The Technology of Sub-Cooling Cryogenic Liquids Below Their Normal Boiling Point (NBP)

The Technology of Sub-Cooling Cryogenic Liquids Below Their Normal Boiling Point (NBP)

Abstract

A practical cryogenic liquid densifier in Plasma Physic & Nuclear Fusion Research School is developed. Density improvement of 10% greater for liquid oxygen (LOX) is expected to substantially reduce the gross lift-off weight of a launch vehicle system by 10%. 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 oxygen or liquid nitrogen down to 68 K. A vacuum pump operates to maintain the ullage pressure in heat exchanger bath below one atmosphere pressure. The sub-cooled liquid nitrogen (LN2) is also used to cool high-temperature superconductors (HTS) in the satellites.

Keywords: Cryogenic- Liquid Oxygen- Liquid Nitrogen- Density- High temperature superconductor

Introduction

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 launch vehicles mass ratios and even cooling high-temperature superconductors (HTS) in the satellites.

Densified propellants are critical to achieving lower launch costs because they enable additional cryogenic propellant to be packed into a given unit volume, thereby improving the performance of a launch vehicle, effectively reducing its overall size and dry weight.

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

Mechanical design

The flow chart of the cryogenic system for sub-cooling the cryogen is outlined in Fig. 3. The thick real line loop with the arrows showing the sub-cooled LOX or LN2 cycle, which consists of a gas–liquid separator, a centrifugal pump, a heat exchanger in the sub-cooler, cryostats, and some control valves for the operation of cryogenic system. The refrigeration station is also shown in the dash line frame, in which there are a vacuum pump, an air heat exchanger for heating up the cold gas-nitrogen pumped from the sub atmosphere LN2 bath which was called as 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 oxygen or liquid nitrogen down to 68 K. This tank has an inner diameter of 0.78 cm, outer diameter 118 cm, height 156 cm, and its capacity is near 0.6 m3. In the sub-cooler there are two major components, a heat exchanger and a reservoir.

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

The locations of some sensors, including the temperature (PT100), the pressure, the LN2 flow rate, and the level, are also presented in Fig. 3.

There are some following characters in the cryogenic system:

1. The electric valve II installed in front of the vacuum pump inlet could adjust the cooling capacity. The maximum cooling capacity is about 6 kW according to the theoretical calculation.
2. The working pressure in 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 & LOX tanks are made of the multi-layer vacuum dewar and their evaporation rate is lower than 3% per day.

The vessels were insulated with layers MLI and Foam or perlit insulation, with a thin metal protective outer cover installed. 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 discharged gas into a 1 inch line leading to the phase separator that was attached to the GN2 gaseous heat exchanger inlet. This heat exchanger increase temperature of GN2 to protect single stage vacuum pump.

The basic design of the exchanger surface area was a group of parallel pancake style coils

(Figure 5) attached to 25 mm supply and return manifolds, submerged in a vertical tank of liquid cryogen. The pancake coils were formed in a “O” shape to provide maximum space above the coils and reduce the fluid velocity at the liquid-gas interface as much as possible to give good separation and minimize entrainment. The manifolds, distributed 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 6 parallel coils with each pass having lengths of 16 meter. Table 1 presents some of the pertinent design and performance requirements.

Both the sub-cooler and cryostat are 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 (Figure 2,3) jacketed. The flanges and exhaust piping were non-VJ but instead were covered by removable mechanical Foam insulation.

The MLI and insulation technology included 3 kinds of insulate material between aluminum foils, 3 different thickness of aluminum foils, and some insulation foam or material such as expanded perlit were studied experimentally.

A spherical receiver was installed into the sub-cooler tank. This spherical tank is used as a receiver to balance the flow rate of liquid cryogen. When need more liquid it supply from receiver and when there is extra liquid it saves in the reservoir. The reason it is installed into the sub-cooler tank is to

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

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. This door is used to sealed charge and discharge cryogenic lines, instrumentations, and vacuum port of cryostat.

Benefits of Densified Propellants

Propellant densification was identified as a critical enabling technology in the development of launch vehicles. The densification of a cryogenic propellant through active sub-cooling allows approximately 8 % to 10 % more propellant mass to be stored in a given unit volume. This allows for a higher propellant mass fraction than would otherwise be possible with conventional normal boiling point (NBP) cryogenic propellants. The enormous benefits involved for using densified propellants not only contribute to vehicle weight reductions, reported to be on the order of 15 – 25 percent according to engineering trade studies [ref. 5, 6, 7 & 8], but densified propellants could also lower launch vehicle capital and operating costs by 11 percent or more and significantly increase mission payload capability for various launch vehicles and mission scenarios.

Sub-cooling LOX from the NBP to 66.66 K leads to 9.7 percent increasing in density. At this reduced temperature there is a significant improvement due to its vapor pressure, which naturally is lowered to 0.034 bar. The benefit derived from a propellant with a lower vapor pressure allows the tank design pressure to be reduced accordingly, thereby permitting thinner walled tanks of less mass to be used on an LV. Finally, lower vapor pressure results in a cryogenic fluid with inherently more stability that is safer to handle. This results from the fact that over-pressurization of a containment vessel becomes more difficult as it would take more time to absorb energy in comparison to a fluid at its NBP because of the additional sensible heat available that the sub-cooled fluid would absorb before it begins to boil. Lower vapor pressure reduces the potential for propellant vapor leaks as the absolute ΔP between tank and the environment is less for a pressurized vessel containing densified fluid.

Table 1 Design Parameters

Figure 1 Testing The MLI insulation

Figure 2 MLI with 3 kinds of insulations

Figure 3 Flow chart of the cryogenic system [3].

Figure 4 Cryostat for cryogen reservation (left) & subcooler (right).

Figure 5  Parallel vertical copper heat exchangers inside the subcooler.

Figure 6 The lower spiral heat exchanger for low flow rates.

References

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