×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Time to heat up lube oil tank

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...

RE: Time to heat up lube oil tank

This is an unsteady state heat transfer problem that requires a differential equation (heat balance) to be solved (integrated). Some editions of Perry's Handbook and Kern's Process Heat Transfer have several cases already solved.

Good luck,
Latexman

RE: Time to heat up lube oil tank

(OP)
If I know the kW rating of the heater outside the tank (through which the oil is passing), would this simplify the analysis. I want to treat the tank as a insulated body.

An approximation is satisfactory because an exact time to reach a specific temperature is unecessary.

RE: Time to heat up lube oil tank

If you Google "batch heating time calculation" you can probably find the equations and/or a website with a calculator. The equations are not too difficult. Yes, you can also approximate this numerically on Excel. I'm pretty sure if you use Search (between Forum and FAQs) in this forum you can find a description how to do that in an old thread.

Good luck,
Latexman

RE: Time to heat up lube oil tank

(OP)
I'll search around. If I know the specific heat of the oil and the kW rating of the heater, it should be a pretty straight forward equation.

Thanks for pointing me in the right direction.

RE: Time to heat up lube oil tank

No problem. Let us know how it turns out, or if you have more questions.

Good luck,
Latexman

RE: Time to heat up lube oil tank

why dont you put an RTD in the tank, run a loop for 30 min., plot T vs. t, and correlate that to a curve.

RE: Time to heat up lube oil tank

If it's an electric heating element, I'd be willing to bet that the heat transfer is only weakly a function of oil temperature. I'd use the mass of oil, its heat capacity and temperature change to calculate the duty you need. The kW rating of the heater should give you the time.

RE: Time to heat up lube oil tank

(OP)
This is what I did..

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

That would be the perfectly ideal time, which requires simultaneous heating and mixing of every molecule in the tank. Since you are not doing that, your actual time will be higher.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

(OP)
Yes I understand, but due to the reasoning behind heating the oil, an exact time is not crucial. Would it be wise to take this time and double it, or even triple it? Just to add some margin

RE: Time to heat up lube oil tank

But the way you describe it, the time is determined by the amount of changeover of the circulating oil. You obviously cannot dump the entire heat load instantaneously into the circulated oil, so your calculation is off. Your equation should be based on the mass transport of the circulation loop.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

(OP)
Understood. The inlet and outlet of the circulating loop are 1-2 feet from each other in the top of the tank, but are at different elevations in the tank. Due to mixing in the oil I was trying to stay away from doing an unsteady state calculation since the exact time is not urgent, but rather an estimate.

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

Yes, you replace the mass with the mass flow, multiply by the specific heat, and multiply by the temperature change you can achieve in this side loop, which gives you a power that is divided into the total mass to get a time. Again, this ignores any transport delays needed for mixing.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

Oh, that was just pitiful shadessad

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)

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

(OP)
If I know the kW rating of the heater would that factor into the equation above?

RE: Time to heat up lube oil tank

(OP)
I ended up with 15 hours using the equation above and 9 hours using the original method I posted, so the 15 hours makes more sense since this is a circulation loop.

Thanks IRstuff for the input!

RE: Time to heat up lube oil tank

The heater's power output is relevant in needing to be large enough to provide the heat flow back into the tank.

Note that the above analysis is EXTREMELY superficial; there are lots of parasitic losses that are not accounted for.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

(OP)
After re-working the problem in Mathcad I ended up getting a much lower heating time using IRstuff's method versus my original method...this doesn't quite make sense to me since IRstuff's method is more accurate and takes into account the flow rate of the circulation loop.

Or am I just overthinking this?

RE: Time to heat up lube oil tank

(OP)
IRstuff. Your equation cancels delta T and specific heat and just leave total mass over mass flow rate. This may be why I am getting such a low number. Any thoughts?

RE: Time to heat up lube oil tank

They should be different delta_T's; if you wanted a final delta_T of 10°C, you wouldn't necessarily use a 10°C delta_T for the input flux.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

(OP)
If we are wanting to raise the temperature of the tank from X degrees to Y degrees, wouldn't that difference in temperatures be the same for both equations?

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

If you wanted a 5° change in the entire tank, would you really only heat up the loop to 5°, and take the hit on warmup time?

Not saying you couldn't, and if you did, then yes, the quantities would cancel out, as you'd expect, since it strictly boils winky smile down to the mass exchange through the heating loop, and the calculation reduces to total_mass/mass_flow, as you'd expect.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

(OP)
can I replace the heat load with the kW rating of the heater in the circulation loop?? Because I need that in the equation somehow...

RE: Time to heat up lube oil tank

Then, the delta_temps would be different, wouldn't they? The first equation tells you how much power you need, which is why I had them separated.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

(OP)
Understood, but we know how much power we have right now.

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

That's assuming that level of power doesn't cook the oil...

which means that you may need to crank up the flow rate.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

IRstuff:

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.

----

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

You can take the heater power, 4kW, and divide by the specific heat of the oil, resulting in a quantity with the units of gm*K/s. This is the mass_flow*delta_temp product that your circulation loop must be able to support. So if the specific heat is 2 J/gm*K, then the mass flow temperature product is 1910 gm*K/s. So if your temperature delta is 10°C, then you need to move almost 20kg/s through the loop. You can do that, but I'm not sure that the convective transfer of the oil is that good.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Time to heat up lube oil tank

since the temperature of the oil will be stratisfied, I would suggest the installation of a mixing paddle to lessen the time to heat the oil and to get a more accurate bulk temperature of the oil.


RE: Time to heat up lube oil tank

(OP)
I think the confusing part is that this skid has a set flow rate and a set heater...can't be changed. So when the oil passes through one time is will not reach the deltaT desired the first time, may take 2-3 loops. But I think I can use a lot of this information posted on here, thanks for the help.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources