Technical Focus No. 9 - Multichiller sequence controller
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Multichiller Sequence Controller

The times they are a changing…

You all know how the song goes, but how true this is when it comes to the latest software developments relating to the management and efficiency of water chillers. Historically many consulting engineers would typically design a chilled water system with a back up chiller should the lead chiller fail (i.e. Duty and Stand by chillers).

Each chiller would have a remote enable feature which is managed by a BMS system. Each chiller also provides a ‘common fault’ signal to the BMS. Therefore if the operational chiller fails the BMS receives a signal and then enables the stand by chiller.

However  if you take a look at the performance curve (Inverter Screw type) you will notice that the efficiency of a chiller is far greater when running in part load.
Using this very valuable information, Aermec realised that it is much more efficient to run two chillers in part load than one chiller up to 100% and consequently developed the ‘MultiChiller Sequence Controller’. 
This intelligent piece of equipment analyses the load on an ongoing basis and manages the efficiency of each compressor to optimise the efficiency of the ‘Chilled Water System’. It also balances the hours amongst all the compressors. Obviously this means that the water pumps (whether integral to the chillers or external) must provide the design flow rate to each chiller in the chilled water system. I get asked many times about the efficiency of running pumps to serve each chiller in the system and give the same answer each time. 
If you look at the kW rating of the water pump and compare it with the amount of energy saved in reducing the refrigeration power consumption, the kW rating of the pump becomes quite insignificant. See sample analysis overleaf…

Sample energy savings…

For example if you have a 562kW Air Cooled Water Chiller with an EER of 3.73 (supply @ 6C, return @ 12.5C in ambient @ 27C) the absorbed power is 150.64kW (Aermec NSI2502XA00). However instead of running a single chiller up to 100%, you should run two chillers @ 50% each (managed by the MultiChiller controller) so the EER of each chiller improves to approx 4.9 @ 50% load. Therefore the absorbed power of each chiller reduces to 57.39 and the total absorbed power for the two chillers reduces to 114.78kW giving you an impressive energy saving of 35.86kW PER HOUR
Of course you need to generate the flow through the additional chiller, so I have selected an additional water pump to deliver 74m3h @ 1.1bar which has a max absorbed power of 3kW. When we account for the additional power consumption for the second pump (for the 2nd chiller), the energy saved is 32.86kWH or 49,290kW per year (based upon two chillers running at 50% load for 6 hours operation per day with each at 50% load, 5 days p/wk and 50 weeks p/yr.). 


If you wish to convert this to a financial saving and assume an energy cost at €0.12kWh this equates to a saving €5,914.80 PER YEAR. As the cost of the MultiChiller controller is approx €3,000 incl commissioning, the payback is just six months. 
Of course as we are using very general information for the cost saving analysis which can differ greatly dependant upon other factors such as varying load and varying ambient conditions. However I believe its still fair to say that this can be considered a ‘no brainer’ to use the MultiChiller controller to manage the efficiency of multiple Aermec chillers.

If you have a live project and would like a more detailed analysis (and can provide a load profile) for the application we can download the climatic profile for the location and Aermec can then input the data into their own ACES (energy simulator) and consequently provide a very detailed analysis of the energy consumption. This ACES program can also be used to compare the energy consumption between differing types of chiller.   


For further information please feel free to call me anytime. 

Best regards,
Peter McMahon
Technical Sales Director
+353 (0) 862634103


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Unit 74 Dunboyne Business Park, Dunboyne, Co. Meath, Ireland

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