2016B Question 08
Describe how the vacuum-insulated oxygen evaporator works.
Examiner Report
47% of candidates achieved a pass in this question.
A description was required, aided by a diagram where necessary. The VIE is filled with cold liquid oxygen, and stored in an insulated container under pressure at a temperature above its boiling point, but below its critical temperature. In addition to the insulation, a steady demand for oxygen keeps the liquid cool, due to the effect of latent heat of vapourisation. A description of how a VIE functions with fluctuations in demand, temperature or pressure gained additional marks.
Some candidates could obtain no marks, because they only described active scavenging or fractional distillation of air. Other common errors include; misunderstanding the effect of latent heat of vapourisation, stating that the critical temperature was more negative than the boiling point, and neglecting to discuss the effect of temperature on VIE function.
Model Answer
Structure:
- Diagram
- Function
- Pros
- Cons
Diagram
Function
| Property | Description |
|---|---|
| How it works | - Storage of O2 - Critical temp -118°C - Boiling point -180°C - Produced en masse by fractional distillation of air - Transported to VIE - Stored as liquid with saturated vapour - Kept outside in gated area - If pressure too low: (e.g. Oxygen taken from pipeline supply) - If mild drop: Vapourisation of liquid O2 - If severe drop: Liquid O2 passed through pressure-raising vapouriser - Vapourisation maintains low temperature - Cold gaseous O2 passed through superheater (just coils exposed to ambient air, not actively heated) - Alarms - If pressure too high: - Safety release valve 7atm - Pressure regulator 4atm - Alarms |
| Pros | - Large capacity compared with cylinder manifold - No active cooling required, very energy efficient |
| Cons | - Expensive - Big |