CAIRE HELiOS Plus Technical & Service Manual page 10

V
Theory of Operation
Standby
When the H300 or H850 contains liquid oxygen, the vent valve
is closed, and there is no flow demand, the pressure in the system
will remain at or near its primary relief valve (PRV) pressure.
As with all vacuum-insulated cryogenic containers, the liquid
oxygen in the H300 & H850 is always evaporating into a gas.
The rate of generation of this gas (called head gas) is called the
normal evaporation rate (NER). When the flow control knob
is in the off position, this gas will build up pressure. When the
pressure reaches the PRV setting, this gas is released through the
primary relief valve into the atmosphere. Figure 6 gives and in-
ternal view of PRV when there is no oxygen flow demand.
Oxygen Withdrawal
Figure 6: Standby (At Relief Valve Pressure)
When the flow selection knob is at any setting other than off and
the pressure in the inner vessel is between the economizer valve
pressure and the PRV pressure, the economizer valve is open.
When the economizer valve is open, gaseous oxygen is forced
from the head space in the inner vessel, through the economizer
valve, and to the conserver. This process "economizes" and
conserves liquid oxygen by withdrawing the head gas first, rather
than allowing it to escape through the relief valve. Figure 7 is a
diagram of the economizer valve when it is open and head gas is
being delivered to the patient.
Whenever head gas is removed from the space above the liquid
oxygen, the inner vessel internal pressure begins to slightly de-
crease. When the pressure drops to the economizer valve setting,
the economizer valve closes. This causes liquid oxygen to flow
up the withdrawal tube and through the warming coils, where
it expands and warms into a gas. Figure 8 is a diagram of the
economizer valve when it is closed and liquid withdrawal is the
source of oxygen to the patient.
If the pressure in the container increases over the economizer
valve setting, the economizer valve opens again and the cycle
repeats. This maintains the correct gaseous oxygen flow to the
patient at all times.
To Flow Control Valve
(No Flow)
Figure 7: R/E Valve Head Gas Flow (Approaching
Figure 8: R/E Valve Liquid Withdrawl (At Economizer
Economizer Pressure)
Pressure)
HELiOS Portables Technical Service Manual • PN 20562190 Rev E
To Flow Control
Valve (Total Flow From
Headspace Gas)
9