Last time we talked about chilled water system upgrade at TWH. The chilled water is pumped to various mechanical rooms to cool air supplied to the hospital, medical equipment such as MRIs and some local refrigeration machines. Then it comes back to the central chiller plant and is cooled down by the chillers. The chillers use refrigerant to transfer the heat from the building to condenser water side. You may be curious where the huge amount of heat removed from the hospital goes. The answer is cooling towers. Cooling tower is a simple but critical device for our hospital operation. It is also the most neglected equipment for many sites.
Figure 1 Basic construction of induced draft counter-flow cooling towers.
There are many types of cooling towers. Figure 1 shows basic construction of a induced draft counter-flow cooling towers used at TWH. On top of this type of cooling tower sit the fan and motor, which draws cool air from outside. Warm condenser water from the chiller goes to distribution pipes and some spray nozzles on the top and then to plastic packs in the middle. The water and air are flowing in opposite directions and this is how it gains the name of “counter-flow”. The plastics are called fill media with cross-corrugated configuration. The purpose of the fill media is to increase the contact area between air and water and to increase the period of time of the contact. Inside the cooling tower, a small portion of water will absorb the heat, evaporate and disappear in the air (that’s why you would see huge white plume over some operating cooling towers in winter). There is also some heat exchange between the warm water and cool air. The cold water will go to sumps located in the bottom of the cooling towers and then return to chillers, and the cycle starts over again. Between the fan and spray nozzles, there are some drift eliminators with S shaped channels. Their function is to catch the large water droplets while let the vapour to escape. This will minimize water loss of the cooling towers. It’s truly amazing that almost all the heat removed from 1,200,000 ft² TWH is rejected to atmosphere through those 3 cooling towers.
Figure 2 TWH cooling towers and unsung heroes, from left to right: shift engineer Mark Rodriguez, chief engineer Isaac Prashad, shift engineer Betrework Wondmeneh. Absent on the photo: shift engineers Phil Green, Sheval Rowe, Waqar Mian, Allen Eddenburn and Jayesh Patel.
To make the cooling tower work properly, there are many things our chiller plant shift engineers and operating engineers need to take care of. They are the heroes working diligently behind the scenes:
- Mechanical drive system: the cooling tower fans are belt driven. Proper alignment and tension must be maintained to run the fan efficiently. In addition fan shaft bearings should be inspected and lubricated regularly.
- Water distribution: water should be distributed evenly on the fill media by the spray nozzles. Those nozzles need to be inspected and cleaned regularly.
- Water treatment: proper pH value must be maintained to reduce corrosion. Biocide and corrosion inhibitor are added to protect the pipes and fitting and control biological contamination such as Legionella. In September 2005, a Legionella outbreak at Seven Oaks Home for the Aged in Toronto cost 21 lives. As many as 135 people were reported affected by the outbreak.
- Blowdown: To reduce scale buildup, water hardness is monitored. When it’s too high a solenoid valve will open to release some water from the system.
- Make-up water: appropriate amount of fresh water must be added to the condenser water system due to evaporation, drift loss and blowdown.
- Strainers and filter: the cooling towers used at TWH are open type. Air-born and water-born debris can enter the system. To protect the pumps and chillers, strainers were installed on the tower sumps. A bypass type sand filter was also installed on condenser water pipes to remove more debris from the water. Those strainers and filter must be inspected and cleaned routinely.
- Cold water basin: there will be some debris accumulation in the basin. It should be inspected and cleaned if necessary.
The cooling towers are sized for design cooling load with reasonable redundancy. How are those cooling towers operated during winter time when there is not too much load? In winter most of our air handling units will use cold outside air for cooling if needed. MRIs and some local refrigeration machines still rely on the chiller plant for cooling. The load is much lower when compared with that during summer. Our building operators would take advantage of the low outside air temperature and run the winterized cooling tower and use condenser water to cool chilled water to the building directly. The cooling tower is an open system and acts like a huge air scrubber so lots of dirt and debris can enter the system. To avoid contamination of the chilled water and causing trouble for medical equipment, a heat exchanger is used to cool the chilled water.
In winter it is observed total load is about 60-200 tons depending on how much medical equipment is running. It can be seen even the smallest 1200-ton cooling tower is way oversized for the load. Because of the relatively small load and high water flow rate required by the tower, the temperature difference between the entering and leaving condenser water is tiny. The MRIs’ require certain supply chilled water temperature for cooling, which determines the leaving condenser water temperature from the tower. The leaving water temperature, cooling load and condenser water flow are relatively constant. It’s not hard to figure out the entering condenser water temperature will not be much higher than the leaving condenser water temperature. The low entering condenser water temperature creates an ongoing issue of not enough heat to melt ice formed on the cooling tower.
Figure 3 Excessive ice buildup on 1200-ton cooling tower
As shown on Figure 3, almost every year there is some ice buildup issue on the 1200-ton cooling tower. Excessive ice buildup can easily cause damage to cooling towers. From energy point of view, it does not make sense to run the auxiliary equipment sized for summer load including a 125 hp condenser water pump and 2×20 hp cooling tower fans for such a small winter load.
A solution proposed is to reduce the condenser water flow by about half and use only 1 out of 2 cells on the cooling tower for winter operation. To provide redundancy to the system, the other cell will be used as backup. With careful planning, a 125 hp VFD has been installed and commissioned on the condenser water pump dedicated to the smallest 1200-ton chiller and cooling tower. Control sequence upgrade will also be made to suit the new winter operation plan. It is expected that the entering condenser water temperature will be raised and ice buildup will be greatly reduced. In the worst case the operators will have the option to reverse cooling tower fan rotation for ice buildup control. In the past the only tool available is a steel ripping bar.
This simple measure will save UHN about 245,000 kWh per year. With generous $24,000 incentive pre-approved by Toronto Hydro and IESO, the payback for the project is very attractive. This is a typical win-win situation for our energy retrofit projects.