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Thread: evaporater leak

  1. #1
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    evaporater leak



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



  2. #2
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    Re: evaporater leak

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

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    Re: evaporater leak

    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/12059657...nual_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.
    Last edited by nike123; 07-04-2009 at 04:30 AM.

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