Industrial Drying 3

[bluffrat]

We have a steam radiator heated dryer that we pass a carpet web through to get dry. I am trying to find what an industry spec or rule of thumb is for relative humidity inside a dryer to accomplish efficient drying of the material passing through it. Any thoughts or direction would be greatly appreciated.

 

[MintJulep]

No such thing.

Different products react differently. For example, crumb rubber will "crust" if dried too rapidly, making it nearly impossible to get moisture out of the center of the product.

What does "efficient drying" mean to you?

 

[bluffrat]

Of course putting an electrical guy on a dryer problem seems as silly as the questions I may ask but I am what the company has so let me try to expound. We have two print speeds on our carpet printing line. When we run to the upper print speed the carpet coming out of our in-line dryer does not get dry. If you open the doors on the side of the dryer, the air coming out is very wet. The dryer is operating at 150 C. I am trying to determine if the air is taking out all of the moisture it can. I have the ability to control the air flow out of the stack (18" diameter) with a damper. The air flow as measured by a hand held Extech hydro anemometer can be varied from 0 - 3.3 m/s at 143.8 C. To be honest, I am not even sure all of the information I need to determine if I have sufficient drying capacity to remove the water from the carpet.

 

[MintJulep]

If you open the doors on the side of the dryer, the air coming out is very wet.

Well you're blowing the air through a wet rug, what do you expect?

In-fact, it probably is not really all that wet. Doubtful that it is much above 40% RH at 150C. It feels wet when you open the doors and mixes with room air because the mixture will be quite high RH.

Simplistically, drying is driven by the difference in vapor pressure between the moisture in the product and the air.

If you want to determine if the air is picking up all the moisture it can, you'll need to look at the relative humidity - that being the amount of moisture in the air relative to the maximum that the air can possibly hold.

But...

Doing things to increase the RH on the air-leaving side of the web will result in only relatively small gains in moisture removal. The reason is that each pound of air really doesn't hold that much water.

More air is probably what you need.

Is the drier zoned? Cool dry air blowing through warm wet product removes more moisture than hot dry air blowing through warm wet product.

There have been a few threads here on drying - use the Google search facility at the top of the page.

 

[dvd]

I assume you have some sort of fan in use and that you are re-circulating the dryer air. You could start by measuring the wet-bulb and dry-bulb temperature of the air after it has been heated and before it passes through the carpet and also after it passes through the product and by the heater. You will see how much moisture is in the air. If the wet-bulb is sufficiently high there won't be many options available without dumping lots of heated air outside as exhaust. You will never get your product drier than the wet-bulb temperature of your air.

Measuring wet-bulb temperature is somewhat tricky, but if you have a small vessel of boiling water very close to your dryer you can insert the thermocouple with sock over it in the boiling water and then immediately place it into the dryer airstream. The wet-bulb temperature can be determined from the minimum of the heat trace; as the temperature will dip down until the wet-bulb temp is reached then rise as the sock dries out and starts giving the dry-bulb temp.

You will actually be removing the final amounts of moisture in the product as it passes over your cooling section and the still-warm product encounters much drier, but cooler, air. The heat and some of the residual humidity will be pulled out then.

I like this old book for it's simplicity and background information. If you study it you will learn something about drying. It is available used for a reasonable price and the engineering library at your local university may even have it in their holdings.

"Mass Transfer Operations" by Robert Ewald Treybal

You need to know how to use a psychrometric chart and understand the implications.

 

[chicopee]

In addition to DVD's comment,you will want to weight dry, wet and partly dry samples at the two print speeds and throughout the process time to figure out your next solution.

 

[HEC]

It does sound similar to a "Yankee Dryer" used in the paper industry. try a google on that for some information.

The circuit of recycled air is to conserve energy required to heat the colder ambient air. You mention that there is a bleed damper from the system and the problem occurs at higher printing speeds. I would suggest the problem is associated with mass flow and mass balance. As the web speed increases, the rate of water removal will also increase. The dryer hood design will be based on a removal rate that will also be associated with the bleed of air out of the circuit. The simple answer for moisture control is to control the bleed from the air circuit. For simplicity this can be related directly to the web speed.


Mark Hutton