Vacuum Flask: Overview

Diagram

Vacuum Flask Diagram

Synonyms

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

Overview

Dewar Flasks & VacuumsVacuum 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 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 heat gain occurs by conductance through the neck of the inner wall and by the electrical wiring or mechanical feed throughs entering the vacuum flask. 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. Typical application durations are 4-24 hours. Unlimited durations can be achieved by actively cooling the flask and removing the small amount of heat that does migrate into the flask.

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

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

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:

  • Less expensive
  • Wider selection, more design options
  • Shorter duration and less expensive project development, quicker to market
  • Easier and less expensive upgrade and evolution paths because of lower cost, quicker development, more design options and newer technology

Thermal Insulator

The thermal insulator's function is to thermally seal the opening of the vacuum flask, displace air to avoid thermal transfer by convection, provide a feed through mechanism 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 ratio of specific heat per volume (J/cm3°K).

In some applications, it is beneficial for the heat sink material to have low a thermal conductivity property because it reduces 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 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 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 ratio of specific heat per volume than conventional heat sink materials.

Shapes and Sizes

Vacuum Flask Shapes

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

Vacuum Flask Openings

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.

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 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 insufficiently specific for design purposes. The application must be defined early in the flask design stage to select the appropriate materials and 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.

Gamma Ray, Low Attenuation

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.

Gamma Ray, High Attenuation

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 strategically 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.