Ensuring that variable-speed pumps really do save energy

Peter Nears
Over-sized variable-speed pumps do not save energy — Peter Nears.
Simply installing variable-speed pumps may not save energy, as experience in Switzerland demonstrates. PETER NEARS explains why correct pump selection is vital, especially when pumps are being replaced.From experience over many years we know that most pump failures can be attributed to over-sizing or poor selection of the product. The client first realises there is a problem when the product fails after a relatively short period of time, when it is generally replaced like-for-like, with the warranty costs being passed back to the original manufacturer. Over the last 10 years or so, both specifiers and contractors have tended to treat the pump as a commodity item — not realising it is one of the most important items in the plant room. We have all seen specifications saying, ‘Contractor to confirm pump duties before installation.’ Does this happen in the real world? Rarely I believe. More commonly old pumps are replaced like-for-like or by one of a similar bore size and, usually, higher head. In my opinion this is a recipe for future trouble. Pumps for condensing boilers Most modern circulators have a choice of manually selected speeds but invariably are left on site at maximum. This often results in over pumping, which will narrow the the difference between boiler flow and return temperatures, so efficiency is not at its design criteria. This is especially relevant today with the trend veering towards using condensing boilers, where a 20 K difference is the norm. Over-pumping a modern condensing boiler will cause it to lose efficiency to the point where it may not condense at all. This is an enormous waste of the additional cost of the condensing boiler and will probably shorten the life of both boiler and pump. Variable-speed drives In a recent independent experiment* in Switzerland, in the Canton of Zurich, equivalent to a local-authority area and one of the most energy-conscious areas of Europe, it was found that significant numbers of large commercial buildings were consuming large amounts of electrical energy via their heating pumps. In Zurich, legislation was brought in to force owners of large buildings to change to variable-speed pump drives. At face value, this appeared to be a sensible piece of legislation. However, when comparing year-on-year consumption adjusted to degree days it was found that many variable-speed drives installed in response to legislation to reduce the energy used by heating pumps in Zurich caused electricity consumption to increase. A fundamental error was made; nobody checked the design and actual duty points before the variable-speed pumps were installed. Put simply, the contractors had matched the new pumps with the old, which of course were considerably over-sized. If variable-speed pumps are over-sized, the computer chip in the pump cannot see small changes in system resistance when working at the end of the pump curve. Consequently, the pumps will always run at maximum capacity and maximum power consumption; a pump with maximum power consumption will always run excessively hot, causing failure of the motor windings. Measurements were taken and most buildings were found to be over pumping. Considerably smaller pumps were then fitted, resulting in a significant reduction in power consumption. The message from many other pump manufacturers is that a variable-speed drive on a pump will solve all problems … but this is only applicable if the pump is correctly sized. Selecting variable-speed pumps The chart shows the operating range for achieving good selection for variable speed pumps.
Pumps with variable-speed drives are at their most energy efficient in area A of the performance curves. Area D represents oversizing, area B includes pumps that are too large, and pumps in area C would tend to run at fixed speed so that the benefits of variable-speed operation are not realised.
The ideal design duty field is in band A, which will provide maximum potential for variable flow rates and energy savings. A pump in band B would be better reselected as a smaller model because the energy savings in that area would be small against any volume changes. A product in band C should be reconsidered as a fixed-speed pump because savings will be negligible, and the capital cost can then be reduced. Pumps that are oversized appear in band D, where the system curve is too flat to detect changes in system resistance, and the pump will then run at maximum output with no potential for energy saving. Night setback or frost-protection speed is shown on the chart as curve min 1. Biral variable-speed pumps are despatched from the factory with a default setting, limiting the maximum output from the pump, because we know that in 99% of cases it will be over-sized. A switch in the terminal box will allow maximum performance to be employed if required. Reducing system noise Selecting a pump slightly under duty does work because all resistance figures are generally rounded up and in recent years, production techniques for pipework and fittings has resulted in improved quality with smoother bores which lower the frictional resistance considerably. When deciding which pump to use, it is also important to remember that a circulating pump operating at low speed is significantly quieter than one operating at high speed. Pumps with very flat curves give little change in head when zones or thermostatic radiator valves close down, thus preventing system noise that often occurs. This also has the benefit of giving a precise match against the design duty plus giving the optimum capital cost. Precise pump selection, using multi-speed or variable-speed drives, is a vital factor in determining the life expectancy of the product, and it is clear that a one-size-fits all approach will not give all the benefits one might expect. Selecting the right pump improves pump reliability and provides the energy conscious with an opportunity to make significant saving in energy use and energy costs over the life of the pump. Peter Nears is product manager for Biral Pumps with Hamworthy Heating Ltd, Fleets Corner, Poole, Dorset BH17 0HH.
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