fan system for a chimney
fan system for a chimney
(OP)
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.
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.





RE: fan system for a chimney
RE: fan system for a chimney
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.
RE: fan system for a chimney
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.
RE: fan system for a chimney
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.
RE: fan system for a chimney
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 ?
RE: fan system for a chimney
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
RE: fan system for a chimney
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.
RE: fan system for a chimney
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.