Understanding Dewars & Micro Bulk Tanks
One of the offerings in our arsenal of gas products is a liquid cylinder. Don’t let the name fool you, liquid cylinders provide either gas or liquid product depending on the facility’s need. People refer to liquid cylinders as dewars, liquid cans, or by an individual manufacturer’s designation. On the gas side, any facility using 5 or more large high pressure cylinders per week will realize the benefits. By converting to a liquid cylinder, facilities see less cylinder handling, smaller footprint, and or a product cost savings
Liquid Cylinders vs. High Pressure Gas Cylinders
Liquid cylinders do not function in the same way high pressure gas cylinders do. For current users or those considering them, the following discusses how they operate.
Liquid cylinders work like thermos bottles to hold gases in their cryogenic state. The cryogenic state can be as low as -400 degrees Fahrenheit (liquid phase). These specially designed cylinders can only maintain the temperature of its contents for a limited time. As the temperature inside the cylinder eventually climbs above the threshold necessary to keep the gas a liquid, the liquid product will convert back to gas. The gas will then expand to approximately one hundred times the volume of the liquid. As with all gas storage cylinders, liquid cylinders have safety relief devices. The safety relief devices on liquid cylinders are at 22, 230, 350psi, or in limited cases some pressure in between.
Liquid to gas transference generally occurs at a constant rate. If the product drawn out of the cylinder does not equal this conversion rate, the pressure in the tank rises. The pressure rises until the relief device opens. The relief valve will remain open until pressure in the cylinder drops back below the relief devices setting. For this reason, these cylinders operate most efficiently when the product demand placed on them is steady, constant, and in excess of the Natural Evaporation Rate of the cylinder. NER = rate that the liquid in a cylinder converts back to gas as a result of heat migration.
Liquid Cylinder Design
Liquid cylinders, like thermos bottles, have two containers. One is inside the other with a small vacuum and insulated annular space in between. Manufactures build liquid cylinders to contain hundreds of pounds of liquid product. Keeping that in mind, they are considerably more sturdy than your typical thermos bottle. There is also transfer tubing in the annular space to serve specific product demand delivery functions.
There are four basic circuit designs on cylinders that dispense both gas or liquid.
Liquid Withdrawal Circuit:
The simplest circuit is the liquid withdrawal circuit, which provides liquid through a tube at the bottom of the inner tank and connects directly to the liquid valve for use. Pressure forces liquid from this circuit up and out of the valve. Maintaining the pressure at no more than 22 psi is optimal.
Gas Withdrawal Circuit:
Liquid from the bottom of the inner tank converts back into gas as it travels through a tube, which coils multiple times around and attaches to the inside of the outer tank. Attaching the gas withdrawal tube to the inside of the outer tank provides sufficient heat transfer to convert the liquid product back into a gas. Internal tank pressure is what drives the liquid through the circuit.
Pressure building Circuit:
As with the Gas withdrawal circuit – tubing connects to the bottom of the inner tank and coils multiple times around. Gas attaches to the inside of the outer tank, where it converts back into gas and routed to the top of the inner tank. Any time gas is withdrawn from the liquid tank at a rate beyond its natural capacity to convert liquid to gas, the pressure builder circuit can be opened to increase the liquid to gas conversion process. The pressure building regulator and valve controls this.
An economizer regulator is used to evacuate gas out of the top of the tank to the gas withdrawal circuit once the pressure exceeds the setting of the regulator. This circuit only works when the gas withdrawal valve is open, and the customer is withdrawing gas.
Contents gauge / Pressure gauge
The normal contents gauge on a liquid tank is imprecise and should only be used as a rough indicator of a tanks liquid volume. The pressure gauge is not an indication of a tanks product volume either, but rather an indication of the tanks internal pressure.
Where are liquid tanks used? The gas or liquid can be used for countless application. Upon request a General Air representative will conduct an on-site training and safety program. In addition, if at any time you have questions regarding the operation of one of our liquid cylinders, call or email our closest store. You will be in touch with someone who can help you.
Liquid cylinders function best and can provide maximum benefits when the delivery demand placed on them is constant and steady.