A researcher at the National Institute of Standards and Technology (NIST, www.nist.gov) is testing a method for improving the energy efficiency of water chillers for cooling large commercial buildings. According Mark Kedzierski, the NIST mechanical engineer performing the research, the method could save as much as 1 percent of the 320 billion kWh of electricity used annually by chillers or an equivalent 5.5 million barrels of oil per year.
The advance builds on past NIST research designed to optimize mixtures of chiller refrigerants with lubricants. As part of this effort, the NIST discovered that some lubricants, when injected in small amounts, can significantly enhance evaporator heat transfer, increasing the efficiency of chillers. After studying the process more closely, the NIST found the most efficient heat transfer occurred when the added oil’s surface tension, viscosity, composition, and chemical characteristics complemented those of the chiller’s base lubricant.
In a recent paper describing the method, the NIST explains how the right additive forms a very thin covering on an evaporator surface, which produces enhanced bubbling during boiling. The improved conversion of the refrigerant molecules into vapor molecules increases the chiller’s cooling capacity similar to a heat pump.
Kedzierski developed rules for the selection of the different types of oil additives according to the type of chiller lubricant, making successful energy enhancement less of a hit-or-miss proposition. Laboratory work is under way testing the energy enhancing potential of several oil and lubricant combinations that have been identified by the rules.
“The leap from a successful laboratory experiment to an everyday large-scale cooling application is a big one,” said Kedzierski, in a prepared statement. “NIST wants to see this theory translated into products germane to manufacturers as soon as possible.”
— Flow Control Staff