Whether you are making ice cream, manufacturing semiconductor wafers, storing biological samples, freezing food or cooling MRI machines, the most effective way to save money on liquid cryogen is by selecting quality vacuum jacketed pipe (VJP) and cryogenic components.
A well-designed VJP system reduces operating expenses by delivering more liquid to the end use point while vaporizing less liquid cryogen. This is accomplished through carefully engineered vacuum jacketed products that are designed to reduce overall heat leak, lessen gas generation and minimize two-phase liquid cryogen flow which improves the performance of the system and uses less liquid cryogen. In contrast, the performance of a foam insulated pipe system worsens with time. After about five years, the foam has deteriorated to the point where it is no longer effective1. VJP is generally cost competitive with foam insulated pipe when all the expenses are considered. Although the initial capital investment for a copper foam-insulated pipe system is commonly less, the pipe installation is labor intensive and the equipment performance deteriorates rapidly with time. The outcome is a long-term investment more costly than anticipated. Whereas with VJP, the first capital outlay may be more but the system assembly is easier with pre-engineered sections requiring less installation time. The higher efficiency and performance stability of the VJP results in lower operation expenses. Over the long term, a VJP system results in a better payback than the copper foam insulated pipe (in many cases, less than 14 months) and justifies the initial investment.
But buyers beware because not all vacuum jacketed pipe and components are created equal.
Making consistent, high-quality vacuum jacketed pipe is challenging. With a little research and some manufacturing experience, it is possible for someone to make their own vacuum insulated pipe. However, making quality, high-performance vacuum jacketed pipe that lasts for many years, with little or no maintenance, requires a thorough understanding of cryogenic and vacuum engineering, highly skilled technicians and fabricators, and years of experience.
Variations in the VJP design and manufacture may significantly change the longevity and performance of the pipe, which directly affects your cryogen consumption, overall operating expenses, and profitability.
Because there are always performance/cost trade-offs inherent in any design/fabrication equation, VJ pipe and cryogenic components should be engineered and manufactured with predetermined quality requirements in mind. Whether it is directly stated or not, each manufacturer has a design/ fabrication philosophy that determines the efficiency and longevity of their pipe1. Design decisions about the material, manufacturing processes, in-process inspection and final testing establish the quality and life cycle of the product. Poor designs offer greater opportunities for higher heat leak, increased gas production and two-phase liquid in the pipe and overall delivery system. “Disadvantages of two-phase flows include higher pressure drop when flowing through pipes and other components, flow instabilities may develop that result in pressure surges and vibrations”2 both cause less cooling capacity at the use point, variable cool-down time and inconsistent liquid delivery. All hinder the performance of your delivery system, increase cryogen use and add to your operating expenses, which reduces your profits.
Superior VJP integrates strategies and features components specifically designed to inhibit heat transfer from conduction, convection, and radiation. The smaller the heat leak the more cost effective the operation of the cryogenic delivery system. Good pipe design includes thermal expansion joints that minimize movement during liquid transfer.
The most critical factor in producing quality vacuum jacketed pipe is establishing and maintaining the vacuum integrity. Without vacuum integrity, the pipe will not perform well for the long term. Guaranteeing a high performing vacuum, one with integrity, begins with defining strict process standards that provide long term results. But life cycle integrity is ultimately ensured during the manufacturing of the product.
Because the manufacturing process is vital to vacuum integrity, the fabrication facility should be an air conditioned environment with low humidity. Moisture is counterproductive to creating a quality vacuum and can be a large obstacle in obtaining a long-term, reliable vacuum. Lower humidity augments the vacuum acquisition by minimizing the number of water molecules needed to be removed and reducing the bake-out time. Bake out time is important to ensure reliability and should be designed to maximize vacuum performance. A quality VJP design will integrate a molecular sieve which improves water absorption during this process.
Another component important to long term vacuum effectiveness is the inclusion of a getter. Getter selection is significant because it helps take up gas within the pipe. Accurate determination of the type, amount as well as location enhances and maintains the vacuum integrity throughout the product life.
A clean work space is another key requirement for production of a quality VJ product with a high vacuum reliability. Any contaminants such as lint, metal shavings, or dirt have the potential to destroy the vacuum. Another potential hazard is finger prints. Fingerprints leave oil, which causes outgassing during pump down and can also potentially damage the vacuum. Therefore, proper handling of the components using gloves during fabrication reduces fingerprints and helps produce a quality vacuum jacketed product.
Other aspects for further design consideration include incorporating nitrogen gas back flush, instituting quality measures for in-process inspections, and establishing final testing requirements. For example, higher quality VJP manufacturers use a high vacuum leak check test at final inspection to predict long term vacuum integrity.
Buyers be wise
Providing your company with the highest return on their investment and the best cryogenic liquid for the application begins with specifying a vacuum jacketed pipe system instead of copper insulated pipe. But outlining the specifications alone does not guarantee that you are buying a high-performance VJP system. Be sure to purchase from a reputable manufacturer. The company should, first and foremost, design their products with quality in mind. Then, fabricate and inspect their products to strict standards which substantiate the quality. By investigating your supplier, you will help ensure the most efficient liquid cryogen transfer, the least costly operation of your delivery system, and best return on your capital investment.
When selecting a vendor, be wise:
- Qualify your suppliers.
- Know your application requirements as well as the specifications for your cryogen delivery system such as bulk tank pressure, pipe size, special machinery requirements and end use point flow rate and pressure.
- Fully understand your needs and system parameters.
- Ask questions about the vacuum jacketed pipe design, fabrication process and internal quality controls and product warranty.
- Request pipe performance verification from your vendors.
Failure to investigate your vacuum jacketed pipe manufacturer may result in a less efficient operation, more liquid cryogen use, higher overall operating expenses and a longer return on your capital investment.
1Rendenbarger, P (2005). High-Efficiency Liquid Nitrogen Transfer Piping, Process Cooling & Equipment
2 Weisend, J (2016). Defining Cryogenics. Cold Facts, 32(4), 21.