Canada is usually associated with our long cold winter. To take advantage of the cold weather, we go out for skating, skiing, snowshoe hiking, ice fishing, snowmobiling, etc., tons of fun activities. Is there anything we can do at hospitals to make full use of the winter? Actually there is. Our hospitals have much essential medical equipment that requires cooling year around. Magnetic resonance imaging (MRI) is one example. It relies on deep cooling to maintain super-conductors inside to generate extremely strong magnetic field and radio waves to do very detailed scanning of patients. The temperature required is so low for super-conductors that they are submerged in liquid helium, which is about -270 °C (-454 °F).
Figure 1: Typical MRI Scanner Cutaway
A typical MRI scanner uses 1,700 litres of liquid helium sealed in an aluminium casing and its low temperature must be maintained by special mechanical refrigerators. If the low temperature cannot be maintained, the helium will boil to gas just like boiling water in a kettle and the gas helium will expand in volume and must be release to atmosphere. This is called quenching. Usually quenching poses no threat to patients or healthcare workers unless there is blockage in the quench pipes. However it not only causes service interruptions, but also is very expensive to reenergize or repair.
Figure 2: MRI quenching
The special mechanical refrigerators for liquid helium rely on chilled water for cooling, which means we must provide cooling to them all the time. How can we use cold air outside to generate cold enough chilled water? With climate change, we get hot spells here and there in the middle of winter. How can we guarantee safe operation of MRIs?
Yes, we can still run a chiller to cool the water down but it doesn’t make sense when outside is -10°C. Plus in winter the space cooling load is gone, whether chiller can handle the low process load is another question.
There is another piece of equipment with which we can reject heat to the cold atmosphere directly at TWH, our cooling towers. To run it in winter and eliminate freezing hazard, the cooling tower must be winterized. As we know cooling towers work on wet bulb temperature instead of dry bulb, even if we got hot spells as long as the wet bulb temperature is certain degrees lower than our target water temperature, it can still provide enough cooling. This is very similar to how kids cool themselves down in a splash pad in hot summer.
Figure 3: Cooling tower works similar to splash pad in certain ways.
Caution must be taken to avoid excessive ice buildup and freezing otherwise the next time you see those cooling tower running is coming spring.
Figure 4: A frozen cooling tower (not at our sites) could be disasterous to cooling system.
Since the cooling tower at TWH is open type, to protect the chilled water system, a heat exchanger was installed to separate chilled water and condenser water from cooling towers. Last winter we tried to run the free cooling system but it looked as if the heat exchanger did not exist – operators saw no chilled water temperature reduction through the heat exchanger. It turned out the pipes for an existing sand filter were clogged and there were debris inside the heat exchanger. Further investigation showed the heat exchanger was not properly sized and the temperature/flow on the spec sheet gave us very little margin for future expansion. As a result, we had to run a chiller all winter long last year.
Figure 5: Existing heat exchanger plate with scale buildup
To fix this we resized the heat exchanger. Old plates were replaced and about 100 additional stainless steel plates were added to existing heat exchanger to increase its capacity and make it future proof. New filtration system was also installed on condenser water side to give us the peace of mind. As mentioned in the past, we already have the pump VFD ready and our plan is to reduce flow through the cooling tower by 50% so that we can run 1 cell instead of 2 cells. This will effectively increase entering water temperature to the cooling tower and help melt possible ice buildup. In the end of the day, everything was back online right before Christmas and we are on free cooling again. That’s a great holiday present to UHN!
Figure 6: A heat exchanger like new.
There are still some existing issues with cooling towers. During commissioning when we tried to run 1 cell, we found out the isolation valves would not close tightly enough due to aging and small amount of water would still go through the valves. If we run one cell, it is expected there will be huge amount of ice buildup in the other cell which could cause damage to cooling tower structure – every drop of water passes through the valve will be frozen before it leaves the fill media. As a result we can only run water through 2 cells and do very limited condenser water flow reduction for the cooling tower. With the pump VFD running at 95%, demand saving is about 20 kW. There is another 60 kW savings from the chiller. That’s about 288,000 kWh savings per year. The most important benefit of this retrofit is possibly to save the wear and tear on the chiller and give our team the opportunity to inspect and service the chiller to make it summer ready. Next step is to replace the valves during shoulder season and make the free cooling fully functional.
As usual we received full support from our facilities management team. Without it this project will never happen. Big kudos to TWH facilities manager Joe Lopes, chief engineer Kranti Sharma and shift engineers!