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# Time to heat up material in a pipe

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## Time to heat up material in a pipe

(OP)
Hello All,

We have a water line that we are looking to bleed down every time it gets up to a temperature set point from the ambient temperature of the room. I am trying to calculate how long it will take to reach that temperature so I can estimate how much water we would bleed off in a day.

The line is 100 ft of 0.5" NPS Sch 40 carbon steel piping, uninsulated. In the summer, the water is typically 80-90 F and the room temp is 125. The temperature set point for bleeding is 120F.

Thanks for any help you can provide.

### RE: Time to heat up material in a pipe

You have a pipe full of water in a hot room.

When the water gets warm you "bleed" it. To where? The drain?

Then fill it up again.

Repeat?

Why?

Just empty it and leave it empty. Save you time, save some water.

### RE: Time to heat up material in a pipe

This souds very similar to this recent post https://www.eng-tips.com/viewthread.cfm?qid=442968

At an end difference of only 5F, I think it will take a loooooong time / never to get to your set point as heat flux is governed in part by delta T

It is though still a variable heat flow so you need to do this in steps of proabably 5F to get total time.

I also wonder why though....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Time to heat up material in a pipe

How about a P + I thermostatically controlled continuous bleed @ 120°F or ~49°C? Once calibrated, the time constant would be factored out.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

### RE: Time to heat up material in a pipe

(OP)
It will bleed to the cooling tower. The reason we are bleeding it is it is for a safety shower, so the water has a max temp for safety reasons.

### RE: Time to heat up material in a pipe

You'll have to do a time step analysis on a spread sheet. So the water is stagnant within a small pipe which are heated in a hot room. The analysis will take on the form of energy entering the pipe equal to the increase of internal energy of the water and pipe. Only consider an incremental length of 1' 5' or 10' since 100' of the pipe is being heated. Also discount the effects of convection within the water as you have a small diameter pipe(Dp). Also discount conduction thru the pipe and the water as both will be assumed at the same temperature which is increasing with time (no temperature gradient). Assuming no forced air movement, therefore, the estimated coefficient of convective heat transfer(Hc) equation was developed as follows: Hc=[0.27 *(Tr-To(t))/ Dp]^0.25 which is referenced in Kent "Power" 12th ed.;pg3-18; To(t) is the temperature of water/ pipe and is time dependent. The general equation is : Hc*A*(Tr-To(t))= ((M*Cv)pipe+(M*Cv)water)*(To(t+dt)-To(t))/dt; The result is T(t+dt)= {[(0.27*pie*Dp*Lp)/ Dp^0.25]/[(M*Cvc)pipe+(M*Cv)water]}*(Tr-To(t))^1.25+T(t))]. Tr room temp; M and Cv are mass and specific heat at constant volume; A is pipe surface area; Lp is the incremental pipe length; dt is the incremental time. BE CAREFUL ABOUT UNITS. I work with English units but you may decide to use Metric. The procedure lends nicely to the use of a spread sheet with the first column as being time and the last being T(t+dt).

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