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Watts needed to heat a water circulating system.

Watts needed to heat a water circulating system.

Watts needed to heat a water circulating system.

(OP)
Watts needed to heat a water circulating system.

I am trying to heat a closed recirculating fluid system from room temp to 194F, in less than an hour.

Currently with 2800 Watts into the 5 gallon tank of deionized water, the system peeks at 180F after three hours with the water flowing.
I'm sure the problem is the heat loss into the room.  We keep adding insulation, but aren't getting closer to the solution.

How can we calculate the watts need to do the job?
The system material is 316SS, I can estimate the mass of the piping system, surface areas etc.
Tank vented, Pressure after pump 12 psi;  flow 6 gpm;    

Any help would really be appreciated.

Thanks,  JWS7
 

RE: Watts needed to heat a water circulating system.

"recirculating" into where, exactly?  If it isn't doing anything, then it wouldn't need to be "recirculating."  Since it is, then the intent must be to transfer its latent heat into something else.  What is that something else?

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Watts needed to heat a water circulating system.

"..........Tank vented,"

Why is it vented? Maybe that's part of the problem.

Like IRstuff, what is the purpose of recirculating?

Please explain in sufficient detail what you are doing so we don't have to guess and give you wrong answers.

RE: Watts needed to heat a water circulating system.

(OP)
Thanks for the response.  
The water circulates to heat a complex rotary valve which is being tested.  The valve is of 316SS, and weighs about 75 lbs.  We are going to be testing the valve through numerous cycles which simulate the actual operation during production in the pharmaceuticals industry.  All the plumbing are the "Sanitary type" (clamp w/ gaskets) fitting.    JWS7    
 

RE: Watts needed to heat a water circulating system.

OK, so how did you account for the heat transfer to the valve?  Given that your water volume occupies less than 1 cubic foot, and therefore has less than 1 m^2 of available heat loss area, it can't be losing much more than about 500W, even with no insulation.

You need to do the math. 6 gpm at only 3°F temperature drop would remove over 2500 W, which is nearly your entire heating capacity.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Watts needed to heat a water circulating system.

(OP)
A centrifugal pump draws H2O from the heated tank, and pushes the water to a series of plumbing fittings and up into a couple ports of the motorized rotary valve.  The water exits the valve through open ports at the bottom, falls into a drain system back to the pump.  Actually we added some needle valves on the exit ports to keep the spec pressure at the valve input.
The tank is covered with Plexiglas and gasketed.  I said vented because I meant it has no pressure.
Heat is lost from all parts of the system plumbing as well as the test valve. We also have a makeup water tank to account for evaporation.  About ½ gallon per 10 hrs.
I am about to add a heater tape with control and wrap it around the test valve, adding 720 watts.   I'm just looking for a formula to estimate the energy needed in watts to see if we are close or not.  Thanks,  JWS7
 

RE: Watts needed to heat a water circulating system.

Any chance of performing a measure of Tout (downstream the tank) and Tin (upstream the tank)?
Have you carried out any test without water flow, that is just heating the volume of water stored in your tank?
 

RE: Watts needed to heat a water circulating system.

(OP)
Yes, the tank heats to 200F with the water not flowing.  When we start to flow, the tank temp drops to 160F, and after a while it goes up and settles at 180F.  Meanwhile, we have two other thermometers in the system, one close to the pump output, and another closer to the test valve input.  These usually register 10F lower as you get farther from the tank.

RE: Watts needed to heat a water circulating system.

Get an Engineer familiar with fluids and thermodynamics. This is a lot more complicated than you think it is.

RE: Watts needed to heat a water circulating system.

There is no "formula," because you haven't even begun to provide sufficient information to do anything beyond sheer speculation.

What goes through the rotary valve?
How big is it?
You claim that your system is "closed" yet you're able to lose water from evaporation?
Where in the chain are these temp sensors and what do they see?

You need to get an engineer conversant with this type of problem.  You're losing 10°F along the way, which is a substantial amount of heat loss.   

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Watts needed to heat a water circulating system.

Ops, I've forgotten to ask how long is the heating process up to 200 F (with no water flowing).
Anyway if you've detected a 10 F temperature drop along your line your heat source is definitely underrated (check previous IRstuff's comment).

You have to heat up your pipeline, fittings, valves (specific heat for stainless steel is approx. 0.17 W/lb/ F)  and compensate heat losses of the whole system. IRstuff has again given you a good figure for the heat loss throughout the tank surface, then considering only natural convection you can assume the heat transfer coefficient for your pipe is on the order of 6 W/(m^2 °C). Evaluation of heat loss though the valve is another matter.
 

