Using Dryer as Exhaust Fan?

[red91sit]

We are working on improving the ventilation to our factory. Our current dryer runs at a constant 45,000 cfm when it is running. I was looking at ducting this into our factory to act as 2 exhaust fans. Being a food grade plant, we maintain positive pressure of roughly .2" wg, to maintain this, we would be adding at the minimum 45,000 cfm of intake fans. My concerns on doing this is the exhaust fan is capable of 20" wg. To prevent this motor from sucking in our walls, I would be adding a pressure sensor to the factory to monitor the pressure. This would kick the dryer fan off if too much vacuum is sensed within the factory. To prevent the operators from running this fan in 'hand' mode, I would wire in a keyed switch to the MCC bucket. I would think I'd want to connect up the new intake fans to VFD's such that they ramp up with the dryer to maintain our pressure, also controlled by the pressure sensor. My other thought would be to wire each intake fan up to a different pressure sensor for redundancy sake.

Thoughts / Comments?

 

[BronYrAur]

Where is the dryer air coming from now? What is this dryer's purpose? If you cycle it on pressure (or anything else) how will it accomplish its drying function?

 

[red91sit]

It's currently ducted to the outdoors. The purpose of this dryer is to dry a food ingredient for final processing. This dryer runs nearly constantly during the season, only shutting down for cleaning, maintenance, and power outages. The pressure would be regulated by the intake fans / PLC (these are HVAC intake fans on the exterior of the building). The dryer fan would only shut down if there was a problem for some reason causing the factory to be under vacuum conditions.

My rational behind this is; the dryer is the main source of heat in the plant, so we don't need the additional ventilation to be running when the dryer isn't running. Which is why it would make sense to use it's exhaust fan in lieu of additional exhaust HVAC fans. I'm currently checking the noise level to make sure sound of the dryer air inlet isn't going to be deafening in our factory.

 

[BronYrAur]

But this will require you to condition the 45,000 of makeup air, correct?

 

[red91sit]

I should have mentioned, this is a food processing facility in Southern Florida. This incoming air will need to be filtered, but that is it for 'conditioning'.

 

[chicopee]

If you connect ducting from the factory to the duct system of the dryer,then the dryer will no longer have the 45000 CFM currently available, is that what you are proposing?

 

[red91sit]

Currently, there is no ducting in the factory. We have several large intake fans near the ground, and several exhaust fans located near the ceiling to duct heat out of the factory. These aren't enough, so we are looking at adding additional intake and exhaust fans.

The dryer pulls air from outside, and rejects it back outside through an exhaust stack.

I'm not concerned about the CFM on the dryer, if anything, it should be higher due to the small amount of positive pressure within the factory.

Just trying to see if anyone has tried this before, or if I'm overlooking any other important details.

Overview of current layout

 

[BronYrAur]

Seems to me that as long as you interlock intake fans so that you are bringing in enough air to support the dryer, there should be no air flow problems. If you are short of ventilation as it is, this idea should give you an additional 45,000 CFM. Air should be brought in far away from the dryer so that it can sweep the room. Dryer intakes should be near the ceiling to pull in hot air. Just make sure you maintain your required positive pressure with enough intake fans. Don't use louvers without fans or you will go negative.

 

[MintJulep]

No no no.

You have to install fans to make this work anyway.

So just install ventilation fans and have a ventilation system and a drier system. Separate.

Don't unnecessarily tie the two together. Too much to go wrong.

 

[chicopee]

I second the motion of MintJulep's proposal, with the additional comments that you ventilate only the work zone for the comfort of your workers and not the entire plant

 

[BronYrAur]

I understand the desire for simplicity but now you would have to add both intake and exhaust fans for ventilation. I think the entire motivating factor for this project is to avoid installing exhaust fans and to use the dryer exhaust instead.

 

[red91sit]

@chicopee. The majority of things are automated in our factory, but being southern Florida, it's unbearable to even walk around in here in the summer time with both dryers, pasteurization, and our steam injection heating the product upto 200'f. Hence the ventilate the whole plant plan.

@mintjuliep - We already have the fans as shown on the CAD drawing, they are a long ways from being enough, which is why we are adding more. I understand the KISS principle, but at the same time, it seems like such a waste to have a 45,000 cfm exhaust fan constantly running that is pulling air in from outside, as well as multiple exhaust fans pulling out the same volume.

@bronyraur - That's exactly the rationale behind this. The airflow in this plant is horrid, the plant is packed with hot equipment. My hope is that with a big centrally located exhaust inlet, we would have a lot more flow covering the plant instead of just blowing around near the walls, with a small amount escaping through the penthouse (that's what we call the top extension bit).

I'm all for other ideas as well! Thanks guys!

