Photo Gallery I4: Plate & Frame Heat Exchangers
Plate and frame type heat exchangers play a critical role in separating one piping system from another or by isolating the transfer of heating or cooling. They are critically important for isolating critical HVAC equipment, such as heat pumps with their extended network of smaller diameter piping, and prevent the more aggressive corrosion activity inherent for open condenser water service. Heat pump systems constructed without heat exchangers, running open condenser water through their elaborate network of small diameter threaded schedule 40 steel pipe, have produced some of the worst results identified in our UT investigations; requiring major pipe replacement in under 10 years.
Early attempts back in the 70s at “free cooling” by directly cross connecting the condenser water system into the chill water system during colder months ultimately proved to not be really “free” at all. The modification included the installation of a large strainer type water filter installed to clean the condenser water before it was injected into the chill water system. Capable of removing only larger particulates exceeding 110 micron, such strainers let through the majority of debris, which for an open system falls in the range of about 50 to 60 microns. All microbiological content passed right through obviously.
For those early “free cooling” adapted buildings, substantial damage to their chill water piping was later realized. This led to the removal of such strainers and their replacement by plate and frame heat exchangers offering excellent high efficiency while achieving true isolation between both systems.
Plate and frame style heat exchangers provide high efficiency due to their very high surface area, but are also more vulnerable to fouling due to the close distance tolerance between plates. Although the plates themselves are constructed of stainless steel, iron oxide has no difficulty adhering onto its surface and accumulating. In addition, all airborne captured particulates are then passed through what can essentially be called a coarse water filter screen. While the closed side should experience low corrosion levels of near 0.5 MPY, it is the open side constriction which typically defines the need for cleaning.
Given the time and expense of disassembling and cleaning such units, effective water filtration to the open side to the cooling tower becomes even more important, with full flow protection required. For that reason, plate and frame heat exchanges represent the one specific application where full flow centrifugal separators are ideally suited. Their ability to remove largest size rust and debris at greater than 70 microns greatly reduces the clogging issue between plates having much larger separation. Many other forms of screen cleaning filters will perform the same function, but at greater cost. Sand filters or bag style basket filters are totally inappropriate given their low flow capability on even a side stream configuration.
For many installations, two heat exchangers are installed in parallel in order to permit regular maintenance and cleaning.
Various other forms of heat exchangers see service in the HVAC field, particularly for steam to hot water and to isolate secondary systems, including the much older shell and tube type. These are less prone to clogging due to the larger diameter tubes, as well as the fact that they are generally installed between two closed systems or where corrosion activity is inherently low.
