MACS Monthly Newsletter - 2020 - SEP2
Dianetti/Milliman
The TXV is installed at the inlet of the evaporator in the
mobile air conditioning system. As the air passes through
the evaporator, heat is absorbed by the refrigerant flowing through the coil, which cools the air temperature and
moisture condenses on the cold outer surface to dehumidify the air. As the refrigerant absorbs heat from the
air passing through the evaporator coils, it changes state
from a liquid to a vapor as it "boils" in the evaporator. In
order to reach maximum evaporator efficiency, the refrigerant is boiling in the range of 32°F to 38°F. The ideal state
of the evaporator is to contain the maximum amount of
liquid refrigerant for heat exchange without allowing
any liquid from exiting the evaporator. The TXV is critical in performing this function.
There are two basic functions of the TXV. The first is to
control refrigerant mass flow to the evaporator to maximize heat transfer and efficiency. Secondly, the TXV controls the amount of superheated refrigerant vapor exiting
the evaporator. Superheat is defined as the temperature
of the refrigerant above its saturated vapor temperature.
At any given refrigerant pressure, there is an associated
refrigerant saturation temperature. The liquid refrigerant
flows through the evaporator as it removes heat from the
air passing through the coil. When the liquid refrigerant changes state entirely from a liquid to a vapor, it has
reached the saturated vapor point.
If the refrigerant is at its saturated vapor point, the superheat level is zero degrees. In other words, when the re-
Dianetti/Milliman
Figure 1: Cross sectional view of a basic block type thermal expansion valve. The valve operation will be discussed
later.
Figure 2: In this example, the refrigerant reaches the saturated vapor point (0°F superheat) before the exit of the evaporator at 40 psig and
45°F (see saturation table in upper right corner). At the exit of the evaporator (as the refrigerant enters the suction line through the TXV), the
pressure is now measured at 39 psig with a temperature reading of 53°F. If we look up the saturation temperature in the refrigerant table for
39 psig, it equals 44°F. Therefore, superheat level as the refrigerant exits the evaporator is 53°F (measured temperature) minus 44°F (saturation temperature at measured pressure of 39 psig). If we subtract 44°F from 53°F, this equals 9°F of superheat (the refrigerant is 9°F above the
saturated temperature).
September 2020
2
MACS Service Reports
MACS Monthly Newsletter - 2020
Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2020
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