Plate Heat Exchanger As Vaporizer and Condenser-1

20-04-2020

Plate heat exchangers (PHEs) have been installed in larger heat pump systems and refrigeration systems since the 1980s. Tech-nicians who deal with these systems need to know some specific details of how they work.

Part one of this article covered PHE design, on-site assembly and service, leak indication, freezing risk, and resistance from vibration, pressure, and seismic occurrences. This week the article continues its discussions of the plate heat exchanger as vaporizer and condenser.

VAPORIZER

Particularly as a vaporizer, the PHE presents the characteristics of a high degree of turbulence and high shear forces in the complex channel geometry, which lead to:

·  The capability for stable capacity regulation;

·  Relatively high heat transfer rates, also in the preheating zone;

·  In direct-expansion systems, high heat transfer rates, even in the superheating zone (pressure drop in the superheating zone is reasonably low since this occurs in a continuation of the relatively large number of channels, which are used for vaporization);

·  The flow is homogenous, leading to effective transport of the vapor-phase, oil, and (if present) inert gas; these will otherwise present high resistance to heat transfer. The high shear with resulting homogenous flow eliminates the development of film boiling, which can otherwise seriously impair heat transfer;

·  High overall heat transfer coefficient; also in freezing duties, using high-concentration glycol, ethanol, or CaC12, or for oil-cooling duties;

·  Low fouling resistances; and

·  Due to the pure counter- (or co-) current operation and low fouling resistance, it is possible to operate at a high vaporization temperature compared to the cooled medium. In some cases, it is possible to select the next-smallest size of compressor compared to that which would be required with other types of vaporizers. In any event, high chill factors can be obtained at a reasonable investment cost .

SUBMERGED VAPORIZATION

Thermosiphon (natural) circulation is obtained when the pressure drop within the exchanger balances the external liquid head; no pump is needed to feed the liquid to be vaporized to the exchanger.

The Plate Heat Exchanger is well suited to this process as the liquid head in the knockout drum is approximately on the same level as the upper PHE connection (outlet). 

REFRIGERANT VOLUME

The total system volume can be kept low and the pipework made in small diameter due to the low refrigerant volume and low circulation ratio (high exit vapor fraction) in the PHE. The knockout drum can also be of relatively small size.

Submerged vaporization with low liquid-phase velocity in the inlet manifold means that a large number of plates can be installed in the frame. The largest vaporizers have about 300 refrigerant channels (600 plates), with a capacity of about 6 MW on R-12 and 8 MW on ammonia.


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