MACS Monthly Newsletter - 2020 - SEP3

Dianetti/Milliman

frigerant is at its saturated vapor point, there is no liquid
remaining. The temperature of the vapor above its saturation temperature is "superheated vapor". If the vapor
is superheated, there is no liquid refrigerant.
Figure 2 illustrates how to determine the level of superheat exiting the evaporator. The example is an R-134a system. A portion of the R-134a refrigerant table is included.
In summary, the TXV responds to the refrigerant pressure and temperature exiting the evaporator to control
the desired level of superheat. It is important to understand that the TXV does not control pressure or temperature. It uses the pressure and temperature of the refrigerant at the outlet of the evaporator to control superheat.
Controlling a reasonable level of superheat exiting the
evaporator is critical to system performance. The desired
level of superheat is determined by the air conditioning
system designer. Controlling superheated vapor at the
outlet of the evaporator is also important to protect the
air conditioning compressor from receiving any liquid
refrigerant at the inlet. "Compressor slugging" must be
avoided as this can cause severe damage to the compressor.
In addition to controlling the desired superheat level, it
is important to control refrigerant mass flow in the system to maintain refrigerant oil flow to keep the compressor lubricated. This delicate balance between refrigerant
mass flow and superheat level requires precise flow control which is achieved by the TXV.

Figure 3: The oriﬁce tube contains a short tube restrictor
which is used to separate the high side from the low side of
the system and regulate refrigerant ﬂow. This device also
contains an inlet screen to protect the short tube restrictor
from contaminants in the system which could clog the tube
preventing refrigerant ﬂow.
conditions, the evaporator flooding is increased causing evaporator pressure and discharge air temperatures to increase along with an overall decrease in vehicle comfort. Since the TXV has the ability to control
a superheated refrigerant condition, performance and
vehicle comfort is maintained.

Mobile Air Conditioning Summary

Role of the Orifice Tube

Another common device used in mobile air conditioning systems on earlier systems is the Orifice Tube
pictured in Figure 3.
Since the orifice tube is a
"fixed" size, it does not have the
ability to sense the condition of
the refrigerant exiting the evaporator. Refrigerant mass flow is
controlled by the size of the orifice tube and the system pressure
drop across this device. The orifice tube system was commonly
called a "flooded evaporator system" which required a refrigerant accumulator/drier at the exit
of the evaporator to protect the
compressor since liquid refrigerant could exit the evaporator.
The Orifice Tube was introduced many years ago in mobile
air conditioning systems as an alternative to the TXV. This type of
system performed well at highway speeds. The orifice tube system does have its shortcomings
during city driving and vehicle Figure 4: This pressure / enthalpy diagram illustrates the refrigeration cycle of a mobile air conidling conditions. During these ditioning system

September 2020

MACS Service Reports

Dianetti/Milliman

The mobile air conditioning system can be summarized by illustrating the refrigeration cycle on the
pressure/enthalpy diagram shown in Figure 4.

MACS Monthly Newsletter - 2020

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2020