Brazed Plate Heat Exchangers are compact, with high efficiency and durability having liquid-to-liquid heat exchangers. These heat exchangers are constructed from stacked plates brazed together that are typically corrugated to add surface area and more heat transfer.
Brazing the plates together instead of bolting them between gaskets is what makes BPHE technology so efficient and compact as 95% of the material of the brazed unit is used for heat transfer. The brazed plate heat exchanger (BHE) consists of thin corrugated stainless steel plates vacuum brazed together using copper as the brazing material. Design Brazing the stainless steel plates together eliminates the need for sealing gaskets and thick frame plates.
With a plate heat exchanger, heat cuts through the surface and separates the hot medium from the cold. Thus, heating and cooling fluids and gases use minimal energy levels. The theory of heat transfer between mediums and fluids happens when: Heat is always transferred from a hot medium to a cold medium.
Plate exchangers typically have a larger coefficient of heat transfer, as they have more contact area between fluids. They have a small footprint that requires little to no maintenance space. They are easy for maintenance, repair, and service. Plate heat exchangers are up to five times more efficient than shell-and-tube designs with approach temperatures as close as 1°F. Heat recovery can be increased substantially by simply exchanging existing shell-and-tubes for compact heat exchangers.
Construction
The brazed plate unit eliminates the end plate, bolts and gaskets from the design. Instead the plates are held together by brazing with copper. This results in a much less complicated, lighter weight and more compact heat exchanger. The simpler design also results in greatly reduced cost.
On the negative side the brazed plate support eliminates some advantages of the plate and frame design, in terms of maintenance the brazed plate units cannot be disassembled for cleaning or for the addition of heat transfer plates as bolted units can.
Most importantly, however, the brazing material is copper. Since most geothermal fluids contain Hydrogen sulphide or ammonia, copper and copper alloys are generally avoided in geothermal system construction. The situation with brazed plate heat exchangers is especially critical due to the braze material and length (a few tenths of inch) of the brazed joints.
Application Consideration
In addition to the material related questions there are also issues related to the standard configuration of brazed plate heat exchangers.
Physical size of the exchangers limits application flow rates to approximately 100 gpm ( although one manufacturer produce units capable of 200 gpm). Maximum heat transfer area is limited to 200 ft^2. Heat transfer rates are similar to those of plate and frame heat exchangers and range from 800 – 1300 Btu/hr ft^2 degree Fahrenheit in most applications.
Heat exchanger equipment cost
The low cost of the brazed plate heat exchanger is its most attractive feature. Since heat exchanger cost us influenced by a most of factors including hot and cold side fluid flows and temperatures. It is most useful to discuss costs in terms of heat transfer area.Cost for units less than 100 ft^2 heat transfer area, it is apparent that brazed plate units offer a significant saving for exchangers in the 2-30 ft^2 size range.
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