Enabling variable-speed pumps to deliver performance

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Oversized variable-speed pumps cannot operate to their full potential. The ideal area for operation in the pump-performance curve is A. Pumps operating in area D are oversized. Area B includes pumps that are too large. Pumps in area C of the curve would tend to run at fixed speed.
With electronic variable-speed pumps being increasingly specified in commercial heating systems, PETER NEARS emphasises that care must be taken to ensure problem-free operation and enable the benefits of energy savings to be achieved.Even in today’s energy conscious environment, most heating pumps continue to be selected oversized, leading to system problems, reduced pump life and wasted energy. Since the introduction of electronic variable-speed pumps, opinion has prevailed that these pumps will regulate themselves to the system, so an exact design is not necessary. This leads to the impression that ‘one pump fits all’ — with just a few pump models required to cover all applications. This misleading philosophy often results in the use of a pump that is too large. Hydraulic design principles To understand this fully it is necessary to consider how an electronic pump functions within the context of the hydraulic design of the system. If a pump is correctly sized and set to the system design criteria using the maximum design resistance, the pump can easily be set to respond to the falling demand and system characteristic curve. When zone valves or TRVs (thermostatic radiator valves) begin to close as heat demand falls, and delivery volumes reduce, the pump can modulate to reduce the flow and pressure in the system. Importantly, the system resistance and its continually changing conditions determine the delivery volume or flow. Influence by the pump control system is not possible when oversized. The larger the pump chosen, the larger the possible delivery volume, which will result in excessive electrical consumption, noisy systems and reduced pump life. Existing buildings When refurbishing older plant rooms the system characteristics are generally unknown. Retaining large-diameter pipework invariably means the new pump will also be too large, leading to excessive volumes being circulated through the system. This in turn results in more noise, poor system performance and unnecessary use of additional energy. In these circumstances the pump will run beyond the extent of its control range and most zone valves or TRVs will have to close before the pump can begin to respond to the changing conditions. Poor control range The effect of using a control range that is too large results in the pump using maximum electrical energy, and consequently, energy savings begin slowly or not at all. Excessive flow rates mean the temperature rise (?T) across the boiler is low. Conventional burners will cycle rapidly, and modulating burners will run at reduced output. The demand for heat may not be matched, resulting in a poorly functioning heating system with reduced system efficiency. Experience is all To determine the correct capacity a detailed pipe analysis must be completed. This should include heat loss from the pipes. If the heat requirement is low and the temperature difference between flow and return is the maximum possible, the result will be a low capacity from the pump. Biral’s experience over many years has led us to equip variable-speed pumps with an electronic speed limiter to counter the problem of pump oversizing; it is factory set at about 80% of full pump capacity. A simple switch in the terminal box puts the pump back to 100%. Electronic variable-speed pumps are an important addition to the range available. A change in system pressure will result in changes in pump performance and significant energy savings. Correct selection and setting to the system characteristic is vital to achieving such cost savings and to prolong the life of the pump. Peter Nears is product manager for Biral Pumps at Hamworthy Heating, Fleets Corner, Poole, Dorset BH17 0HH.
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