Sudden Dropoff in heat transfer : Steam coil

[seasar]

All,

I would appreciate any ideas or thoughts you may have regarding a phenomenon I am experiencing.

A large vessel with 6,000 gallons of a water/grain mixture (a mash cooker) has a steam coil supplied with 20,000 lbs/hr and the supply valves is ~ 30% open.
The vessel starts at 170 degrees with the intent of inducing a boil...at just over 200F the steam flowrate drops off drastically (down to 10,000 lbs/hr) while the supply valve opens to 100% attempting to make setpoint.
The pressure in the vessel is 14.7 psia.

The weird part is that this only occurs when using grains from the 2007 crop year. Can there be something in the fluid mixture that suddenly changes the heat transfer across the coil? There is no burnt residue on the coil when the vessel is emptied.

 

[imok2]

Could the moisture content be greater then normal in the 2007 crop thereby creating a greater latent heat transfer then what is supplied by the lbs of steam supplied for a drier grain? Whenever you have a change of state it requires more heat content then a change of temperature.
Just a thought!

 

[MintJulep]

Where is the temperature sensor?

What is the steam condition at the entry to the coil?

Where does the steam go when it exits the coil? What is the pressure there?

What type of controller?

Does the controller have "recipes" for different products, grains, etc.?

 

[seasar]

Where is the temperature sensor? An average of three sensors that protrude into the liquid near the wall of the vessel.

What is the steam condition at the entry to the coil? Slightly superheated, 70 psig

Where does the steam go when it exits the coil? What is the pressure there? Into several, properly functioning, bucket traps. There isn't a pressure indicator downstream of the coil.

What type of controller? PLC with a PID loop controlling the valve off the steam flow meter.

Does the controller have "recipes" for different products, grains, etc.? No. For this portion of the cycle the set point for the controller is the same for all recipes.

 

[MisterDonut]

Is the vessel at atmsopheric pressure? Assuming so, then at just over 200F, the mash could be boiling, particularly close ot the coil. The mash liquid/solid mix may be holding vapor pockets near the coil, which would decrease the heat transefer. Is there a mixer in the vessel?
Just an idea that came to mind...

On a similar application, I used a steam injection nozzle with success.

 

[StoneCold]

I would try increasing the amount of water to grain. If that does not appeal to you how about reducing the steam pressure. My guess is that you have film boiling that is knocking down your heat transfer rate.

Regards

Stonecold

 

[seasar]

There is a mixer/agitator in the vessel.

 

[MJCronin]

Ceasar...

Either you have "classic" heat exchanger
"stall", or the 2007 crop has some kind of bizarre agricultural curse on it. Have you really experimented with crops from other years ? How about crops grown with farm subsidies from the Bush administartion... Is this really and truly a viable variable...??

In HX stall, the coils get flooded with condensate, the little controler goes wide open, trying its best to do its job, but to no avail...heat transfer from condensate to the mash is just not as good as from saturated steam...

The classic solution is to install a "fair size" drain pot at the bottom of the coil and allow the steam trap to draw from the new pot

http://www.spiraxsarco.com/resource...densate-removal/heat-exchangers-and-stall.asp

Let us know your final decision and tell us more about condensate drainage

-MJC

 

[imok2]

Are the grains ground before mashing to expose the starch granules and help them remain in suspension in a water solution. If not that could be the reason the load drops off and as was said "stall "develops. The grain should be ground into a meal -- not a flour! -- that will pass a 20-mesh screen. On a hammermill, however, a 3/16" screen will suffice.

 

[chief]

A low steam flow rate with your control valve full open indicates a restriction on your exhaust steam side.The steam traps are of course the first suspects. Install a pressure gauge at your condensate bucket traps , before and after to monitor this effect. The sizing and routing of the condensate line also is an issue to investigate. Check out GEM steam traps for an alternative to bucket type. The Gem are inherently fail safe.

Offshore Engineering&Design