Online flow measurement through Heat Exchanger using diff pressure 1

[chem98]

I am a fresh process engineer having two years of industial experience. I have been given a task of effectivness rating of various heat exchangers at site. To evaluate U & C factors I have to measure CW flow through each heat exhanger where i know the presure drop of CW side only while inlet & outlet temperatures of both sides. Thermal & Mechanical deigns of all exchangers are known through vendor data sheets & GA Drawings.

Can anybody help me out to resolve this issue..

Ur responses 'll be highly appreiciated.

Greetings,
Chem98

 

[hacksaw]

it is always good to do that at least once. Who knows you might be successful.

you'll soon discover that such measurements can only be useful of records are kept from the date of first installation when the HEX as new.

good luck

 

[rmw]

Pressure drop across large surface condensers is a very accurate way to determine flow.

If your heat exchangers are sophisticated enough that they have good pressure drop values, not just some design engineers SWAG, then you can use it.

You should set up some type of verification test, (catch a timed sample in a bucket for example) or in the case of a single Hx, if there is one on a single pump, verify the flow that the pressure drop would indicate with the pump curve, and the electrical horsepower (assuming the pump is motor driven), or get ahold of an acoustic flow meter to measure flow at given operating conditions.

If you can do some empirical testing, and verify some values on Hx data sheets, then you can assume some degrer of confidence level, assuming, of course that your Hx's are not so fouled on the CW side that the build up is not affecting the flow rate, and the pressure drop also.

Be creative. Good luck.

rmw

 

[chem98]

Thanks hacksaw for reply, actually we have chemically cleaned five HX's in the annual overhaul (a month before) and found scale & sludge both on CW side (after one year service). So I am willing to get reference data which would be compared to quarterly sets. We have vendor's AS BUILT performance data to compare with the first dats set with too.

Well RMW, ur advice have a lot worth to me, but the problem is i am working in an industy not in an research or educational complex, I hope u would understand that I cann't do the experiments adviced by u with HX's having CW flow requirements of ~ 3000 te/hr (normal operation). We do have ultrasonice flow meter & have used it one two among five HX's but it cann't measure flow in less than 10" OD line. So three HX's r still remaining unattended.
I have heard that people use diff pressure across exchanger to find flow throuh it. I have done simple pressure drop calculations (in reverse) but the difference of 0.05 bar (least count of PG's available) leads to calculated flow variations of ~ 500 te/hr. And it cause considerable error. So please get me out of this situation.

 

[hacksaw]

The pressure drop method is only good for 20%, and you want 2% or so (guessing here).

There are tracer methods, ultrasonic, hot-tap insertables, etc.

To keep things in perspective, you have one of the most commonly encountered problems in a operating plant. It is do able, but no one wants to spend the bucks, so you have to run around collecting data that know one will believe...

Have have fun, while you are doing it, but don't let the important work go undone. It's a job that is reserved for the new guy anyway. Join the club...