Design of 2-Stream (He/He) Plate-Fin Heat Exchanger for Helium Refrigeration and Liquefaction Plant

Abstract

Some of the key components of Helium Refrigeration and Liquefaction (HRL) plant are heat exchanger working at cryogenic temperature. Effectiveness of these heat exchangers is one of the important parameters which decide the efficiency of helium refrigerator/liquefier. To achieve high effectiveness, it is necessary to use plate-fin heat exchanger, which provides very high heat transfer surface area per unit volume of heat exchanger. Such heat exchangers also have benefit of low pressure drop of fluid flowing through it. These cryogenic heat exchangers will be placed within a vacuum chamber having vacuum of about 10⁻⁵ mbar and hence compact heat exchangers are preferred to reduce the size and cost of the vacuum chamber. As per the chosen thermodynamic configuration of indigenous HRL, it will have 8 heat exchangers. It can produce liquid helium at around 4.5 K and can operate as refrigerator-cum-liquefier with equivalent cooling capacity of about 2 kW at 4.5 K.

In the present study, design, analysis, optimization and validation of the heat exchanger operating at the lowest temperature zone between 7 to 4.5 K is presented. In this temperature zone, there is large non-linear property variations in the helium fluid which is considered in the design work. It has 2 streams (He / He) in counter flow configuration. These 2 streams of helium flow are at 2 different pressures: high pressure (HP) helium gas stream of 4 bar coming out of the 3rd turbine after expansion from 13 bar to 4 bar and the low pressure (LP) helium vapour stream of 1 bar going back to the compressor suction from the liquid helium (LHe) Dewar. The tentative flow rates of LP and HP streams are 90 g/s and 100 g/s for nominal operation. The proposed material for heat exchanger is Al 3003 and proposed fin type is serrated fins over plain rectangular fins considering lower pressure drop, reduced size, weight and cost. Based on International Standard and Apollo Standard, optimization is done. Design is validated with existing plant and Aspen Tech software. This design work is a step towards the indigenous development of liquid helium plant.