Image Credit: Terra Universal, Inc.
Cryostats and dewars are used to store or maintain materials at very cold (cryogenic) temperatures. Often suppliers distinguish cryostats as vessels designed to control and maintain low temperatures by incorporating refrigeration systems; dewars are considered to be simply cold storage devices. However, the two terms are sometimes used interchangeably, and can include all types of storage vessels with high thermal insulation. These devices are important for storing cryogenic solids or liquids and maintaining the temperature of materials (hot or cold).
Heat capacity (C) defines the relationship between temperature change (dT) and internal energy change (dU) for a given substance by the following equation.
C = dU / dT
A material's heat capacity is nearly constant at most temperatures. At cryogenic temperatures, however, the heat capacity of a material drops rapidly as it approaches absolute zero (0 Kelvin). This principle accounts for the ease of cooling in dewars and cryostats at low temperatures, but also means that small heat leaks can cause large temperature changes in these vessels.
Cryostat and Dewar Design: Mitigating Heat Transfer
Cryostat and dewar vessels are designed to drastically reduce heat transfer between the inside of the container and the environment. Heat transfer can take three forms: conduction, convection, and radiation.
- Conduction is heat transfer through physical contact between two surfaces. This is reduced in dewars and cryostats by utilizing a double wall to separate physical contact between the inside and outside of the container.
- Convection is heat transfer through the movement of fluids (gases or liquids). This is reduced in dewars and cryostats by evacuating the space between the double wall. This creates a vacuum in which there are few air molecules to move heat between layers.
- Radiation is heat transfer from electromagnetic waves emitted from an object due to its temperature. This is reduced in dewars and cryostats by lining the walls of the flask with a thin layer of reflective material such as silver; the more reflective the material, the less thermal radiation it will absorb.
In addition, the tops are sealed with a sample stick, screw top, or cork depending on the design application. Vessels with cooling systems commonly incorporate a cryogenic fluid bath such as liquid helium to continuously cool the material and maintain the low temperature.
Selecting Cryostats and Dewars
A number of factors should be considered when selecting cryostats and dewars, including the type of refrigeration incorporated in the design, performance specifications, and physical parameters.
Types of Cooling Systems
There are four main types of cooling systems that cryostats and dewars can incorporate: closed-cycle, continuous-flow, bath, and multi-stage.
- Closed-cycle products pump cryogenic vapor though a chamber. They use a mechanical refrigerator that consumes a relatively significant amount of power, but they do not need to be refilled with vapor - making them suitable for continuous-duty applications.
- Continuous-flow cryostats are also cooled by liquid helium which is continuously replenished by a steady flow of cryogenic liquid. They do not require electrical power but expend large quantities of liquid when operating.
- Bath cryostats are similar to vacuum flasks, but are filled with liquid helium that boils off. The boiled helium vapor effectively cools thermals shield around the bath.
An example of a complex bath cooling cryostat design is shown in the figure below:
Image Credit: Scientific Products
- Multi-stage cryostats have a coldplate along with additional cooling stages to achieve temperatures lower than liquid helium.
Dewars and cryostats can be performance rated based on their temperature range. This range specifies the temperatures the vessel was designed to maintain effectively.
Other specifications may depend on the type of system. For vacuum designs with no additional cooling mechanisms, performance is described by:
- Material loss - amount of cryogenic liquid material evaporated from the vessel, commonly measured in liters per day (L/day).
- Static holding time - time vessel can hold a cryogenic liquid before requiring a bleed of some of the evaporated material, measured in days.
For designs with additional cooling or heating mechanisms, performance is described by:
- Cooling power - power required to heat or cool the vessel by a certain temperature increment, simplified to watts per Kelvin (W/K).
- Power consumption - amount of power consumption of system compressors and other equipment, measured in kilowatts (kW).
- Temperature stability - amount of temperature fluctuation (K, °F) during cooling or heating operation.
Dewars and cryostats can be defined based on their physical specifications. These include:
- Capacity - the amount of material the vessel can store.
- Neck inner diameter (I.D.) - the inner diameter of the vessel's neck or opening.
- Height - the height dimension of the vessel.
Some specific uses of cryostats and dewars include applications such as infrared detector assemblies, thermal imaging systems, and optoelectronic instrumentation. In MRI machines, cryostats maintain the magnet's superconductivity. In histology, they are used to cut or slice tissue under cryogenic conditions.