Damper curves

[zmis]

Among other things we build stacks with installed control dampers to maintain static draft. Customer wants a damper curve (pressure drop vs % open). I feel like there's an appropriate place to start but that I can't find it. We don't do any in place testing so measuring pressure drop at various rates and temperatures and % closed isn't going to work. Perry's doesn't have anything that I can see. Google hasn't come up with much besides HVAC and although it's been somewhat handy I really just need something in the ballpark, not something exact. I'm not sure that's even possible without field data.

Any suggestions?

 

[Compositepro]

Damper are essentialy butterfly valves with a lot of leakage, which you will only notice at shut-off.

 

[zmis]

We use the damper to maintain draft at a set location in the unit and setting varies dependent on fuel firing rate etc which produces a given quantity of flue gas at a corresponding temperature. Usually multiple wafer opposed blade dampers but on smaller units we do sometimes have a single "butterfly" style damper. In either case there is a fair bit of adjustment that does affect the draft the burner sees. I understand the customer's preference for a curve they can reference when setting the damper, I'm just not sure it can be produced from my desk as opposed to gathering their own field data after startup.

 

[EmmanuelTop]

I don't think it is possible to generate an exact (or close to exact) curve based on general data, because the flow vs. pressure drop will be different for each heater - depending on the height of the stack, the amount of combustion air supplied (which again depends on type of the heater - forced draft, induced draft, balanced draft, natural draft, etc.), in-leakages of air, resistances inside the heater (e.g. fouled convection section etc.). The whole thing gets complicated further by the fact that the available pressure difference is very small (in the range of a few kPa for natural draft furnace) so the sensitivity and hence the relative error become significant.

Everybody in the world uses Oxygen analyzers for adjusting damper position (= draft). I would never rely on any equation (even if supplied by the heater manufacturer) in order to adjust position of the damper.

Dejan IVANOVIC
Process Engineer, MSChE

 

[sshep]

Hey zmis,

What may make the client happy is a curve of flowrate vs dP, for a set of % open curves.

I think you can mathematically produce such a set of curves if you have even a single data point and some reference assumptions. The first step is to relate % open to area. dP = f(area, flow). The most simple results should be mathematically reasonable. Correcting for non-circular cross-section as the damper closes is a possible enhancement and an area to literature search.

The usefulness of having such curves is debatable, but the effort may be minimal.

best wishes,
sshep