Getting the best from variable-speed pumps installed in parallel

Using variable-speed pumps in secondary chilled-water systems can reduce pumping energy by more than two-thirds. In the traditional arrangement, the variable-speed drive is programmed to maintain a pressure at the location of the differential-pressure sensor. As demand for cooling reduces, the 2-port valves start to close, and the differential pressure across the load increases. The drive then slows the pump to maintain the set value.
Variable-speed drives for pumps are generally accepted — but how can their benefits be maximised — especially when several pumps are installed in parallel. WAYNE ROSE discusses the issues to be considered.The benefits of variable-speed pumps in variable volume systems are widely accepted and the advantages of lower energy cost, accurate comfort control, reduced system component wear and reduced plant room noise levels have been discussed in some depth. The underlying principles governing improvements in pump energy efficiency are also gaining greater understanding among professionals outside building-services — particularly the ‘cube law’ which dictates that a 20% reduction in speed equates to a 50% reduction in power absorbed. The focus is not whether to install variable-speed pumps, but how to derive best efficiency from a system. This raises additional factors that have the capacity to reduce or increase the efficiency of the system. • How best to manage pumps connected in parallel. • Location of differential-pressure sensors for optimum performance • Compatibility of individual system components and synergy of control logic Pumps in parallel Using variable-speed pumps in secondary chilled-water systems can reduce pumping energy by more than two thirds, making it a highly attractive solution for new and retrofit installations. When the maximum cooling requirement of the building is large, and hence the power of the pumps, it can be advantageous to split the duty across several pumps in parallel. Variations in flow rate can then be achieved by switching pumps on and off to meet demand using only the amount of energy required to meet the momentary requirement. Splitting the duty across a number of pumps gives security of supply in the event of pump fault or routine maintenance, as additional pumps are available to satisfy demand. When pumps are selected for operation in parallel, the combination of system characteristic and pump head/flow curve are such that sufficient flow increases are possible as each pump is staged on and that no pumps are operating at the end of their curve as pumps are staged off. A common control philosophy for this type of arrangement is ‘cascade control’, with one variable-speed pump used until it cannot provide the required flow or pressure — at which point a fixed-speed support pump is staged on. From an operational standpoint, mixing variable-speed and constant-speed pumps saves energy, and capital costs are contained because there is only one variable-speed drive. There are, however, issues to be addressed. Accuracy of control is reduced, as a ‘staging bandwidth’ is generated around the set point to prevent hunting of support pumps. Energy consumption is not optimised as the variable-speed pump is often operating in an inefficient area of its operating region. Cascade control on chilled water applications can also increase the risk of pump damage if the variable-speed pump is operating at a low speed and cannot overcome the back pressure on its non-return valve caused by a fixed-speed support pump. This ‘back-pressure’ problem can also occur on systems where each pump is variable speed and all pumps do not run at the same speed when staged on. By operating all pumps under variable-speed control it is possible to stage pumps in and out so that hydraulic efficiency is maintained and reduced wear on pumps is achieved by operating them in a stable area of their operating range. This is referred to as ‘best-efficiency staging’ and gives superior energy savings over other control methodologies. In a best-efficiency- staging system, a master controller stages slave VSD pumps in and out to suit demand and exports a reference signal to ensure that all pumps are running at the same speed. The system considers pump and system data to determine whether pumps are staged in or out: i.e. is it more efficient to run three pumps at reduced speed rather than two pumps at full speed? Typical multi-pump systems in the past have required flow-measurement equipment to determine staging requirements. However, with a best-efficiency staging system, only pressure measurement is required. Locating differential-pressure sensors The next issue is where best to position sensors. With any variable-volume system controlled on differential pressure, the location of sensors will have an important impact on energy efficiency of the system. In this type of system, the pressure created depends on the sensor location and the VSD’s set point. If the sensor is at the pump, you must provide full design head, giving only minimal energy savings over a fixed-speed system. If the sensor is out in the system, you automatically take advantage of the variable head losses as flow decreases, and come closer to cube-law energy savings. Some systems require the monitoring and control of two loads that differ in size and pressure drop: i.e. two zone control. This type of control is also possible with best-efficiency staging systems. Recent advances in technology have provided system designers with alternatives to the ‘trial-and-error’ of determining optimum sensor position. One solution involves sensorless pumps, in which the inverter, attached to the body of the pump itself, enables the pump to calculate its own speed requirements based on the load on it at any one time. During operation, the power and speed of the pump are monitored by the inverter. Embedded in the memory of the speed controller are pump-performance curves for 10 different speeds. This data is programmed during manufacture of the pump and includes power, pressure and flow across the flow range of that specific pump. So, as long as the inverter can identify the power and speed of the pump, it can determine the hydraulic performance and position in the pump’s head-flow characteristics. The speed controller then regulates the pump to ensure that only the required energy for its current base-load is used. Another approach is to interface sensors directly with selected conventional zone control valves in strategic locations, where they reflect the operation of similar adjacent zones and measure overall system status. Purpose-designed control technology then interprets load requirements and minimises pump speed, whilst ensuring all load requirements are satisfied. Speed is retained to within ±5 rev/min to maximise energy savings. The sum of the parts Lastly, a key factor in under-performance of HVAC systems utilising variable-speed pumps is the ‘mismatching’ of individual system components. Increasingly, particularly in large projects, the move has been towards pre-engineering and packaging of complete systems, often from a single manufacturer, to ensure consistency and synergy of operational parameters across the entire system. In summary, by concentrating on the factors discussed above, potential benefits, include improved pressure control, greater security of supply and reduced energy cost — with 10% to 15% savings feasible compared to similar systems. Other advantages can include reduced maintenance, fewer components, reduced on-site costs (through building the package at the factory), less site commissioning and single-source accountability for the entire system. Wayne Rose is product manager with Armstrong Holden Brooke Pullen, Peartree Road, Stanway, Colchester, Essex CO3 5JX.
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