Some extensive background information to start
I'm attempting to reduce water consumption at 25+ production facilities, and I've identified that the largest consumption of water (60+%) comes from evaporation cooling for all the unit operations in the process. This is done at the facilities via an evaporative cooling tower. The towers are all the same base design, and relatively similar;
- 2, 3 or 4 cells
- counter flow induced draft (see picture)
- design capacity 45-85 MW heat load, with most being 65-75.
- the largest discrepancy is that each tower was designed for 10 degree Fahrenheit (5.5 C) Delta-T, but with a wet bulb reference temp that encompasses 350/365 days statistically per year for that respective latitude / longitude (facilities are spread out across the US Midwest, so that number varies). This is considerable, but shouldn't matter for what I'm about to ask (I include on the off chance it does)
I understand the basic workings of a cooling tower (wet bulb temp, efficiency, cycles of concentration, make-up, blow down, drift etc) fairly well. With that, I understand that the amount of evaporation is proportional and fixed to the heat load. A such, ones first opportunity to reduce water is to recover/minimize blow down and drift. The facilities already have 0.1% drift on average, and run anywhere from 10-25 cycles of concentration in addition to recovering the blowdown for process water.
Despite this, the cooling towers still represent over 60% of water usage, so the next step is looking at the evaporation side. During the summer, the evaporative load is fixed and cannot be reduced. However, during the winter we already see reduction in water consumption. This stems from a combination of latent + sensible heat changes during the winter, as opposed to exclusively latent heat changes during the summer. As much as 25% of the heat load at certain facilities is dissipated via sensible heat change during the deep winter (January February). This is a commonly referred to phenomenon in the cooling tower industry and even has it's own correction factor in some simplified equations.
This has led me to believe that there is potential to control this effect and reduce water consumption even further. Adding to this evidence is the graph below. In this graph, one can see that for a substantial portion of the year, the basin temperature goes as low as 55 degrees F. The cooling water for the process does not need to be any colder than 70-75 degrees F.
Considering that the basin temperature is consistently so much colder than necessary, my gut reaction says "we have more cooling taking place than necessary". While we don't want to reduce total cooling MW's, one would want to minimize mass transfer driving force (to stop evaporation), and maximize temperature driving force (to increase cold air to hot water heat transfer). Doing this would shift more of the heat load from latent to sensible heat. I was thinking this could be achieved by slowing the circulation fans down with VFD's. Lower air flow rate for the same water flow rate, air gets saturated sooner to reduce evaporation, and the water circulating would be warmer to hopefully increase heat exchange between air/water. In effect, what I'm hoping will be the case is being able to operate the cooling towers as a "wet cooling" unit during the summer, but utilize it as a "partial dry cooling" unit when possible to save water, but without spending the capital to purchase an actual dry cooling unit.
Now my questions
- I cannot find ANYTHING online about doing this to reduce water consumption. Am I missing something blatantly obvious that shoots down the idea of trying to control the heat load towards sensible vs latent heat? (effectively trying to "dry cool" instead of "wet cool", but using a cooling tower designed for wet cooling)
- If the idea isn't completely bunk, how would I go about modeling this with mass & energy balances &/or a psychrometric chart to get a ballpark number for how much water could be saved? Given that I know: Historical wet bulb temp & temp, Hot in temp, cold out temp, fan amperage (er go fan speed), water re-circulation rate, blow down, make-up and cycles of concentration