What Is A Dewar Cylinder?

What is a dewar cylinder?

A dewar cylinder, also known as a dewar flask or cryogenic vessel, is a specially designed container used to store and transport extremely cold substances, such as liquefied gases or cryogenic liquids. It is named after Sir James Dewar, a Scottish chemist and physicist who invented the dewar flask in the late 19th century. These cylinders are widely used in various scientific, medical, and industrial applications where the preservation of low temperatures is crucial. In this article, we will explore the construction, working principle, uses, and safety considerations associated with dewar cylinders.

Construction of a Dewar Cylinder

A typical dewar cylinder consists of two concentric vessels made of high-quality materials that provide excellent thermal insulation. The inner vessel, also known as the inner cylinder or inner flask, is usually made of borosilicate glass or stainless steel and holds the cryogenic substance. The outer vessel, known as the outer jacket or outer cylinder, surrounds the inner vessel and acts as a protective layer.

Between the two vessels, there is a vacuum space known as the annular space. The vacuum minimizes heat transfer by conduction and convection, as there are no molecules to conduct heat or transfer it through air currents. This design allows for long-term storage of cryogenic substances without excessive evaporation or temperature fluctuations.

Working Principle of a Dewar Cylinder

The working principle of a dewar cylinder is based on the conservation of thermal energy and the prevention of heat exchange with the surroundings. The combination of the vacuum insulation and the reflective properties of the inner vessel helps to limit heat transfer.

When a cryogenic substance is stored in the inner vessel, it has an extremely low boiling point, usually below -150°C (-238°F). The pressure exerted by the evaporating substance is relatively low, enabling it to be safely stored at atmospheric pressure.

The vacuum insulation of the dewar cylinder ensures that the heat transfer mainly occurs through radiative heat loss. The silver or metallized coating on the inner vessel reflects radiant heat back, minimizing its absorption and consequently reducing the temperature rise of the stored substance. This design provides a highly efficient thermal barrier, preventing the substance from warming up quickly.

Uses of Dewar Cylinders

Dewar cylinders have a wide range of applications in various fields due to their ability to store and transport cryogenic substances efficiently. Some common uses include:

1. Scientific Research: Dewar cylinders are extensively used in laboratories for the storage and transportation of liquid nitrogen, liquid helium, and other cryogenic substances. These substances are essential for low-temperature experiments, sample preservation, and cooling sensitive materials or instruments.

2. Medical Applications: In the medical field, dewar cylinders are crucial for cryopreservation and cryosurgery. Cryopreservation involves freezing and storing biological samples, such as sperm, eggs, embryos, or tissue samples, for future use. Cryosurgery, on the other hand, uses extreme cold to destroy abnormal cells or tumors.

3. Industrial Applications: Certain industries, such as semiconductor manufacturing, require cryogenic substances for various processes. Dewar cylinders are employed to store and supply these substances for cooling, cleaning, or preserving sensitive equipment or materials.

4. Food and Beverage Industry: Dewar cylinders are also used in the food and beverage industry, especially for the storage and transportation of liquid nitrogen. Liquid nitrogen is used for flash-freezing food products, creating novelty ice creams, or producing frozen cocktails, among other culinary applications.

5. Energy Storage: Some advanced energy storage systems, such as cryogenic energy storage, utilize dewar cylinders to store liquid air or liquid nitrogen for large-scale energy storage. This technology aims to store excess electricity produced during off-peak hours and release it during periods of high demand.

Safety Considerations

While dewar cylinders are designed to efficiently store and transport cryogenic substances, there are several safety considerations that must be taken into account:

1. Cold Burns: Cryogenic substances can cause severe frostbite or cold burns upon skin contact. Proper personal protective equipment, including insulated gloves and goggles, should be worn while handling dewar cylinders.

2. Pressure Build-up: The evaporation of cryogenic substances inside the dewar cylinder results in pressure build-up. Excessive pressure can cause the cylinder to rupture, leading to potential hazards. Regular release of excess pressure or venting is necessary to ensure safe operation.

3. Asphyxiation Risk: Cryogenic substances displace oxygen and can lead to oxygen deficiency in poorly ventilated areas. Adequate ventilation must be maintained while working with dewar cylinders to prevent asphyxiation.

4. Fire Hazards: Certain cryogenic substances, such as liquid oxygen or liquid hydrogen, are highly flammable. Appropriate precautions must be taken to avoid ignition sources, and fire safety measures should be in place.

Conclusion

In conclusion, a dewar cylinder is a specialized container designed for the storage and transportation of cryogenic substances. Its construction, with an inner vessel, vacuum insulation, and an outer jacket, provides excellent thermal insulation, minimizing heat transfer. Dewar cylinders find wide-ranging applications in scientific research, medical fields, industrial processes, food and beverage industry, and energy storage. Despite their usefulness, safety precautions are necessary due to the potential hazards associated with extremely low temperatures and pressure build-up. With proper handling and awareness of safety considerations, dewar cylinders are vital tools that enable the use of cryogenic substances in various critical applications.