enormous technological importance, since evaporators are widely used as heat exchangers: in the generation of process steam, in refrigeration machines and heat pumps and in component cooling and temperature control.

For a given temperature difference, the area-related evaporator capacity depends only on the heat transfer coefficient a, which can be enormously increased by structuring the evaporator surface. Requirements on the structures are

• a high volume-related surface and good thermal conductivity of the structure and
• a high number of bubble nucleation sites and sufficient porosity for bubble removal.

Powder metallurgically produced structures - possibly with additional coatings - are optimally suited for this application.

We offer you the full service:

• experimental analysis of different evaporator structures under defined conditions,
• direkte Strukturoptimierung anhand der Ergebnisse mit werkstoff- und fertigungstechnischem IFAM-Knowhow.

Geometrically irregular structures as well as regular ones are ideal for structuring evaporator surfaces. Material competences at Fraunhofer IFAM Dresden include the following areas:

• metal powder coating of surfaces,
• application of metal fibre, metal foam and metal wire structures (open cell metals) and
• structuring of evaporator surfaces using metallic 3D screen printing or electron beam melting.

Especially metallic 3D-screen printing allows the production of very precise structures on flat surfaces.

Regardless the type of structuring of an evaporator surface, the heat transfer characteristics must be analysed in experiments due to the complexity of physical processes. For this purpose test samples are manufactured and thermally tested

• with flat evaporator surface as in the picture or
• with cylindrical surface (equivalence to vessel boiling on the outer tube shell, flooded shell-and-tube evaporator).

Two evaporator test plants are available in the thero-technical laboratory of Fraunhofer IFAM Dresden:

• an atmospheric pressure system as an open system with water as fluid for the examination of plane samples and
• a flexibly applicable evaporator test plant as a closed system (see picture)

-       for rough vacuum pressures up to 10 bar,
-       for refrigerants that can be handled in the laboratory with suitable boiling temperatures and
-       for flat and cylindrical sample geometries.

The samples are placed in the refrigerant preheated to boiling temperature and electrically heated, evaporating fluid is condensed. The heat transfer coefficients can be determined from the area-related heat output and the measured wall and fluid temperatures.

Based on the measurements, it is easy to recognize the influence of individual structural parameters on achievable heat transfer coefficients under practical conditions. On this basis, the evaporator structures can be optimized step by step.