10
IOM / @DNOVA_THS-0508
1. Introduction :
The simultaneous presence of liquid and vapour makes it necessary for both to be in a state of saturation [ Gibbs
law ], as shown in fig. 1). In conditions of thermal equilibrium, the pressure in the tank corresponds to the T of the
surrounding environment; a withdrawal of refrigerant charge will cause pressure drops, which will be associated
with
- withdrawal of refrigerant charge
pressure drop in tank
- pressure drop in tank
T drop change of state
- T drop change of state
evaporation of part of the liquid, causing the liquid itself to
cool
- cooling of liquid
thermal exchange with ambient air, further evaporation of
remaining liquid; the original pressure is restored in the tank
after a certain amount of time
2. Vacuum and charging machine
3. Vacuum cycle
In general it is preferable to apply a “long” rather than “hard” vacuum: reaching low pressures
too abruptly may in
fact cause any trapped humidity to evaporate instantaneously, thereby freezing part of it.
The figure represents a vacuum cycle and an optimal subsequent pressure rise for the refrigeration devices we
manufacture.
As a rule, if there is suspicion of an extensive presence of humidity throughout the circuit or system as a whole,
the vacuum must be “broken” with anhydrous nitrogen and then the steps must be repeated as described; this
operation facilitates the removal of trapped and/or frozen humidity during the evacuation process.
P
Heat content h
T tank/ambient
Saturated gas
Saturated
liquid
Fig. 1
P
[Pa]
Time
200 s
6
150
Fig. 3