evaporater leak

[jebar]

hi every body I want to know procedure for servicing aleak in the evaporater of air cooled water chiller.please,

[cleanNcold]

Depends on the style of evaporator and manufacturer. I would contact the manufacturer to get their recommended procedure.

[nike123]

If it is brazed plate type, change it, since leak is probably caused by evaporator freezing and it is damaged at many places.


http://rapidshare.com/files/120596572/Alfa_Laval_Technical_Manual_4thEd.pdf

6. Leakage in a BPHE.
6.1. Causes of leaks.
♦ Corrosion. This can occur on the entire surface, but
tends to predominate on the lower part, close to the
porthole. This is because some liquid can remain in the
porthole of a closed-down unit and corrosive compounds
then concentrate here. Corrosion in a BPHE is
usually pitting/crevice corrosion on the stainless steel.
♦ Freezing. This occurs on the water side at a point
where the wall temperature is lower than the freezing
point. The point with the lowest temperature in a DX
evaporator is normally some distance from the refrigerant
inlet, but freeze damage normally occurs on the water
side close to the refrigerant inlet, as the conditions
for forming a closed-off space are greater here.
Excessive torque and/or forces on the connections.
The connections are kept in place by the double action
of being expanded into the frame plate and brazed to it.
However, excessive force can cause ruptures.
♦ Vibrations from the surrounding equipment. This can
cause fatigue in the material, therefore with rupture of the
plates and/or brazing. The problem will most likely occur in
combination, e.g. with excessive forces on the connections.
♦ Water hammer or other pressure shocks. The
amount of water flowing in a circuit represents a considerable
kinetic energy. If the movement is suddenly
stopped, e.g. by a fast-closing valve, the energy has to
be captured somewhere. This may result in deformation
and rupture of the confining surfaces.
In a BPHE, the most likely damage is a deformation
and, after repeated shocks, rupture of the end plate opposite
the water inlet nozzle. Thus, a valve in the water
pipe after the HE must never close rapidly. There is less
danger on the refrigerant side since the vapour phase is
compressible and absorbs the shocks.
♦ Thermal shocks. If cold water suddenly enters a very
hot BPHE or vice versa, the thermal stresses between
the first channel plate and the end plate might cause
ruptures, as the thin channel plates expand/contract
more rapidly than the end plate.
♦ Material defects. Most defects are discovered during
pressure or helium tests of the BPHE, but some, e.g.
slag inclusion in the plate, might turn up later.
♦ Manufacturing defects. As with material defects, most
are detected at the testing of the BPHE. Sometimes a
crack caused in the pressing might be covered with
copper during the brazing and show up later, mostly in a
corrosive environment. The brazing might be incomplete
and, most likely in combination with some other factor,
might be the cause of leakage.
♦ Exceeding the test pressure and/or the design temperature.
This is rare though, as the BPHE will stand a
pressure of at least five times the design pressure and
the melting point of copper is 1083 °C.
6.2. Leak seeking.
6.2.1. General.
Most leaks in a BPHE are on the heating surface because
of one of the factors described above. This means that the
leakage will most likely be internal, i.e. one fluid will leak
into the other. Apart from possibly causing damage, the
exact place of an internal leak is difficult to pinpoint. It is
however important to know exactly where a leak is situated,
in order to discover the cause.
For this to be done successfully, knowledge of the operating
environment and an investigation of the BPHE is necessary.
6.2.2. Inspection of the system.
♦ Check the pump-down procedure & temperature in an
evaporator. Is the pressure controlled in the condenser?
A decreased condensing pressure, most likely in the
winter, could force the evaporation temperature to decrease.
Is there a separate pressure controller for the
liquid receiver?
♦ Check the shutdown and startup procedure and the
temperature changes, if thermal shocks are suspected.
Is cold fluid suddenly entering a hot BPHE or vice
versa? Which is switched on/off first; the cold, the hot
side or both simultaneously? Will a shut down BPHE
reach the entrance temperature of one of the media?
♦ Check for vibrations from other equipment. How are the
pipes attached to the BPHE? Are there any bends or
bellows, which can take up forces or vibrations?
♦ In case of parallel-connected compressors or BPHEs,
sudden pressure and temperature surges could occur
when one machine is suddenly started or closed down.
Do all the BPHEs have their own pressure controller?
♦ Are motor valves or solenoid valves used on the water
side? Beware of solenoid valves after the BPHE. These
could cause liquid hammering.
♦ Are valves, which achieve semi-continuous operation by
modulating the opening time, used? Such valves could
open for one second, close for five seconds, change to
opening for five, and closing for one second, and so on.
These could cause pressure and temperature transients.

