Veneer Dryer 1

[shyst]

Our steam plant operates a co-gen facility. We are presently using superheated steam (320 psig & 650 F)in our veneer dryers & condensing steam turbine. A recent study suggested that we desuperheat the steam in the dryers to increase the efficiency. A desuperheater presently installed in the line to the process was decommissioned several years ago, as the operators found that production in the dryers could be maximized using superheated steam instead of saturated steam. Does anyone have any thoughts before we recomission the desuperheater?

 

[MintJulep]

Was the "recent study" specific to your plant? What was being looked at in the study.

Possibly the study may have been looking at the dryer only, and not your entire system?

Maximum efficiency and maximum productivity may not coincide. Which is more valuable to you?

 

[shyst]

The study was specific to our plant. Consultants were hired to point out general areas that would provide opportunities to save money and/or increase production.
One thing I forgot to point out is the condensate return is sent to a tank at 280 psig and pumped directly to the steam drum make-up line.
Maximum veneer production would be the most desirable outcome.

 

[MintJulep]

Does the report offer any technical substantiation for the recommendations, or only a list of recommendations?

Were the consultants aware that you used to have a desuperheater in front of the dryer and that you're colloquial experience is that you are better off without it?

Do you have the opportunity to discuss the report with the consultants and ask questions?

 

[shyst]

The report offers very little in the line of technical substantiation for the recommendations.
The consultants were not aware that the desuperheater had been decomissioned because of the higher production rates attained some twenty years ago.
I have not had an opportunity to discuss the recommendations with the consultant nor will I get the chance, as the report was commissioned by a non-technical superior.

 

[MintJulep]

Tough spot to be in I guess huh?

You can:

1. Blindly implement the consultant's recommendations and hope for the best with the excuse in your back pocked of "I was just doing what the consultant told us to do."

2. Apply your own knowledge and judjement to the recommendations, and apply only those that you believe are warranted.

3. Tell your non-technical superior that the consultant's report is worthless as-in, and that he will have wasted his money unless you get the opportunity to ask questions and have the consultant substantiate their recommendations.

 

[JKEngineer]

Studies have been done in the paper industry that suggest that desuperheated steam is more effective in the drum dryers in the paper machine. Note that I said effective, not efficient. Overall heat transfer coefficients using only saturated steam are higher and seem to outweigh the decrease in temperature driving force. All of that says that the desuperheated steam in those cases allows more production by allowing higher heat transfer.

Your data and experience disagree with this. You find that production is higher with superheated steam.

I assume that the dryer and the steam turbine are in parallel. It sounds like the desuperheater would service only the dryer. It should not be used on the feed to the turbine; it will hurt efficiency and possibly cause damage to the turbine.

Like you and the others, I am puzzled about why desuperheated steam would increase efficiency. If the dryer is achieving good steam utilization, and is not blowing uncondensed steam out at the exhaust (this is an indirectly heated system, isn't it --the steam is in coils that heat air that does the actual drying -- not direct steam to veneer contact?) then the efficiency OF THE DRYER should not be sensitive to the inlet steam condition. Perhaps the overall plant steam cycle needs to be examined. It is possible that the efficiency gains are being realized with desuperheated steam because of other parts of the steam system. Does that sound likely?

Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[rmw]

How to start.

In this type of heat transfer, such as in a veneer dryer, the manner in which the majority of the btu's get transferred to the air drying the veneer is by the condensation of the steam, wherein the latent heat of the steam as it condenses is transferred to the process.

When the steam is superheated, and in this case by almost 225F, sensible heat transfer has to occur to bring the steam to saturation temperature before condensation can occur. Unless your dryer coils are arranged in such a way that the velocities of the steam in the 'sensible heat transfer' portion of the dryer are very high, then usually the Reynolds number, etc., relationships do not favor good sensible heat transfer in devices made for condensation heat transfer. Yours could be the exception, based on the plants experience. However, I suspect otherwise. 650f down to 427F is 223 btu per pound at a typical U value of 100 btu/sq-ft, while the latent heat available is closer to 800 btu per pound at a 400-600 btu/sq-ft range. Off by a factor of 3-1/2 multiplied by 4-6.

So, the desuperheater brings the steam temperature down as close as possible to saturation, so that the steam can go into the coils at saturation. However, the air on the veneer side of the dryer will not be as hot, as it will be an approach temperature based on 427F, and not 650F, which will not in most probability be intuitively understood by your operators, who might equate drying air temperature coming off the coils with the actual heat transfer capability of the coils.

A patent was issued mid last century for a method using the throttling calirometer principal to control a desuperheater right at saturation, and it was used extensively in the paper mills in our area.

Paper makers can tell in a paper drying roll just where the superheated steam changes to saturated steam, because the paper sheet drys much better where the condensation is taking place. Hence, saturation control is crucial to them.

Now, these veneer dryer types of coils, in my experience do not have a very good means of taking off the non condensables entrained in the steam, (or introduced by the condensate used for desuperheating) and that will hamper good heat transfer, but more especially with condensation heat transfer.

The consultants are absolutely right in their recommendation, as were the plant designers when they put the desuperheater in, assuming it was properly specified, applied, and installed. (A whole other discussion)

Note, for the sake of discussion, that devices that are compelled to handle, and transfer heat from superheated steam sources, like utility turbine size feedwater heaters utilizing turbine extraction steam, which have their desuperheating zones maximized for performance (so that they can be minimized in size) only get heat transfer rates in the 100-200 btu/sq-ft range, as compared to 6-7-800 in their condensing zones. They would certainly do better if at all possible.

However, the operators might just be just as right too, if all is not working as designed. Yours is to examine all aspects of the system, and make sure that the entire system is working as it should. My major concern is about the ability of the system to get the air (uncondensables) removed, as these types of coils are not built like conventional condensers, which have specially designed non-condensable removal zones, very carefully done.

Visit with your trap supplier(s) to get their recommendation regarding this, as well as looking at putting a bleed in above the condensate level in the coldest part of the coil to (sweep) remove the air.

This has been long, but now way are you getting the maximum, (I did not say best) heat transfer from this dryer with superheated steam.

Good Luck,

RMW

 

[shyst]

I just wanted to update the status of this post. We put the desuperheater into service and manually controlled the temperature entering the veneer dryers temperature control valve, a few degrees above saturation. Operated in this fashion for over 4 hours with no appreciable difference in steam flow, or veneer dryer operation. Since the veneer dryers are old and have been mechanically altered so many times over the last 20+ years, I have to assume that one of these changes (perhaps the lack of steam traps on the outlet) has altered the designed operation. Thanks for all the insight that has been offered.