Freeze Prevention
Freeze Prevention
(OP)
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.
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.





RE: Freeze Prevention
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,
RE: Freeze Prevention
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
RE: Freeze Prevention
RE: Freeze Prevention
RE: Freeze Prevention
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
RE: Freeze Prevention
RE: Freeze Prevention
RE: Freeze Prevention
I used (40+32)/2 = 36 degF for the 3E Plus program
I used se 40-32 = 8 degF for the HVAC formula
RE: Freeze Prevention
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.
RE: Freeze Prevention
RE: Freeze Prevention
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?
RE: Freeze Prevention
RE: Freeze Prevention
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?
RE: Freeze Prevention
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
RE: Freeze Prevention
There it is, for what it's worth.