Plate Heat Exchanger: How Does It Work?

November 14, 2000
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Since 1984, plate heat exchangers (PHEs) have been installed in larger heat pump systems and refrigeration systems. Knowing how they work is critical to being able to troubleshoot and service these systems.

In comparison with coaxial and tubular heat exchangers, the size and geometry of plate heat exchangers make them suitable for installation within the system’s framework. In one and the same PHE frame, it is possible to install both the condensing and the vaporizing sections in order to save space, weight, and costs of installation.

Gas cooling (desuperheating) with heat recovery can be incorporated in a section of the condenser pack using a separate cooling medium, with condensation in the following section using a different coolant.

Due to the low internal volume, both the condenser and the vaporizer can be emptied into the connecting vessels; a separate recipient is not necessary.

For submerged vaporization in a PHE, a circulation ratio of 1.2 to 1.5 is common. The refrigerant pump — if one is used — can, therefore, be chosen with a relatively small capacity, as can the liquid pipework. Not of least importance is that the vapor-liquid separator can be of small size and correspondingly low weight and cost.

By minimizing the CFC/CFCH inventory, the choice of heat exchanger type has great significance for the overall economy of the refrigeration or heat pump system.



SERVICE, EXTENSION, AND ON-SITE ASSEMBLY

The twin-plate PHE can be relatively easily opened for inspection, cleaning, and maintenance while the plates are hanging in the frame. Chemical cleaning is very effective due to the high turbulence in the channels formed by the corrugated plates. The small holdup volumes demand less cleaning solution and shorter circulation time.

The brazed PHEs cannot be opened. Chemical cleaning methods are, however, effective for the same reasons as cited earlier, using a cleaning agent of a type and composition suitable for the type of fouling involved.

Increased capacity, change of refrigerant, change of temperature profiles, and improved chill factor can be achieved by the addition of plates (welded cassettes) or a change to a different thermal characteristic. To combat a change in corrosion conditions, others can replace cassettes in a different material.

The twin-plate can be assembled in place, piece by piece. For example, in deep-level mine installations, the shaft’s dimensions and lift capacity may prevent the transport of the assembled unit to its final location, despite the PHE’s low weight and volume. For retrofit installation of a PHE, the limited space requirement and the possibility of on-site assembly renders this type of unit highly attractive.



LEAK INDICATION

In the unlikely event of leakage in a PHE, this is almost certainly an external leak, which is easy to detect. The gaskets seal only against the surroundings; therefore two gaskets (the space between which is open to the atmosphere) always separate the media. A gasket failure cannot, therefore, result in internal leakage and contamination.

With twin-plate units, it is thus a question of changing the ring-joints only, which are glue-free and thus easy to remove and replace with the plates hanging in the frame. Leakage in a tubular unit is usually a question of internal leakage, resulting in a need for repair welding, the plugging of tubes, or fitting (by rolling or welding) of new tubes. Both sides of the tubular unit must be emptied for this repair work.

To the PHE’s disadvantage, both sides must be drained before the unit is opened. Refrigerant volumes, however, are low, permitting rapid emptying to the separator, receiver, or to a separate vessel which can be relatively small. The evacuation of the system after maintenance is, for the same reason, a relatively fast procedure. The same applies to inert gas purging.



FREEZING RISK

In connection with maximum energy extraction from heat sources in heat pumps, a study has been made to determine how close to the freezing point it is possible to drive the system — even to the extent of using the latent heat of freezing as an energy supply.

The results gained from these tests can be summarized:

  • It is only with difficulty that the water channels are totally blocked by a reduction in vaporization temperature when the water circulation is maintained.
  • Since water is still circulating in partially frozen channels, the system is rapidly thawed when refrigerant vaporization temperature is restored to design level.
  • If complete freezing is achieved by stopping the water circulation, it has been demonstrated that the gaskets and plates are not damaged. This is explained by the construction being sufficiently flexible to accommodate expansion on freezing. The expansion forces are not propagated.
  • Tests have shown the formation of ice crystals in the stream leaving the exchanger. In the long run, ice will be formed within the unit, reducing the heat transfer capacity; the effect should only be used temporarily.
  • The high turbulence and wall shear stress in the PHE is of importance for the formation of the ice deposit.
  • The vaporization temperature can be relatively high due to the close approach temperature characteristic of the PHE. Risk for the initiation of ice formation can therefore be avoided at the design stage. In mine cooling and foodstuffs installations, the possibility to cool the liquid virtually to the freezing point is often utilized.