 

[willard3]

Do you have a fan curve for this exhaust fan? What kind of fan is it, FC, BI, Vaneaxial?

The fan type may influence how you control it.

 

[317069]

You said: "We have several large intake fans near the ground, and several exhaust fans located near the ceiling to duct heat out of the factory. These aren't enough"

- why not enough?

 

[red91sit]

@Willard3 - it is a BI, but this will NOT be used for pressure control. This would only be throttled down if for some reason the building was no longer under pressure. The intake fans would be throttled to maintain the positive pressure we are aiming for in the factory.

Dryer exhaust fan curve FYI

@317069 - we have added several heat intensive processes, as well as an additional dryer. Due to these changes, the plant is now uncomfortably hot.

 

[317069]

as I understand(correct me please), there is a 45000cfm dryer use outside air to dry products then discharge air to outside again, you want this dryer to take this 45000 cfm from the factory instead of outside and use it to dry products and to ventilate the factory space at the same time.
it is a good idea but the question is about the factory air being exhausted, does it have the same parameters od the outside air such as: temperature, humidity...etc. if it does not, would it effect drying process.

 

[chicopee]

Since you have the fan curve, you need now to provide the existing system curve and then add an addition system curve for the proposal and then determine the resultant system curve and seen where it intersects the fan curve to determine feasibility and electrical load. Remember that the resultant system curve should deviate around +or- 10% of the original intersection.

 

[dbill74]

One question to ask if it hasn't already, is whether the dryer's intake can be modified to pull air in from the factory floor. If it can, would it void any warranties assuming it is still under warranty. If you do add ductwork, the fan now has to overocme the additional friction lossess associated with the new ductwork; from the fan curve, you don't have a lot of room to work with without sacrificing air flow. Which raises the question of can the dryer do it's job with less than 45,000 cfm of air? Contacting the dryer's manufacturer is recommended to ensure it can be modified to do what you want it to.

You mention adding several heat intensive processes. Where are these relative to your dryer? If they are not close to the dryer (lots of new ductowrk) or there is workspace between these new processes and the dryer, having ventilation air moving over these processes towards the dryer is not going to solve your worker comfort problem since you are now heating this ventilation air and your workers with your new processes. The work spaces between the new processes and dryer will likely be warmer than outside (not desirable for south Florida).

I think you would be better off leaving the dryer as is, adding the new supply fans as intended (probably do not need 45,000 cfm of new) and add exhaust fans or gravity vents in the roof above the new heat generating processes. Intent being to direct air heated by the new processes away from your workers and their work areas.

Not knowing exactly what you are producing or how interrelated the drying process is with the new process, keeping the dryer's ventilation separate from the rest of the building allows you take it out of service (planned or otherwise) without affecting your worker comfort.

For worker comfort, have you considered adding A/C to high traffic areas or inspection locations? It wouldn't have to be a big VAV system with central AHU trying to cool the entire plant, but just enough in places where your workers need to stand for more than a minute or two to provide a cool spot. Ductless mini-split systems in select areas with a local occupancy sensor would probably work and be a welcome relief to your workers.

 

[red91sit]

317069 - I'm not to concerned with minor differences between inside/outside air temp/moisture. We experience larger changes on a day to day basis just due to the changing weather patterns. The temperature on the drying system is controlled by the outlet air temperature. Depending on feed rates, inlet temps typically climb to roughly 600f.

dbill74 - whew, lot to reply to here;

One question to ask if it hasn't already, is whether the dryer's intake can be modified to pull air in from the factory floor. If it can, would it void any warranties assuming it is still under warranty. If you do add ductwork, the fan now has to overocme the additional friction lossess associated with the new ductwork; from the fan curve, you don't have a lot of room to work with without sacrificing air flow. Which raises the question of can the dryer do it's job with less than 45,000 cfm of air? Contacting the dryer's manufacturer is recommended to ensure it can be modified to do what you want it to.

This is a fairly old, heavily modified drying system, no warranty issues to worry about here. The new ducting would actually be shorter, as we could cut it down lower in the 'penthouse'.

I think you would be better off leaving the dryer as is, adding the new supply fans as intended (probably do not need 45,000 cfm of new) and add exhaust fans or gravity vents in the roof above the new heat generating processes. Intent being to direct air heated by the new processes away from your workers and their work areas.

I'm starting to lean more this direction for simplicity's sake. We're actually going to be adding over 100,000 cfm of new ventilation. The new dryer, is insulated with 2" of insulation, but is still expected to loose roughly 1/2 million btu/hr. With our new process, summer running is expected to increase, so we want to get this place nice and ventilated.

As far as area specific cooling, this is less of an option as a good portion of the process involves operators walking around monitoring various aspects of the process. This might come in handy in our packaging area where we do have employees staying in the same location for longer periods of time

Thank you all for the input!