Basic Intro to Liquid Cylinders - Earlbeck Gases & Technologies

Throughout history, humans have used containers to store, transport, and dispense items. Regardless of the era, it is generally the case that what’s inside a container is more important than the container itself. However, when it comes to cryogens, the considerations and specifications behind their storage cylinders give these containers special notoriety.

In order to properly choose and use liquid cylinders, it is important to understand their make up as well as any considerations for their care and storage.


What are Liquid Cylinders and What are They Used For?

A liquid cylinder, sometimes called a “liquid flask” and most commonly known as a “dewar” is a double-walled, vacuum-installed container used for storing cryogens. A dewar acts like a thermos in that it keeps atmospheric gases well below ambient temperatures (sometimes as low as -400 degrees F) so that they are stored in their liquid states. 

Commonly stored cryogens include liquid nitrogen, liquid argon, liquid oxygen, and liquid helium. Depending on the design and features, dewars can dispense the elements as gas, liquid, or both. Although dewars used for temporary storage may be made out of foam insulation, most are made from metals such as aluminum or steel.

Cryogens themselves are used in many applications, such storing food, cooling superconductors, and freezing lab samples. They are also used in MRI machine functioning, producing cryogenic fields for rockets, and performing cryosurgeries. 

The benefit of dewars is that just one of them can function like many gas cylinders. This superpower is achieved by the dewar’s ability to house pressure far larger than its natural productive capacity. Because their functionalities can be equivalent to 20 high pressure cylinders, it is easy to surmise that even a single dewar can save precious storage space while significantly cutting back on costs.
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Anatomy of a Liquid Cylinder

The construction of a liquid cylinder can be simple or complex depending on its purpose. 
Although common working pressures include 22, 180, and 230psi, dewars can be custom designed to accommodate other pressures. Although a cylinder is the most-common shape, dewars can also be crafted into other dimensions. Additionally, they can be outfitted with handles, legs, various drain locations, outlets, and caster mountings – whatever is needed to help the dewar fulfill its intended use.

Regardless of the shape they ultimately take, most dewars share the same, internal anatomy. The most visible part of a dewar is the outer vessel, which is the metal surface that makes up the dewar’s shape and protects the inner vessel where the liquid tube and vent tube are often housed.

The combination of the outer and inner vessels allows the dewar to function. Opening a pressure building valve at the top of the tank draws liquid through the liquid tube and through a pressure building coil. As the liquid travels through the tube, it transforms into its gaseous state by the warmer temperature outside of the inner vessel. This process, sometimes known as the Vaporizer Circuit, increases pressure within the canister that can be harnessed to dispense the gas.

Many cylinders have an Economizer Circuit which prevents gas losses from venting by storing extra pressure that may build up when the cylinder remains unused for a time.  When the internal tank pressure reaches a specific psi, a regulator allows gas to flow through an internal vaporizer in order to reduce inner-tank pressure. When pressure normalizes, the regulator closes and the cylinder functions normally.

At the top of most dewars is a system of gauges and valves responsible for monitoring and releasing the gas. As its name suggests, the pressure gauge indicates the pressure inside the inner vessel. Typically attached to the pressure gauge is a pressure control valve.
The gas-use valve and liquid-use valve draw gas or liquid forms out of the dewar for use.  The pressure building regulator controls the rate of pressure build while a liquid contents gauge measures how much liquid is left in the tank.


Care and Considerations

It is beyond question that caring for cryogens and their storage containers requires careful consideration. 

First, it is important to have a working knowledge of the different types of gases and what each might do under certain conditions. For example, since frost often form outside of dewars due to prolonged usage or a pressure valve being left open, it is advisable to keep a drip tray or pan under the dewar to avoid water damage. 

Second, it is important to know and follow each cylinder’s specifications and instructions exactly. Cryogens should NEVER be stored in containers that are not specifically designed for them because each cylinder has different venting and temperature regulation abilities.  This logic also applies to any hoses, valves, regulators, gas detectors, or other containers that may be involved in manipulating the cryogens. While there may be room for error in storing/transporting some substances, cryogens are not among them.

Third, it is important to know how to protect the person who will interact with the cryogens and liquid cylinders. Safety equipment, including glasses, gloves, face shields, closed toed shoes, lab coats or other protective clothing, etc. should be used at all times. Whenever possible, the person working with the dewars should receive any training or instruction that is available on use and safety/emergency practices. Please consult with your gas supplier for the latest training on safe handling procedures.
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Final Thoughts

When proper care is given to operating liquid cylinders, their economic, industrial, and scientific benefits can be enjoyed.  While it may require extra effort, keeping educated about the anatomy of the cylinders, the nature of the atmospheric gases that they house, and current safety practices will ensure that dewars and their contents achieve their purposes.

Vacuum insulated container

A cryogenic storage dewar (or simply dewar) is a specialised type of vacuum flask used for storing cryogens (such as liquid nitrogen or liquid helium), whose boiling points are much lower than room temperature. It is named after inventor James Dewar, who developed it for his own work. They are commonly used in low-temperature physics and chemistry.

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Cryogenic storage dewars can range widely in size and may take several different forms, including open buckets, flasks with loose-fitting stoppers, and self-pressurising tanks. All dewars have walls constructed from two or more layers, with a high vacuum maintained between the layers. This provides very good thermal insulation between the interior and exterior of the dewar, which reduces the rate at which the contents boil away. Precautions are taken in the design of dewars to safely manage the gas which is released as the liquid slowly boils.

The simplest dewars allow the gas to escape either through an open top or past a loose-fitting stopper. More sophisticated dewars trap the gas above the liquid, and hold it at high pressure. This increases the boiling point of the liquid, allowing it to be stored for extended periods. Excessive vapour pressure is released automatically through safety valves. Dewars are also designed to be resistant to any sort of puncture to preserve the contents, as cryogens are costly to produce, and some (like helium) are in limited global supply.

The method of decanting liquid from a dewar depends upon its design. Simple dewars may be tilted, to pour liquid from the neck. Self-pressurising designs use the pressure of the gas in the top of the dewar to force the liquid upward through a pipe leading to the neck.

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Selected expansion ratios Cryogenic
fluid Expansion
ratio[1][2] nitrogen (LN2) 1:696 helium (LHe) 1:757 argon (LAr) 1:847 hydrogen (LH2) 1:851 oxygen (LO2) 1:860 neon (LNe) 1:

Cryogens present several safety hazards, and their storage vessels are designed to reduce the associated risk.

Firstly, no dewar can provide perfect thermal insulation and the cryogenic liquid slowly boils away, which yields an enormous quantity of gas. This is known as the liquid nitrogen evaporation rate. In dewars with an open top, the gas simply escapes into the surrounding area. However, very high pressures can build up inside sealed dewars, and precautions are taken to minimise the risk of explosion. One or more pressure-relief valves allow gas to vent away from the dewar whenever the pressure becomes excessive. In an incident in at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were sealed with brass plugs. As a result, the tank failed catastrophically and exploded.[3]

Secondly, if a dewar is left open to the air for extended periods, atmospheric chemicals can condense or freeze on contact with the cryogenic material. This can introduce contaminants. If these materials freeze, for example, water vapor becoming ice, they can block the openings, leading to pressure buildup and the risk of an explosion.

Contact us to discuss your requirements of Liquid Dewar Cylinder. Our experienced sales team can help you identify the options that best suit your needs.