CONTROLS HEATING AND COOLING
observed at low load, with one on-line chiller having an evaporator discharge temperature of 4.5C, mixing with the flow through the off-line chiller to achieve a mixed flow temperature of 6C. An evaporator flow temperature of 4.5C
Settings should be appropriate for the stable and economic operation of systems
carefully selected, and individual items with significantly higher pressure losses avoided, as these can dictate the maximum pump operating head. I have encountered systems where the majority of the loads had losses of 15 to 30 kPa, but a couple of loads were around 100 kPa. To minimise energy use on variable flow
systems, additional differential pressure sensors can be installed to control the individual parts of the circuits, to meet the required differential pressure only when these items of plant are operating. However, care needs to be taken to ensure stable operation of all items of plant when the system differential pressure changes. Selecting components with similar (low) pressure losses on each circuit is by far the most effective method for energy efficient controllable operation.
Chillers One problem with chilled water (CHW) systems that I have identified on a couple of sites recently is parallel connection of a pair of chillers with a common pump-set (see Figure 4). In both cases the chillers were modern, large energy efficient chillers, but had flow temperature sequence control
6C 4.5C
CHILLER 1 ON_LINE
7.5C
CHILLER 2 OFF_LINE
7.5C Figure 4: A pair of chillers connected to a common pump set
CHW SYSTEM 25% LOAD
will only satisfy a 25% load, assuming a constant flow system and 6C to 12C system temperatures. 3C would be necessary for 50% load with only one chiller operating to provide a mixed temperature of 6C, although chillers are typically set for a minimum control temperature of 4C to avoid the possibility of freeze-up. Running a chiller at lower CHW temperatures will typically have a reduction in coefficienct of performance (CoP) of around 3% to 6% at 4.5C instead of 6C, and 6% to 12% at 3C. The overall reduction in seasonal efficiency could be significantly greater, dependent upon the selected chillers and load profiles, plus there will be additional pumping losses compared with individually pumped chillers. I understand in both cases the chiller manufacturers had recommended the hydraulic layout and control system! It is normally more efficient to run modern multiple chillers together down to loads of around 20% to 25%, dependent upon chiller selection. However, individual chiller turndown is limited and stability of operation has to be considered at low loads. Significant energy savings and stable, reliable operation can normally be achieved with a controllable system design incorporating individual chiller primary pumps, a common header or buffer vessel, heat load control and variable flow secondary circuits. Reset of evaporator flow temperature at low loads can also save further energy in many applications.
Conclusion The above is a selection of common controllability issues, most of which are easily avoided with suitable input to system design, resulting in controllable, energy-efficient operation. Robust control strategies are necessary to achieve optimum energy efficient, low carbon operation. Simple solutions are preferable, wherever possible, although modern plant with direct communication via BACnet, for example, offers further opportunities for more efficient operation. CJ
l Graham P Smith is director of Birling Consulting. He was a technical adviser for and a principal author of CIBSE Guide F: Energy Efficiency in Buildings -1998.
www.birlingconsulting.co.uk www.cibse.org/bookshop
26
CIBSE Journal December 2011
www.cibsejournal.com
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