Page 468 - Mechanical Engineers' Handbook (Volume 4)

7 System Components 457

is increased, the superheat leaving the evaporator will increase. This increase in flow will
increase the temperature and pressure (P ) of the charge within the bulb and capillary tube.
1
Within the top of the TXV is a diaphragm. With an increase in pressure of the thermostatic
charge, a greater force is exerted on the diaphragm, which forces the valve port to open and
allow more refrigerant into the evaporator. The larger refrigerant flow reduces the evaporator
superheat back to the desired level.
The capacity of TXVs is determined on the basis of opening superheat values from 2
to 4 C. TXV capacities are published for a range in evaporator temperatures and valve
pressure drops. TXV ratings are based on liquid only entering the valve. The presence of
flash gas will reduce the capacity substantially.

Electronic Expansion Valve
The electronic expansion valve (EEV) has become popular in recent years on larger or more
expensive systems where its cost can be justified. EEVs can be heat-motor activated, mag-
1
netically modulated, pulse-width modulated, and step-motor driven. EEVs can be used with
digital control systems to provide control of the refrigeration system based on input variables
from throughout the system.

Constant-Pressure Expansion Valve
A constant-pressure expansion valve controls the mass flow of the refrigerant entering the
evaporator by maintaining a constant pressure in the evaporator. Its primary use is for ap-
plications where the refrigerant load is relatively constant. It is usually not applied where
the refrigeration load may vary widely. Under these conditions, this expansion valve will
provide too little flow to the evaporator at high loads and too much flow at low loads.

Capillary Tube
Capillary tubes are used extensively in household refrigerators, freezers, and small air con-
ditioners. The capillary tube consists of one or more small-diameter tubes, which connect
the high-pressure liquid line from the condenser to the inlet of the evaporator. Capillary tubes
range in length from 1 to 6 m and diameters from 0.5 to 2 mm. 17
After entering a capillary tube, the refrigerant remains a liquid for some length of the
tube (Fig. 19). While a liquid, the pressure drops, but the temperature remains relatively
constant (from point 1 to 2 in Fig. 19). At point 2, the refrigerant enters into the saturation
region where a portion of the refrigerant begins to flash to vapor. The phase change accel-
erates the refrigerant and the pressure drops more rapidly. Because the mixture is saturated,
its temperature drops with the pressure from 2 to 3. In many applications, the flow through
a capillary tube is choked, which means that the mass flow through the tube is independent
of downstream pressure. 17
Because there are no moving parts to a capillary tube, it is not capable of making direct
adjustments to variations in suction pressure or load. Thus, the capillary tube does not pro-
vide as good as performance as TXVs when applied in systems that will experience a wide
range in loads.
Even though the capillary tube is insensitive to changes in evaporator pressure, its flow
rate will adjust to changes in the amount of refrigerant subcooling and condenser pressure.
If the load in the condenser suddenly changes so that subcooled conditions are no longer
maintained at the capillary-tube inlet, the flow rate through the capillary tube will decrease.
The decreased flow will produce an increase in condenser pressure and subcooling.
The size of the compressor, evaporator, and condenser as well as the application (re-
frigerator or air conditioner) must all be considered when specifying the length and diameter
of capillary tubes. Systems using capillary tubes tend to be much more sensitive to the