Dewars are used for sample storage, transport of cryogens from source-to-point of use, and temporary benchtop storage of cryogens for immersion of samples and flasks.
Under no circumstances should cryogens be stored or transported in a container that is not designed specifically for cryogenic storage or transport. Insulated beverage containers, glass labware, and other non-approved vessel are not designed to vent properly or to withstand cryogenic temperatures and must not be used.
Cryogenic liquids such as liquid nitrogen and argon are dispensed from pressurized dewars that vary in capacity between 60 and 265 liters. These large, pressurized dewars can be used for dispensing liquid cryogens, but can also be used when high volumes of gas are required for lab processes.
The equipment described below is a common type of liquid nitrogen dewar used on Penn’s campus. For information about pressurized dewars for other cryogenic liquids, contact EHRS.
Liquid nitrogen service is provided to the labs in one of the two ways described below.
Industry-standard pressure settings for liquid nitrogen dewars classify them as Low Pressure, Medium Pressure, or High Pressure depending on the operating pressure of the dewar. The pressure rating for the safety relief vent are generally set by the manufacturer as follows:
Low Pressure 22 psig Medium Pressure 230 psig High Pressure 350 psigLow-pressure dewars are used for liquid withdrawal. Medium and high-pressure dewars are used for gas withdrawal. Dispensing liquid from a medium or high-pressure dewar is not recommended as it may cause splashing and/or transfer-line whip and increases the risk of exposure and cryogenic burns. Only low-pressure dewars may be used for dispensing liquid nitrogen.
Low-pressure dewars are used for supplying cryogenic liquid only. Pressure-relief valve setting: 22 psi
In a low-pressure liquid nitrogen dewar, the pressure inside the vessel is maintained through the normal heat-leak of the cylinder. As the liquid warms, it evaporates and expands to almost 700 times its liquid volume, which increases the pressure inside the vessel. When the pressure reaches 22 psi, it will automatically vent through the pressure relief valve. If the relief valve fails, the emergency rupture disk will vent the excess pressure so that the cylinder does not fail.
Medium and high-pressure dewars are used for supplying gas. Pressure-relief valve setting: 230 psi and 350 psi
Medium and high-pressure dewars are used for delivery of gas. The two types of dewars function very similarly but differ in the amount of liquid cryogen they can contain. The gas delivery pressure is not the same as the vessel’s operating pressure. Delivery pressure must be controlled using a separate regulator connected to the gas-use fitting.
Medium and high-pressure tanks increase internal pressure by means of a vaporizer coil in vacuum insulated outer jacket of the dewar. When the pressure build valve is opened, liquid enters the coil and is warmed, evaporating and expanding to gas.
When the internal pressure of the cylinder reaches the pressure-relief set point (230 psi for medium-pressure, 350 psi for high-pressure), the relief valve will open and vent gas to reduce the pressure. If the relief valve fails, the emergency rupture disk will vent the excess pressure so that the cylinder does not fail.
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Consult the equipment manufacturer’s operating manual for the recommended pressure settings and flow rates for your equipment. If flow rates are too high, the gas from the dewar will be very cold and may result in excessive ice accumulation on the dewar’s controls and valves and damage to the attached equipment.
Medical grade CO2 has 99.99% purity rating. Both Beverage and Food grade CO2 both have a 99.95% purity rating. Industrial CO2 has a 99.90% purity rating. The other .09 – 1% is made up of impurities such as hydrocarbons or nitrogen. The slight difference between Industrial-grade CO2 and Food grade CO2 is the type of test that are done to qualify CO2 as Beverage gas compared to Industrial-grade. Food grade CO2 is in fact Beverage grade CO2, they are equivalent. Its all about the purity of the gas. All CO2 is made the same (to the best of our knowledge). Its all about how the CO2 is stored, handled and transferred. The FDA has put compliance laws on Food aka Beverage grade CO2, making distributers store the gas in specific tanks just for Food or Beverage grade. Food grade CO2 is pure enough for human consumption based on FDA compliance. Food-Grade, anything (in the US) means special requirements on transport and handling, which implies more expense. So even though the same plant may produce it, the pipes leaving the plant have to be Food grade, the tanks the product goes into have to be Food grade. The non-food grade version will go through less expensive /less-maintained/less-cleaned pipes and into similarly treated containers. Medical grade CO2 is not reasonable to get for consumption purposes.
