Mulitple tank recirc pump question

Kevin Bouwman

Colin,

I am from Iowa but am getting to know some of your past customers. Several Alberta dairymen have relocated to my trade area.

It is easy to see that you know what I am up against. I have seen many hot water systems and their installers brought to their knees by the demands of a dairy. It really is more about how much fully hot water you can get out at once than it is about how you make it hot. I too have a 100 gallon refrigeration heat recovery tank. I no longer count the heat they collect in my equations because too often maximum demand happens when the unit has been drawn empty buy an earlier demand. They save operating cost but they cannot be counted on to contribute to stored supply. I notice that your twin 60 gallon IDW installation amounts to 30 gallons less than your max demand and the twin 80's exceed it by 10 gallons. This still seems like you are cutting it close. Would you recommend a twin 100 gallon install for a 180 to 190 gallon demand or would a twin 120 gallon be alot safer? Your series vs. parallel advice makes a lot of sense to me. Your concerns about rollover with a single IDW connected to additional storage is exactly why I posted in the first place. Stopping the recirc pump during a dump draw is not impossible but it is not trivial either. I would love to know that I could put it a pump that would not need to shut off.

Mark, I love your columns and I you have given me a lot of additional options to consider. I will need to make a decision soon and have just had it complicated by an addtional heat recovery method to consider. I have calculated that if I ran all of my waste water from the cleaning processes into a pit, installed an evaporater in the pit connected to a compressor and water cooled condenser, I could pump enough of the heat back into the tanks to cut the total water heating cost in half. The main drawback being the increased installation cost.

Thank you both for your input, Kevin

Kevin Bouwman

I need to build a DHW (Dairy farm Hot Water) system that can deliver 200 gallons of 160+ degree water in 15 minutes and then recover within 4 or 5 hours. I will have a 100,000 btu electric boiler for a radiant floor heat system that I intend to use with a 40 gallon indirect heater and two 120 gallon storage tanks with a recirculation pump. Because I need the 200th gallon flowing out of the system at 160 or better it is important that the recirculation pump does not over mix the three tanks. If I design the potable hot water recirculation loop such that the BTU of incoming cool water is equal to or below the recovery capability of the indirect water heater I should not see the potable outlet temp of the system drop, correct? If I do this, then as the temperature of the incoming recirc water rises, the boiler will start to cycle because the warmer water will not absorb as much total heat, correct? Is it ever done to use some type of variable speed pump to match the heat absorbing capacity of the incoming recirc water to the boilers heating capacity? Is much attention ever paid to sizing a pump for circulating water with multiple tanks?

[Deleted User]

Wow...

Guess I better leave my calculator running...

Remember, math is NOT my strongest suit.

Experience tells me though that you will not be able to draw 100 perent of a tanks stored volume without breaking up the straticiation, and causing some dilution of the final discharge temperature. Conversely, the only way to guarantee top to bottom unstratified temperatures is through constant homoegenization (mixing, not allowing stratification to occur). If I'm reading your thread correctly is exactly the opposite of what you want to do.

Let's run the numbers first.

You need 200 gallons of 160 degree F water. Assuming that you can get 80% of a stored tanks voulme out before dillution and mixing occur, this means that you will need 200/.8 = 250 gallons of stored hot water.

Assuming incoming well water at 40 degrees F, this represents a btu demand factor of 280 gallons X 8.33 lbs per gallon X 120 degree F delta T = 279,888 direct btu's. This does not include heat losses from the piping system or the tanks. You'd be wise to add something to that to insure coverage. Lets add 10% to that for CYA reasons, so your btu demand is 279,888 / .9 = 310,986 btus, divided by your allowable time frame of 4.5 hours = 69,108 btu's per hour. The major limitng factor in all of this is going to be the heat exchangers ability to produce this much energy on the load side of the heat exchange process. I'd recommend you consider the use of a Thermomax, or Triangle tube, or flat plate heat exchangers to get the job done.

Now, back to your delivery issue. It's going to be pretty tough to guarantee that the tank temperature from top to bottom will remain at 160 degrees F. If you can get the tanks there, when it comes time to dump the tanks, you'd be wise to shut down the pumps that are moving the water between the source and the load to avoid inadvertent mechanical dillution. This could be done with a flow switch on the cold inlet to the tanks. As soon as the dairy starts the dump load call, the circulator is shut down until the load has been delivered. Once the load ceases, recovery takes over to rebuild the tanks.

Alternately, you could keep the tanks at an even higher storage temperature (180 to 200 F), and use a mixing device to lower the temperature to the required output temperature. This would cause your standby losses to be much higher, but would do a better job of guaranteeing that the 200th gallon is dead nuts on at 160 degrees F.

