Reviewing F-Chart Data from Duke
You may recall my trip a few weeks ago to the retrieve the flow meter data from the mechanical room of the Bryan Center on the campus of Duke University. I’ve been swamped with several other projects since then but finally had enough time to take the data and put it into my F-Chart program. Remember, the starting point for all solar hot water systems is to determine a client’s current consumption of hot water in gallons per day. It turns out that the Bryan Center is using about 4700 gallons of hot water each day. So how does this number drive the overall size of our proposed system? Take a look at the video below to find out. I’ve also provided a copy of the actually F-Chart here for you to follow along with.
Leave your questions or comments below!
Dr. Ben
Hi Dr. Ben,
We developed very similar drainback systems in Denver in 1979-1983. Evaporative water loss with an atmospheric tank was one issue we didn’t completely solve. I just reviewed your 1981 patent 4416258 and noticed the pressure venting pipe (40).
Did this pipe solve the problem completely? It looks like there would still be some “breathing” that would allow vapor loss.
Kevin, you question is a good one about evaporative losses in a non-pressurized solar hot water system.
In designing the Fluid Handling System, I was faced with the question of how to have an atmospheric pressure tank (non pressurized/ non ASME) with a vent that didn’t evaporate large quantities of water. The answer is that we can eliminate the standby ” evaporative” loss, but we cannot eliminate the losses that come from the expansion and contraction of the vapor volume inside the tank and piping system. This is done by an “immersion” vent. The vent runs from a low “cold” part of the tank upward into the air space in the top of the tank. The molecules of air and water in the pipe try to rise with the heat gradient inside the pipe, but cannot since they are trapped at the top of the tank. The immersion vent effectively stops stand by evaporation losses.
The expansion and contraction losses are reduced by having the initial steam from the collectors strike a large body of water (the tank water) before they migrate over to the vent pipe. I even came up with a ratio of square footage of “quench” water in the top of the tank to the collector area. If that ratio is over 40 (collector ft2) : 1 (tank ft2), then high evaporation losses occur. If the ratio is under 40:1, water loss can go years before any water makeup is required.
A second very important parameter is the load cycle of the system. Suppose a system sits idle through summer weather. The tank will arrive at the maximum temperature and shut down. The collectors will soar in temperature. If a little energy is removed from the tank and the temperature drops below the cutoff deadband, the collector pump with turn on again. With very hot collectors, a large amount of steam is generated before the water cools the collectors below the boiling point. The volume expansion during this time will blow steam out the vent pipe.
This condition can occur in residential systems when the family goes on vacation and forgets to turn the system off. In a commercial system, the facility could have an employee holiday and be idle. In both cases, the system should simply be turned off.
However, with low ratio systems, this has never been a problem. A simple visual check once in a while and a manual refill are enough.
We did find out, however, that some government military bases have stated that they don’t do maintenance (“we have air filters that haven’t been changed for two years”), so we developed an automatic refill option for commercial system.
The bottom line is we anticipated the loss of water due to steam volume expansion during low duty cycles, developed a metric to indicate a potential severity of the problem, and offer an automatic water makeup for anyone who doesn’t want to check the water level occasionally.
There is one other method for eliminating water loss in a drain back system. That is to make it pressurized. There are a number of small residential systems (check AETs offerings) that are put together very much like a glycol system but have a small drain back reservoir in the line and a heat exchanger in the water heater.
This doesn’t scale well, because going to large pressure vessels for the drain back reservoir gets costly very fast. This method also requires a heat exchanger between the collectors and the tank, which the non-pressurized version does not. A direct heat transfer from the collectors into the storage tank is much more efficient than having an exchanger in the line.
I Hope this windy reply answers your question.
Hello Dr. Ben,
I just wanna say that i am very amazed in your blog. I wish i could also write blog like you do. Thanks! I learned a lot. Keep posting Dr. Ben. Congratulation!
Dr. Ben,
First off, thank you so much for all the clear and concise information, and what all you have contributed to the industry both here in NC and elsewhere. I am just getting to explore your website and have been very impressed thus far.
In hopes of quizzing your brain: I’m seeing the scenario you describe above with low loads during summer months. To add insult to injury this is a combined DHW and space heating system – 4 AE40’s. The system is drainback, and I put in a relief valve on the collector outlet to protect against high pressures. Needless to say, the valve is blowing (temperature is surely throwing it and not pressure – 210F and 150 psig). Considerable steam is coming out at times, but the collector Tmax prevents any pumping from occurring. I’m considering pressurizing the system a bit more (a 20 – 30 psig steam valve?) and seeing how much I can reduce this, but I’m anticipating this may still be an issue. Any thoughts or ideas?
Thanks,
-David
David,
If you have a non-pressurized system, then throw the TPRV (temp & pressure relief valve) away, it is not needed. When the system is off, the collectors may go above 300F. That is normal.
If you have a pressurized AET drainback module then put a PRV (not a TPRV) on the drainback tank for safety. It may operate when you generate steam as described below.
The only problem you will have is when the tank temp falls below the high limit deadband and turns the system on again during the day when the collectors are hot. It will generate lots of steam until it cools the collectors down.
This should vent out harmlessly from a non-pressurized tank, but may trip a PRV. You will have to watch your water level in the tank so you don’t loose too much water. You can also just turn the system off if the tank is hot enough, especially when you go on vacation.
I wouldn’t add any pressure to the system. You could instead install a higher pressure PRV.
Good luck,
Dr. Ben