What is air conditioner
expansion valve?
The expansion device is the
fourth major component in air
conditioner units. It ’s also known
as meter devices.
Air conditioner expansion valve
is the divided point between the
low side and the high side of the
air conditioner units. Another
dividing point is air conditioner
compressors.
The meter device is located
indoor (air handler) units with
the evaporator coils. It ’s small
and hard to see, unless you open
the evaporator compartment.
The meter devices process is
showed between points 5 and 6
in PH charts.
Types of air conditioner
expansion valve
Have a few types of ac
expansion valve in air
conditioner units:
Thermostatic expansion valve
1. Capillary tubes
2. Automatic expansion valve
I’ll discuss the operating
principles of two types of air
conditioner expansion devices,
the thermal expansion valve and
the capillary tube.
Thermal Expansion Valve
The thermostatic expansion
valve (TEV or TXV) is used for
refrigerant flow control and
operates at varying pressures
resulting from varying
temperatures. This valve
maintains constant superheat in
the evaporator.
Thermal expansion valve has a
sensing bulb, which is connected
to TXV by a length of capillary
tubing. The capillary tube
transmits sensing bulb pressures
to the top of the TXV valve ’s
diaphragm.
Capillary tube metering device
A capillary tube is a refrigerant
control; its common types of air
conditioner expansion valve. The
capillary is simply a length of
tubing with a small inside
diameter which acts as a
constant throttle on the
refrigerant entering the
evaporator.
A fine filter or filter drier installed
at the inlet of the capillary
prevents dirt from blocking the
tube.
A recent development in the
design of capillary tubes for air
conditioning system uses
capillary tubes with a larger
insider diameter and a longer
tube length. A larger diameter
tubes are less likely to become
plugged with dirt and other
impurities than a smaller
diameter tube.
The long length provides the
necessary resistance to create
the desired pressure difference
across the metering device.
The capillary tube equalizes the
pressure in the system when the
unit stops. This pressure
equalizing characteristic of the
capillary allows a low starting
torque motor to be used with
the compressor.
Typically, a capillary does not
operate as efficiently over a
wide range of conditions as
does the thermostatic expansion
valve. However, because of its
counterbalance factors in most
applications, its performance is
generally very good.
Refrigeration systems using a
capillary tube doesn ’t require the
use of a liquid receiver since all
the liquid is stored in the
evaporator during the off cycle.
However, a suction accumulator
is often found in the suction line
to prevent any non vaporized
refrigerant from reaching the
compressor.
This will prevents damage to the
compressor when excessive
liquid refrigerant enters the
evaporator on a low evaporator
load condition.
How does capillary tube works?
The capillary tube can be
described as a fixed length of
small bore tubing connecting the
high pressure side (condenser)
of a refrigeration system to the
low pressure side (evaporator).
Capillary tube works by
restricting and metering the
liquid flow, the capillary tube can
maintain the required pressure
differential between the
condenser and the evaporator.
Because of friction and
acceleration, the pressure drops
as the liquid flows through the
tube.
In order to reduce the
temperature of the liquid to the
saturation temperature of the
evaporator, some of the liquid
must turn into a vapor in the
capillary tube, “flash”, just as it
does with all refrigerant controls.
*Notes: All air conditioner
expansion valve works in similar
fashion. It shapes, size, capacity,
and manufacture are different,
but it operation principle are
alike.
System Design Factors
The capillary tube diameter and
length must be such that the
flow capacity at the design
pressures (condensing and
evaporating) equals the
compressor pumping capacity at
these same conditions.
For example, if the tube diameter
is too small (resistance to high)
the liquid refrigerant flow will be
less than the pumping capacity
of the compressor with the
evaporator being “starved” and
the suction pressure being low.
Less liquid will enters the
evaporator and the excess will
build up in the condenser
reducing the effective
condensing surface and
increasing the condensing
temperature and pressure. This
pressure change tends to
increase the flow in the tube and
at the same time reduces
compression capacity.
The system will now balance at
different from design capacity
with a reduction in compressor
and system capacity.
If the capillary tube resistance to
the refrigerant flow is too low,
(diameter of tube too large) the
flow rate will be greater than
pumping capacity. This results in
the flooding or overfeeding of
the evaporator and flood back of
liquid to the compressor.
The refrigerant system uses a
capillary tube selected for
capacity balanced conditions. A
liquid seal is present at the
capillary inlet but not excess
liquid in the condenser. The
compressor discharge and
suction pressures are normal
and the evaporator is properly
charged.
How does air conditioner
expansion valve works?
AC expansion valves work by
controlling the amount of
refrigerant flows to the
evaporator coils. It acts as
restriction to provide a specific
amount of refrigerant flows into
the evaporator coils.
This is the basic principle behind
any metering valves.
There are different types of
expansion valves used in air
conditioner units, but the
function of the metering device
used in any central air
conditioner units are two fold:
First: It controls the amount of
liquid refrigerant entering the
evaporator coils. The amount of
liquid refrigerant entering the
evaporator must equal the
amount of refrigerant boils in
the evaporator coils.
Second: It maintains a pressure
difference between the high and
low pressure sides of the system
to permit the refrigeration to
vaporize.
The pressure difference allows
the ac Freon to vaporize at low
pressure and temperature in the
evaporator; while at the same
moment, the refrigerant in the
air conditioner condenser
condenses at a high pressure,
high temperature in the
condenser units.
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