recycle temperature
recycle temperature
(OP)
I have a pump system, which may go into 100% recycle for many hours. I would like to calculate what the tank temperature will be after 10 hours. At time = 0 T= 120 F, tank volume = 5000 gallon, cp = 1.0 Btu/lb/F, the recycle flowrate = 5 gpm.





RE: recycle temperature
Calculate WHP by the formula 8.33xHxQxSG/33000, H is in feet and Q is in gpm
Check
http://www.mcnallyinstitute.com/01-html/1-04.html
thread407-96694: quiz - pump efficiency calc without flow
thread124-102433: Calculate Heat gain in a fluid from pump
and many more threads on this site.
RE: recycle temperature
RE: recycle temperature
P: fluid density
q: Heat rate (input)per a volumatric flow rate
V: volume
Cp: specific heat
Try it
RE: recycle temperature
At time = 0, cp = 1btu/lb/F, m = lb/hr, volume = 6000 ft3
energy balance is: in-out= accumulation, so:
Lb/hr*btu/lb/F*(T2-T1)= IN = 1000*(1)*(120-120)= 0
Out = -(1000*(1)*(T2-120))= d(mcpdelta T)/dt
= m dT/dt +(T-120)dm/dt => dm/dt = 0
Material balance:
0-1000 = 0 => dm/dt = -1000 Past this step I am lost, but I give it a try:
1000*(T-120)= mdT/dt = 6000*62.4*dT/dt+6000*62.4*Tdm/dt
1000ln(T-120) = 6000*62.4*ln(T-120), I am lost.
RE: recycle temperature
This depends. If the recycle line is very short and goes directly from the discharge to the suction, then, yes, it will keep going up and up. On the other hand, if the recycle line involves the pump pulling from the bottom of a tank with a discharge line going back to the top of a tank, with significant lengths of piping between and a large volume of water, the temperature may not go up that much at all. You can still use the general equation given by Powerman81, but your values will change.
As an input point, the Nuclear Regulatory Commission issued guidance back in the late 80's about operation of pumps required to mitigate accidents if they went on a long-term recycle. Two specific guidance documents, available on the NRC website (www.nrc.gov) are Information Notice 87-59 and Bulletin 88-04. In general, utilities provided responses back on the Bulletin indicating that a recycle flow rate between 10 and 25% of total flow was adequate to ensure continued pump operation without overheating. The differences arose from the length of pipe and the recycle path. (To find the Informatin Notice and Bulletin, select Electronic Reading Room from the list of the left side of the page. Then select Collections of Documents by Type and then Generic Communications.)
Patricia Lougheed
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RE: recycle temperature
RE: recycle temperature
RE: recycle temperature
In a simple way, you can calculate 42.4 btus of heat rising the temperature of 5000 gallons of water. The temperature increase will be 42.4/(8.33*5000)= 0.001F
Now, run the pump for 10 minutes and the temperature rise will be 0.01F.
Your final simple formula can be written as ΔT = 0.001*(BHP-WHP)*t (where t is minutes of pump running)
RE: recycle temperature
A star for you for recognizing that "it's just not that complicated." Who needs differential equations for a straight line anyway?
Speco
RE: recycle temperature
Thanks for the undeserved star. As you exactly pointed out, this is a straight line function.
ratash,
Q*dt = mCpdT should solve your requirement. The time rate of change of heat input is constant and it comes out of the integrand. Finally, it boils down to ΔT = Q*t/(mCp) and this is what my simplified equation gives you.
RE: recycle temperature
RE: recycle temperature
Your formulation begins by subtacting the WHP from the BHP, and then uses the remainder as the heat input to the fluid. I have seen this done before, and I always wondered about it. If all of the flow is recirculated to the source tank, shouldn't the entire BHP go into heating the fluid? It seems like the WHP goes into friction along the pipe (and hence heat) and any "left over" pressure energy is dissipated in the tank.
Thanks
RE: recycle temperature
Obviously, the upper limit for heat up calculation is BHP and lower limit is (BHP-WHP). ASHRAE suggests using BHP for recirculation systems incase of heat load calculations.
However, this is quite conservative approach. For example, (1) The inefficiency of the pump and the friction at the pipe walls do impart heating to the fluid. However, WHP is not totally offset by friction. The liquid does have kinetic energy which, ultimately, gets dissipated in the tank.
(2) WHP should take care of the static head of the fluid.
(3)Some portion of energy is being used to displace the air from the tank.
(4)Heat is lost to the atmosphere from piping and tank.
So, I think this is a risk based approach. Do you have any comments on this?
Regards,
RE: recycle temperature
If your situation is big pumps in a small system, and a few extra degrees (or Btu/lb) are important, then maybe considering all of the BHP going into heat might be an appropriate approach. If, on the other hand, the effect is small relative to other assumptions or approximations you're making, then maybe you can let it go...
thanks for listening