Condensation in pipe lines-pneumatic conveying system

[rkarthik]

I am experienced with a pneumatic conveying system - Dense phase system for fly ash. I have frequent jamming problems on the flyash pipe line. The system uses two 110 KW compressors - 45 psi max at 150 deg C.This is a dual tank operation and if one finishes conveying the other one starts. I have found jamming is more frequent during winter season especially when the temperature fall to nearly 4 deg C. The conveying pipe line is 950 mts long.The temperature of the pipeline is almost atmospheric after 300mts of pipeline, and its where the jamming occurs. Can anyone help me in finding out whether condensation is a problem in the system.what would be the most vulnerable condensation point in the pipeline.

 

[ccfowler]

What is the nature of the compressed air supply? Is the 150 C the temperature of the compressed air being supplied to the conveying system? Is there a serious effort to remove moisture from the compressed air?

Most flyash tends to be quite "thirsty," so it will pick up moisture from the air supply. As the ash picks up moisture, it can get quite sticky. I've seen some flyash that picked up ambient moisture, became obviously moist, and could be readily picked up by hand and formed into "flyash snowballs." As I recall, this observation was made at ambient temperatures around 40 to 50 F. This was a particularly fine fraction of flyash that was produced by a mechanical separation process. (After making "flyash snowballs" with this stuff, my hands were not just moist, they were WET.)

My guess is that the problem may involve the finer particle fractions within the stream preferentially grabbing the available moisture and getting sticky along the "sufficiently cool" pipe wall.

 

[TKengine]

I would try to solve diferential equotin and find out temperature profile along the pipeline and there I would find the point where condensation ocures... the problem is how to determine local coeficient of heat exchange bitween the pipe and ambient air.

 

[KenRad]

Do you have air dryers downstream of the compressors? You need to achieve a dewpoint that is no higher than the lowest ambient air temperature to which the pipe is exposed, or you will have condensation. A properly sized refrigerated dryer will get you down to a 4C dewpoint, and condensation will no longer be as issue.

---KenRad

 

[rkarthik]

There is no dryer in the system.We have found out the temperature profile along the pipe line and it some times reaches 4-10 deg.
The following are the specs
a. material temperature - 30 to 45 deg C
b. Ambient temperature - 2 min deg C to 45 max deg C ( during winter and summer resp.)
c.Currently its winter and the amb temp is around 2 to 12 deg C.
d. 100 % relative humidit.
e. compressors outlet temp min 100 deg and max 150 deg c.No dryer in the system at comp. outlet nor receiver is there.
f. type of compressor - reciprocating compressor.

Will heat tracing along the pipe line or boosters along the sensitive points in the pipe or insulation in the pipe will be useful.your replies will be highly appreciated.

 

[KenRad]

rkarthik,

With no dryer in the system, your compressed air is always saturated with water, ready to condense as soon as its temperature is lowered. Insulation alone will only delay the formation of condensate, but it will form at some point. And if you use heat tracing, you will have to use a high enough wattage to maintain the compressed air at a minimum of the compressor/receiver discharge temperature. I believe that the only way to ensure dry air at the point of use is to bring the dewpoint down with some kind of dryer. For a 110kw compressor (assuming about 600 CFM), you should be able to get a packaged refrigerated dryer for $10k - $15k dollars US. Without some sort of dryer, I believe that condensation will be inevitable.

---KenRad

 

[ccfowler]

I think that KenRad is on the right track for this. If you have high abmient humidity and don't dry the compressed air after the compressor, you will continue having moisture problems. You can probably install the necessary chilling and drying equipment and operate it for less than the cost of heat tracing installation and operation.

Good luck!

 

[rkarthik]

Dear Mr.ken rad & Mr.cc flower
thanks for the advice. but i am at the suppliers end. the compressors flow as measured near the system is around 700 cfm. The orginal designer is not accepting for dryer system. so from our end we have identified some critical points where the jamming occurs and it also coincided the lowest temperature points on the pipeline. can we suggest some boosters like injectors on those vulnerable areas will a protable compressor which suits there cracking pressure.And secondly while running in dual tank ( the system is designed for dual tank mode) than in single tank the jamming is less frequent.But for maintenance reasons we have to run in single tank also.But surely we are thinking of dryer and will any heating of the input flyash to the system will be helpful?

 

[quark]

Are you required to do the conveying at an air temperature of 150[sup]0[/sup]C? Otherwise if you can cool down the air using an aftercooler at your operating pressure, you cna reduce the jamming problem.

The main problem may occur at potential pressure dropping part of the piping like valves, bends etc. Even if you are cooling the air to ambient temperature (i.e pressure dew point at ambient temperature) it's atmospheric dew point further reduces. So there will be less likely chance of condensation even if you are reducing the air pressure to atmospheric.