fan system for a chimney 1

[kagadpencil]

hi,

with a 20 foot chimney for our over we are getting too much excess air. We need to bring the excess air by about 300%. Is installing a fan system on the outlet of chimney the solution ? I was thinking with original chimney we have too much excess air -- that means we have enough draft. Isnt it true that fan system is installed to overcome pressure loss? I think we have enough draft in the system already -- What is the best way to control or bring down the excess air ? we want to be cost-effectively control the % excess air going into the oven.

 

[sshep]

You are correct in reasoning that a fan is NOT the answer. You need a damper which you can close to adjust the air. With an actuator you can even make closed loop control.

 

[sshep]

Not sure about the layout of your system, but aren't there some air registers which can be adjusted on (or upstream of) your burners? Adjusting these could solve the problem for you. Because attention was drawn to the stack, I previously did not consider to advise about control on the other end. It seems hard to believe that some sort of air control along those lines was not originally provided.

Registers and dampers are the common control elements on natural draft systems (since the variable height stack has not yet been invented). While opinions vary on the proper control pairing of excess air and draft using dampers and registers, dampers more often lend themselves to automatic control of excess air, although register adjustments are still required to properly distribute combustion air and set a suitable draft.

A fan would (normally) only be needed to provide motive force for an air preheat or waste heat recovery system.

 

[kagadpencil]

you are exactly right SSHP.

we plan to install a heat exchanger to recover some of the heat that is going out of the chimney. The avg. temperature of the flue is over 700 F and a lot of heat is being wasted. Since heat exchangers are not in place at present, we an orifice with beta = 12/20 to simulate the heat exchanger pressure drop. I now need to come up with the sysetm curve for the present set up. The fans have already been installed in hundreds of such setups so there is no point in thinking and rueing about the "correct" solution of baffles, dampers etc. The job now is -- acccount for all the pressure drops and come up with the system curve (delta P vs. flow rate ACFM or SCFM)

I had a few more questions:

1. since our chimney height is over 20 feet, do I need to consider Bernoulli theorem in calculation of delta P for the system.
2. When I consider the positive draft due to chimney height I am basically accounting for the buoyancy body force that arises due to the weight difference in the air inside the stack and air outside the stack -- that does not mean I also acccount for change of elevation of gas -- I still have to use Bernoulli for that - right ?
3. I am hoping that that the pressure drop in the system is more that the +ve draft of chimney -- what if it is not ? how do we plot the system curve then ?

looking forward to some helpful tips ! -- thank you for all your help so far.

 

[sshep]

The velocity component of the calculation is often significant as are the height and density difference between stack gas and air. I think the potential energy due to elevation is usually insignificant.

I found the following formula in the GPSA manual for available draft in natural draft stack systems, which is the natural draft less friction and velocity. Note that these losses are "added" because draft is a negative number.

Available Draft= 0.192*Hs*(Rhog-Rhoa) +
0.0029*V^2*Rhog*(4*f*(Hs/D)+1)

H=height of stack, ft
Rho=density of air or stack gas, lb/cuft
V=velocity, ft/s
f=Fanning friction factor
D=diameter, ft

Perry gives good advice for chemical engineers for fan selection and sizing: don't actually choose one from vendor tables (i.e. static pressure vs flow) without a guarentee proposal from the supplier's technical support. Fan selection and installation details can have more pitfalls than pumps. These guys will also be able to do an evaluation of the upstream and downstream duct system.

 

[kagadpencil]

ok... now I understand the challenges in this project. But to be honest with you, I am not the first engineer to work on this project. The installations were done last year, but some installations are giving problem. The goal is to control the % excess air in the system using the variable speed fan at the end of the duct system. In the duct I have chimney draft effect, pipe frictional losses, orifice loss, sharp exit loss when chimneys release flue gases in the plenum chamber, sharp entrance loss when a short pipe sucks flue gases from the plenum chamber to the centrifugal fan. By taking different ACFM values I need to find pressure drop in the system and get the system curve. Then by finding the fan curve intersecting this system curve I need to find the correct RPM value of fan which will give us the right % excess air ( ACFM is a function of % excess air). I have a conceptual question:

1. I am dealing with hot flue gases (800 F ) that are not at std. conditions. Thus do I need to convert the fan curves to flow conditions ? -- In other words, do I need to multiply the delta P values with the density ratio (density at 800 F / density at 70 F) before I try to find the intersection with the system curve ?

Please tell me if I am missing something here. Is my approach correct ? Do you suggest any other approach ?

 

[friartuck]

Hi

Just picked this query up

Di I understand that you actually already have an induced draft fan installed rather than just having natural draft.

We use Draft Stabilizers...these work on the principle that the boiler/boilers discharge into a header flue which in turn goes into the main riser chimney. At the end of the horizontal chimney run, the draft stabilizer is fitted. This is a circular disc pivoted at the centre (Slight above centre actually). The disc is free to tilt but is also weighted. When the draft gets too high, the disc pivots and lets some room air into the chimney and thus avoids excess air being drawn through the boiler or process. (IVO make these)

They are cheap and simple to set up.

They caould be used on natural draft or ID systems.

Friar Tuck of Sherwood

 

[kagadpencil]

yes, the fans are already in place - so no point in thinking about the correct solution. But the good thing is we have an orifice fitted into the chimney to simulate the heat exchanger that will be installed in the future to recover some of the heat in the flue gases. From my calculations orifice pressure loss is about an inch of water as opposed to the chimney draft of 0.1698 inch water. Thus fan is necessary in the system. The goal now is to find the algorithm to find the correct RPM of this fan to get the correct ACFM.

I have the calculations ready to find the pressure loss for different ACFM for the system -- Thus I can get the system curve at actual flow conditions -- task now is to find the point of interestion with the fan curve ( which is under standard conditions) -- QUESTION: HOW DO I CONVERT MY SYSTEM CURVE WHICH IS FOR ACTUAL FLOW CONDITIONS TO STANDARD CONDITIONS? Clearly the point of intersection of the present plot of delta P vs. ACFM and fan curve is not going to give me the correct operating point.

please give me some guidance.

 

[sshep]

The starting point is to leave your flow axis alone and replot the curve using head instead of inches of water pressure (or plot a new pressure curve using density ratio). I think horsepower is also directly proportional for density change. This will correct the curve for the density difference between standard and actual conditions.

Since the fan exists and does not appear to be working at design, you can already appreciate the wisdom of Perry in giving the advice I referenced in a previous post above. This is advice (and the affinity laws applicable to your situation) is given in chapter 6 of my Perry's Chemical Engineering Handbook 6th Edition, especially "selection of fans". Best of luck straightening out these problems.

Incidently, is this to be forced or induced draft relative to your new exchanger? Although either configuration is possible, typically I have seen these systems to have the heat transfer equipment upstream of the fan (induced). This means the fan will be handling a significantly cooler (and higher density) gas when the project is complete. If this was the original basis it could help explain why the interm system is not performing as expected.