Freeze Prevention

[gibsi1]

I'm looking to run a ~3600' 3" line to transfer contaminated water. We will be using a lot of line that is currently not insulated or steam traced. The cost of the entire project would be greatly reduced if we could forego adding the tracing and insulation. Due to pump automation, we cannot have operations blow N2 through the line after each transfer.

We're looking to provide a constant flow in the winter months, via a cooling water or filtered water jumpover. Our current cooling water lines, ranging from 8" to 36" are not insulated or traced, and use minimum flow jumpovers to keep fluid velocity up. Is this practice acceptable for a 3" line? How can I determine the minimum velocity needed? We would also be looking at an elevation change of 50-60ft.

 

[CRG]

1. Is this practice acceptable for a 3" line?

I don’t know your design conditions; however if you calculate the flow, you can make the determination.

2. How can I determine the minimum velocity needed?

First calculate the heat loss. 3E Plus is a free program that may help with this task.

http://www.pipeinsulation.org/pages/download.html

You will need to make an assumption of the average fluid temp to simplify the calculation. You may consider (Tin + Tfreeze)/2 as one option; however, it does not leave you a safety margin should you loose circulation. Engineering judgment is required to determine the average temp of fluid that is appropriate for your design conditions.

The other method is to break the line into short segments and do the calculation in parts reducing the Tin for each segment by using the heat loss calculation of the upstream segments to calculate a new Tin for each downstream segment.

Next to determine the flow; I would use the dimensionally inhomogeneous formula the HVAC guys use for water:

q=500*GPM*deltaT

q is the heat transfer from the water in BTU/hr
GPM is the flow of water in gallons per minute
deltaT is the change in the temperature of water from inlet to outlet of the piping system

Note: some engineers here do not like the above equation being called a formula, see thread378-123332

You will have q from the 3E Plus program, and already have determined what you want deltaT to be. So simply solve for GPM.

Once you have done the above calculation, you can determine if it is practical to use cooling water or filtered water for freeze protection. Also, you should consider how accurately your math model reflects actual conditions and add the appropriate safety factor for flow.

Regards,

 

[gibsi1]

Thanks for the response! I found some calculation software and formulas at

http://www.thermomegatech.com,

which they use to size freeze protection valves. For future reference...

GPM = [A1*A2*(0.5*Tw-Ta+16)]/[40.1*d^2*(Tw-32)]

GPM = gallons per minute of water flow
A1 = Transverse internal area of pipe, ft^2 (Crane410, Navco, etc)
A2 = Exposed pipe surface area, ft^2
Tw = Temperature of resupply water, *F
Ta = Minimum ambient air temperature, *F
d = ID of pipe, ft

 

[CRG]

I would consider wind speed a very important factor in determining the heat transfer rate of any exposed pipe or valve. The formula given by

http://www.thermomegatech.com

does not consider this factor as a variable in the equation. Unless the formula is adjusted for your design wind velocity, I would not use it.

 

[sailoday28]

Don't forget to include the energy input to the water due to the inefficiency of the pump.

 

[gibsi1]

Well, the calculated results are vastly different.

The Thermomegatech formula gives 95GPM for 65*F resupply water and 291GPM for 40*F resupply water.

The 3E software and listed HVAC "formula" gives 27GPM.

Not really sure where the problem is. I would've assumed the 3E numbers to be higher since wind speed was accounted for. The numbers I am using are as follows:

3600ft of 3" SCH 40 (ID=3.068" OD=3.5")
Cooling water resupply temp = 65*F
Filtered water resupply temp = 40*F
Ambient temp = 0*F
Wind speed = 20mph

 

[gibsi1]

The water temperatures I am using are downstream of the pumps. We are talking enormous systems with multiple pumps putting out 5750GPM... each!

 

[gibsi1]

The 3E software does not take the temperature of the resupply water into account, which is why the numbers are so much lower. Using lower process temperatures results in less flow required because 3E is assuming you want to maintain that temperature.

 

[CRG]

I get 239 GPM for 40 degF resuply water.

I used (40+32)/2 = 36 degF for the 3E Plus program
I used se 40-32 = 8 degF for the HVAC formula

 

[TD2K]

http://www.thermomegatech.com/freezepro/index.htm#

Running your numbers gives me about 290 gpm (no correction factor for wind speed) with 40F supply water, 0F ambient and 3600 feet of 3" sch 40 pipe.

I don't have a feel for this though the flow seems awful high to me. Then again, there's a lot of feet of pipe there and not a lot of temperature drop available to supply the sensible heat due to heat losses to the ambient air.

I'll have to do some more playing around with this.

 

[CRG]

Wind has such a significant effect on the film coefficient, I suspect that it is factored into the formula listed on

http://www.thermomegatech.com.

The problem is that gibsi1 did not state what the formula wind speed was; and it is possible that thermomegatech did not publish the wind velocity that their formula is based on. So that is why I questioned thermomegatech’s approach. If you looked at the heat loss for still air at 0 degF and compare it to 20 mph air at 0 degF, you will find the heat transfer rate is over 5 times the calm condition for 20 mph wind. Note, that this large effect of wind velocity is most pronounced when there is no insulation on surfaces with high heat conductivity.

 

[gibsi1]

Yes, I'm assuming that ThemoMegaTech has a high wind velocity factored in. I should note that I am calculating this for 4 months out of the year (1440hrs).

I don't see how you are coming up with those 3E numbers. The cooling water supply runs at 65*F in the winter. We obviously need the water above freezing when it exits the pipe, so we'll use 35*F for the exit temperature. That gives an average operating temperature of 50*F. Plug that into 3E, along with 1440hrs of operation, 0*F ambient temperature, 20mph winds, and 3" pipe. That spits out 530,600Btu/ft/yr. Multiply that number by 3600ft to get 1.91 billion Btu/yr. That converts into 218,055Btu/hr. Plugging that into the HVAC forumla with a delta T of 30 (65-35*F) gives you a flow of 14.5gpm. How are you getting such high numbers?

 

[gibsi1]

If I use 8760hrs for the entire year, then I can match your numbers. Do I have to use an entire year for calculations, since maybe using my reduced hours is spreading the heating load out across 365 days, instead of the 120 days that we'll be needing this? I also just noticed that I was using the wrong number of hours. I guess what I'm figuring is the worst-case scenario, which is 0*F and 20mph+ winds, so I would need to calculate it on a yearly basis (8760hrs) to find the flow needed for that case. Is this correct?

 

[gibsi1]

Hmm... wish you could edit your own posts.

Why does the 3E software result in larger diameter pipe requiring more flow? Wouldn't the exact opposite be true because the large pipe has more volume and more heat to give up during transfer?

 

[zdas04]

The underlying physics of all of this arithmetic is that you are trying to get the water to scurry from a warm place through a cold place and back to a warm place before it can give up enough BTU's to freeze.

This "scurrying" implies a moderately rapid velocity (not volume flow rate). With bigger pipe you need a bunch more volume flow rate to achieve the (slightly) slower velocity required of the increased volume.

David

 

[TBP]

I've got a book of nomographs "Handbook Of Data Sheets For The Solution Of Mechanical Systems Problems" (I'm not making that up...) that has in it a chart for the required bleed rate to freeze protect bare water lines. It assumes bare pipe, a water temp of 40*F, and an air temp of -20*F. For a 3" line, it indicates a bleed rate of 2.5 USGPM per 100 feet of pipe. For wind velocities in excess of 15 MPH, the bleed rate should be doubled. According to this chart, the flow should be 90 GPM, or 180 GPM if windy conditions are expected.

There it is, for what it's worth.