The feasibility of the manufacturing of a printed circuit type heat exchanger produced from graphite

Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010The development of high temperature heat exchangers will play a vital part in the success of High Temperature Nuclear Reactors (HTRs). Manufacturing such heat exchangers from metals is becoming increasingly difficult as the operating temperatures keep rising. Above 1000'C most metals loose their strength and have high creep rates, while certain ceramic materials (including graphite, in the absence of oxygen) are able to operate at these temperatures. A literature study was done in order to identify the major problems regarding the use of graphite for heat exchanger construction as well as to investigate to what extent graphite has been used for heat exchanger construction in the past. Following from the literature survey, it was decided to design and manufacture a Printed Circuit Heat Exchanger (PCHE) from isotropic graphite to gain experience regarding the use of graphite as a heat exchanger material. This heat exchanger was then tested in order to learn about the operation of a graphite heat exchanger and to determine its effectiveness. A model ofthe heat exchanger was also constructed in order to determine what the performance of such a heat exchanger should theoretically be. It was found that the single greatest hurdle standing in the way ofgraphite being used as a heat exchanger material is its high gas permeability. This causes mixing between the two fluid streams as well as leakages to the environment, which have a negative effect on the heat exchanger's heat transfer capability. The methods used to establish a seal between the consecutive plates of the PCHE are also affected by the permeability of the graphite. Coatings on the surface of the graphite might be able to reduce its permeability and can also inhibit the high temperature degradation of graphite in the presence of oxygen. Manufacturing very small flow channels for the PCHE is limited by the availability of small enough end mills. Alternative manufucturing techniques is needed to economically construct a graphite PCHE. It was also found that the heat transfer effectiveness of the heat exchanger is influenced negatively by heat losses to the environment through the outer surface of the heat exchanger. Effective insulation around the heat exchanger or a graphite material :with higher heat conductivity perpendicular to the flow direction might solve this problem. This study concluded that if diffusion bonding techniques, effective coatings and a graphite material with increased heat conductivity perpendicular to the flow direction are used, manufacturing a printed circuit heat exchanger from graphite is feasible.Master