Measuring Flow in a Hot Oil Vapor Line (Dowtherm)

[Quiny96]

Current situation: There is an excessive amount of Dowtherm hot oil vapor moving through a heat exchanger, causing pressure in the condensate line and collection tank.
Proposed solution: Place a hot oil vapor trap on the outlet of the heat exchanger to capitalize on latent heat and minimize vapor in the condensate return line.
Dilemma for solution: Design specs on the heat exchanger are unavailable due to its age (80 years) and the manufacturing company no longer existing. Sizing the trap has been difficult due to this lack of data.

Does anyone have cost effective ways to measure/test the amount of hot oil vapor leaving the heat exchanger?
Obviously installing a flow meter rated for the temperature and pressure range is the simple way to do this, but this will be costly.

 

[TBP]

Has anything changed in this system - flows, temperatures on either side of the HX? Is it possible that one side or the other (or both) of your 80 year old HX is fouled and just needs cleaning?

 

[Compositepro]

There should be a vapor trap, like a steam trap, in the condensate line. Seems that it is not functioning.

 

[TBP]

I've seen large, low-pressure steam HXs use loop-seals rather than traps. Traps would have been terrifyingly expensive. I've also installed steam HXs with control valves on the condensate outlets, instead of traps. There worked very well, providing a variable capacity trap function. After all, a steam trap is simply an automatic valve.

I can only assume that this 80 year old hot oil system used to work properly, and now does not. What changed?

 

[1503-44]

clamp-on ultrasonic flow meter?

https://duckduckgo.com/?q=clamp+on+ultrasonic+flow+meter&t=brave&iax=images&ia=images

 

[Quiny96]

The dowtherm system has definitely changed over time. I have only been looking into it for a month or so now, but it is apparent the system is being ran more aggressively than designed for (CVs on full flow).

There are currently no vapor traps on the condensate return line, but CVs on the HX exit and loop seals are some interesting options I hadn't considered.

Ultrasonic flow meter is along the lines of what I was looking for. I'll have to look into their accuracy.

Thanks all for helpful suggestions!

 

[btrueblood]

I would not expect any flow meter to give you good results, if the flow is 2-phase (bubbles of vapor and slugs of liquid).

 

[1503-44]

It might require some "creative calibration", but is probably your only cheap option.

 

[Compositepro]

Focusing on measuring vapor flow in this line is a distraction from solving the problem. There should be no significant vapor flow in the line. If there is no vapor trap, the tank and condenser must be at the same pressure. Is your condensate tank hot due to oil vapor condensing in it? Then you may need a non-condensible gas in the tank to keep out the oil vapor. If the tank pressure is too high, then you may have to much non-condensible gas in the condenser, which is reducing its condensing capacity, thus increasing pressure.