The Benefits of Using Liquid Dewar Cylinder

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.

The old saying goes something like:

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To know someone is to love them.

If people count on you to supply liquid nitrogen or other cryogenic liquids for storing biological samples or processes like Liquid Chromatography Mass Spectrometry (LCMS) and Molecular Beam Epitaxy (MBE), the saying can apply to your relationship with your dewars and liquid cylinders.

Because, most of the time, things are good.

But when things go bad (you unexpectedly run out of liquid, liquid stops flowing, or your floors get water damaged), the more familiar you are with the operations and maintenance of your dewars and liquid cylinders, the faster you can make things better.

This post will reacquaint you with a key piece of equipment – dewars and liquid cylinders – so that you can quickly solve problems and resolve issues that keep you away from your numerous #1 priorities.

Tomāto, Tomăto … Dewar or Liquid Cylinder

Many people use the word “dewar” to describe a “liquid cylinder”, and vice versa. There are some key differences.

What Is a Liquid Cylinder?

Liquid cylinders are pressurized containers specifically designed for cryogenic liquids. Liquid cylinders let you withdraw liquid and/or gas.

A liquid cylinder has valves for filling and dispensing the cryogenic liquid, and a pressure-control valve with a brittle rupture disk as backup protection.

What Is a Dewar?

Dewars are non-pressurized vessels, like a Thermos Bottle. They typically have a loose-fitting cap or plug that prevents air and moisture from entering while allowing excess pressure to vent.

Laboratory dewars have wide-mouthed openings and do not have lids or covers. Laboratories primarily use these small containers for temporary storage.

The remainder of this post focuses on Liquid Cylinder operations.

Know Your Gauges, Circuits, and Valves

To become familiar with the important parts of your liquid cylinders, take a look at the diagram below:

Pressure Gauge

The Pressure Gauge is probably the one you will look at first and refer to most frequently. It indicates gas pressure inside the inner tank.

Since cryogenic liquids are actually liquefied gases, the pressure within the tank will constantly increase as the laws of physics transform the cold liquid into warmer gas. This pressure will help you withdraw the liquid or gas from your cylinder.

But for most applications, the pressure inside the tank must be artificially maintained. A Pressure Building Circuit can automatically do that.

Opening the Pressure Building Valve located at the top of the tank takes liquid from a line that runs from the bottom of the inner tank and passes it through the Pressure Building Coil attached to the inside wall of the outer tank.

As the liquid passes through the Coil, it is vaporized by the heat of the outer tank. The resulting gas is fed through the Pressure Building Valve and Pressure Building Regulator into the inner tank, causing the pressure to rise.

Gas-use Valve and the Vaporizer Circuit

When the pressure has been built, you can draw gas from your cylinder by opening the Gas-use Valve. Opening this valve lets the pressure in the tank force liquid up a withdrawal line, and then down into a vaporizer coil. Once again, heat is conducted through the outer tank walls to the vaporizer.

As the liquid moves through the coil, it is vaporized by this heat. The resulting warm gas flows up through the Gas-use Valve out to the user system to complete the Vaporizer Circuit.

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Generally, a single-stage regulator is attached directly to the Gas-use Valve to reduce the supply pressure to match your application’s requirements.

Economizer Circuit

If you don’t use the cylinder for several days, pressure will continue to rise at a rate of 30 psi per day because a small amount of heat will leak into the inner tank.

This heat vaporizes a small amount of liquid and causes the pressure to slowly rise. The pressure may build up to the design of your Pressure Control Valve. The valve will then open and vent gas to the atmosphere.

To minimize losses from this venting, the cylinders have an Economizer Circuit. The Economizer Circuit comes into action when the pressure reaches 140 psi.

At this point, the regulator allows gas from the top of the tank to flow through the internal vaporizer out of the Gas-use Valve to your target system. This reduces pressure in the inner tank and minimizes losses from venting.

When pressure normalizes, the Economizer Regulator closes and the cylinder then delivers gas by drawing liquid through the Vaporizer Circuit. The Economizer Regulator should have a set pressure of 15 psi higher than the Pressure Building Regulator.

Pressure Control Valve and Rupture and Burst Discs

The Pressure Control Valve is mounted on the same stem as the Pressure Gauge. Often set to open at 230 psi, the Pressure Control Valve works in conjunction with a Rupture Disc in the inner tank.

As a secondary relief device, there is also a Burst Disc on the outer tank to protect the space between the inner and outer tanks from high pressure.

Liquid-use Valve

To withdraw liquid from your cylinder, first close the Pressure Building and Gas-use Valves. Then open the Liquid-use Valve to allow head pressure in the tank head to force liquid up to the withdrawal tube and out the Liquid-use Valve.

Liquid withdrawal should be done at low pressure to prevent flash losses. During the transfer, if the pressure in the tank is higher than the normal liquid withdrawal pressure, open the Vent Valve to lower the pressure. Before withdrawing liquid, liquid is typically withdrawn at less than 15 psi.

When filling an open container, if a greater liquid withdrawal pressure or rate is required, a qualified service agent can adjust the Pressure Building Regulator.

Don’t Get Caught Dry: Liquid Contents Gauge

A Liquid Contents Gauge is in the center of the tank. This can be a float-type gauge that provides an approximate indication of the tank’s contents.

If you want more accurate measurements, try a gauge that uses differential pressure to determine liquid levels. These modern devices also contain graphical digital displays to give you precise measurements. They also often have the intelligence to eliminate the need for lookup charts.

In addition, many of these digital liquid content gauges have telemetry capabilities to make it easier to monitor the levels of key cylinders.

Watch Out for Frost and Water

Because the Pressure Building Vaporizer contains cold liquid, it cools the outer tank. It’s perfectly normal for frost to form on the outside of the cylinder. During a prolonged high draw, the gas-withdrawal temperature falls considerably, and the outside of the cylinder will be very heavily frosted.

This frost eventually turns to water that can damage flooring or seep into your facility’s interstitial space to create more damage to other systems.

A Drip Tray can save you a lot of headaches and hassles. Place your liquid cylinder and/or its vaporizer on a pan or tray to catch the water as the frost evaporates. The higher the tray’s lip, the less worry you’ll have about water damage.

So What?

The more you know about your dewars and liquid cylinders, the less you have to worry about them.

If you think something has gone wrong, just remember:

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• The Pressure Gauge indicates the pressure inside the inner tank. Opening the Pressure Building Valve increases tank pressure to normal operating levels.
• The Gas-use Valve allows gas to flow from the tank.
• The Economizer Circuit minimizes product loss.
• To draw liquid, close the Gas-use and Pressure Building Valves, and open the Liquid-use Valve.
• If you need to know exactly how much liquid is in your cylinder, use a Digital Gauge.
• Frost and ice are nothing to be scared about. But use a drip tray or pan to avoid water damage.