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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
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