At Toronto Western Hospital, the central chilled water system provides building cooling and process cooling for the whole site. It’s capable of providing astonishing 4800 ton cooling capacity. Each ton of cooling capacity is the energy needed to melt 2000 lbs of 0°C ice within one hour. That’s enough to cool about 1,200 houses! The 3 chilled water pumps are the largest at UHN rated at 200 horsepower (hp), 250 hp and 250 hp respectively.
Every summer, Angelo Bruni, Facilities Manager and his team would scratch their heads about the chilled water system. When the chiller plant was built about 12 years ago, only 1 of the 3 chilled water pumps was equipped with a variable frequency drive (VFD) for free cooling in winter. Without VFDs, people can only run the pump at full speed, approximately 1750 RPM. However the system downstream has numerous control valves to regulate the flow so that an appropriate amount of cooling goes to various spaces. More or less cooling means hot or cold complaints, something Angelo would avoid at any costs.
Since the cooling load changes all the time but the chilled water pump speed is fixed, the pumps can’t match the actual load very well. To solve the issue, the design engineer at that time proposed a bypass line between the supply and return main of chilled water pipes and a differential pressure (DP) sensor not too far from the chiller plant. The bypass valve would kick into gear by the building automation system if there is excessive water pressure for control valves downstream. The solution works but there are a few potential issues:
- Operating pumps can only run at full speed for mechanical cooling regardless of actual load. The pumps cannot match cooling load very well.
- A triple duty valve on each pump outlet is partially closed by the original water balancer. Any partially closed valves will add restriction to the water flow.
- Whenever the bypass valve opens, chilled water will run from the supply main to the return main directly. That water and the energy to pump it is totally wasted since it’s not actually cooling anything.
- There are certain times that water pressure is too high even with a fully open bypass valve. It becomes difficult for the control valves downstream. In fact, building operators observed 12 passing control valves on air handling units at various buildings. Our technician had to enable steam heating coil in the middle of summer!
- The system is designed for peak load, which only happens about 1% of the time. Under part load, the water pressure will become excessive due to the location of the DP sensor. Water flow and pressure head will determine how much pumping energy is needed. Excessive pressure means waste of energy.
- One end of the bypass line is incorrectly connected to a chiller inlet. This creates a low entering water temperature issue for that chiller and sometimes even causes the chiller to trip.
These charts show that it is more efficient to control water pressure at part load using a remotely located DP sensor.
In other words, the chilled water system operation is very inefficient. It’s like driving a vehicle on 401 with gas pedal pressed all the way to the floor (pumps running at full speed), a defective transmission system that only transfers part of the engine output to the wheels (bypass valve opens) and brake used to control the speed to follow traffic (control valves must close further to maintain proper water flow for cooling coils, etc. Triple duty valve is partially closed). It’s ridiculous for anyone to drive his or her vehicle like that, but that’s how our chiller plant operates on a daily basis. Angelo knew there was quite some waste but unfortunately his team does not have an alternative.
After thorough investigation, the Energy & Environment team (that’s us here at Talkin’ Trash) determined there was huge energy saving potentials to convert the existing constant speed pumping system to a variable flow chilled water system. As part of UHN’s retro-commissioning project and energy management plan, funds were approved for construction and an incentive was pre-approved by Toronto Hydro on behalf of IESO. Starting from November 2014, the contractor installed new VFDs, remote DP sensors and flow meters, and upgraded the BAS. The plan is to modulate pump speed to make sure all the control valves are happy with water pressure that matches load changes. It is possible that chilled water flow required by the buildings may drop below the chiller minimum threshold. To protect the chillers, the existing bypass valve would open to increase the flow whenever a low flow situation is detected by flow meters. That’s the only time the bypass valve would open and it seldom happens.
The bypass pipe tie-in issue was also corrected without shutting down the chiller plant so people in the buildings didn’t even notice what extensive work was being done. The construction team used liquid nitrogen to freeze the pipe and stopped the flow before cutting and connecting to the chilled water return main. All the triple duty valves on pump discharge were opened fully to reduce their restriction on water flow to minimum. The installation, testing and commissioning were completed just in time before the summer started and cooling is really important.
To secure available incentives, the Energy & Environment team did pre- and post-project monitoring and verification (MV) as required by the provincial government SaveOnEnergy incentive program. All the pump motors were monitored for 2 weeks during the hottest days in summer before and after the installation. As seen in the 3rd party MV report, the energy savings is about 200 kW and incentives would be about $120,000. It’s also easier to control chilled water after the retrofit since the control valves downstream do not need to fight with excessive differential pressure. As of this summer onward, Angelo and his team can operate the chiller plant with confidence that the system is properly controlled and energy is not wasted.
“The results of this project are very pleasing. We are now saving approximately 200 kWh of energy each hour we operate our system during the summer peak months. As the cost of electricity continues to increase, especially during peak hours when we need to run of chilled water system, the amount of actual cost savings also continues to grow. These saving are not a one-time cost saving , but an ongoing operational cost savings. “ Angelo said with a grin.