Exhaust stack sizing 1

[radaes]

Just wondering if there is a standard rule of thumb for sizing the diameter of an exhaust stack (something along the lines of "for every X cubic feet/hr, increase diameter Y inches"). I'm trying to vent a small enclosure (on the order of 4x4x4) of exhaust gasses and want to make sure there's a decent draft.

Thanks (and if you think I should post this elsewhere, please let me know)

cheers,
rad
"Remember, if you leave it to the last minute, it'll only take a minute"

 

[AEBuck]

Not that I will have an answer, but a few more details on the application are necessary. Exhaust gas temperature? Length of horizontal and vertical stack segments? Type and quantity of stack fittings? Never seen a rule of thumb for this ... perhaps for good reason. Look at stack sizing guidelines for natural draft boilers. Maybe you'll discover a correlation. Just how important is this stack?

 

[imok2]

For kitchen exhaust ducts a "rule of thumb" is 1500/2200 ft/min to terminate 40" above roof line. 16ga galvanized or 18 ga stainless steel.

 

[lilliput1]

For lab exhaust we duct the exhaust to a vertical fan discharge, using the fan discharge size up to minimum 10 ft above the roof then taper the tip 15° to a size that would correspond to 3000 FPM. Stack must be minimum 20 ft from an intake. The stack must have minimum 3 guy wires at 120°apart. The guy wires must not be tighten too much or it will act to compress the stack. They are needed to resist 100 mph wind.

 

[lilliput1]

For lab exhaust we duct the exhaust to a vertical fan discharge, using the fan discharge size up to minimum 10 ft above the roof then taper the tip 15° to a size that would correspond to 3000 FPM. Stack must be minimum 20 ft from an intake. The stack must have minimum 3 guy wires at 120°apart. The guy wires must not be tighten too much or it will act to compress the stack. They are needed to resist 100 mph wind.
Sizing stack in this manner minimizes duct friction. Note the transition & exit velocity loss would be about equal to 1 velocity head or .57 say 0.6"wg at 3000 FPM

 

[AEBuck]

Did I miss the point? When "draft" is mentioned, does that not imply natural draft? For the other applications mentioned subsequently - yes, that's the way to do it.

 

[radaes]

AEBuck,

yes, it's supposed to be natural draft. I'm trying to avoid having to include a draft inducer (staying outside-power independent).

I'm using IR heaters on a vertical heat exchanger, with a smaller HX placed horizontally above to scavenge some of the waste heat (both from the exhaust products and the hot air). To do that, I need to keep the air moving - hence the exhaust stack. Height doesn't really matter, I'm more concerned with the size of the hole.

imok2 and lilliput1 - thanks for the input, just not quite what I was looking for.

Thanks for everyone's suggestions,



cheers,
rad
"Remember, if you leave it to the last minute, it'll only take a minute"

 

[lilliput1]

Height does matter with natural draft. Natural draft is created because the density of the hot air is less than that of the ambient air. The natural draft must create enough lift to overcome the friction loss through the vent pipe and fitting. Note that heat loss through the vent pipe will reduce the draft available.

 

[rogzog]

Don't forget about the combustion air. Assuming we are talking about a gas fired heater,for example, if the unit is getting all of the combustion air from inside the building with no infiltration of air from outside, you need two openings to an air source sized 1 sq. inch per 1000 Btu/h or a minimum of 100 sq. inches each. Usually one at 12" above the floor and one 12" below the ceiling.

Other rules apply for other conditions. This acording to Uniform Mechanical Code 1997 Edition Table 7-1. Flue size is described as no smaller than the outlet of the appliance.

But a space of 4'x4'x4' obviously is not a typical mechanical enclosure. I wouldnt know if this would apply to your situation. But I think you need some free air source to get any draft up the flue.

rogzog

 

[AEBuck]

From "Fan Engineering" published by Buffalo Forge Company (a must have for the serious engineer), pages 22-15 onward:

"Any structure that confines a vertical column of gas is capable of producing a stack effect. The magnitude of that effect (or the draft created at the base of the structure) if there is no flow can be calculated from the difference in weight between the column of gas and a similar column of the ambient air or gas."

Assuming the molecular weight of the gas is equal to that of ambient air, the equation for theoretical draft reduces to ...

Pd = 0.2554 * Pt * L * [(1/Tamb) - (1/Tmg)]

Pd, theoretical draft, In WG
Pt, barometric pressure, In HG
L, stack height, Ft
T, temperature of ambient air or moist gas, degrees Rankine

"The rate of gas flow that can be sustained by any stack effect depends on the energy losses that must be overcome, including the friction loss through the stack itself and the exit loss." Obviously this would also include losses starting from the source of make-up air and continuing through any obstructions in the enclosure of interest.

The text goes on to state "... for chimney-sizing purposes, a stack draft loss of 5% of the theoretical stack effect usually yields an economical diameter."

From an included table for theoretical stack effect in In WG per 100' of stack height, assuming Pt = 29.92 In HG and SG gas = 1.0, and using average flue gas temperatures from 300F to 800F:

At an average ambient air temperature of 40F, theoretical stack effect ranges from 0.52 to 0.92 In WG per 100'.

At an average ambient air temperature of 80F, theoretical stack effect ranges from 0.41 to 0.81 In WG per 100'.

For example, the actual stack draft loss for a 25' stack at 550F in 60F ambient should not exceed 0.71 * 25/100 * 5% = 0.009 In WG. Reduce this by any safety factor that seems appropriate.

Using the moist gas mass flow rate and density, calculate all losses in the flow path using actual cubic feet per minute.

You do need to account for stack heat loss, thus use average stack gas temperature.

With this array of variables, there is no "rule of thumb" for every situation.

You do need to read this reference or something similar. I had to ... I am not this smart! Bottoms up.

 

[radaes]

AEBuck,

you get a star - that's more detailed info than I was hoping for. Thanks!

rogzog - the enclosure is located outside, so I think air supply is taken care of

thanks for everyone's help on this



cheers,
rad
"Remember, if you leave it to the last minute, it'll only take a minute"