RE: Watts needed to heat a water circulating system.

" The water exits the valve through open ports at the bottom, falls into a drain system back to the pump"



Looks like a wide open system to me.

Is that correst?

More like a cooling tower.


Why do you need an open system for this?

Well, if that is the best you can do structurally and you are willing to add more heat , then there is no need to hire an engineer for this.

I calculated that given your steady state losses of 2800W at 180 F and making the reasonable assumption that losses are proportional to (T-Tamb) and a system time constant of 1 hour, you would need 4500W to raise the water to 194 in 1 hour, using a 70 F starting temperature.

RE: Watts needed to heat a water circulating system.

(OP)
Thanks Zekeman.  The exit port falls into the return bucket because the test valve is motorized and rotating.  The guard around this zone where I lose some water as the condensation drips out.  The tanks heats to 200F in less than an hour.
I appreciate your 4500W estimate.  This info what I was looking for.  My current plan is to add 720 W heating tape to the test valve OD, ;  and use a 1500W space heater on the metal bench top aimed up toward all this upper plumbing.  That totals about  5020W.  This should be enough to get our testing going.  
Next I'm facing a filtering issue; because the point of the test is to measure particulate matter, and I cannot keep the 10 micron filters clean enough to keep the flow up to 6 gpm.  The water system needs to be cleaner before we can measure accurately the PM.  I wonder if there are experts somewhere on this forum for this subject.   Thanks,  JWS7
 

RE: Watts needed to heat a water circulating system.

JWS7,

If it takes approx 1 hour to reach 200 F (assuming 68 F as starting temperature), heat loss in your tank are on the order of 1200 W. It seems a bit high to me, anyway I suppose you are sure your heat input is 2800 W.

Frankly I doubt only another 4500 W could compensate the heat loss of the line (according to the info you've passed on to us, that is 10 F temperature drop detected with 6 usgpm flow). I would go for 12 kW overall heat input.
 

RE: Watts needed to heat a water circulating system.

(OP)
Thanks ione.  The tank is actually a heated water bath  (rated to 200F)  with 600W heater and rated at  2048 output BTU.  We added two  1100W immersion heaters.  It has insulation below in its cabinet, but not great insulation.  We had a pipe welded to the tank bottom for the pump suction line.  We changed the SS lid for Plexiglas so we could close-fit the fittings.   
Another issue is the SS tank is showing some rust spots, and this is putting contamination into our system that we cannot tolerate.  We covered the numerous spots with JB Weld, but we may need to have the tanks coated with PFA or something.
Your 12KW estimate is scary.  My assistant says the static tank heats to 180F in 45 min, but it take another hour to reach 200F.  Maybe we need to improve the tank insulation.
 

RE: Watts needed to heat a water circulating system.

"that is 10 F temperature drop detected with 6 usgpm flow). I would go for 12 kW overall heat input."

Ionne

Let's see, the 10 degree drop @6 gpm cannot square with  180 degree equilibrium and 2800Watts.


Jws,

Is 10 gpm with a 10 degree drop an equilibrium condition, and exactly where are the measurements made.

 

RE: Watts needed to heat a water circulating system.

(OP)
Hi  Zekeman.  Yes, the 6 gpm and the 10F less at subsequent points seemed steady for three hours or so.  The tank was 180F.  The next measurement point is about 5 tube- ft away after a few large SS valves and tees.   The last measurement point is after some more valves, tees, and hose another 5 feet away; just as the water enters the test-valve manifold system.  The test valve itself is probably lower temp; I will need to measure it with a heat gun for the next test run.

RE: Watts needed to heat a water circulating system.

Easy experiment would be to run a smaller heater in your tank (maybe try one of those 1100W heaters you got), wait for it to get to steady state with the pump running at 6 gpm and record the temperature and time.  You have one data point, steady state temp on the x-axis and wattage on the y-axis reaching steady state in a certain amount of time.  You already have a second data point for 180 deg steady state with 2800 W in a certain amount of time.  Extrapolate the Wattage to 194 F and multiply by the desired time you want to heat it in.  I think that might get you in the ball park with an oversizing factor to be sure without doing the tedious heat transfer calcs.


Are you controlling the temperature somehow to the heater?  You will overshoot if you want to heat it in one hour and keep having to turn the heater on and off manually if not.

What do you guys think?
 

-Mike

RE: Watts needed to heat a water circulating system.

