Vacuum Flask: Overview

Diagram

Synonyms

Dewar, Dewar Flask, Vacuum Bottle, Heat Shield, ThermoFlask, ThermoHousing

Overview

Vacuum flasks are thermally insulated double wall vessels. The narrow space between the two walls is evacuated which dramatically reduces thermal transfer. The near absence of air molecules minimizes thermal transfer by convection. Reflective surfaces in the vacuum space minimize radiant thermal transfer and the separation between the inner and outer wall minimizes thermal transfer by conductance. In summary, a vacuum flask thermally decouples a flask's payload from the external environment. The most common application of vacuum flasks is the storage or transmission of cryogenic fluids such as liquid nitrogen or liquid natural gas which can be maintained at extremely very low temperatures for extended durations. National K Works specializes in designing and manufacturing vacuum flasks exclusively for thermally insulating instruments from hostile temperature environments.

Vacuum flasks provide very effective thermal insulation. The insulation is passive and the largest portion of thermal transfer into the vacuum flask occurs by conductance through the neck of the inner wall and by the electrical wiring or mechanical feed through entering the vacuum flask (thermal leakage). The small amount of conductance eventually raises the inner temperature within the vacuum flask which limits the payload's duration within hostile high temperature environments to a finite duration.

An additional form of heat gain in the vacuum flask is the heat generated by the electronics within the vacuum flask. Since the vacuum flask thermally decouples the contents of the flask from the external environment, the heat dissipated by the electronics can not escape from the vacuum flask and consequently increases the inner temperature within the vacuum flask which also limits the payload's duration within hostile high temperature environments.

The duration can be extended by increasing the specific heat (thermal mass) within the vacuum flask in the form of separate heat sink material or by increasing the payload assembly's total specific heat (changing materials and/or quantity of material). Typical application durations are 1 to 24 hours. The duration is a function of the ratio of heat gain (by thermal leakage plus heat dissipated from the electronics) to specific heat (thermal mass) within the vacuum flask. In other words, the duration can be extended by increasing the amount of specific heat within the flask. Unlimited durations can be achieved by actively cooling the flask and removing the small amount of thermal leakage into the flask plus the heat dissipated by the electronics.

External flask temperature applications can exceed 1,000C (1,832F).

Common payloads protected within a flask are electronic assemblies, detectors, sensors, batteries, motors, cameras, explosives and temperature sensitive chemicals.

High temperature vacuum flask applications include insulating instruments within industrial processing ovens (glass, metal, electronics, paint, cement), food processing, pharmaceutical sterilization, oil and gas wells, geothermal wells, and power generation (nuclear reactors, boilers, steam lines).

Flask vs. high temperature electronics

For cost and time savings, many of our customers prefer to use standard temperature rated electronics housed within a vacuum flask instead of sourcing or developing special high temperature electronics without a vacuum flask. Benefits of standard temperature electronics over special high temperature electronics are:

• Standard temperature electronics are drastically less expensive.
• Standard temperature electronics offer electrical engineers a wider selection of components permitting more design options and access to the newest technology.
• Standard temperature electronics decrease project development time which allow projects to come to market sooner (more revenue) and consequently decrease engineering costs.
• Standard temperature electronics allow less expensive future upgrades and easier future evolution paths.

Thermal Insulator

The thermal insulator's function is to thermally seal the opening of the vacuum flask, displace air to reduce thermal transfer by convection, provide a feed through passage for inputs into the flask, mechanically secure the payload within the flask and provide a method to extract the payload from the flask.

The insulator's material should have low thermal conductivity properties, sufficient mechanical properties to support the payload's weight within the flask and sufficient temperature rating to withstand the external environment's temperature. Common materials are Teflon, PEEK and Ceramic.

Heat Sink

The heat sink's function is to reduce the rate of temperature rise within the flask by absorbing and storing the thermal energy leaking into the flask and the thermal energy dissipated by the payload within the flask. Increasing the quantity of heat sink within the flask; increases the duration.

The heat sink's material should have a high volumetric heat capacity (J/cm3°K). In other words, the material should have high specific heat (J/K) for its volume (cm3).

In some applications, it is beneficial for the heat sink material to have low a thermal conductivity property because it reduces the rate of thermal transfer within the flask environment while in other applications, it is beneficial for the heat sink material to have a high thermal conductivity property such as when a localized component is dissipating heat within the flask. High thermal conductivity allows heat to be rapidly pulled away from the heat dissipating component and stored within the heatsink. In this case, the high thermal conductivity property of the heat sink prevents concentrated "hot spots" within the flask's environment.

Common heat sink materials are aluminum, stainless, brass, copper, Teflon, PEEK. Phase change alloys are also used because the latent heat provides significant thermal absorption but these alloys have a lower volumetric heat capacity than conventional heat sink materials.

Ranges of Application

Shapes are limited by the practicality of manufacturing. Round is the least expensive shape because round tubes are commercially available and if not available are readily manufactured to necessary sizes.

Sizes are limitless and lengths can exceed 9 meters. Long lengths require supports to centralize the inner tube within the outer tube.

Openings on Both Ends

There is the option to have openings on both ends of the flask. The through passage opening incorporates an expansion joint to compensate for the differential in thermal expansion between the outer and inner walls while maintaining the vacuum integrity.

Vacuum Flask Materials

Thermal conductivity

The inner tube is a path for thermal conductance along its length consequently the inner tube's material should have low thermal conductance.

• Stainless, nickel alloys and titanium have low thermal conductivity and are the most common materials.
• Aluminum and copper have high thermal conductivity and are poor material choices for a vacuum flask.

Non-magnetic

Some instruments such as magnetometers are sensitive to interference from magnetic materials. The term "non-magnetic" is not specific enough and too general for design purposes. The application must be defined early in the flask design stage to select the appropriate vacuum flask materials and magnetic testing specifications to match the application. Various applications for non-magnetic materials are:

• Absolute magnetic measurement
• Relative magnetic measurement (quantifying magnetic change in an environment)
• Electromagnetic induction triggers

Magnetic Shielding

Some photomultiplier applications are sensitive to magnetic fields and mu-metal is used in the flask to improve performance by optimizing magnetic shielding while minimizing overall size..

Low Attenuation for Gamma Rays

Some detector applications are sensitive to the flask materials' gamma ray attenuation properties. Materials with lower gamma ray attenuation properties improve the detector's response by affording increased count levels and count qualities. Titanium has a low atomic number which reduces its gamma ray attenuation and consequently improves detector responses. In this case, the entire flask must be titanium material because of the difficulties of joining titanium to other materials such as stainless.

High Attenuation for Gamma Rays

Tungsten has a high atomic number and very high gamma ray attenuation properties. It is used to selectively shield detectors from gamma rays and also collimate gamma rays to the detectors. Tungsten is built directly into the flask to minimize the overall size, maximize the quantity of shielding and optimize the source to detector spacing.

Optical window

We design and manufacture flasks with transparent windows for cameras and optical sensors in high temperature environments. Applications include high temperature protection for video cameras, proximity sensors, bar code readers and lasers.