NuChill® Chilled Water Pump Upgrade
Following the successful NuChill® chiller replacement project undertaken in 2015-16 ThermOzone had identified that the existing chilled water circulating system was not providing very effective service, and was less than efficient.
Following discussions with the project Consultants, Mssrs Arup, we conceived a simple solution that resolved two issues, and has seen the project become self funding within a few years.
The client on Leadenhall Street operates from a medium size office building in the heart of the ‘Square Mile’ of the City of London. The chillers project here was our first full retrofit NuChill® project. It made sense to optimise the overall chiller performance by paying attention to the mode of chilled water flow control.
To properly understand the issues identified and how we resolved them is broken down into several stages…
Primary Loop System
Modern buildings using chilled water as the mechanism to provide a cooling source for the various services around the building need to optimise how the system is laid out. A variety of services require a continuous supply of cold (chilled) water, typically around 10 - 12oC. However, getting the water to diverse secondary systems at an even temperature and with sufficient flow can be difficult. The typical resolve for this is a dual loop system. The first or primary loop simply circulates water using a pump (and normally a standby pump) around a loop whereby the chillers, normally at least two, but any number can be provided, are connected in parallel and downstream of the pump.
From this loop at some point after the chillers’ outlet pipes have re-joined, further take off pipes or headers will provide water to separate ‘secondary’ pumps which in turn send the water to individual services, e.g. Air handling units, Fan Coil units and chilled beams.
These systems are generally quite diverse, and operating a Primary / Secondary loop system makes the task of cooling the water in a controlled manner far more manageable.
The flow around these secondary services picks up waste heat energy from the building then returns the warmed water back to the primary loop, and this water is in turn re-circulated through the chillers for re-cooling. This is a continuous process. It is also self regulating - if the secondaries draw less then the flow in the primary loop will simply cool at the return to the on line chillers, reducing the load by the chillers own internal control.
Mixed flow restriction
An inherent problem with parallel flow chillers serving the primary loop is that to achieve a steady flow temperature to the secondary system headers means either all chillers must be running, or if one or more shuts down when the load is light, then the running chillers must compensate by further (over) cooling the water passing.
The causes a further problem because having a variable setpoint for individual chillers is complex to control, and the further the water is cooled the more energy is consumed by the chiller.
Indeed there is also a fundamental limit to how cold the water can become before freezing becomes a significant risk. Practically for most systems this is 6oC, but why should a system that is overall required to supply water cooled to 10oC have to over cool some of the water to 6oC.
A further explanation of Mixed flow temperature dilution
If the primary flow is say 100 litres per second, entering the chillers at 14oC, with a desired mixed flow temperature of 10oC, and this is split into two equal parts flowing through each chiller. Only Chiller 1 is running, but water is circulating through both chillers.
|Chiller 1 & 2 return temperature||14oC|
|Chiller 1 exit or flow temperature||10oC|
|Chiller 2 exit or flow temperature||14oC|
|Mixed flow temperature||12oC average of each chiller flow temperature.|
Thus is can be seen that with both chillers running the Mixed flow will be 10oC as desired, but as the load falls off and one chiller stops, the mixed flow then rises to 12oC, which is less than desirable because the secondary services still require water fed at 10oC.
Thus to compensate Chiller 1 now has to cool the temperature to 6oC, reducing the average Mixed flow to [(6 + 12) / 2)] 10oC.
The more chillers that are assigned to a primary loop the more critical this problem becomes, and some chillers with multiple sub-systems can see compounded mixed flows. Indeed one site we recently observed was requiring water at 6oC with four chillers in a parallel connection. To achieve 6oC Mixed flow but with just one chiller operating would have required it to supply water back to the system at -12oC, a somewhat difficult task, and even with anti freeze -12oC is a difficult and expensive temperature to cool to.
Back End Valves
There is however a simple solution to this problem - the Back End Valve. This is a motorised valve that can shut off the flow when the chiller is not required. This means the mixed flow now equals the temperature from the individual chiller/s remaining on line. This removes the need to overcool the water, increasing the overall operating efficiency, and also requires less effort from the primary circulating pump, saving pump energy.
Pump control - PID loop
As a chiller shuts it back end valve, less water circulation is required in the primary loop. Indeed the total flow in the loop to the secondary headers is less important than a maintained constant temperature.
A proportional–integral–derivative controller (PID controller) uses a feedback loop signal to accurately control an output, in this case the speed of the chilled water primary circulating pump. The logic is provided inside the pump Variable frequency drive Inverter, and requires just one control input variable to achieve accurate a constant control based upon a continuous pressure increase across the pump.
As an individual Chiller’s Back End Valve closes, the flow around the system becomes restricted, imposing a greater back pressure to the pump outlet. The pressure sensor detects this increase and the PID control loop within the Inverter smoothly and instantly adjusts the pump speed to reduce the pressure difference to the setpoint assigned. There is no requirement for any complex control logic between the Chiller’s and the pump, the water flow and pressure inter-reaction provide al the control input required.
A separate run / stop logic is required to ensure the pump cannot run ‘dead headed’ i.e. in a situation where no chillers are required, and this is a simple task for the BMS system assigned to stop and start the overall systems, using end switch contacts on each Back end valve. At least one back end valve end stop “Open” must provide a positive signal to allow the pump to run.
A separate control loop is also provided within the BMS to rotate Duty and Standby pumps and to run the standby pump in the event of failure of the duty pump.
So in consideration of the above the actual pump set up at the site was from some elderly belt driven fixed speed pumps. The permanent 3 phase current draw was ~19 amps (~11.5 kW). Closing the Back end valves would see the pump having to work harder to overcome the increased back pressure, so use of Back end valves was not practical without including output control.
Modern Inverter pump selection tables showed we could achieve similar flow requirements for far less effort, and the pump re-selection suggested a run current of just 6 amps (~3.5kW) would support the same flow requirement of all three chillers at this site. It is recognised a significant energy loss is caused by the belt drive, however this was the expectation at the maximum flow position with all three chillers back end valves open. As the load reduces to two (95% of the time) or one chiller (30% of the time), the pump energy would also reduce, although not by so much.
Armstrong offered a Pump with very similar geometry. This minimised the modifications required to the pump connections, and this was installed complete with a suitable VFD Inverter with built in PID control capability, plus a simple pressure sensor allowed us to achieve accurate control from this single input. Please note the single sensor works on a principle that the low side pump inlet pressure is held constant by the pressurisation system.
The whole thing can be integrated separately to the BMS system as an added feature, although for initial set to work and proving the BMS works were of secondary importance.
As this is a live and operational site we had to achieve integration with minimal loss of service, indeed because each pump set had isolation valves this was quite straightforward.
Some minor pipe adjustments were necessary but all achievable within a single day for the hot works (welding) works.
The electrical supply and control for the original pumps was largely re-used. The contactors were effectively removed and replaced with the Inverter as Pump power controller and starter. The original pump run and fault signals were used via a control relay to provide the start signal, with the VFD Inverter providing the fault output function.
© Trevor Dann - December 2017