The Filter/Dryer is often fitted with a
"sight glass" through which the a/c technician can determine much about
the amount of refrigerant contained within the a/c system (the
"refrigerant charge"). In fact, one of the important aspects of good a/c
system service is to insure that this level of charge is correct for
optimum a/c unit operation. Although only an approximation, a good "rule
of thumb" to determine if an air conditioner is properly charged, is to
add refrigerant to the system just until there are no more bubbles
visible in the sight glass. (A more comprehensive method is presented in
Appendix I.)
The reason a bubble-free liquid refrigerant emerging from the
Condenser is a good indicator of proper refrigerant charge level is that
just enough refrigerant has been added to be sure that at least some
liquid refrigerant exists in the last stages of the Condenser. If there
is not enough liquid in the system (bubbles still visible) then there
can be no possibility of subcooling all of the refrigerant; this denies
the Condenser the opportunity to deliver its maximum performance. That
is, a deficient charge will enable some of the refrigerant to literally
'slip-through' the Condenser. Low refrigerant charge will also cause the
Evaporator to operate at lower pressure and temperature than optimum and
therefore also decrease system performance.
On the other hand, excess charge levels cause increased refrigerant
pressure loss through the Condenser because liquids are generally more
resistive to flow than the vapor which is much lighter. This pressure
loss must be made up by the Compressor because it has to deliver a
higher discharge pressure ("head pressure") for the refrigerant to flow
through the loop. This, of course, increases the amount of energy
required to operate the air conditioner and, therefore, decreases its
efficiency. [Most air conditioning systems operate with about 15 Fº
subcooling.]
