Liquid Oxygen Saturation Principles - Puritan Bennett Tyco HELIOS Reservoir Technical Manual

HELiOS Liquid Oxygen System Technical Manual
1.4

LIQUID OXYGEN SATURATION PRINCIPLES

Oxygen, in its normal state, is a colorless, tasteless, and odorless gas that is non-flam-
mable, although it greatly accelerates combustion in high concentrations. It constitutes
about 21% of the Earth's atmosphere by volume. Oxygen in higher concentrations is
medically beneficial to patients suffering from certain respiratory diseases.
Oxygen, like most gases, will condense into a liquid with an increase in pressure or
decrease in temperature. As a liquid, oxygen is pale blue in color and is about 860 times as
dense as its gaseous form. At atmospheric pressure (14.7 psia), oxygen condenses into its
liquid form at a temperature of about -297°F (-184°C). Liquid oxygen (LOX) is an efficient
form of oxygen to meet a patient's portable, ambulatory oxygen needs. A volume of liquid
oxygen, when vaporized, yields about 860 volumes of gaseous oxygen (Figure 1-5). As
you can see, a relatively small volume of liquid oxygen provides a much larger volume of
gaseous oxygen for a patient to use.
In medical liquid oxygen systems, liquid oxygen, and the gaseous oxygen resulting from
its vaporization or boiling, is stored under pressure. The elevated pressure, typically 22
psig (152 kPa), enables oxygen to flow to the patient at a selected, prescribed rate. To
sustain this oxygen flow to the patient, the liquid oxygen must be in a state that allows
vaporization to readily occur. In other words, the liquid oxygen must be in a state of
saturation. Let's take a look at what liquid saturation is all about.
A saturated liquid is one that absorbs the maximum amount of heat possible at a given
pressure without vaporizing into a gas. If additional heat is added, the saturated liquid
begins to vaporize (boil) while remaining at a constant temperature until all of the liquid is
vaporized. A common example of a saturated liquid is water at its boiling point of 212°F
(100°C) at sea level. The constant addition of heat to the boiling water does not cause it to
become hotter, but instead causes part of the liquid water to turn to water vapor
(Figure 1-7).
Figure 1-7: Saturated (Boiling) Water at Sea Level
The saturation (boiling) point of a liquid depends not only on temperature but also on
pressure. If the pressure in a container of saturated liquid increases, the temperature
required for saturation to occur will also increase. This leaves the liquid unsaturated, that
is, capable of accepting more heat before it will boil (Figure 1-8).
B-701693-00 Rev. A Introduction to the HELiOS System - 1-7