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Back in June of 2002 when I first started writing for this magazine (yes, it’s been that long) I wrote a piece on pump controls. Well, since that time, domestic pumps have come a long way.
I remember in the early days of my career (mid 80s and 90s) when I worked in New York City, the typical building had pumps on individual concrete pads with constant speed controls. There were typically at least three feet between the pumps, and the control panel was on the wall. Those days are long gone.
Twenty and thirty years ago, the cost of variable frequency drive (VFD) controls carried a large premium compared with constant speed controls. This premium was typically only justified for chilled water and condenser water systems, where horsepower varied greatly with flow.
To elaborate, constant speed controls run a pump at one speed, 60 Hz frequency. A pump has one design operating point of pressure and flow. Granted, the pump moves around on its pump curve in response to changes in flow and pressure, but it runs at one speed, 60 Hz, using constant (maximum) horsepower.
This design still makes sense in New York City, because tall buildings are required by code to have house tanks — water tanks that provide storage on top of the building. Surely you have seen photos of New York where these tanks pepper the skyline, but newer buildings generally have them hidden inside, out of view.
In this New York City design, the pumps operate at one single point as they fill the house tank, providing the average demand of the building. The tank is then sized to make up the difference between the average demand and peak demand. In this scenario, constant speed pumps still make sense.
Outside New York City, most buildings do not have domestic storage tanks, and pumps operate in a direct-pumped fashion, having to respond directly to the peaks and valleys of building demand. In this configuration, constant speed pumps do not make sense because you don’t want the pumps operating at peak horsepower all the time. This is where VFDs come into play.
Because domestic pumps serve mostly to overcome the static head of a building, only a fairly small percentage of the power used is dedicated to overcoming friction. However, with the dramatic variations in flow, VFD controls allow the pumps to speed up or slow down as flow varies, but only to the extent possible while maintaining pressure at the top of the building. I have seen VFD systems that fluctuate between 40 and 60 Hz. In contrast, chilled water pumps serve almost entirely to overcome friction as the flow varies in this closed loop system. Consequently, chilled water VFDs can slow down to as little as 22 Hz, which is the commonly held minimum for VFD controls. Since the affinity laws state that horsepower varies with the cube of the pump speed, reduction of pump speed will save a great deal of energy.
Other than VFD controls, another advancement in domestic pumps is that they are almost always purchased as packaged systems. Two, three or more pumps are factory-assembled on a skid complete with pump check and shut-off valves, and the control panel completely wired and ready for installation. This saves the contractor (and hence the owner) a great deal of field labor, as well as real estate given the much smaller footprint compared with individual pumps mounted on concrete bases. Note that the skid systems generally do not come with shut-off valves for the package, only for the individual pumps, so it is important to indicate those on your drawings.
In speaking with Jim Weil of Weil Aquatronics, a local pump expert, he enlightened me with this additional information about recent pump developments.
ASHRAE 90.1 is the energy standard for buildings. It is used more in HVAC than in plumbing, and depending where your project is, it may be just a guideline, or it may be the local energy code. Here in California, our state code is stricter than 90.1, so for the most part, it does not apply. That said, 90.1 requires that pumps be controlled by a sensor at the farthest fixture, rather than at the discharge of the pumps, to maintain constant pressure at the most remote point.
This effectively takes into account the actual water usage and friction losses in the building. Most manufacturers use an algorithm based on the Darcy-Weisbach equation, which relates pipe friction to the flow rate to simulate this. The controller automatically reduces the boost pressure for lower demand flows. I personally locate the pump control pressure transducer at the top of the building whenever the pressure at the top floor is sensitive. So often the top floor has the most expensive penthouse units, and as the rules of fluid mechanics hold, it also has the lowest water pressure in the building. With the pressure sensor located at the top of the building, it helps prevent that penthouse level from also seeing the pressure losses due to friction in the system piping below.
Also in 90.1, pressure-reducing valves are no longer allowed to be used as pump discharge check valves, as they are seen as a waste of energy. Pumps should control pressure purely through their operating speed. Also required is that the pump system completely shuts down when there is no demand. This should not be controversial, you would think. Most manufacturers (with one notable exception) argue that a hydropneumatic tank is required to keep the pumps off as long as possible. So, full compliance with the letter and intent of ASHRAE 90.1 would seem to mandate a tank. In my opinion, a hydropneumatic tank in a VFD pump system serves more to smooth out the pump control than it does to provide water supply volume.
Electronically commutated motor (ECM) technology, also known as the brushless DC electric motor, is a permanent magnet design that is inherently more efficient than shaded-pole motors. ECMs can achieve motor efficiency levels of up to 94 percent, which surpass NEMA Super-Premium motors that meet the NEMA 12-12 efficiency levels of today. ECMs are controlled electronically by a microprocessor and electronic controls, which provide the ability to program and control the speed and/or torque of the motor, without the efficiency losses of a VFD. Combined with electronic controls, ECMs can maintain efficiency across a wide range of operating speeds. Currently available up to 10 HP, booster pump manufacturers are combining ECMs with the latest high efficiency drives to achieve combined efficiencies of up to 80 percent.
I hope everyone has a very happy Thanksgiving!