As published in Cold Facts magazine, Summer 2006
Applications for Transfer Hoses
The transfer of cryogenic fluids such as liquid helium, hydrogen, nitrogen,and oxygen is a daily common laboratory and commercial occurrence. In many instances it is convenient to use flexible vacuum insulated hoses to facilitate the transfer. These hoses allow the transfer to occur with minimal loss of the cryogen due to evaporation from heat transfer.
Applications include moving cryogens from portable dewars to end use points such as laboratory biological dewars. Filling of super magnets and MRI’s is also typically done using special flexible vacuum insulated transfer hoses.
Bendable transfer piping is used in wide ranging applications from food freezing to elastomer deflashing and the manufacture of computer chips. Small sections are almost always used in rigid vacuum insulated piping systems to attach the rigid pipe to the end use point.
In the laboratory where cryogens are transferred over short distances, the losses of product due to heat leak and subsequent evaporation, as well as pressure-drop due to flow restrictions, are not a significant issue. In large commercial operations these issues are magnified and make proper transfer hose design and selection critical.
Wisely choosing a flexible vacuum insulated transfer hose or pipe requires a basic understanding of hose construction. High quality hoses are typically insulated with multi-layer insulation and evacuated to a warm vacuum settle pressure in the 10-4 Torr range. Good quality hoses also include a vacuum getter material to provide long-term vacuum reliability. Minimum wall thickness of the inner hose is maintained to provide rapid cool down and good flexibility. Manufacturing techniques vary from manufacturer to manufacturer and therefore also the heat transfer and cool down performance of the hoses.
Frictional resistance to flow is generally greater in flexible transfer hoses since they do not provide the smooth bore associated with rigid vacuum insulated piping. This is due to the greater surface area and the additional turbulence created in the flow stream. Care must be taken to size the transfer hose to ensure that the required flow is maintained given the input pressure and required output pressure.
Since a cryogenic transfer hose may sit idle for periods of time, evaporation of the contained fluid can occur. For this reason a system with elevation changes cannot be considered a siphon system. Loss of head due to change in elevation must be considered in any cryogenic transfer piping system. This loss can make the resistance to flow of bendablepipe systems even more critical. Additionally, pressure drop due to flow restriction in fittings and valves is a critical consideration in any transfer system. Accessories to the system can include valves, gas traps, and vapor vents which contribute to pressure drop. Each system must be carefully evaluated for pressure drop with all items that resist flow considered.
In commercial bendable piping systems heat transfer is also a very important consideration. Significant amounts of liquid cryogen can be lost through evaporation if heat transfer is not kept to a minimum. Liquid helium transfer is far more critical with regard to cool down and heat transfer than the other cryogens due to its relative high cost and low boiling temperature.
Another consideration for the bendable piping system is that it must be designed based upon the duty cycle it will encounter. Some systems experience repeated cool down cycles. For these, a low mass inner hose will require less cryogen to be evaporated to reach operating temperature. For systems where cool down occurs infrequently, high mass inner hoses can be used without significant loss of cryogen. The higher weight inner hose may provide more durability in applications where hoses may be subjected to environmental damage or high pressure.
System pressure considerations must be made in any piping application. Flexible vacuum insulated transfer hoses are commercially available in different pressure ratings based upon material thickness and diameter. Where high-pressure use is needed the inner convolute hose may be encased in a stainless steel wire mesh to limit deflection and increase resistance to high pressure.
Some manufacturers offer bendable transfer pipe with a smooth Teflon inner tube. While the tube slightly reduces the inside diameter of the pipe, the smooth bore reduces resistance to flow because of the reduction in turbulence and low friction characteristics of the inner wall. The bendable pipe with the liner remains flexible when warm but loses flexibility under cold working conditions. This makes it suitable for pipe applications but not for typical transfer from mobile dewars.
Pros and Cons of Rigid vs. Bendable Vacuum Insulated Pipe
It is well known that transferring cryogenic liquids through non-insulated or poorly insulated pipes can create safety issues including the formation of liquid air (an extremely potentially volatile liquid).
The virtues of vacuum insulated pipe over other forms of pipe insulation are well known. Vacuum insulation provides a high degree of resistance to heat transfer and is very durable, surviving many years of service without degradation of insulative characteristics.
In general flexible hose and bendable pipe offer somewhat less resistance to heat transfer. Thermal conductivity increases in the bends of the bendable pipe due to compaction of the insulation between the inner and outer hoses. The tighter the bend radius the greater the heat transfer.
Bendable pipe typically is rated for lower pressure use than rigid pipe due to the necessary decrease in wall thickness to achieve flexibility. Bendable vacuum insulated pipe usually has less resistance to damage from misuse and handling.
Bendable vacuum insulated pipe has greater resistance to fluid flow than rigid pipe of the same diameter due to internal turbulence.
Bendable vacuum insulated pipe offers several important functional advantages over rigid insulated pipe. By its flexible nature it may be used in a variety of applications not possible for rigid pipe. Applications include the filling of MRI magnets from portable dewars. Filling of biological dewars is many times done using flexible vacuum insulated hose. Research super conducting magnets are filled much like the Medical MRI units.
In industrial applications bendable pipe can be used to connect liquid nitrogen food freezers and deflashers to liquid bulk storage tanks. By using bendable pipe the installation cost can be dramatically reduced due to shorter installation time. Production downtime is also reduced.
Bendable insulated pipe is a good choice when changes in plant layout occur frequently or change is planned. Many times the bendable pipe can simply be rerouted without the need to make new custom-fit piping.
Bendable vacuum insulated pipe can be made available with an inner liner to reduce the effects of resistance to cryogen flow, making it a viable alternative to rigid pipe. Bendable vacuum insulated pipe can be manufactured with the inner hose encased within a steel wire mesh to increase its working pressure and significantly raise its rating. Bendable pipe is also commercially available with optional outer protective coverings ranging from polymers to braided and spiral stainless steel. These options make the flexible hose more durable and less susceptible to environmental damage.
Bendable vacuum insulated pipe and flexible hose is a viable low cost alternative to rigid vacuum insulated pipe. When low cost rapid installation is of primary concern and insulative qualities are required to prevent excessive loss of product, bendable pipe is a good alternative to rigid vacuum insulated pipe. Bendable pipe also offers the user the opportunity to change the pipe layout without having to rework expensive piping systems.