♦ Does the water contain excessive amounts of chlorine
ions or other corrosive compounds? Try to get a water
sample. Be aware that water in alimentary installations
is often chlorinated.
As the chlorine is consumed, more is added, and soon
there is an excessive amount of chlorine ions. This
could lead to pitting.
6.2.3. External inspection.
Strip the BPHE of insulation and inspect the exterior.
♦ Is there a bulge on the cover plate, opposite the water
inlet nozzle? This could indicate liquid hammering.
♦ Is there any hint of deformations on the sides?
♦ Are the nozzles firmly attached?
♦ Check both sides for fouling and traces of corrosion.
♦ Check for transport or installation damages.
6.2.4. Locating the leak.
The simplest method is to fill the BPHE with water on one
side and connect a pressurized air supply to the other.
The bubbles emerging from one or two of the nozzles will
reveal a leakage. In case of an external leakage, the
BPHE has to be immersed in water, or a soap solution
could be applied at the suspected location.
If the leak is close to a porthole, (1 in figure 06 B) it might
be easy to locate. The exact location of other leaks (2 in
figure 06 B) is harder to determine.
Figure 06 shows a method, which could be used. This
method, however, assumes that the leak is large, i.e. the
pressure drop over the leak should be minimal. More sophisticated
methods use helium or another tracer gas to
detect a leakage.
6.2.5. Cutting the BPHE.
When the leakage has been located, the BPHE has to be
cut into pieces for inspection. Before that is done, the position
of the leak has to be pinpointed.
Once a piece has been cut out, it might be difficult or impossible
to establish if there is a leak at all, let alone its
position. This is particularly true if a leak needs a certain
pressure for the bubbles to emerge. This could happen if
the edges of a crack have to be bent apart by the air pressure
before the crack opens and air can leave.
Start the cutting as indicated in figure 06 B in the order A,
B, (& C, if the leakage is close to the port). Use an endless
saw and keep the piece cooled and lubricated. Reversing
saws are less suitable as they deform the plates.
The ideal situation would be to be able to cut out a piece,
which clearly shows the type of damage. This could happen
in case of freezing, which normally deforms a number
of channels in addition to rupturing one of them.
Sometimes the leak is more elusive, but normally it is possible
to cut out a piece where the leak is situated. If the cut
out piece clearly shows leaks or deformations, then proceed
to § 6.3; otherwise the brazing of the piece has to be
dissolved in nitric acid; see below.
6.2.6. Dissolving the BPHE in nitric acid.
In order to dissolve the copper brazing, the piece is immersed
in nitric acid. Nitric acid readily dissolves copper
(and silver) but leaves the steel unaffected. There are
some precautions to be taken.
♦ The cut out piece should be as small as possible. This is
not always easy, but the BPHE should at least be cut as
section A - A' in figure 06 B. This is because the copper
brazing around the ports is extremely hard, not to say
impossible to dissolve.
If the BPHE is not cut, the result of the process will be that
all the brazing is dissolved, except around the ports. As
each plate is attached to the preceding plate at the ports of
one side and to the succeeding plate at the ports of the
other side, the result is a plate pack kept together at the
ports. If cut along A-A', the plates separate into pairs, the
plates of each pair brazed to each other at the left or the
right ports, but each plate available for inspection.
The difficulty of dissolving the port brazing makes it virtually
impossible for internal leakage to occur through
corrosion of the copper. The port brazing is the only
place where brazing separates the fluids, but before this
is corroded; the brazing at contact points and the edges
has long gone.
♦ The brazing at the contact points will dissolve in a few
hours. The edge brazing will dissolve in about two days.
Thus, cut or grind away the edges if possible.
♦ Stir the nitric acid every now and then to get fresh acid
to difficult-to-reach places.
Observe local regulations for the use and disposal of
nitric acid. If there are none, use the recommendations
in figure 06 C.
6.3. Examining the result.
See Figure 06 D for some typical damages. Note that:
♦ Corrosion damages show seldom any deformations of
the plates.
♦ Freezing can rupture the plates, so that the edges of the
rupture are displaced.
♦ Channels, which are bulging outward, are normally a
sign of freezing.
♦ Thermal stresses usually shear of the plates at the contact
points.
The most difficult cases of damages to examine are combination
damages. The copper corrodes and the weakened
contact points cannot stand the pressure.
A similar effect can happen for thermal stresses or fatigue
from vibrations. The contact points are weakened or destroyed
and cannot hold the pressure.
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