When talking about cylinders we must first consider high pressure vs. low pressure. HPA (High Pressure Air) tanks are pressurized up to the tank’s rating of psi or psi (Pounds per Square Inch). The pressure is then regulated through the tank’s regulator down to 850psi (High Output) or 450psi (Low Output). High pressure vessels are generally referred to as tanks, whereas Low pressure containers are generally referred to as dewars. Many people use the word “dewar” to describe a “liquid cylinder”, and vice versa. There are some key differences and they come in a wide variety of options.
While dewars might seem complicated at first, in reality they’re safe and easy to use once you understand the basics. If you’re interested in talking with an expert about how liquid CO2 trimming and sifting can radically improve efficiencies at your operation, contact us today to talk to one of our post-harvest processing experts.
When utilizing The Original Resinator in a commercial application, CO2 Cylinders similar to the Liquid Cylinders pictured above are very popular. This is do to the nature of large volume production needs, cost savings and convenience. Typically there are three valve handles, a pressure gauge, and a CO2 level indicator located on the top of the Liquid CO2 Tank. These valves handles should be labeled as Gas/Vent, Liquid, and Pressure Builder. We only use the Liquid supply to operate The Original Resinator. We will cover each of these below.
Before we dive into the tank valves we will briefly review the two types of outlets found on Liquid Cylinders, the CGA-622 and the CGA-320 outlets. Tanks outfitted with a CGA-622 or a female end, will need a CO2 liquid cylinder adapter, an essential component for hooking up your Resinator to a low-pressure liquid cylinder. If your cylinder has a CGA-322 outlet then your Resinator coil assembly included with your machine purchase is all you will need.
The top of a liquid cylinder tank – a standard CO2 source for Resinator machines – can be a confusing sight at first. Each outlet and valve serve a particular purpose, so each are important to understand. Since each outlet and valve should be clearly labeled, let’s zero in on the most critical one for a Resinator machine: the Liquid outlet. This is the source of our liquid supply of CO2. This outlet is the one you’ll use for all Resinator machines, but it’s important to know that you may need a particular CO2 tank adapter to hook up to the outlet properly. (Adapter not provided with Resinator purchase.) Until recently, Dewar/Liquid Cylinder tanks came standard with liquid CO2 outlets called a CGA-320, allowing you to then attach a hose between the outlet and Resinator machine. That’s changed. Liquid cylinder tanks are now coming standard with a female CGA-622 outlet, and therefore require a new CO2 tank withdrawal adapter, the CGA-622 x 320, to fit properly to a hose. If your liquid cylinder has a CGA-622 this Liquid Dewar adapter is now a necessary part of a secure setup so that your machine functions properly. It’s a minor change, but it’s important to make sure that you’re using the correct fitting when dealing with liquids and gases at such pressures.
*CGA-622 X 320 Liquid Cylinder withdrawal adaptor.
(Pictured below and available through our online store.)
Once you have identified the Liquid valve and are using withdrawal adapter (if necessary), it’s time to attach the hose. The CO2 supply hose is threaded onto the valve outlet, being sure to use the appropriate sealing washer, and tightening to mitigate any leaks. Being that the Resinator is not a closed loop system, a few CO2 leaks are nominal. Although leaks will contribute to raised CO2 levels, which should always be monitored. When ready to operate the Resinator, open the Liquid valve fully by turning counter clockwise. The pressure builder is a very important component in achieving the proper Resinator operating pressure. We can monitor this operating pressure by observing the pressure gauge located on the top of the Liquid CO2 Tank. Ideally the operating pressure of the Liquid CO2 Tank is 330-350 psi. This is achieved by opening the pressure builder valve prior to operating the Resinator.
Tell help you see the forest from the trees, take a look a the diagram below to become familiar with the important parts of your liquid cylinders. Be sure to get request
If you don’t use the a cylinder for several days, pressure will continue to rise at a rate of apox. 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 psi setting. 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 the 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 deliveries gas by drawing liquid through the Vaporizer Circuit. The Economizer Regulator should have a set pressure 15 psi higher than the Pressure Building Regulator.
The Pressure Gauge is probably the one you will look at first and refer to most frequently. This gauge indicates gas pressure inside the inner tank. Since cryogenic liquids are actually liquefied gases, pressure within the tank will constantly increase as the laws of physics transform the cold liquid into warmer gas. Fortunately, 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 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. We recommend operating your cylinder at 350psi to avoid any clogging/freezing issues while under operation with your Resinator.
If you want to learn more, please visit our website Liquid Dewar Cylinder.