Temperature measurement of mixed stream 2

[Suddo]

I am currently working on a process where two streams of the same liquid at different temperatures are mixed. There is a temperature indicator at the outlet of the mixing position, however, it shows erranous readings. It is suspected that this is because the two streams have not mixed properly. we have tried to change the location of the TT by several metres, still the result is not satisfactory. Is there a method to measure this mixed stream temperature correctly?
I can provide more details if needed.
Difference in temp > 30 degree celsius
Density difference (due to temperature change) is about 30 kg/m3
The flow is quite turbulent, 650 m3/hr in an 18" pipe

 

[Suddo]

@rotw, this is what I am trying to do now, estimating Q_loss while taking the farthest temperature point.

@LittleInch, yes, the absence of flowmeters renders this method useless. We have the individual temperatures though.

Thanks for the suggestions.

 

[TiCl4]

Sudhanshu,

If temperature measurement of the mixed stream in inconsistent and cannot be rectified without installing an in-line static mixer, I would do as LittleInch has suggested and calculated a mixed temperature based on the two inlet streams. You said you have temperature measurements, but no flow measurement, and you need to keep the process running for the time being.

There is a way to get your individual flow measurements right now - clamp-on ultrasonic flow meters. A quick search reveals that these are good for your pipe size, but suffer some accuracy loss as compared to wetted versions. The ones I found had errors that were 1-2% of reading for clamp-on transducers. I don't know if these have sufficient accuracy for what you need. I have not used these before, so I do not speak from personal experience.

 

[LittleInch]

clamp on Ut's are good, but need a reasonable straight length ~ 10D and NO BUBBLES

If it's liquid then a simple orifice meter should be used.

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

 

[Suddo]

@TicL4, thanks for this input.
Do these flowmeters work with an insulation too? Insulation thickness is around 150mm.

 

[jari001]

Unless the vendor has some proposed correction factor for different types and thicknesses for the insulation, you will have to be able to clamp the probes directly onto the pipe. Even if they had some correction factor, 150 mm insulation is a lot and I think that might impact the measurements (whether it being wave absorption or the speed of the sound wave changing non-uniformly).

I believe there are different transmitters for measuring clean fluids v.s. particulate laden/bubbly fluids, so you'll need an idea of the contents of your fluid so a vendor can properly recommend the type of device that's best for you. You should also make sure that the device itself is safe for pipe surface temperatures, I've only ever used these types of flow meters for ambient temps (62F to 80F).

Some quick info from Google about interference from linings and temp gradients -

https://www.omega.com/en-us/resources/ultrasonic-flow-meters[/URL]]The accuracy of an ultrasonic flow meter relies on proper mounting. Large temperature changes in the pipe or a significant amount of vibration may affect the alignment of the transducers and acoustic coupling to the pipe. In most cases, ultrasonic flow meters won’t work at all if the pipe is lined with materials such as copper, concrete, plastic or fiberglass. These factors must be accounted for during installation. In addition, to provide an accurate volumetric flow rate, all ultrasonic flow meters require the pipe to be full.

 

[LittleInch]

No. UT meters need very good contact with the metal, but orifice plate meters only need a small tube coming out of the insulation.

I'd forgotten you're looking at circa 300 Deg C...

Looking back at the posts, I still think you're not going to get good mixing without some changes to the pipework or placing your TT a long way downstream.

I still think if you take off a section of insulation as a strip say 150,mm wide and then heat gun or surface TT measure all the way around the pipe then do the same thing at say 1 m intervals all the way down you'll soon see a pattern developing and where the differences around the pipe start to become much reduced. My bet is still on somewhere past 40D given you have apparently a near 1:1 volume.

you could try someone with CFD capabilities - you always get really nice looking colour plots which impress management but I wonder about their accuracy sometimes....

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

 

[georgeverghese]

What did your instrumentation engineer have to say about applying thermal conducting paste on the TT probes / thermowells ? This will iron out any inconsistencies in the contact between TT probe and thermowell for the 2 locations.

 

[Suddo]

@LittleInch, Currently we are looking at the TTs way downstream. I was already working on the CFD plots idea, although was only using google images to illustrate the point. A TT at more than 40D, ( It's inserted horizontally on the pipe side, and that too at an angle to the flow, don't know why), also shows a reading that is confusing. The reading is 2-3 degrees lesser than what we get at 150m downstream, which is not possible. What sort of inline mixers do you recommend?

@georgeverghese, I think I didn't mention this but there are two heaters in identical configuration, and both have this problem. So the probability of both of them being inconsistent was ruled out by me.