I have an application where I need to consume 20 gallons of hot water every 8 minutes for a total of 7 times (for a commercial washing machine). The temperature needs to be 160 degrees Fahrenheit for each fill. What size commercial water heater would I need to get to make this work? Below is what I think would work...
75 Gallon Rheem Commerical Water Heater @ 75,100 BTU (recovery rate is 75.8 gallons per hour)
Can someone confirm?
Theory only works perfect in a vacuum.
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I assume by the "total of 7 times", you mean 7 times per hour??
Assuming this thing is going to actually be feeding 20 gallons every 8 minutes (all day), it would be best to ignore the inital stored volume and concentrate on what the burner can produce.
So, if this thing is working full time, we can assume 20 gal/8 minutes = 2.5 gpm (~20 lbm/min)
Next, you need to look at your temperature rise. During the summer, you are looking at probably 100F rise (160F - 60F). In winter, you may be more like 120F or 125F rise.
1 BTU will raise 1 lb of water by 1 F.
So, 20lbm/min x 125F rise = 2500 BTU/min or 150,000 BTU/hr
This means if you have 150,000 BTU (output) you will never run out of hot water assuming nothing else substantial is tapping into this tank at the same time.
Now, if this think isn't going to run back-to-back or you have more time between loads, you can get by with a smaller burner. You could also potentially go with two tanks (with say 75k burners) instead of a single tank with a huge burner.
Some commercial washing operations have also been able to reap a real performance boosts from drainwater heat recovery heat exchangers, provided that there is some drain flow concurrent with filling. For a single batch fill/drain later it doesn't work, but for multiple washers running staggered timing it can be very cost effective. The ideal is a fully balanced flow (say in a shower, where the drain flows for over 95% of the period that the hot water is being drawn), but big operations with many washers, there's a strong temporal overlap, if not 90%+. It's some thing that has to be designed, for a commercial app, unlike a residential shower.