مقالیه Design and Manufacture of Cryogenic Sub-Cooler

12th Cryogenics 2012, IIR International Conference in Dresden in Germa

Design and Manufacture of a Cryogenic Sub-Cooler for High Temperature Superconductors

Mahmoudian S.M.R. *, Alinejad N. *, Zandkarimi M. *, Zarvandi M. *

ABSTRACT

The technology of sub-cooling cryogenic liquids below their normal boiling points (NBP), and thereby making the fluid denser, is one of the key process technologies necessary to meet the challenge of cooling high-temperature superconductors (HTS). One of the most reachable methods for fusion is Magnetic Confinement Fusion (MCF). In this method in order to optimize energy consumption, superconductor cables are used.

1. INTRODUCTION

A practical prototype cryogenic liquid densifier is developed in Research School. Temperature improvement of 11% compared with liquid nitrogen (LN2) is expected to substantially improve cooling properties for high temperature superconductors and minimise risks of geysering in vertical pipe lines and cavitations in cryogenic pumps. The densification production unit was configured with a high-efficiency sub-atmospheric boiling bath heat exchanger to cool the working fluid. A nitrogen boiling bath at 66 K is used to provide the heat sink to cool liquid nitrogen down to 68 K. A vacuum pump operates to maintain the ullage pressure in the heat exchanger bath below one atmosphere. The sub-cooled liquid nitrogen is also tested to cool high-temperature superconductors in a superconducting magnetic energy storage (SMES) system.

The densification units and project plan were originally established to be integrated with the cryogenic pre-cooling and sub-cooling properties of LN2 in the Plasma Physic & Nuclear Fusion Research School.

1. Mechanical design

The flow chart of the cryogenic system for sub-cooling the cryogen is outlined in Fig. 1. The thick real line loop with the arrows shows the sub-cooled LN2 cycle, which consists of a phase separator, a centrifugal pump, a heat exchanger in the sub-cooler, cryostat, and some control valves for the operation of the cryogenic system. The refrigeration station is also shown in the dashed line frame, in which there are a vacuum pump, an air heat exchanger for heating up the cold gaseous nitrogen pumped from the sub-atmospheric LN2 bath which was called the sub-cooler, and an electric valve II for adjusting the flow rate of nitrogen gas so as to adjust the cooling capacity.

A nitrogen boiling bath at 66 K is used to provide the heat sink to cool liquid nitrogen down to 68 K. This tank has an inner diameter of 0.78 m, outer diameter 1.18 m, height 1.56 m, and its capacity is near 0.6 m3. In the sub-cooler there are two major components, a heat exchanger and a phase separator.

The pressure of the sub-cooler could be pumped down to 0.014 MPa corresponding to an LN2 saturated temperature of 65 K.

The locations of some sensors, including the temperature (PT100 & T type thermocouple), the pressure, the LN2 flow rate, and the level sensors, are also presented in Fig. 1.

The cryogenic system features some of the following characteristics.

1. The electric valve II installed in front of the vacuum pump inlet could adjust the cooling capacity. The maximum cooling capacity is about 1.4 kW according to the theoretical calculation.
2. The working pressure in the LN2 circular system could be changed from 0.1 MPa to 1.0 MPa.

In addition, the sub-cooler, the gas–liquid separator and the LN2 tanks are designed as a multi-layer vacuum dewar and their evaporation rate is lower than 3% per day.

The vessels were insulated with layers of MLI and Perlit in high vacuum. The pressure relief system was set for a nominal 3.5 bar relief pressure, even though the design pressure of the vessels and coils was 16 bar. The stage two vent discharges gas into a ¾ inch line leading to the phase separator that was attached to the GN2 gaseous heat exchanger inlet. This heat exchanger increases the temperature of GN2 to protect the vacuum pump.

The basic design of the recuperative heat exchanger was a group of parallel “V” -shaped coils (Figure 2) attached to 19 mm inner diameter supply and return copper manifolds, submerged into the sub-cooler (Figure 3 & 4). The “V” -shaped coils were filled with an “O” -shaped coil installed at the space into the “V” -shaped coils. The “O” -shaped coil is in series with the parallel “V” -shaped coils.

The manifolds distribute the heat load uniformly throughout the liquid bath. All coils were formed from extruded “O” -shaped copper tubing. The tubing was 16 mm and 19 mm OD net with 1 mm thick walls. The heat exchanger tube bundles comprised 30 parallel coils with each pass having lengths of 1.6 meter. Table 1 presents some of the pertinent design and performance requirements.

Both the sub-cooler’s and cryostat’s vacuum jackets were fabricated from stainless steel, ASTM A312, type 304/304L. With the exception of process flanges and exhaust piping, all cryogenic piping and components not previously mentioned were MLI jacketed. The flanges and exhaust piping were non-VJ but instead were covered by removable mechanical foam insulation.

The insulation technology was studied experimentally, including 3 kinds of insulating material, such as 3 different thicknesses of aluminium mylar, and some insulation foams, plus material such as expanded Perlit.

A spherical stainless steel 304 receiver was installed into the sub-cooler tank as a phase separator. This spherical tank is also used to balance the flow rate of liquid cryogen. When more liquid is needed, it is supplied from receiver, and when there is extra liquid it can be saved in the reservoir. It reduce the fluid velocity at the liquid-gas interface as much as possible to give good separation and minimize entrainment. The reasons it is installed into the sub-cooler tank are

1. To reduce heat losses through piping and walls.
2. To minimise the footprint of the total system.
3. It will work as an extra heat exchanger even when the system is off.
4. It saves money needed for external wall and insulation between them.

The cryostat is a cylindrical vacuum insulated container with 66 cm inner diameter, 88 cm outer diameter, and 40 cm height with a removable O-ring sealed door. Cryogenic lines, instrumentations, and vacuum port of cryostat are sealed through this door.

1. Conclusion

This project gives some experimental information about cryogenic technology using liquid nitrogen that will be useful for future Iran fusion (MCF) program, a tokamak that will operate with high temperature superconductor cables, called Persian Gulf tokamak.

Table 1 Design Parameters.

Figure 1 Flow chart of the cryogenic system [1].

Figure 2 The lower spiral heat exchanger for low flow rates.

Figure 3 Installation of heat exchanger and phase separator into the sub-cooler.

Figure 4 Installation of heat exchanger and phase separator into the sub-cooler.

References

• F. Fan, L.H. Gong, X.D. Xu, L.F. Li, L. Zhang, L.Y. Xiao. Cryogenic system with the sub-cooled liquid nitrogen for cooling, Journal of Cryogenics, Vol. 45 2005, pp. 272–276.