Spray To Dry ?


From original questioner:

ive been experiencing what seems to be dry steam . as i hit eq and conditioning steps ,i introduce spray, mc's lows around 6% , set up eq to run170/141,144 ,a 6%,6.4% emc .only to come back to find lower than setpoint emc moisture content ! i've checked ,i think , the obvious, bulb accuracy,vents hanging open, ect. it seems to be intermitant,although it does happen in all 20 kilns. had a hurst boiler tech on site , said boiler ops looked good . tend to run approx. 100 psi fairly consistant. has anyone else experianced this?

From contributor Ge

This is common with higher pressure steam in well insulated kilns. Although the steam pressure is reduced to atmospheric when it enters the kiln, the steam still has the heat of 100 psi. This extra heat is called superheat.

Perhaps the best way to get rid of this heat is to use a desuperheater just before the steam enters the kiln. A desuperheater is a device with a small chamber that has liquid water in it. The steam enters the chamber prior to entering the kiln. The water uses any superheat to make additional steam. More water is added, as needed. The end result is steam at 212 F that is "wet" or saturated that goes into the kiln.

Older kilns were not well insulated, so the superheat would be lost through the walls and roof mostly.

Alternatively, you can reduce the steam to several psi near the boiler and run a separate spray line to the kilns. This line would not be insulated so the superheat is lost on the run from the boiler to the kilns (hopefully, a long run; short runs will not work well). This cooling may not work too well with the large number of kilns you have, however, unless you have several steam supply lines running from the boiler to the kilns for spray steam.

When the superheat problem is not too big (maybe with a 15 psi boiler), you can use the heat to increase the dry-bulb. So, if you finish the main schedule at 160 F and equalize at 170 and 140, use the superheat to get the DB up to 170 rather than the heat in the heating coils. Likewise, for conditioning at 180 and 170, use superheat to get to 180 F.


From contributor ke

Gene,thank you for the response .there have been several options discussed with management , including desuperheaters,prv's at the kiln,(globe valve's presently) .among other options.With the argument , we've not had any problem until approx. six weeks ago,and thats what throws me!kilns performance was pretty strait forward up to then .

From contributor Ge

It will be more common when an adjacent kiln is operating at a high temperature, as that slows the heat loss.

I would check for an open trap, as that would keep the hottest steam coming into the kilns and therefore more superheat. Also, maybe you had a return water issue a few months ago that limited steam flow, but now that is no longer, so you are getting hotter steam. Obviously, something changed and I am just guessing at a few possibilities.

Again, increasing the DB using the superheat will work, as mentioned in the past paragraph of my response above. It will even work at 100 psi, but you may have too much heat for total success.

I do on-site consulting if you need additional help.