This article is about a project we completed in the summer to improve low delta T at TGH. The project installed new chilled water valves with integrated flow meters (“Energy Valves”) and controls on seven air handling units in the Toronto General Hospital Eaton Wing. If you’re only interested in the savings, you can skip to the end!
Low delta T is a common issue that impacts the performance of cooling systems in many buildings, including TGH. Delta T refers to the difference in temperature between chilled water supplied to the building for cooling and water returned from the building back to the plant to be chilled again. Follow these links for a refresher on building cooling and the TGH Deep Lake Cooling chilled water plant.
Why is low delta T bad?
- If the water is coming back from the building too cold, it means we are pumping too much water and wasting electricity to run the pumps.
- Chillers tend to operate more inefficiently at low delta T, wasting even more electricity.
- In the case of TGH, we have minimum requirements for temperature of water we are allowed to return to the Enwave district cooling loop. In order to meet those requirements, we typically must run a large chiller to boost our return water temperature and drop our supply water temperature.
What causes low delta T?
- Bypass water. In many cases, older systems use bypass valves to divert water past loads and back to the chilled water plant when there is low cooling demand. This design was common prior to variable speed pumping systems, when pumps were all constant speed and their flow could not easily be reduced. Secondly, if valves or controls are not operating properly, chilled water may be bypassing cooling loads and returning directly to the plant unintentionally.
- Overflow of cooling coils in air handling units (AHUs). In many hospitals chilled water coils are oversized due to large design safety factors. The valve supplying chilled water to the coil is typically only controlled to maintain a discharge air temperature for air conditioning. On a cooling coil, once design flow is reached adding additional chilled water flow to the coil may not provide significantly more cooling. For example, supplying 100 gpm of CHW could be providing the same air supply temperature as 75 gpm but the controls don’t care as long as supply air temperature is satisfied. This problem is what we are trying to address with the installation of energy valves.
Energy Valve Project
To test the impact of the energy valves before larger implementation, we replaced 7 aged pneumatic chilled water valves on AHUs in the Eaton wing and monitored their performance. First though, how do the energy valves improve our delta T?
The energy valve has a built-in flow meter and supply/return temperature sensors. Therefore, the energy being delivered by the coil can continuously be measured. The valve is used to control discharge air temperature the same way as before, but now we have additional data to add secondary control to improve performance.
Firstly, maximum flow as per the original cooling coil design can be set on the valve to prevent overflow of chilled water. Additionally, coil flow can be maintained regardless of varying pressure in the system. After initial installation, the valves were run for a month as normal to generate a performance curve unique to that chilled water coil. The performance curve is used to determine the maximum power point, after which the coil doesn’t provide significantly more cooling. This point can be used to further limit unnecessary chilled water flow. As long at the discharge air temperature is met, the valve can now work to reduce flow to meet a higher setpoint for delta T.
There are other non-energy benefits to this system as well. Primarily, building operators now have access to much more information about the cooling coils. When issues arise, it is easier to trouble shoot a problem when you have information.
We installed energy valves on seven AHUs out of eighteen total AHUs in Eaton Wing. Eaton Wing has a chilled water flow meter and supply/return water temperature sensors for the entire wing. These meters were monitored for one cooling season before and after the project. With the energy valves commissioned, the total flow to Eaton wing was reduced on average by about 500 gallons per minute and the delta T to the building increased by 3 degrees F. This is 500 additional gpm capacity that can now help prevent overheating in operating rooms on the hottest days.
Based on the reduced water flow, it was fairly straightforward to calculate the electricity savings from pumping power. Additionally, knowing the increase to delta T as well as the efficiency of our chiller, we were able to calculate the electricity savings from running the chiller at a lower load.
Overall Eectricity Savings: 358,443 kWh
CO2 Emission Savings: 14 Tons (equivalent to driving 56,000km in a gas car)
Cost Savings: $41,000 per year
Based on these results, energy valves will be made standard for all new construction and major renovations of air handling units at UHN. This project was completed with an excellent design team from HH Angus and extremely professional installation by Kelson Mechanical. Belimo and JCI were also highly involved in the training and commissioning of the new valves.