ASHRAE Journal - August 2011 - 40

What Is Cavitation?
Cavitation is the formation of vapor bubbles within a pump followed by their rapid collapse. The formation and collapse of vapor bubbles produces a distinct audible signature that sounds as if the pump is circulating gravel. Although cavitation can occur in the process of pumping any liquid, refrigerant pumps are more susceptible to cavitation because fluid being pumped is at or near saturation conditions (i.e., its boiling point). When cavitation occurs, the pump loses its ability to consistently move liquid and to develop pressure lift. The loss of pump capacity (flow) can starve evaporators leading to a loss of refrigeration capacity. The action of vapor bubbles collapsing in the pump causes erosion of the pump impeller with subsequent degradation of pump capacity and efficiency. For semihermetic pumps, the reduction in refrigerant flow can result in a reduction of electric motor cooling leading to premature motor failure. Preventing cavitation requires keeping the pressure of the liquid refrigerant above the saturation pressure corresponding to the refrigerant’s temperature as it enters the pump. This is accomplished by ensuring the refrigerant being pumped has adequate net positive suction head (NPSH). reduce down (close) the hand-expansion valves on the liquid overfed evaporators being served by the pump. Adjusting down on, or more, hand-expansion valves will increase the discharge head on the pump; thereby, decreasing its flow rate and the corresponding NPSHr. This process should focus on the largest evaporators first since they have the greatest impact on liquid demand. If cavitation-free pump operation cannot be achieved by this approach and the recirculator is equipped with a capacitance probe for liquid level sensing/ control, it may be possible to raise the vessel’s liquid operating level to increase the available net positive suction head (NPSHa). However, raising the operating level will reduce the capacity of the recirculator vessel for surge (i.e., unexpected return of liquid) from the system. Another possible reason for pump cavitation is excess parasitic heat gain between the vessel and the pump suction. Visible frost on the suction piping, as shown in Photo 1 is an indication of a loss of insulation integrity, which increases the heat gain to the refrigerant. To remedy this it will require reinsulating the pump suction piping.
Whether you are an HVAC&R system designer, architect, building owner, building manager/operator, or contractor who is charged with designing a green building, the ASHRAE GreenGuide aims to help you answer your biggest question—“What do I do now?” Using an integrated, building systems perspective, the ASHRAE GreenGuide covers the need-to-know information on what to do, where to turn, what to suggest, and how to interact with other members of the design team in a productive way. Information is provided on each stage of the building process, from planning to operation and maintenance of a facility, with emphasis on teamwork and close coordination among interested parties. This third edition of the ASHRAE GreenGuide is meant to be an easy-to-use reference with information on almost any subject that should be considered in green-building design. The GreenTips found throughout this edition highlight techniques, processes, measures, or special systems in a concise, often bulleted, format. Also, information is provided in dual units—Inch-Pound (I-P) and International System (SI)—so that the content is easily applicable worldwide. References and resources mentioned are listed at the end of each chapter for easy access. In this edition, you’ll find the following new information: • Guidelines on sustainable energy master Photo 1: Liquid recirculator package planning excessive ice acshowing • Updates on teaming strategies cumulation on insulation indicating the failure ofaffect building design sys• Information on how issues related to carbon emissions the insulation and operational decisions tem with corresponding increases in liquid refrigerant heat gain. • Strategies for greening existing buildings

ASHRAE GreenGuide

The Pump Curve
A pump curve is a compact, graphical representation of a pump’s performance (Figure 3). Let’s review the basics of reading a pump curve for a liquid refrigerant pump. In this case, we will consider an open-drive pump operating with ammonia (specific gravity of 0.7). The horizontal axis of the pump curve shows the pump’s developed flow rate
40 ASHRAE Journal

expressed in •gal/min. Theonvertical axis of the pump curve Additional information building energy modeling and follow-up measurement and verification shows the generated pump pressure differential (head) ex• Compliance strategies for key ASHRAE standards pressed in psi. new chapter on water efficiency •A The lines that projectincluding those featuringfrom the vertical axis to • New GreenTips, horizontally green strategies for chilled-water plant and boiler plant design the right sloping downward represents this model’s pump performance with varying, but discrete, impeller diameters ranging from 8 in. (200 mm), the lowermost curve, to 10.375 in. (264 mm), the uppermost curve. The dashed lines that run on a diagonal from upper left to lower right represent the required pump power ranging from 1.5 to 5 hp (1.12 to 3.73 kW). The of Heating, Refrigerating and semicircularAmerican SocietyEngineers, Inc. pump efficiencyISBN: 978-1-933742-85-4 lines represent the range from Air-Conditioning 1791 50% to 65%. Tullie Circle 781933 74285 4 Below theAtlanta, GA 30329-2305 of the plot (i.e., the 9pump curve) upper portion 404.636.8400 (worldwide) Product code: 90324 11/10 www.ashrae.org is an additional plot of the pump’s NPSH requirement with a 10.375 in. (264 mm) impeller diameter operating with a discharge pressure of 23.7 psig (265 kPa) connected to an ammonia recirculator operating at a pressure of 8.8 in. Hg (–40°F [–40°C]). Since the system is operating in a vacuum, we must first determine the pressure developed by the pump. The saturation pressure in the recirculator vessel is 8.8 in. Hg (–40°F [–40°C]) or –4.3 psig; therefore, the total head or pressure developed by the pump is 28 psi (294 kPa). The flow delivered at that pressure differential (i.e., head) can be determined by drawing a horizontal line from the 28 psi (294 kPa) hash mark to the 10.375 in. (264 mm) diameter impeller pump curve (blue line). The intersection of this pressure with the pump curve represents the pump’s operating point. Projecting a vertical line from the pump operating point down to the horizontal axis gives the flow delivered by the pump. In this case, the flow is 140 gpm (31.8 m3/h). From this operating point, other operating characteristics of the pump can be obtained such as the pump efficiency (~64%) and operating power required (~4 hp [~3 kW]). The last piece of information is the NPSHr, which is 2 ft (0.61 m) in this case.
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• Updates on newly developed green-building rating systems and standards

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August 2011

ASHRAE Journal - August 2011

Table of Contents for the Digital Edition of ASHRAE Journal - August 2011