Air conditioning basics – Fieldpiece HG2 - HVAC Guide System Analyzer User Manual

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Air Conditioning Basics

e Evaporator, Condenser, Restrictor (Th

rottling

valve) and Compressor are the four basic components
of an air conditioner. Following one pound of refriger-
ant through the system shows the function of each com-
ponent.

Subcooled liquid refrigerant at high pressure enters

the restrictor and is throttled to saturated refrigerant at
a lower pressure. Th

e restrictor can be of either a fi xed or

TXV/EXV type. Th

e fi xed type must be charged to a tar-

get superheat that varies with indoor and outdoor con-
ditions. Th

e evaporator capacity varies with the indoor

heat load on a fi xed restrictor. Th

e TXV/EXV regulates

the size of the restriction to maintain a constant super-
heat. Th

is essentially adjusts the capacity of the evapora-

tor responding to the indoor heat load. TXV/EXV sys-
tems must be charged to subcooling.

Aft er the restrictor, refrigerant enters the evaporator

at a low temperature and pressure and boils (evaporates)
into a gas by absorbing heat from the indoor air. Th

e re-

frigerant stays at the same temperature and pressure un-
til all the refrigerant evaporates into a gas. Aft er the re-
frigerant becomes a gas, it will become superheated and
the temperature will change. Th

e superheat measure-

ment is the best indication of refrigerant charge level in
a fi xed restrictor system and a TXV/EXV system will
keep the superheat constant. Th

ere must be superheat

present to ensure liquid does not fl ood the compressor.
Superheat measurements are taken on the suction line
between the evaporator and compressor.

e compressor takes this low temperature, low

pressure, slightly superheated refrigerant and compress-
es it into a much higher temperature and pressure.

e highly superheated gas enters the condenser

and rejects heat into the outside air. Th

e refrigerant

condenses back into a liquid. Once all of the gas is con-
densed into a liquid then additional removal of heat
causes a temperature drop known as subcooling. TXV/
EXV systems are charged to subcooling since superheat

is controlled by the throttle valve. Subcooling measure-
ments are taken on the liquid line between the condens-
er and TXV/EXV. Th

en the subcooled liquid enters the

restrictor and the cycle starts again.

Figure 33. The Refrigeration Diamond.