" Zekeman.  Yes, the 6 gpm and the 10F less at subsequent points seemed steady for three hours or so.  The tank was 180F.  The next measurement point is about 5 tube- ft away after a few large SS valves and tees.   The last measurement point is after some more valves, tees, and hose another 5 feet away; just as the water enters the test-valve manifold system.  The test valve itself is probably lower temp; I will need to measure it with a heat gun for the next test run."

This is very confusing.You use terms like tube-feet and now you say several valves, etc. As I stated earlier, the equilibrium 180 @ 2800 Watts is at an severe odds with the delta T of 10 degrees and 6gpm which doesn't make sense to me and resulted in the wildly different estimates of mine and Ione.... unless you are talking 2 different experiments. It should not be so difficult to draw what you are trying to say, since your explanations are not clear.

Accordingly, I would respectfully ask you to show us a sketch of what this thing is doing, including all of your heat inputs and flow rates,pump location, tank runs of piping and the valve(s) being tested. I think you have  simple problem that is not being presented properly.

Very confused

Zeke

  

RE: Watts needed to heat a water circulating system.

There's a gigantic disconnect:

4.1868(J/gm*K)*.9718(gm/cm^3)*10(°F)*6(gal/min) = 8.56kW

By this calculation, your tank should never get much above room temperature.

TTFN

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RE: Watts needed to heat a water circulating system.

(OP)
OK.  I appreciate the knowledge and talent here, and your help.  The system has other features, but I didn't want to complicate the original question.   I have a non-disclosure agreement with the customer, so I was trying to be a little vague.  Sorry.  I could show you photos and schematics,  but then, you know what would have to happen...   Basically, there are actually two tanks, one at 86F and one at 194F.  There are 1 1/2" ball valves to select the supply and return plumbing to one tank or the other, depending upon the test cycle.  Also the pressure has two different requirements, so I have selectable pressure relief valves, with the 1 1/2" SS ball valve to shut of the lower pressure choice again depending upon the cycle.  Also the plumbing has tees for pressure gages, and tees for temperature gages at certain locations.  These all add mass and surface area to the system.
The performance issue I am addressing here is getting to the required temperature using the hotter tank.  I was asked the distance between components, and the physical distance might be close, but the feet of tube (or pipe) the water travels to get there, I called the tube-feet.  Sorry for the confusion.
In the end, if I could understand the formulas you are using to estimate these Watts, I could try to better estimate the variables to narrow down the answer.  Thanks,  JWS7
 

RE: Watts needed to heat a water circulating system.

It seems IRstuff and I are tuned on the same wavelength, but frankly I still have difficulties to figuring out your scenario.
If I'm not mistaken (but probably I am), you take water at 194 F from a tank to heat up water stored in another tank at 80 F. Water from the second tank (the colder) is then circulated back to the hotter and so on.

An enthalpy balance is required:

Mh*cp*Th + Mc*cp*Tc = Mtot*cp*Te

Where:
Mh = mass of water stored in the hotter tank
Mc = mass of water stored in the colder tank
Mtot = total mass of water (Mh + Mc)
cp = water specific heat
Th = hotter tank temperature
Tc = colder tank temperature
Te =  equilibrium temperature (unknown to be computed)

It is understood that without keeping on heating the equilibrium temperature Te will always be lower than Th=194 F

You have now to evaluate power required to heat up the whole water mass Mh from Te temperature to Ttarget target temperature (this also involves another parameter, time t required to complete the heating process).
You have then to consider power required to heat up pipes, fittings and valves:

Msteel *Cpsteel *(Ttarget – Te)

You have then to add wattage to compensate heat loss through the system (you can consider 6 W/m^2/°C for heat transfer coefficient).
 

RE: Watts needed to heat a water circulating system.

jws7,

You can't come here asking for free advice and not give us a clear picture of the system you have.Worse than that ,you have wasted all of our time trying to help you with this "nondislosure" problem And to ask for equations is simply over the top. How can we give you equations? You haven't given us a straight problem.

It's like asking a doctor to diagnose a problem without giving him any symptoms.

I strongly suggest you bring in an engineer to unravel this problem  and mercifully end this dialogue.

I think the administrator should close this thread out.


 

RE: Watts needed to heat a water circulating system.

Beating a dead horse it seems...........

Get an Engineer familiar with fluids and thermodynamics. This is a lot more complicated than you think it is.  

RE: Watts needed to heat a water circulating system.

(OP)
Sorry Zekeman.   I thought that my original question was straight forward.  The extra tank is bypassed during the cycle in question, so I didn't think it needed to be  mentioned.   I am trying to heat the test valve, not the 2nd tank of water.  
I suppose you are right.  We will just end this inquiry.  Sorry, but I do appreciate your efforts.  JWS7
 

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