VIE - Vacuum insulated evaporator ALL YOU NEED TO KNOW

Vacuum insulated evaporator

vacuum insulated evaporator (VIE)

The vacuum insulated evaporator (VIE) is  liquid oxygen storage tank  which has  vacuum insulated

wall built to keep the temperature of oxygen below −160°C.  Wall of tank is made up of two layers . Outer layer is built with carbon steel shell and inner wall is made up of stainless steel shell .  it may be installed on weighing tripod to measure the amount of oxygen in it.

Uses

VIEs provide the piped oxygen supply in most hospitals.

How it works

General principles

Liquid oxygen is produced by fractional distillation of air than it  is delivered to the hospital  and stored in the VIE. Critical temperature of OXYGEN IS −119°C if temperature of oxygen is increased that will result in conversion of liquid form to gas to prevent this  VIE keep temperature of oxygen between -160 °C to -180°C .  

The VIE does not require electricity to work . it depends upon insulation and evaporation to maintain low temperature . Insulation is provided by the vacuum wall, which

reduces  conduction and convection of heat into the chamber. The small amount of heat which

does enter the VIE causes some of the liquid oxygen to evaporate. Evaporation uses energy in the

form of heat and therefore the temperature of vie remains low .

Low and high use situations

The pressure in the VIE is approximately 700 kPa (7 Bar at -160°C )

1st scenario : if  all the taps of oxygen is closed , it will result in slow evaporation of oxygen and increase in temperature of tank .( IT WILL EXPLODE IF NOT PREVENTED )

To prevent an explosion in this situation, a

pressure relief valve RELEASES  unused oxygen into the atmosphere.

2ND SCENARIO : IF DEMAND IS VERY HIGH  amount of vaporization will be large as a result oxygen temp. will fall  resulting  in the reduction of vapor pressure and therefore reduced supply.

 In this

scenario  a valve is electronically opened, allowing liquid oxygen to enter an evaporator

coil exposed to ambient temperature. This pipe is also known as a super heater.

(super heater use  heat from surrounding only .) 

Oxygen leaving the VIE is very cold and have more pressure then pipeline . to  increase the temperature  and reduce the pressure of oxygen it is passed through another super heater and a pressure regulator which brings it to normal temperature and pressure 400 kpa (4bar).

  

Vacuum insulated evaporator

Measuring the contents
The amount of oxygen  can be calculated from its mass. this is done by weighing it using

a tripod weighing scale –  The VIE’s empty (tare) weight is subtracted from the measured weight of oxygen filled VIE 

Alternatively, the oxygen contents may be calculated from
the difference between the vapour pressure at the top of
the VIE and the pressure at the bottom of the liquid oxygen.
Using these pressures, it is possible to calculate the height of
the fluid column and, by knowing the VIE’s cross-sectional
area, the volume of liquid oxygen remaining.

Advantages

 Storing liquid oxygen is highly efficient in terms of space. It expands to 860 times its volume

as it vaporizes to 20°C.

 The VIE does not require power to store oxygen in a liquid state.

 IT REDUCES THE COST OF  both to deliver and to store as a liquid OXYGEN .

Disadvantages

 Initial equipment costs are much higher 

 A backup cylinder manifold and/or second VIE is required in case of interruption to the

oxygen supply.

 IF OXYGEN IS NOT USED REGULARLY  a significant amount of oxygen will be RELEASED IN ATMOSPHERE TO MAINTAIN THE TEMP. 

Safety

 The VIE must be kept outside the building because of the fire risk.

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