    RESISTANCE

    Elimination of induced vibration: The “fishbone” plate corrugations in the PHE (twin-plate and brazed) — when the plates are assembled with the pattern pointing in opposite directions on alternate plates — result in a large number of contact points. This can be in the order of 500 to 1,000 per sq m. Maximum distance between contact points is about 10 mm.

    The plate pack is sufficiently rigid without detracting from the flexibility, which is required to avoid damage on freezing, to completely avoid induced vibration in such short, unsupported spans. For tubular constructions, extreme care must be taken both in design and operation if flow-induced vibration is to be avoided. Tube damage due to vibration is a very common cause of failure in tubular heat exchangers.

    Pressure range: The corrugated-plate construction permits test pressures of up to about 50 bars (more than 700 psig). For brazed units, the standard design-test pressure is 30/45 bars (453/653 psig); destructive tests have shown that a brazed PHE has a bursting pressure of about 150 bars (2,200 psig).

    Seismic and shock loads: The frame construction can be augmented where such loads are to be expected. Units in the nuclear, offshore, and marine industries are often constructed to these requirements. The frame can also be used to absorb the forces from connecting pipework; maximum permissible tension, compression, and shear forces, together with bending moments and twisting forces, can be advised on request.



    THE PLATE HEAT EXCHANGER AS VAPORIZER AND CONDENSER

    Refrigerant vapor is fed into the upper connection, and the condensate (together with residual vapor and noncondensables, if present) is withdrawn from the lower connection. Condensation is thus in the “falling film” mode.

    The plate corrugations result in a complex channel geometry of high-shear characteristics, so that the flow regime is in the shear-dominated region; the condensate film is highly turbulent and offers low resistance to heat transfer.

    At the pressures involved in refrigerant condensation, the high-shear characteristics do not result in unacceptable pressure drop. However, at lower pressure or under vacuum, the PHE is less suitable as a condenser. A type with smooth surfaces, large cross section, and short flow path (giving lower pressure drop characteristics and operating in the gravity controlled region) is preferable, although generating higher resistance to heat transfer in the thicker, laminar, condensate film. The spiral heat exchanger is a typical example of a type suitable for condensation duties at low pressure or under vacuum.

    Condensate subcooling in the PHE is predictable due to the virtually pure counter-current operation. The condensate is in contact with the cooled surface at all times, in contrast to shell-side condensation in a tubular exchanger, where the condensate drops off the tubes and runs along the bottom of the shell. If substantial degrees at subcooling are required, a separate PHE with an individually regulated coolant supply is recommended (see Table 1).

    Pure counter-current flow, high overall heat transfer coefficient, and low fouling resistance render the PHE a compact condenser with features of high interest in heat recovery and overall efficiency contexts. The condensation temperature can lie close to the coolant temperature — say within 2° to 3°C of coolant outlet temperature. This has a positive influence on the choice of compressor, the efficiency, and the heat pump factor. It is possible to carry out an optimization, since the PHE can be designed and the whole system adjusted in small stages until a totally optimized solution is obtained.

    The turbulent flow on the coolant side creates shear forces which assist in keeping the surfaces free of deposited fouling material. The use of corrosion-resistant material also means that small fouling resistances can be used (see Table 2).

    Regulation of coolant (water or glycol) down to a turndown of 10% to 20% is possible with full turbulence maintained. Relatively small design coolant flow-rates are possible.

    NEXT WEEK: More on vaporization, freeze risks, and indirect systems.

    Stromblad is with Alfa-Laval Thermal, 5400 International Trade Drive, Richmond, VA 23231.

    Publication date: 11/12/2000

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