3 replies to this topic

[jaberne]

Hello,

 

I'm a first year EIT currently trying to calculate the length of 1/2" glycol tracing line necessary to cover 1/2 of the surface area of a horizontal drain vessel and maintain the temperature of the liquid in the vessel at 45 degC. This is my process so far:

 

1. Vessel OD is 1.092m and length is 3.048m, found the volume and surface area of the vessel assuming 2:1 elliptical heads.

 

2. Specifed a maintenance temperature ™ of 45 degC and an ambient temp (Tamb) of -40 degC (the vessel is outdoors).

 

3. Found the heat lost by the vessel based on 51mm mineral fiber insulation (using a table = .775 W/m2/C), a delT of 85 degC and the surface area of half the vessel at 6.521 m2. (Qloss = Qins*delT*As)

 

4. Adjusted for insulation using an Insulation Adjustment Factor from table - Ia = 1.16. And adjusted for wind. Total heat loss is 627.85 W/m.

 

5. Used Q=mcdelT to calculate the mass rate of glycol needed. Assumed 4 tracers on the vessel - one for each head and one for each half of the body. HYSYS model specifies glycol is at 80 degC and 540 kPag in and 74 degC and 34s kPag out for heat tracing in that area.

 

6. Divided the mass rate by the number of tracers and the relative area they cover. Used the density of glycol to calculate the flow rate and the cross sectional area of the tubing to calculate the velocity.

 

7. Now I'm stuck for how to calculate the length of tubing required assuming 4 passes. I went back and calculated the overall heat transfer coefficient but I'm not sure which direction to go next.

 

Please advise. Any suggections are greatly appreciated.

[jaberne]

Oh and I tried to run it through a line sizing spreadsheet and get a calculated delP/100m using the Moody Frictiuon factor, then back calculate the length of the pipe but it was much too high >1000m. Might need to adjust my pressure drop.

[Art Montemayor]

Jaberne:

 

I believe you are going about the required heat tracing in a wrong manner.  I say this, because I’ve been down this same road many times before.  You should not be trying to circulate a glycol solution through a ½” tube in order to keep the liquid contents of a horizontal vessel at 45 ^oC.  Even though this is a relatively small vessel (1 meter diameter x 3 meters long), you have a difficult physical orientation of the tubing runs that you have to cement to the vessel wall and this is going to cause a very large pressure drop in the circulated hot glycol.

 

If possible, the best answer to maintaining the contents warm is to simply weld a jacket under the vessel and keep it full of circulated hot glycol.  If you are not allowed to weld on the vessel, then apply a heating panel to the underside of the vessel.  This topic has been discussed before in this Forum: http://www.cheresour...nels#entry62281

 

I recommend you download the literature I uploaded in that thread and study the simple application of a preformed panel.

[jaberne]

Thank you for your quick reply. I will recommend that option.