Finally Found!

I'm so excited! FINALLY, after 4 years of searching (admittedly, it wasn't a full time search), I think I have found the key to maximizing ROI on most rainwater jobs for commercial buildings!

If attended one of my rainwater presentations in the past, you've remember page like this:

Where I tried to come up with some reasonable estimate of water used for cooling tower makeup (CTMU). It's my assertion that CTMU is one of the best uses for rainwater in commercial buildings, and provides the best payback. It's relatively easy to pipe, even in a retrofit. The rainwater is cleaner than the water in the cooling tower, so treat the water in the tower, not the incoming rainwater.  

But estimating demand correctly is key to maximizing payback.  Too small and you might buy water that you could have stored. Too big, and you bought more tank than you needed (and you dedicated more space than you ought). So trying to realistically estimate demand, I created a table like this:

Assuming a run time based on maximum and minimum average temperatures in a month. It wasn't the best possible estimate, but it was better than anything else I could find. Using these numbers, we came up with a total building demand that looks like this:

Which showed that for buildings 5 - 10 stories and taller, all available water from the rooftop could be used for cooling tower makeup. That's important, because we could use all available rainwater, but not have the extra expense of extra piping to all the fixtures. Just one riser to the cooling tower, which in many buildings is the way it was designed anyway.

Well, I was never happy with the calculation. I knew that there had to be something better. I mean, an outside air temperature equates to a certain cooling load requirement in the building, which after you go through some efficiency losses, equates to a certain amount of water that needs to evaporate (taking the heat with it) in the cooling tower. It's been a long time since I studied that in school.

Well finally!  I've found a resource to calculate the ACTUAL number of gallons of water used in a cooling tower. Not the maximum flow for the size, but how much is required at a given outside temperature. At 70°F outdoors, the cooling tower doesn't have to work nearly as hard as at 100°. First, the chiller doesn't reject as much heat, because the heating load is smaller. Second, the cooling tower is more efficient.  I always knew that there had to be a way to calculate it, but couldn't find the equation. But here it is! Thanks to SPX cooling towers for creating the page.

We also need to calculate the actual cooling load, but that's much simpler. Stay tuned for more reliable payback calculations!