Design and Application of Cryogenic Valves

by Louis J. Arcuri, Valcor Scientific (CSA CSM), director of sales and marketing,

Intersection of Cryogenics and Everyday Life

Food Industry–Our most frequent encounter with the byproduct of cryogenic gas usage is in our food chain, where much of the frozen food we buy is flash-frozen in cryogenic freezers that use liquid carbon dioxide (LCO2) or liquid nitrogen (LN2). Vegetables, poultry, fish and beef are all flash-frozen in immersion cryogenic freezers, and commercially baked goods are cooled much more quickly and readied for packaging using LN2 vapor. Frozen ice cream novelty treats are flash-frozen in an LN2 bath and beverages like beer, soda and sparking drinks are often carbonated with LCO2 from bulk storage systems.

Cryogenic solutions are popular because they’re effective, relatively inexpensive and don’t affect the color, taste or flavor of the foods and beverages with which they come in contact. With the advent of microbulk systems, cryogenic cooling is available to more food and beverage processors than ever before, especially small customers who could not otherwise afford the large bulk tanks often seen outside of large-scale producers.

Environmental Test Chambers–These use solenoid valves to control the flow of LN2, enabling testing at temperatures to -320°F. Environmental chambers can simulate the cold of deep space or arctic ambient conditions and are used for testing all manner of components, devices and systems.

Surgery–Combinations of cryogenic gases are used for cryosurgery. LN2 is directly applied to many skin maladies for freezing and removal, such as warts, moles and skin tags. Liquid argon (LAR) is often used to create a freezing point at the tip of a probe. LAR treatments include freezing of tumors and cancerous tissue. Cryoablation treats arterial fibrillation in cardiac patients. Cryofreezing minimizes damage to the surrounding healthy tissue.

Cleaning–LCO2 is frequently used for cleaning and deburring. CO2 snow cleans surfaces quickly and efficiently, leaving no residue. CO2 cleaning is frequently used in optics manufacturing, painted surface preparation and some semiconductor devices. Dry ice, or frozen CO2, is used as an abrasive to deburr parts that are too delicate for traditional glass beading or sandblasting techniques.

Flavors and Fragrances–CO2 is also a great solvent, often used in a supercritical state to extract the essential oils that are associated with flavors and fragrances. It is used to decaffeinate coffee and tea, and sometimes even to dry-clean fabrics.

Show Biz–Finally, we all enjoy the special effects created by LCO2 fogging machines. Whether at a rock concert or on a theater stage, the low-hanging CO2 fog creates atmospherics that enhance our entertainment experience. New CO2 jets provide plumes of smoke reaching high into the air, providing a backdrop for dramatic LED and laser lighting effects.

Durability, dependability and long life are expected in all such applications, so manufacturers of cryogenic instruments should use the best construction methods available to handle the demanding requirements of controlling cryogenic media under pressure.

Cryogenic valve design can be divided into two basic categories: process control and instrumentation. Process control applications are often associated with line sizes of ½” and larger, with relatively low cycle life and operating pressures of 300 PSIG or less. Included here are manually operated and actuated designs of varying types where opening and closing speeds (response time) are generally not important. Line sizes below ½” are usually used in instrumentation applications and are typified by higher cycle life and operating pressures. Solenoid-operated, these are compact, low power and fast.

Let’s focus on the importance of these smaller instrumentation valves for a moment, imagining, for example, the disruption, delay and cost added to the test of a mission-critical aerospace component resulting from failure of a solenoid valve in an environmental test chamber. Or the disappointment of fans if the special effects fogging cannons fail to punctuate the lead guitar licks at a rock concert.

A direct-acting solenoid valve designed specifically for cryogenic media provides a dependable and cost-effective solution for the control of cryogenic media, especially in today’s market, where budgets are tight and applications demand 100 percent uptime.

Many instrument grade solenoid valves sold for cryogenic service are modified from general-purpose industrial designs, and most incorporate an elastomeric pressure boundary seal and poppet seal disk. For LCO2 applications, this construction will usually work for a while, but LCO2 is by nature a great solvent and will hasten the breakdown of elastomer seals, leading to internal or external leakage. This construction is not at all suitable for LN2 applications where the elastomers will quickly harden and shrink on contact, no longer capable of sealing at all.

The best construction for cryogenic valve applications uses non-elastomeric seals. Seals of polytetrafluoroethylene provide a better seal overall as they are immune to the solvent-extraction properties of LCO2, are capable of remaining resilient at LN2 temperatures and have no cure date.

A properly designed poppet seal is not flat, but tapered to ensure bubble-tight shutoff at cryogenic temperatures. It should provide years of trouble-free service and over a million cycles of life. Body materials available include brass and stainless steel. A brass body is generally sufficient for most applications. It should be machined from a forging for strength and to prevent porosity in castings. In some cleaning applications, a stainless steel body is preferred to brass, providing the most inert, corrosion-resistant flow path for the cryogen.

The solenoid coil should be rated for continuous duty service, even at cryogenic temperatures. A coil draws more current as it cools, in turn creating heat within the coil windings. The enamel insulation will break down if the heat exceeds the magnet wire insulation temperature rating, leading to coil failure. Look for an H-class coil temperature rating to ensure continuous duty service. To enhance long service life, especially at high frequency cycling, the plunger should incorporate one or more rider rings. These rings surround the plunger body, preventing contact with the core tube, eliminating friction between the two components that create wear and blocking associated particles from entering the flow stream.

Finally, a direct-acting design will provide the fastest cycle time, enabling fine temperature control in smaller environmental chamber applications. Direct-acting valves also work at low pressure differentials, enabling tight shutoff even in dead-end flow conditions.