As for the rest of your statemnt regarding "If I design the potable hot water recirculation loop such that the BTU of incoming cool water is equal to or below the recovery capability of the indirect water heater I should not see the potable outlet temp of the system drop, correct?" I'm assuming that what you're saying is that you want the incoming cold water to hit your heat exchange/source tank first, and you want it to have the abitlity to act as an instantaneous water heater to insure against cold dillution. This is where is gets tricky. 200 gallons draw in 15 minutes = 13.3 GPM. 13.3 GPM at a 140 degree F rise would be 932,960 btu/H transfer capacity, which is going to be impossible to hit due to your 100,000 btuH boiler capacity. If my interpretation of your staement in wrong, then disregard and clarify.

As for the second part of this statement "If I do this, then as the temperature of the incoming recirc water rises, the boiler will start to cycle because the warmer water will not absorb as much total heat, correct?" Not sure what your trying to get at, but a btu is a btu and it does the same thing to water at 40 degrees F as it does if the water is at 159 degrees F. Heat exchanger effectiveness is affected by water temperature differential, but is usually sized based on worst case scenario, and will work substantially better if at a more ideal condition.

Maybe the storage tank route is not the best or most efficient way of looking at it. Maybe you should be looking at it from the instantaneous position.

Using electricity, you'd need 932,960 /3.413 = 273,354 or 274 KW of electric power, or if using natural gas, and a Takagi Mobius TM1 at 90% efficient you'd need 1,036,622 btu's input to guarantee delivery. With their TM1 rated at 3.5 GPM for a 150 degree F rise, you'd need 5 of them piped in parallel to do the trick.

Not sure what constraints you have, but now you know your options...

Questions?? Corrections??

ME

Kevin Bouwman

Mark,
Let me add a little more information, a cutoff temp for the system of 170 to 180 is very practical for the process. Let's call the 40 gal indirect heater tank #1, the 120 gal storage tank in the middle of the recirc loop tank #2, and the second 120 gal storage tank tank #3. I intend to have the cold water enter tank #3, pass from the outlet of #3 to the inlet of #2, from the outlet of #2 to the inlet of #1, and out of #1 to the point of use. Tank #1 would be a Megastor with a 100,000 btu input capacity. Tanks #2 and #3 would be highly insulated commercial storage tanks with 1-1/4" bottom inlets and 1-1/4" top outlets. I very much want to avoid trying to do this as an instantaneous heater.

> As for the rest of

> your statemnt regarding "If I design the potable

> hot water recirculation loop such that the BTU of

> incoming cool water is equal to or below the

> recovery capability of the indirect water heater

> I should not see the potable outlet temp of the

> system drop, correct?" I'm assuming that what

> you're saying is that you want the incoming cold

> water to hit your heat exchange/source tank

> first, and you want it to have the abitlity to

> act as an instantaneous water heater to insure

> against cold dillution.


What I mean here is that if I pump water out of tank #1 with the recirc pump into the inlet of tank #3 at too high a flow rate I will have mechanical dilution...can I avoid this by pumping slowly? If I pump slowly enough that when the water in tank #2 is 40 degrees it only takes 100,000 btu to raise it to 180 won't it take only 50,000 btu to raise to 180 when the water in tank #2 is 110 degrees causing the boiler to cycle off half the time? What I am curious to know is if it is possible, practical, or worthwhile to consider running the pump as a variable speed so the flow rate is inversely proportional to the temperature difference so that I can keep a mass of water coming into tank #3 that can just absorb all the boiler capacity while minimizing mechanical dilution?

On a related note; how big a contributer is cold water inlet velocity to mechanical dilution in a water heating appliance? Can/Should this be addressed in an application like mine by oversizing the supply line to the appliance? What should the target velocity be?

[Deleted User]

HMMMmmm...

Interesting concepts for sure. My gut tells me the tanks 2 and 3 should be piped parallel reverse return with #1 being the heat source. Thats the way I've been doing it for many years. I've never personally piped anything in series, so I can't say that one is better than the other. No empirical data for or against your method, just personal experience.

As for variable speed pump, the biggest problem would be finding a control to give you what you want. Nothing on the market to my knowledge. You could experiment with a variable speed fan switch on some of the 00 or 15 thru 26 series pumps.

I've always piped them parallel because I try and recover tanks equally, which means the heat source will always see the coolest water available, thereby creating the greatest potential for heat exchange. Also, on draw, both tanks would contribute equally to the load, and the flow rate into the tank would be 1/2 of the system flow rate on draw.

