Deriving value from variable-speed pumps
One of the major players in the market for speed-controlled pumps has been looking at the benefits of this technology in chilled-water air-conditioning systems.Large chilled-water air-conditioning systems have traditionally utilised large constant-speed pumps to circulate the chilled water, with throttle valves to control the flow. The development of variable-speed pumps makes possible new approaches to circulating water in the various parts of such an air-conditioning system — enhancing performance and considerably reducing running costs.
Considerable benefits A case study carried out by Grundfos Pumps highlights the very considerable benefits that can be realised. The project involved the replacement of fixed-speed pumps in an existing system with variable-speed models. The renovation of the air-conditioning system in a 23-year-old building involved the replacement of all the 3-port valves with 2-port valves and replacing fixed-speed pumps with variable-speed models. An area of 23 000 m2 is cooled for 7000 h a year by 450 fan-coil units and 35 air-handling units. The total cooling capacity is 1200 kW. A cost comparison was made with the use of constant-speed pumps selected to run at their best efficiency point. While the initial capital cost of the more sophisticated system was about three times greater than the fixed-speed solution (about £12 000 compared with £4000), the vastly reduced energy consumption gives a predicted life-cycle cost over 20 years for the variable-speed system of just 40% that of the fixed-speed design (Fig. 1). Such is the vast improvement in energy efficiency that the simple payback for the extra capital and maintenance cost is probably less than a year.
Fig. 1: Such are the energy savings made possible by speed-controlled pumps compared with fixed-speed units in a real-life refurbishment project that payback was achieved within a year.
Opportunities There are many opportunities for using variable-speed pumps to improve the performance of chilled-water air-conditioning systems and minimise energy and life-cycle costs — as highlighted in technical information developed by Grundfos Pumps. All the ideas are based on the fact that the relatively small difference between flow and return temperatures in chilled-water systems compared with heating systems necessitates larger pumps. The accurate control of circulation is thus the key to achieving comfort and efficient operation. For example, at the heart of a chilled-water system is the evaporator. To maximise energy-efficient operation, water should flow through the evaporator at a constant rate. Much more effective than the combination of a fixed-speed pump with a throttle valve to control flow is a speed-controlled pump set to run at constant speed at the correct duty point. In a large system with several chillers operating in parallel, speed-controlled pumps make it much easier to achieve constant flow at all times (Fig. 2).
Fig. 2: for maintaining a constant flow through the evaporator, a speed-controlled pump set to run at constant speed at the correct duty point is a more effective solution than a fixed-speed pump with a throttle valve.
Out in the distribution system, the demand for chilled water varies throughout the day and throughout the year. Variable-speed pumps and 2-port valves provide a more effective solution than fixed-speed pumps with 3-port valves to divert part of the flow. Even if heat withdrawn from a building by the air-conditioning system is recovered for other uses, such as space heating elsewhere or generating domestic hot water, a means of rejecting excess heat is required. Cooling towers are much more cost-effective and environmentally friendly than air-cooled condensers. The performance of a tower depends on ambient conditions, but what does not change is the need to supply water to the condensing circuit at a constant temperature of 27°C. Pumping requirements thus vary considerably, and the most effective and energy-efficient approach is a variable-speed pump controlled by temperature (Fig. 3).
Fig. 3: To deliver water from a cooling tower to the condenser at a constant 27°C a variable-speed pump controlled by temperature is an effective solution.
Variable-speed pumps can also be applied to circulating water through terminal units. To cool air for distribution through ductwork, a common approach is to pass the air across a coil through which chilled water is passed. For maximum efficiency, a constant flow of water should be maintained through this coil. To minimise whole-life costs, a variable-speed pump set to run at constant speed at the correct duty point provides a better solution than a fixed-speed pump with a throttle valve (Fig. 4).
Fig. 4: The combination of a throttle valve and constant-speed pump to deliver a constant flow to a cooling coil in an air-handling unit can effectively be replaced by a variable-speed pump.
Where fan-coil units and chilled ceilings are used as terminal units, it is necessary to ensure comfort and avoid condensation that the water temperature supplied should be higher that that delivered by the chiller to the main circulation system. Since the required temperature varies according to the load characteristics, the use of individual room controls to vary the flow make it beneficial to install speed-controlled pumps (Fig. 5).
Fig. 5: A speed-controlled pump with 2-port valve and bypass makes it possible to vary the flow to chilled-ceiling panels (and fan-coil units) at a higher temperature than in the main distribution system.
Future changes Pumps with electronic speed control enable the flow of chilled water to be matched to system loads, maximising energy efficiency and minimising energy consumption. Operating profiles can be adjusted in response to seasonal and climatic changes. Future changes in needs are also readily accommodated. Variable-speed pumps in chilled-water applications can be controlled by variations in pressure, temperature, flow, differential pressure and other measurable parameters. Not only do such pumps achieve energy savings of 20 to 50%, but they also enable other components in a system to function at maximum effectiveness.
Grundfos Pumps Ltd is at Grovebury Road, Leighton Buzzard, Beds LU7 8TL.
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