Time to heat up lube oil tank
Time to heat up lube oil tank
(OP)
We are filtering these rectangular lube oil tanks and want to use the skids to heat up the oil during cold startup.
I pulled out the old Heat Transfer book but I think I'm over thinking this. If we are taking cold oil out from the bottom of the tank, heating it (specific temp.), and discharging just below the oil level, how do I determine the time for the entire contents of the tank to reach that specific temp.?
I know the geometry of the tank, thermal resistance of walls, heat capacity of the oil. The troubling part is determining how the oil mixes when the suction and discharge are relatively close (just different elevations in the tank). I'm starting to think a general approximation would be sufficient...
I pulled out the old Heat Transfer book but I think I'm over thinking this. If we are taking cold oil out from the bottom of the tank, heating it (specific temp.), and discharging just below the oil level, how do I determine the time for the entire contents of the tank to reach that specific temp.?
I know the geometry of the tank, thermal resistance of walls, heat capacity of the oil. The troubling part is determining how the oil mixes when the suction and discharge are relatively close (just different elevations in the tank). I'm starting to think a general approximation would be sufficient...





RE: Time to heat up lube oil tank
Good luck,
Latexman
RE: Time to heat up lube oil tank
An approximation is satisfactory because an exact time to reach a specific temperature is unecessary.
RE: Time to heat up lube oil tank
Good luck,
Latexman
RE: Time to heat up lube oil tank
Thanks for pointing me in the right direction.
RE: Time to heat up lube oil tank
Good luck,
Latexman
RE: Time to heat up lube oil tank
RE: Time to heat up lube oil tank
http://www.chromalox.com/catalog/resources/technic...
RE: Time to heat up lube oil tank
RE: Time to heat up lube oil tank
Converted 4 kW to BTU/hr. Multiplied this by the specific heat of the oil to get X.
Divided X by the total weight of the oil to get Y.
Divided Y by the temperature difference I am trying to achieve to get Z.
This left me with Z in units of 1/hr
Just took the inverse of this and got it in hours. Again this is simplifying the problem drastically. But I was conservative in the temperature range based on the ambient conditions. I could in theory add some sort of safety factor to this to account for the mixing of different temperature fluids in the tank and the convection heat transfer from the air.
Does my methodology seem correct? I can post exact numbers and units if that helps.
RE: Time to heat up lube oil tank
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RE: Time to heat up lube oil tank
RE: Time to heat up lube oil tank
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RE: Time to heat up lube oil tank
If I know the flow rate of the cirulation loop and the volume of the tank, is it possible to somehow include this in my calculation? Just doesn't seem to be a spot to include this...
RE: Time to heat up lube oil tank
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RE: Time to heat up lube oil tank
mass_flow*specific_heat*delta_temp = heat_flow_in (Watts)
total_mass*specific_heat*desired_delta_temp = thermal_mass (Joules)
thermal_mass / heat_flow_in = time (seconds)
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RE: Time to heat up lube oil tank
RE: Time to heat up lube oil tank
Thanks IRstuff for the input!
RE: Time to heat up lube oil tank
Note that the above analysis is EXTREMELY superficial; there are lots of parasitic losses that are not accounted for.
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RE: Time to heat up lube oil tank
Or am I just overthinking this?
RE: Time to heat up lube oil tank
RE: Time to heat up lube oil tank
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RE: Time to heat up lube oil tank
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RE: Time to heat up lube oil tank
Our desired delta is the the difference between X and Y, then what is the other delta T represent?
RE: Time to heat up lube oil tank
Not saying you couldn't, and if you did, then yes, the quantities would cancel out, as you'd expect, since it strictly boils
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RE: Time to heat up lube oil tank
RE: Time to heat up lube oil tank
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RE: Time to heat up lube oil tank
Wait, I think I'm getting this now, lol. If I know the current power of the heater I can calculate what the delta T will be at the flow rate of the heater circulation line. This in turn can be applied to the total mass of the tank (this delta T will be the temperature rise we want) and eureka we have the time figured out...
RE: Time to heat up lube oil tank
which means that you may need to crank up the flow rate.
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RE: Time to heat up lube oil tank
Could he take it (the double-ended dynamic thermo-dynamic and flow mixing problem in two simpler steps?
Make the approximation that the lube oil is flowing through the (known capacity) electric heater at a small but steady rate that will heat up the oil coming out of the heater to a single almost-constant value, regardless of what the input temps into the heater are ... at least at the bigenning of the problem.
Then take this steady (but low-percentage of the total tank flowrate) amount into the larger tank.
Heatup rate of the large tank then becomes function of (Heat gain into tank from steady oil flow) - (heat loss from tank into room due to delta T between tank and room)/(heat capacity of tank & oil inside tank).
Heat problem (in the larger tank) then becomes a dynamic (logarithmic) problem of a-steady-amount-of-hot-oil-into-large-tank problem.
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Now, realistically, when I use an electrically-heat oil heater just like this during an oil flush to heat up power plant lube oil tanks, I don't really measure the heatup rates nor do I specifically measure the oil flowing through the heater. 10 to 15 hours is sufficient to heat a 1200 MegaWatt oil tank from room temperature to 130 degree F flush temperature using a kilowatt-sized in-line heater with a 2 inch lube oil hose going through the heater and back into the tank. (A cold plant in winter in the mezzanine is about 45 F to 60 F. A warm plant in the summer is 65 to 75 degrees oil temperature.)
In either case, once the oil is up to a flush temperature of around 130 degrees, we just turn each heater coil off sequentially on and off and keep it (output temperature temperature of the heater element) between 130 to 140 degrees. Tank temperature (system oil temperature) will be about 5 to 8 degrees lower. Don't really "calculate" things - just keep a watch on the heater output thermostat since each plant's oil room and pipe geometry and ambient (outside) temperature is different every time and for eery flush.
RE: Time to heat up lube oil tank
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RE: Time to heat up lube oil tank
RE: Time to heat up lube oil tank