As we stated in the last issue, the first danger
is associated with an approximate charge. The superheat depends
on the temperature of the air at the evaporator inlet.
To enable us to better understand the next problem,
let's examine the arrangement below ;
Water is ejected
into the open air (at atmospheric pressure) through a capillary.
If
the water is at a pressure of 3 bars, an amount M1 of
water is ejected in a given time (DP between the terminals
of the capillary is then 3 bars).
If
the water supply pressure is reduced to 1 bar (DP across the
capillary is then only 1 bar), it is easy to see that the
amount M2 of water being ejected in a given time becomes
smaller.
So,
when DP across the capillary terminals falls, the mass flow of water
being injected also falls.
Naturally, the
same effect is produced if R22 is used instead of water.
Therefore, the
liquid mass flow through the capillary is larger if the pressure
difference between HP and LP increases.
In practice,
the more the HP increases, the larger the increase in the
amount of refrigerant being injected into the evaporator will
be.
This
effect has consequences for the operation of the system.
To
understand what these are, let's examine the diagram opposite.
Let's
assume that with an ambient temperature of 20°C the HP is
14.3 bars (+40°C) and the LP is 4.1 bars (+1°C).
This
mean that DP between the terminals of the capillary is 10.2
bars. At this point the compressor suction side temperature
is 8°C, which gives us a superheat of 8 - 1 = 7°C.
Whatever
the cause (fouled condenser, increase in air temperature at
the condenser inlet etc.), let's now imagine that the HP increases
to, say, 18.5 bar. Since the HP increases, the compressor
draws in less vapour (see: Effect of pressure on mass flow,
Refrepair manual page 41) andthe LP also increases
to, say, 4.6 bar.
DP
across the capillary rises from 10.2 bar to 13.9 bar.
This causes a noticeable increase in the amount of liquid
injected into the evaporator. The last droplet of liquid to
evaporate therefore moves closer to the compressor, and the
superheat measured at the compressor inlet falls (in the example
it's only 6 - 4 = 2°C).
This,
then, is the second danger associated with an approximate charge.
The superheat depends on the HP value.
When
a capillary expansion valve is fitted to an air-conditioning unit,
another problem can prevent normal operation being achieved. The
problem concerns the rotation speed of the fan (A/C units are usually
fitted with multi-speed motors). Remember that with this equipment,
high speed is used in summer to prevent blowing air that is too
cold into a room. Low speed is usually used for heating in winter
for warmer air to be blown out.
Imagine
that the air-conditioning unit is set to maintain an ambient
temperature of 20°C. Since the apparatus is running at high
speed, a large volume of air is flowing over the evaporator.
Everything is operating satisfactorily, and the superheat
at the end of a cycle is quite normal.
If
the client now switches the equipment to low speed (often
because he finds the high-speed operation too noisy) the flow
of air over the evaporator decreases. Since there is less
air passing over the evaporator, the liquid refrigerant evaporates
more slowly and it slowly 'creeps' towards the compressor.
The superheat then decreases dangerously.
There
is then a good chance that the compressor will experience
destructive liquid hammer.
Indeed,
most of the causes of lack of evaporator capacity studied
can cause dangerous liquid slugging in this type of installation,
whether in air conditioning or commercial refrigeration applications.
This is because there is no thermostatic expansion valve controlling
the amount of liquid being injected (see:Lack of
evaporator capacity - some practical aspects, Refrepair manual
page 114).
D) When is a system fully charged?
The
best way of charging a system equipped with a capillary expansion
device is doubtless to charge it with the precise mass of
refrigerant specified by the manufacturer.
This
requires that all the refrigerant remaining in the equipment to
be recovered. Then a vacuum must be pulled on the system, and the
precise quantity of refrigerant indicated on the identification
plate should be introduced from a charging cylinder or using a balance
of adequate precision.
On
site, an engineer confronted with a lack of refrigerant generally
doesn't have immediate access to a charging cylinder or balance.
On occasion, the identification plate is missing or illegible. If
it is impossible to take the equipment to the workshop for the work
to be made on site, priority must be given to repairing any leaks.
When
charging is performed, refrigerant must be carefully introduced
(preferably in the vapour phase), whilst simultaneously measuring
the changes in the superheat at the compressor inlet. With commercial
refrigeration equipment it will often be possible to visually observe
frosting gradually moving along the evaporator as the air temperature
falls.
Depending
on the operating conditions when the refrigerant is being charged
into the system, an engineer needs to remember that the superheat
can fall dangerously if:
The ambient
temperature falls (it is generally higher than normal when
charging takes place).
The HP
increases (an artificial increase in HP can be produced, for
example, by partially blocking the condenser with cardboard to
check if the superheat stays at an acceptable level).
The evaporator
cooling capacity falls (this is normal in air conditioning
if a filter is blocked). In commercial refrigeration, the evaporator
is normally frosted.
Note
that too large a refrigerant charge will result in operation with
the suction side superheat being too small. Excess refrigerant can
then cause destructive liquid slugging which may damage the compressor.
On
the other hand, too small a charge will cause a system to operate
with much too high a suction side superheat. The pot of the hermetic
compressor will then be poorly cooled and the compressor motor will
overheat. Then we must hope that the internal safety devices will
operate properly, otherwise the motor is very likely to die an early
death!
Finally,
let's remember that installations equipped with a capillary expansion
device have a reduced refrigerant charge because the equilibration
of pressures when the compressor stops means that the evaporator
could become completely full.
Only
experience will allow an engineer to estimate the precise moment
when the refrigerant charge is correct. If there is any doubt at
all, it is always advisable to return later to the site to make
any necessary adjustments to the charge.
E) Problems encountered when replacing the capillary.
Unfortunately,
capillaries can sometimes become partially or completely blocked.
This usually happens after a compressor burnout, or after
sub-standard work has been carried out on the system.
If
the capillary is blocked, there will be very little liquid
in the evaporator. The cooling capacity will therefore be
reduced, and the superheat will be high. This will cause an
abnormal temperature rise in the hermetic compressor body.
Warning:
a lack of refrigerant charge produces exactly the same symptoms.
However, if there is a lack of refrigerant in the system, there
will also be a shortage of refrigerant in the condenser, and the
sub-cooling will be small. On the other hand, if the capillary
is blocked, the refrigerant missing from the evaporator will be
in the condenser, and the sub-cooling will be quite normal.
Another
extremely valuable index can be used to diagnose a blocked capillary
with certainty. If the compressor stops, the pressures will quickly
equalise. If the capillary is blocked, this pressure
equalisation
will not occur. If there is a partial obstruction, then the worse
the blockage, the longer this equalisation takes to occur.
Be
careful not to confuse a blocked capillary with a lack of refrigerant.
