In a pumping system, the objective, in most cases, is to transfer a liquid (e.g., circulate liquid through a piped network as a means of heat transfer). Pressure is needed to make liquid flow at the required rate and overcome losses in the system. Losses are of two types: static head and friction head. Static head, in its simplest form, is the difference in height between the supply and the destination of a liquid or the pressure in a vessel into which a pump discharges, if it is independent of flow rate.
A closed-loop circulating system without a surface open to atmospheric pressure shows only friction losses. Most systems, however, have a combination of static and friction head. The ratio of static head to friction head over the operating range influences the benefits achievable with a VSD. Static head is a characteristic of installation. Reducing static head when possible generally reduces installation and pumping costs. Friction-head losses must be minimized to reduce pumping cost, but after unnecessary pipe fittings and length are eliminated, reduction of friction head requires larger-diameter pipes, which add to installation cost.
“This is the dilemma that most HVAC professionals face: balancing the installed upfront cost against the longer-term life-cycle operating cost,” Bryan Payne, PE, Taco’s southeastern sales manager, said. “The great news is that drives allow the use of smaller pipe sizes, now pushing design velocities up over traditional constant-speed systems. For many months of the year, it’s quite common for piped systems to operate at flow rates that have very low velocities and pressure drops, yielding significant savings to building owners. Designing with drives for all system pumps allows the best of all worlds: lower installed costs and lower operating costs.”
When a centrifugal pump is installed in a system, the effect can be illustrated graphically by superimposing pump and system curves. The operating point is where the two curves intersect.
“Many pumping systems require a variation of flow or pressure,” Gene Fina, senior product manager for Taco, said. Referring to Figure 2, he added: “Pumps would run at maximum speed on a design day, but this occurs only rarely. A vast majority of the time, the system needs reduced flow and reduced pressure drop. This allows the pump to run at slower speeds and to track the system curve.”
According to Fina, either the system curve or the pump curve must be changed to get a different operating point.
Single pumps installed for a range of duties are sized to meet the greatest output demand and, thus, usually are oversized and operate inefficiently for other duties. Consequently, there is an opportunity for energy-cost savings through the use of control methods, such as variable speed, that reduce the power driving the pump during periods of reduced demand.
“Here in Florida, ... new ASHRAE requirements for outside air have increased the need to introduce fresh air, which, 10 months of the year, is very high in moisture, needs to be cooled and reheated,” Bill Dalhoff, LEED AP, vice president of Jacksonville, Fla.-based manufacturers’ representative Florida Hydronics Inc., said. “Reheat coils and capacity increase, larger boilers are needed, and now there’s also greater system flow. Of course, at the high end, with winter design conditions, it’s no surprise that we see systems with greatly varying flow needs—say, 10 percent of load to 100 percent—making the use of variable-speed pumps a perfect fit.
“It’s now almost expected that secondary-loop building pumps have variable-speed drives,” Dalhoff continued. “Now, the quickly building trend is the use of variable drives to balance constant-speed pumps on the primary side for chillers and boilers. This makes so much more sense than to use constant-speed pumps with manual balancing valves that impart false head and require higher-horsepower pumps. Though this ultimately achieves the same thing, the smarter, far-more-cost-effective solution is to use variable-speed technology. Payback to the building owner is amazingly fast. Though a starter may cost $550 and a variable-speed drive perhaps $700 or so, the upcharge can typically be recovered within three or four months.”
Mike Kiani, PE, CPD, CIPE, principal mechanical engineer for MK2 Engineers, a Fairfield, Calif.-based consulting engineering firm, said he recommends VFDs for virtually all pumping applications.
“Building owners are now quite open to it and see the value in the minimal upcharge,” Kiani said. “Variable-speed pumps and drives that were prohibitively expensive and at the bleeding edge have now become utilitarian, entirely sensible for broad use—even for constant-volume systems—because of the huge operating-cost advantages and reduced maintenance.
“I compare it to the difference we see today between cars with carburetors and those with fuel injection,” Kiani added. “The automotive industry shifted to new technology for very good reason, including greater fuel efficiency, reliability, and (throttle) response. Variable-speed pumps are designed to track the load closely, an ability that constant-speed systems simply don’t have.”