Of all
the expansion devices in use, the capillary is certainly the simplest.
It consists of nothing more than a simple length of refrigeration
tubing of small diameter, which results in a very low cost. In addition,
there are no mechanical components, and no adjustment is required.
This produces excellent reliability and long term performance.
Despite
the precautions that we'll be discussing in this chapter, the many
advantages of the capillary expansion device explain its presence
in a diverse range of small capacity equipment, especially where
it is mass produced. This includes air-conditioning units, domestic
refrigerators, small heat pumps, small commercial cabinets, etc.
A)
Operation
The
object of this chapter is to highlight the precautions that
need to be taken when servicing a refrigeration system fitted
with a capillary tube. Let's start by examining such a system
with a hermetic piston compressor:
The
vapour leaving the evaporator is usually taken into the top
of the compressor (point 1). This area is therefore
fairly cold, whilst the bottom of the hermetic chamber (the
"pot") is warm. The vapour drawn in then passes over the compressor
motor to cool it. The compressor oil is located at the bottom
of the pot (point 2), and the vapour at the compressor
discharge (and therefore the bottom of the compressor) is
hot.
Sometimes a problem can be identified by
simply touching
the compressor pot, but take care you don't get burned, as zone
2 can be very hot!
The
sub-cooled liquid emerging from the condenser (point 3) then passes
through a filter or filter-drier (point 4). This filter is
essential as it prevents the most serious capillary fault:
Obstruction by any sort of contamination in the system (e.g. copper
filings, abrasive materials or brazing debris) would prevent the
passage of liquid refrigerant, and so result in the symptoms of
a lack of expansion device capacity. The expanded liquid emerging
from the capillary (point 5) then passes through the evaporator,
and the superheated vapour produced returns to the compressor.
Exercise:There is no liquid receiver shown at the condenser outlet. In
your opinion, should there be a liquid receiver in the system? Why?
Solution
to the exercise. Firstly, we must understand that
a capillary expansion device is only a short length of small
diameter tubing with an orifice that is permanently
open.
In
operation, HP is exerted at the capillary inlet, but
at the outlet, there is LP. As the capillary is permanently
open, when the compressor stops, nothing to prevents the HP
liquid from continuing to travel through the capillary (and
into the evaporator), until the LP and HP have equalised.
Therefore,
when the compressor stops, the condenser empties into the
evaporator, which fills with liquid.
If
a liquid receiver is introduced, its contents could migrate
into the evaporator when the compressor stops and fill it
completely, especially if the evaporator is 'cold'. There
would then be extensive liquid slugging when the compressor
starts up.
A
liquid receiver is therefore never installed in a system fitted
with a capillary expansion device.
Note
that the evaporator must be designed so that liquid cannot
flow under gravity towards the compressor when it stops. This
is the reason for the evaporator being supplied via the bottom
pipework in the diagrams.
B) Advantages
of pressure equalisation at compressor cut-out.
The
current flowing through a compressor is directly dependent
on the HP value (see: the influence of HP on current consumed,
Refrepair manual page 43).
We've
just seen that the LP and HP pressures equalise when the compressor
stops. When the compressor starts again, the HP doesn't rise
immediately, but slowly increases until it reaches
its normal operating value. This means that after start-up,
the current passing is at first small, and then progressively
increases as the HP increases. So the motor start-up is straightforward,
there is no excessive resistance to operation and the current
drawn by the motor is limited. Because of this ease in start-up,
the equalisation of the pressure due to the capillary when
the compressor stops allows the use of a smaller motor.
From
an economical point of view, this is obviously of great importance
when a single-phase motoris usedinmassproduction (domestic refrigerators,
comfort A/C units, etc.).
C)
Problems with refrigerant charge.
The
refrigerant charge is doubtless the most difficult problem
associated with this type of system. To help us understand
why this is so, lets examine how a small A/C system correctly
charged with R22 operates.
When
the air reaching the evaporator is hot, (say 25°C), there
is rapid evaporation of the liquid refrigerant. The last molecule
of liquid evaporates quickly (point A) and the
superheat is somewhat high (for example about 15°C). The top
of the hermetic pot is relatively hot (e.g. 35°C), and the
bottom of the compressor is very hot (e.g. 60°C).
Let's imagine, a
little later on that
the air arriving at the evaporator is at 20°C.
The
air being colder than before, evaporation is less intense,
and since the same amount of R22 is passing through
the capillary, the last molecule of liquid moves towards
the evaporator outlet (point B). The superheat
then gradually falls with the fall in air temperature until
it reaches, say, 7°C at the end of the cycle. The top of the
pot is then warm (e.g. 30°C) and the base of the compressor
remains very hot.
Let's
now imagine that the service engineer wants to add some refrigerant
to the air conditioning unit during a site visit. Since he
has no charging cylinder or balance, he decides to charge
the system slowly with vapour.
With
the external temperature at 25°C, he charges the unit until
he obtains a normal superheat (e.g.7°C). The temperature of
the pot is normal and the unit is 'producing cold'. Our engineer
therefore leaves the site expecting no problems…
However,
as the ambient temperature falls, the superheat gets smaller
If
the thermostat is set to cut out at 20°C, it is likely that
liquid slugging will occur. Note that as the superheat decreases,
the lower the pot (top and bottom) temperature becomes smaller,
compared to normal.
This,
then, is the first danger associated with an approximate charge.
The superheat depends on the temperature of the air at the
evaporator inlet.