I'm sure that a Siggy could develop an FEA given enough time to prove or disprove your theory.

As for entering velocity, the more subtle and spread out the flow is, the better the stratification, and what I refer to as the cold piston effect, to push the hotter water out of the tank without enhancing dillution. I think that State Industries has a method called hydro jet whereby the inlet pipe is connected to a circular inlet pipe inside the tank with holes drilled all around the ring to enhance the piston effect.

A lot of open solar tanks have a baffle, or bubbler built into the top of the tank to enhance thermal stratification of hotter water at the top, and colder water at the bottom.

As far as flow rate is concerned, given a fixed exchange, the slower the flow, the larger the delta T, the faster the flow, the lower the delta T. From a heat exchange transfer view, you want to maintain as little delta T as you can, thereby keeping the average temperature differential across the exchanger greater. THe greater the delta T between source and load, the more transfer you will get.

Again, my gut tells me parallel reverse return with high extraction flow rates, ceased extraction during dump draw, but its just a gut feeling. My method would require the use of a 3 port tank. 1 six inches from the bottom, another 6" above that, and the outlet tapping on the very top.

Myself and Kelly Weibold, an instructor at RRCC were going to do this very same piping experiment at the school, and meter it to see which one actually delivered more gallons of hot water. Then he took a job as the plumber for a local prison, and dropped out of site for a year or so. He's back now, but I don't think we'll be able to get you any answers soon enough for this project.

ME

Colin_2

Hot Water at Dairy

I am relatively new to the heating bus.(1yr) but previosly spent 11 yrs as an installer/technician for a dairy equipment company. Because of relationships built I have had the opportunity to apply my new trade in a few of these installations. Our dairies (Ontario CAN.) tend to be smaller, but what we found can be applied on a larger scale. The Largest we did was one that required approx. 150 gal of 180* water. We wanted to prove the system to the customer based on fuel saving so we put a meter on his existing 80 gal, 250000 btu commercial water heater for a time 1st and then converted to new syst with 100000 btu condensing (propane) boiler and IDW's. We started with 2 60gal IDW but found we ran out of hot water. The problem occured ouncew a day when the milk cooler was being washed at the same time or immediatly following the milking syst.The water syst. on the farm is very good, and the syst. is dumping water at 30-40 gpm. We piped the 2 IDW's in series on the domestic and boiler sides both.(hotest boiler into the hotest IDW.) not enough hot water. Tried parallel piping on both boiler loop and domestic. No better, if anythig worse. (when the hot water was gone, it was gone). Since the boiler is also supplying small heating zones, office milkroom etc. we did not want to upsize boiler, and it is also less costly to add storage as opposed to extra btu. We switched the 2 60s for 2 80gal IdW and now the syst. works good. (piped in series both ways). Saving approx 30% on fuel. At the rate they are consuming about 10$ per day. You do the math. We have the sensor in #2 tank (the cooler one) and shut down at about 170*. the #1 tank ends up being about 185*. We went into the 2nd tank with sensor because we want it to respond as quick as possible to incoming cool water. Another notable thing is that our syst. recovers heat from the milk cooling syst. and the 1st 100gal+ coming in is already preheated to 100-120*. We tried 1 syst with an IDW and a storage tank, and found you need to be very careful about rolling over your tanks and losing your stratification. We prefer additional IDWs in series as this increases your T.D. and also gives you more heat exchanger surface at these hoter water temps . The series piping also seems to keep the return water temps lower longer for increased eff. with cond./mdulating boiler. Hope this helps!

[Deleted User]

I'm going to follow my gut...

It's never let me down:-)

Let us know what you did and how it works Kevin.

ME

mike parnell

heat exchangers

there is a heat exchanger that claims you wont run out of hot water,its something i am planning to use for my own house system,http://www.ergomax.com/.I have seem these on display at farm shows,might be worth looking into,ive going with it because my plumber doesnt think i can fill a 80-100 gallon jacuzzi with out running out of hot water,its worth a try,less $$$ than a megastore my plumber wants....mike

Larry Weingarten

heat recovery

Hello: If you have a stream of warm waste water perhaps one (or more) GFX heat exchanger/s would be of use. Sized right, they collect 60% of the "waste" heat. Also, I like the idea of series tank connection if flow is OK, because you'll get more undiluted hot water.

Yours, Larry

Paul Pollets

Recovery

You could obtain faster recovery using a dedicated steam boiler and Viessmann HoriCell indirect tank(s) The horizontal tanks are rated for steam and don't have issues with stratification. I've installed several for residential use and the recovery rate is under 5min for 53g @ 125 deg F./10gpm flow

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