Heat exchangers boost HVAC efficiency
As industry and government standards up the ante for energy efficiency in commercial buildings, the use of heat exchangers is on the rise in heating and cooling systems. Heat exchanger design and effectiveness are helping buildings operate more efficiently through waterside economization.
Most large commercial buildings require cooling year round as internal loads often are located in the core of the building and require cooling regardless of outdoor temperature. Airside economizers 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 in a process known as free cooling.
Waterside economizers have become more important since the October 2013 deadline for states to adopt commercial building codes that meet or exceed ASHRAE Standard 90.1-2010, Energy Standard for Buildings Except Low-Rise Residential Buildings. ASHRAE 90.1 outlines the minimum requirements for energy-efficient design and construction of buildings and their systems. Since 1992, the Energy Policy Act has required the U.S. Department of Energy (DOE) to review each new version of ASHRAE 90.1, while also requiring all states to adopt building energy codes that are equal to the standard.
In October 2011, DOE issued a positive determination that ASHRAE 90.1-2010 saves more energy than previous standards and notified states to update their codes to meet or exceed ASHRAE 90.1-2010 by October 2013. States currently are in various stages of requiring these more stringent thresholds. Due to their success in reducing energy consumption, guidelines for air and waterside economizers were expanded in ASHRAE 90.1-2013 to include the majority of commercial settings.
There are numerous design variations 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 and ability to handle close temperature approaches, and because they can isolate the closed chilled water system from the open water system.
Plate and frame heat exchangers are used in other HVAC applications, such as district cooling and heating, thermal storage, pressure interceptor, and heat pump.
A Closer Look
Before considering the specifics of HVAC system design that include waterside economizers, it is helpful to examine the inner workings of a plate and frame heat exchanger.
Plate and frame heat exchangers consist of thermal plates with gaskets that are compressed together within a rigid frame to create an arrangement of flow channels. Each thermal plate is relatively thin, typically ranging from 0.4 to 0.8 mm thick, and stamped with a chevron pattern to form corrugations. Different corrugations provide different heat transfer rates and pressure drops.
For most designs, a hot liquid will be on one side of the plate and a cold liquid on the other. The fluids travel in the opposite direction of one another, referred to as counter-current flow. This maximizes the logarithmic mean temperature difference (LMTD) between the hot and cold fluids. The LMTD is the logarithmic average of the temperature difference between the two fluids and determines 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 and the less heat exchanger surface area needed, resulting in a compact heat exchanger.
Fouling, the degradation in heat transfer over time caused by material deposits on the walls of a heat exchanger, typically is not as significant a factor with plate and frame heat exchangers. High turbulence and high heat transfer rates have a self-cleaning effect so salts, particulates and other matter from the fluid itself do not build up on heat transfer boundary walls.
In other types of heat exchangers, such as shell and tube, fouling can significantly affect thermal and hydraulic performance, which drives up energy costs and requires maintenance. To overcome fouling factors, HVAC system designers often oversize equipment—up to 80% in some cases—which means systems are not running at optimal efficiency.
Because of the high heat transfer rates generated by the corrugated thermal plates and the counter-current flow pattern of the fluids, plate and frame heat exchangers are suited for applications with a temperature cross and/or close temperature approach. A temperature cross occurs when the cold fluid outlet temperature is higher than the hot fluid outlet temperature. Approach is the difference in temperature between the inlet temperature of one fluid and outlet temperature of the other.
Closer temperature approaches allow for more efficient building operation because less energy is required as a waterside economizer transfers heat from a warm fluid to a cold fluid via the heat exchanger. In a plate and frame heat exchanger, the minimum recommended temperature approach is 2°F. Although a plate and frame heat exchanger can, in theory, be designed for a closer approach, it will have a great impact on the size and cost. For example, going from a 2°F approach to 1°F results in doubling the size of the heat exchanger, and it will cost more. Also, many system installations cannot support measurements closer than 2°F.
Seal of Approval
Third-party certification is another important consideration in selecting heat exchangers. Certification provides assurance that the equipment will perform accurately and consistently and ensures all manufacturers are designing to the same tolerances.
The Air Conditioning, Heating and Refrigeration Institute’s (AHRI) Liquid to Liquid Heat Exchanger (LLHE) certification program is based on AHRI Standard 400 for gasketed plate and frame type heat exchangers. ASHRAE 90.1-2010 or newer requires waterside economizers to comply with ANSI/AHRI Standard 400.
The scope of the AHRI LLHE certification program includes gasketed plate heat exchangers that use water or sea water as the operating fluids and where the heat load is less than or equal to 240 million btu per hour and both flow rates are less than or equal to 20,000 gal per minute.
As more states enact stricter regulations for HVAC energy efficiency and building owners realize cost savings and operational efficiencies with free cooling practices, waterside economizers will play an important role in sustainable HVAC system design.