What Relative Humidity for Max Effcy Drying? 1

[ovenguy]

I am drying a very wet product at aroung 250 to 300 F and when I increase the exhaust rate in the dryer it dries faster, but uses more gas. When I decrease the exhaust rate, the RH in the dryer increase to as much as 99% and the product gets dry with a minimal exhaust rate, but my dryer has to run longer to accomplish this. Am I optimizing the efficiency by running at a very high RH to insure the exhausted air is fully saturated, or is my thinking flawed? What RH would provide the optimal energy efficiency? The dryer processes 20,800#/hr of water! So any % increase in efficiency makes an big impact.

 

[JKEngineer]

It sounds like the product is introduced to the dryer and then batch dried. Is this the case or is it a continuous product?

Staged drying should be helpful. If a batch process, arrange the internal flow to simulate counter current operation. This would probably need consideration of splitting the batches, i.e., introducing the product in steps. Put the fresh (half) of the product in the outlet end of the oven, move the partially dried product to the inlet end of the oven and deliver the dried half to the receiver. (Where inlet and outlet refer to the gas flow, which is the reverse of the product flow.) For a continuous system, go counter current.

Feel free to contact me if you want more substantive help or calculations.

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

http://www.KleinfeldTechnical.com

 

[quark]

You are right. It is always better to have the drying medium fully saturated provided your product quality is not offset. When you do fast drying, sometimes, case hardening of the product takes place and this may affect the drying.

At energy front (for efficiency) you can do little calculation to check overall power consumption in both the cases by doing enthalpy balance and add power consumed by the drying medium fan.

Regards,


Believe it or not : A Neutrino is so mass less(and electrically neutral) that, a conservative estimate states, it can pass through a 50 light year thick lead block.

 

[patrickraj]

If u r able to saturate (100%RH)the air without affecting the product or process that is the maximum efficiency,where the power consumption will be low due to low air requirement. Sometimes when the air RH% goes up, the product will start picking moisture.

 

[ovenguy]

Thanks guys. The process is continuous, and we have the heat/airflow ducted to come first from the left, then from the right as the product passes through the dryer. I am considering recovering the heat from the exhaust blower (290F or so) and using it to preheat the incoming air using a heat exchanger. Has anyone done this at such low exhaust temps? My only experience at heat recovery has been at much higher temps.

 

[JKEngineer]

Ovenguy -
Heat recovery has been practiced in the paper industry for paper machine dryers. I don't remember the temperatures off-hand, but they are in the ballpark. The trick is to recover latent heat if possible.

I assume that in addition to the cross flow that you describe, the net flow of air is against the flow of product. True?

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

http://www.KleinfeldTechnical.com

 

[quark]

Heat is recovered in HVAC systems at much lower temperatures (for cooling and heating as well). You can use plate type air to air HE or rotor type.

Below is a link for one good manufacturer and his selection software.

http://www.hoval.com/english/defaults/download/down_waerme_d.html

Just take care of condensation(ideally you should only exchange sensible heat).

Good Luck,


Believe it or not : A cobweb with it's strands a pencil width, can catch hold of a concord. All questions directed to Discovery Channel.

 

[amkh]

Dear Ovenguy

Two major factors affect energy balance of your case, Volume of exhaust and temperature of it.
The amount of moisture which air can hold depends on pressure and temperature.

If exhaust directly vented to atmosphere, then pressure is determined by elevation above sea level of the site. Low pressures can hold more moisture than higher ones, at the same temperature

Moisture content at higher temperatures will be higher then low temperatures.

Saturated exhaust means that in some part of drier, no drying take place, consequently, as you have mentioned, production rate will reduce. Low production rate, with lower gas consumption necessarily does not mean lower energy consumption or optimization.

My suggestion is to maintain RH about 90 to 85 percent and change exhaust volume or its temperature. From energy point of view, if drying process permits, the more exhaust temperature approaches to mean environment temperature, the more sensible heat has been converted to latent heat.

amkh

 

[amkh]

Dear Ovenguy

I understand that exhaust temperature is 290F = 143.333C and Relative humidity is around %100. Relative Humidity, RH = Pw/Pws and Pw = pressure of water vapor in the air , and Pws= pressure of saturated vapor(steam) . RH=100 means that Pw=Pws and Pws at this temperature vapor pressure is 3.969117 bar (59.41769psi), that is the air should be compressed and have much more pressure than water vapor, so take consider this fact in choosing recovery unit.
amkh

 

[ovenguy]

Dear Mr. Kleinfeld,

The flow is unfortunately side to side, perpendincular to the product flow. This line was installed in a piecemeal way over many years with one sideflow zone after another added. In all, the dryer is 1,600 feet long. I have seen the style where the flow is counter to the travel of the product, thereby insuring the driest air hits the product last. I presume this is probably the best?

 

[JKEngineer]

Ovenguy -
Except for the comments about case hardening, which may affect what is practical, then, yes, having the driest air hit the driest product will be best from the standpoint of energy and speed.

The system can approximate counter-current flow even with the cross current approach you describe. I assume that the run direction consumed by each cross flow is relatively small with respect to the 1600 foot length of the dryer. If the exhaust from the driest end is used as the feed to the next and so on, you have approximated counter current operation. For a given air flow and energy cost, this will give the fastest drying.

Feel free to contact me directly through my website below, if you want some more substantive help.

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

http://www.KleinfeldTechnical.com

 

[XELR8]

Ovenguy,

The relative humidity trick is interesting. I'm curious how you control this dryer. The advice above is good.

You need about 20.2MM Btu/hr to evaporate 20,800 lbs/hr of H2O. Plain and simple.

You could greatly increase your drying efficieny through many mechanical means but that would depend on a visual inspection.

What I believe you are accomplishing is steam drying. As you increase temperature and lbs H2O/ lbs Dry air you can evaporate an infinite amount of water at high temperatures. At low temperatures moisture is bad for drying but at higher temperatures 400-500 F and a little water say .5, you can greatly increase your efficiency. This is based on higher thermal conductivity and partial pressures.

Overall, it sounds like your doing well by controling RH and using less energy.