  Two cryogenic bath-type heat exchangers for ultra-pure noble gas applications
were developed with particular emphasis on noble gas liquefaction in cryogenic
distillation systems. The main objective was to construct heat exchangers for
xenon from materials that do not emanate radon and that fulfill ultra-high
vacuum standards. Therefore, only high-quality copper and stainless steel
materials were used. Especially, large-area oxygen-free copper fins with high
conductivity in a new design ensure efficient heat transfer. One bath-type
Xe-Xe heat exchanger was designed with a diameter of 50 cm to achieve a xenon
condensing capacity of at least 100 kg/h. In order to guarantee the necessary
heat transfer between the two xenon reservoirs, this heat exchanger features a
specially manufactured stainless steel flange with a copper plate welded
inside. We first tested our concept on a dedicated bath-type heat exchanger
with a reduced diameter of 30 cm using liquid nitrogen to liquefy the xenon. A
model based on conservative assumptions such as film boiling on the nitrogen
side and film condensation on the xenon side was developed and applied to
caluclate the expected heat transfer for our design. We were able to
demonstrate an adjustable xenon liquefaction rate of up to 113 kg/h limited
only by our measurement procedure at a cooling efficiency of $(0.98\pm 0.03)$
for the LN$_2$-Xe heat exchanger.
