FALL 2017

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B Y J O H N B O Y E R & M I K E K I S S E L L ook inside some of today's most energy-efficient commercial buildings and you'll likely find an HVAC system using a waterside economizer. Economizers are favored for sustain- ability features because they reduce the need for mechanical refrigeration, resulting in significant energy savings. The importance of waterside econo- mizers has grown steadily since the Oc- tober 2013 deadline for states to adopt commercial building codes that meet or exceed ASHRAE Standard 90.1- 2010. Due to their success in reducing energy consumption, guidelines for air and waterside economizers were expanded in ASHRAE 90.1-2013, the Energy Standard for Buildings Except Low-Rise Residential Buildings, to include the majority of commercial set- tings across the country, though states are in various stages of requiring these more stringent thresholds. Most large commercial buildings require cooling year round as internal loads are often located in the core of the building and require cooling regardless of outdoor temperature. Airside econo- mizers use cool outside air directly as a means of cooling the indoor space, whereas waterside economizers use cooled water indirectly as a means of cooling the indoor environment. This process is known as free cooling. There are numerous design varia- tions of waterside economizers, but the most common setup consists of a cooling tower and heat exchanger to indirectly cool the chilled water loop, which is used to reject heat from the building via hydronic coils. Plate and frame heat exchangers are ideal for this process due to their high heat transfer rates, ability to handle close temperature approaches and because they can isolate the closed chilled water system from the open water system. In addition to waterside economiz- ers, plate and frame heat exchangers are used in other HVAC applications, such as district cooling and heating, thermal storage, pressure interceptor and heat pump. INNER WORKINGS OF PLATE AND FRAME HEAT EXCHANGERS Before considering the specifics of HVAC system design that include waterside economizers, it's helpful to understand how a plate and frame heat exchanger works. The plate and frame heat exchanger consists of a series of thermal plates with gaskets compressed together within a rigid frame to create an ar- rangement of flow channels. Each ther- mal plate is relatively thin, typically ranging from .4 mm to .8 mm thick, and stamped with a chevron pattern to form corrugations. Different corruga- tions provide different heat transfer rates and pressure drop. For most designs, a hot liquid will be on one side of the plate and a cold liquid on the opposite side. The fluids travel in opposite direction of one another, referred to as counter-current flow. This counter-current flow maxi- mizes the log mean temperature differ- ence between the hot and cold fluids. The LMTD is the logarithmic average of the temperature difference between the two fluids and is a driving force to the size of the heat exchanger. Plate and frame heat exchangers operate with highly turbulent flow regimes due to the chevron pattern on the thermal plates. Turbulence is good for heat transfer — the higher the turbulence, the higher the heat transfer rate between the hot and cold fluids. The higher the heat transfer rate through the heat exchanger, the less surface area needed, resulting in a compact heat exchanger. For a typical water-to-water appli- cation, plate and frame heat exchang- ers can have heat transfer rates three to five times higher than other types of heat exchangers, such as a shell and tube. This results in three to five times less surface area being required to perform the duty. Due to the high heat transfer rates generated by the corrugated thermal plates and the counter-current flow pattern of the fluids, plate and frame type heat exchangers are well suited for applications with a temperature cross and/or close temperature ap- proach. A temperature cross occurs when the cold fluid outlet tempera- ture is higher than the hot fluid outlet temperature. Approach is the differ- ence in temperature between the inlet The Great Economizer The basics of plate and frame heat exchangers The innovative plate design on this heat exchanger allows for more heat transfer, while using less space. HVACPproducts.com HVAC & Plumbing Product News \ Fall 2017 14

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