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Temperature of a Fluid after Heat Losses

08/10/2015 12:44 PM

Hi,

I've been inquired by one client which is requesting to change the thickness of a pipe insulation. One insulation has heat loss of 8.5 w/m while the second is around 7.5 w/m. The temperature of the fluid (heated water) inside the pipe is around 60ºC.

I have no clue how to measure if this difference is significant or not. Is that possible to measure the inlet temperature against outlet temperature? Could it be measured per meter?

Any equation or tip will be very appreciated.

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#1

Re: Temperature of a fluid after heat losses

08/10/2015 12:49 PM

It depends with the length of pipe of course, 0.5 inch will not matter much

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#2

Re: Temperature of a fluid after heat losses

08/10/2015 12:52 PM

You will need to look at how the heat loss ratings of the insulation was developed. If it was assumed that the pipe has 100 degC water in it and the ambient temperature was 25 degC then you have a 75 degC differential that creates the watt per meter loss.

If your water is 60 degC and the ambient temperature is 40 degC, then the cost of the thicker insulation MAY not be worth the cost in terms of energy saved over the lifetime of the insulation or plant.

That's how engineers earn their money.

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#3
In reply to #2

Re: Temperature of a fluid after heat losses

08/10/2015 1:00 PM

It would be even less of a reason to change out if the said pipe is 20 cm in diameter and carrying 2000 LPM flow rate.

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#4

Re: Temperature of a fluid after heat losses

08/10/2015 1:05 PM

Is that possible to measure the inlet temperature against outlet temperature? Could it be measured per meter?

Yes, you can measure the inlet / outlet difference if you have the measurement equipment available and you know how to use it.

Yes, you can also measure it per meter, however it is going to be prohibitively expensive to install thermo-wells every meter.

  • How long is the piping run?
  • What is the pipe diameter?
  • What temperature is the water......
  • At the inlet?
  • At the outlet?
  • What is the design temperature on the heated water piping?
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#6
In reply to #4

Re: Temperature of a fluid after heat losses

08/10/2015 1:20 PM

The question was not well formulated, sorry. I don't want to measure meter by meter. I just want to find a way of calculate how many temperature difference I would have in a meter of pipe.

The temperature of the fluid, let's say, in the inlet of the pipe is 60degC. Considering the heat loss of 8,5W/m, how much would be the temperature in degC after 1 meter?

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#8
In reply to #6

Re: Temperature of a fluid after heat losses

08/10/2015 1:49 PM

There is a lot more information needed before we could even begin to calculate your heat loss.

  • What is your inlet temp.?
  • What is your outlet temp.?
  • What is the ambient temp.?
  • What is the piping length?
  • What is the pipe diameter?

I don't want to measure meter by meter. I just want to find a way of calculate how many temperature difference I would have in a meter of pipe.

  • Inlet temp - outlet temp / by the length of your pipe will get you your heat loss per meter of pipe.
  • If you are unable to do that calculation then refer to post #7
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#7
In reply to #4

Re: Temperature of a fluid after heat losses

08/10/2015 1:25 PM

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#5

Re: Temperature of a fluid after heat losses

08/10/2015 1:18 PM

1. Your first assignment is to define 8.5 w/m in terms of heat dissipation per unit area.

2. Failing this, quit your job today. You are not well suited for thermal analysis.

3. You could always look at this as the ratio of 8.5 vs 7.5 to get a relative, ballpark, quick and dirty wild a$$ed guess.

I suggest #2.

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#9

Re: Temperature of a Fluid after Heat Losses

08/10/2015 4:04 PM

How fast is the water flowing through the pipe? The quantity of water that flows into the pipe per second equals the cross section of the pipe x flow rate.

You know the rate that heat is being lost from the length of the pipe (watts per second) from your insulation heat loss numbers (7.5 or 8.5 w/m). (1 watt = 1 Joule/sec). Using the specific heat of water, you can find the temperature drop with the amount of heat being lost during the time the water traverses the length of the pipe.

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#11
In reply to #9

Re: Temperature of a Fluid after Heat Losses

08/11/2015 5:19 AM

Not watts per second. Watt is a unit of power, = 1 joule/sec as you say a little later.

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#13
In reply to #11

Re: Temperature of a Fluid after Heat Losses

08/11/2015 5:59 AM

Thanks, you're correct. I should proof read more carefully.

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#10

Re: Temperature of a Fluid after Heat Losses

08/10/2015 9:59 PM

Start here...

https://books.google.com/books?id=hlUsBgAAQBAJ&pg=PA139&lpg=PA139&dq=8.5+w/m+heat+loss+pipe+run&source=bl&ots=rAC9PHKrdF&sig=S8LjwnotLwLPWUgzhjvk_ry_aoY&hl=en&sa=X&ved=0CCQQ6AEwAWoVChMI1sP_wPSfxwIVxO6ACh0eCAaz#v=onepage&q=8.5%20w%2Fm%20heat%20loss%20pipe%20run&f=false

...or you might find the answer in these tables somewhere...

http://www.engineeringtoolbox.com/heat-loss-pipes-tanks-t_15.html

The heat loss from insulated copper pipes or tubes to surrounding air are indicated below. The heat loss is based on temperature difference 55 oC (99 oF) with insulation thickness 25 mm (1 inch) and conductivity coefficient k = 0.043 W/m oC (0.3 Btu in/ft2 hr oF).

Nominal boreHeat loss for fluid inside pipe

(mm)

(inches)

(W/m)

(Btu/hr ft)

223/488
2811010
421 1/211.512
54214.515
672 1/21617
7631920
  • 1 W = 3.4 Btu/hr
  • 1 ft (foot) = 0.3048 m

Seems rather negligible to me....but I don't know the length of the run or the ambient temperature...or if the pipe is copper for that matter....

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#12

Re: Temperature of a Fluid after Heat Losses

08/11/2015 5:45 AM

Your insulation is not fully specified it should be of the form x w/m°C (Watts per metre per degree Celsius) where the temperature referred to is the differential temperature between the inside and outside of the pipe.

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