This is a bit of an older project, completed in 2018, but I thought it was worth writing about since the savings are great and we have data to report. Plus, I don’t think we have a blog written on cooling tower water softeners yet!
Cooling towers are critical components in most conventional chiller-based building cooling systems (although deep lake cooling projects have allowed us to largely eliminate cooling tower operation at TGH and PMH). Typically, warm water is pumped from the condenser side of the chiller to the top of the cooling tower, where it is sprayed over fill media and through a fan-forced air stream. The large surface area of the fill media combined with the air stream causes some of the water to evaporate. When some of the water evaporates, the latent heat of evaporation is removed from the remaining water stream, lowering its temperature. The cooled water is then returned to the chiller condenser to pick up more heat from the building air conditioning and the cycle continues. Images below show a screenshot from the KDT BAS showing the cooling towers and an image of another cooling tower at UHN’s Bickle Centre.
There are two reasons why cooling towers use a lot of water. Firstly, all the water that gets evaporated needs to be replaced in the system, known as “makeup water”. This is usually accomplished by an automatic fill valve that maintains a constant water level in the cooling tower (similar in concept to how your toilet stays full). Secondly, when the water evaporates, residual mineral contents of the water become more concentrated (think of the Dead Sea becoming more salty as the sea evaporates). When the concentration of certain minerals reaches a high level, it can precipitate out of solution and get deposited throughout the system (for example, scale can form on chiller condenser tubes and cause poor chiller performance and costly maintenance). To combat this effect, water is continuously purged from the cooling tower, known as “bleed off” or “blow down”.
The ratio of makeup water to bleed off water is known as the cycles of concentration. Higher cycles of concentration means less bleed off water and therefore less overall water consumption. However, as I mentioned, the higher you go, the more you risk scale buildup. In the past, the cooling tower at KDT was operating at an average of 3.1 cycles of concentration based on several years of meter readings. This means for every three units of makeup water, about one unit goes down the drain.
In order to reduce the risk of scale buildup, a water softener was installed to decrease the water hardness entering the cooling tower. With lower levels of calcium carbonate in the makeup water, the facilities and water treatment team were able to increase the cycles of concentration to 6.0 cycles. Now, we are only sending one unit of water to drain for every six units of makeup water. The chart below shows the reduction in water consumption:
The left orange axis shows water consumption decreased roughly 30% or 4,700m3 per year after the project was implemented in 2018! That’s a reduction of almost two Olympic sized swimming pools each year! The right blue axis shows cooling demand each year, proving that the reduction is independent of weather effects year over year. The related cost savings of this project are also quite eye-popping:
- Water cost savings: $19,000
- Makeup water chemical treatment savings: $27,500
- Added operating cost of softener: $1,700
- Total Cost Savings: $44,800
With an implementation cost of approximately $13,000 this measure had a payback under 4 months! Credit goes to the facilities management team at KDT led by Rick Ysidron for implementing the project. If you are looking for water savings and cost savings, your cooling tower might be a good place to start.