Hot Air Dryer Supply / Exhaust Air Handling

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Thread Starter

Paul Kraemer

My company designs and builds coating and web handling equipment on which a moving web is coated with a solvent based adhesive and then passed through a hot air dryer. The hot air dryer has three temperature zones. Each temperature zone has a dedicated supply blower and heater to deliver heated air to a supply plenum that serves the corresponding temperature zone. Each supply plenum has several adjustable air nozzles that direct heated air at the coated web (the product). The three supply plenums are located at the top of our dryer, but are separate from one another. We have a single exhaust plenum located at the bottom of the dryer and that runs the entire length of the dryer (to serve all three supply zones). A single exhaust blower removes solvent laden air from the dryer through the exhaust plenum.

In our standard control setup, we run the exhaust blower at a fixed set point (RPM). We run the three supply blowers at fixed ratios of exhaust blower speed. We set these ratios so that the exhaust blower will remove just a little more air than the supply blowers are supplying, so that pressure will be slightly negative inside the dryer as compared to outside in the room where the dryer is located (so that solvent laden air will not enter the room with the operators).

The air flows that the coated web is subjected to are critical to product quality. Our customers have done air flow studies that have shown that for a given exhaust blower speed (RPM) and given ratios for Supply #1, #2, and #3 blower speeds, the air flows that the coated web is subjected to at the various air nozzle locations are predictable and repeatable (assuming that the adjustable air nozzles remain configured the same way).

Our exhaust blower typically sends the exhaust air into a thermal oxidizer that removes the solvent before exhausting to atmosphere. Until now, I believe that we have been able to rely on the fact that the static pressure seen by our exhaust blower (created by the oxidizer) is more or less constant.

One customer is now planning to install a bypass around the oxidizer that will be used for certain water based products for which the oxidizer will not be required. They tell me that activating the bypass could cause a drastic change in pressure and they are afraid that this will impact the air flows seen by their product. This customer has asked if rather than controlling our exhaust blower to a fixed RPM, if we could use pressure feedback to do closed loop pressure control. I haven’t thought through the details, but I’m sure we can do this with our PowerFlex700 AC Vector Drive.

I am concerned about how this will impact our control scheme for the Supply Blowers. I assume that our current scheme where the supply blowers follow the exhaust blower RPM at a set ratio will no longer be appropriate (now that the exhaust RPM will vary as necessary to maintain the pressure set point). I am leaning towards rather than using ratios of exhaust blower speed, simply allowing entry of Supply Blower speed set points as fixed RPM’s.

The Supply Blowers we buy are rated to deliver a certain CFM for a particular static pressure at a particular RPM. With the exhaust blower controlling the static pressure, I hope that we can consider the static pressure to be constant, in which case I would hope that there would be a direct/repeatable relationship between supply blower RPM and the air flow seen by the product (regardless of the exhaust blower RPM it takes to create the desired pressure).

Can anyone here think of any reason why this would not be the case?

I am also concerned about the possibility that running the supply blowers at fixed rpm’s while allowing the exhaust blower to adjust its rpm to maintain the desired pressure might create the possibility that pressure inside the dryer might become positive in certain situations, causing solvent laden air to enter the room. Should I consider adding controls to monitor differential pressure between inside / outside the dryer?

Any advice will be greatly appreciated.

Thanks in advance,
Paul
 
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Paul Kraemer

thanks for your response Curt,

Do you mean that rather than running my Supply Blowers at fixed RPM's, you would control the Supply Blowers to maintain a set differential pressure between each Supply Plenum and my single Exhaust Plenum? This should allow me to eliminate the possibility of creating a positive pressure inside my Dryer (which would cause solvent laden air to get into the room / operator area).

What I'm unsure about is this...Each of my three supply plenums are separate from each other on their shared sides (between zone #1 and #2, and between zone #2 and #3), but each supply plenum has several adjustable openings (my air nozzles) into the shared drying chamber.

I assume that I would want pressure in each Supply Plenum to be greater than that in the Exhaust Plenum. I will, however, most likely want different pressure settings for each Supply Plenum. For product quality, the most important thing is repeatable air flows through the air nozzles that direct air from the Supply Plenums at the product. Do you think that my air flows through these air nozzles would be just as repeatable if I control differential pressure as opposed to if I controlled my Supply blowers to a fixed RPM?

Also, should I worry about my three supply zones "competing" with each other considering the fact that all three blow air into a shared drying chamber? For example, if I control pressure so that Supply #1 Pressure and Supply #2 Pressure are both greater than Exhaust Pressure, but Supply #2 Pressure is greater than Supply #1 Pressure. Is it possible that the lower pressure in Supply #1 would pull Supply #2 air in that direction before it goes to the exhaust?

Thanks again for your help,
Paul
 
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Paul Kraemer

Thanks for your response Curt.

Do you mean that you would control the differential pressure between each of my three supply plenums and my common exhaust plenum?

Controlling the supply blowers this way would definitely give me the ability to prevent the possibility of a dangerous situation where pressure inside the dryer would become positive compared to the room, allowing solvent laden air to escape the dryer into the operator area.

My concern is whether this control method would give me the same repeat-ability for the air flows directed at our product web. My three supply plenums are located at the top of my dryer and separated from one another on their adjoining sides, but each plenum has adjustable openings (my air nozzles) into a common drying chamber. While the drying chamber does not have any physical separation between my temperature zones, the air nozzles can be positioned either above or below the product web, and can be angled to direct air flow either towards or away from the product web at any angle. The air nozzle openings can also be widened or narrowed to allow either more or less air to pass through. The air passing through these air nozzles will pass towards/around the product web before it gets pulled into the common exhaust plenum.

For product quality and consistency, I need repeat-ability of the air flows seen by the product web. Would you expect that I would get just as good repeat-ability of the air flows out of my air nozzles (assuming I keep them positioned/adjusted in the the same way) as I would if I ran the supply blowers at a fixed RPM?

thanks again for your help,
Paul
 
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Curt Wuollet

IIRC, the quantity that was changing was the exhaust velocity. You already have your supply plenums set at the ratios and distribution that you want, I wouldn't change that. I would control the exhaust blower to maintain the same pressure when the oxidizer was bypassed as when it is in the circuit. If you measure the DP when the oxidizer is in and all things are running smoothly and then turn down the exhaust blower to get the same DP when the oxidizer is bypassed, every thing should work the same, yes? Much easier.

Regards
cww
 
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