CO2 Bouyancy vs Diffusion 2

[AbbyNormal]

Consider a gas furnace in the basement of a home. Atmospheric vent, with a draft hood.

If the products of combustion were to spill would the CO2 tend to rise up to the upper levels of the home because it is hot, or would it soon cool off and collect at the floor level, stratified?

Or would it just mix evenly into the room air of the basement?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

I suppose the same could be wondered about any carbon monoxide produced as well

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[mbeychok]

AbbyNormal:

At the same temperature and pressure as air, carbon dioxide is a denser gas (i.e., heavier gas) than air because it has a higher molecular weight than air. The only way it will rise above the air is if it is hot enough.

From the Ideal Gas Law:

[ρ] = M[•]P/(R[•]T)

where:
[ρ] = gas density in kg/m³
M = gas molecular weight = 29 for air and 44 for CO₂
P = gas pressure, in atmospheres
T = gas temperature in kelvins (K), and K = 273 + [°]C
R = the universal Ideal Gas Constant = 0.082057 for the above units

Assuming an air temperature of 293 K (i.e., 20 [°]C), the density of air is 1.205 kg/m³ at 1 atmosphere of absolute pressure

To have the same density, the CO₂ would have to be at a temperature of 445 K (i.e., 172 [°]C)

Thus, for the carbon dioxide to rise to the upper levels of the house it would have to be at a temperature above 172 [°]C to have a density lighter than the air. As soon as it cooled to less than 172 [°]C, it would sink to the ground floor level.

The molecular weight of carbon monoxide is 28 which is very slightly lighter than air, so it would tend to rise above the air and remain there even after it cooled off.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[25362]

The macroscopic picture presented by mbeychok is correct when considering CO₂ in isolation.

When considering complete combustion with excess air, CO₂ is but one of the outcoming gases, hot nitrogen, oxygen and water being major participating molecules, beside various minor components. Thus the gaseous average mass density wouldn't be far apart from that of hot air.

At the molecular level gas diffusivity is a complex subject with many variables playing a role.

The diffusional volume (cm³/mol) of CO₂ being 50% larger than that of nitrogen and 110 % greater than that of water molecules.

Just consider that at room conditions (T~300K, p~101.3 kPa) a molecule collides with other molecules at a rate of about 10⁹ times per second to appreciate the complexity of the question in hand.

Not having any connection with -or knowledge concerning- HVAC, I assume an approximate correct answer bringing the time factor into account should be based on experience and practical results.

 

[AbbyNormal]

thanks for the responses

so it appears that initially it is possible for some buoancy due to the temperature, then the gases would cool and those heavier than air would 'fall to the floor'

There must also be some diffusion processes at work and movement from areas of high concewntration to low concentration, other wise in an 10 foot high room we would be all crawling around trying to breathe oxygen in the layer two feet above the floor while the ntirogen occupied the top 8 feet of the space.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[25362]

Some points to ponder:

[•] Ventilation is a sine qua non condition to avoid asphyxia.
[•] Gaseous mixtures don't de-mix by themselves.
[•] Thermal convection currents could create sufficient turbulence to make the CO₂ concentration about equally distributed with time.

 

[bimr]

http://www.newton.dep.anl.gov/askasci/chem03/chem03364.htm

 

[mbeychok]

AbbyNormal:

The following quotations from the Consumer Products Safety Commission at

http://www.cpsc.gov/cpscpub/pubs/466.html

may be useful to you:

"CO alarms should be installed according to the manufacturer's instructions. CPSC recommends that one CO alarm be installed in the hallway outside the bedrooms in each separate sleeping area of the home. CO alarms may be installed into a plug-in receptacle or high on the wall. Hard wired or plug-in CO alarms should have battery backup. Avoid locations that are near heating vents or that can be covered by furniture or draperies. CPSC does not recommend installing CO alarms in kitchens or above fuel-burning appliances."

and

"Install a CO alarm that meets the requirements of the current UL 2034 or CSA 6.19 safety standards. A CO alarm can provide some added protection, but it is no substitute for proper use and upkeep of appliances that can produce CO. Install a CO alarm in the hallway near every separate sleeping area of the home. Make sure the alarm cannot be covered up by furniture or draperies."

Milton Beychok
(Visit me at www.air-dispersion.com)
.