chimney design 1

[kagadpencil]

hi,

I am reading ASHRAE standards. I see that for calculation purposes, density of flue gases in chimney can be calculated as that of air at the same temperature! -- This inspite of flue gases having totally different composition -- How correct is that ? Please give some comments for this rookie engineer.

 

[25362]

Air is composed mainly by nitrogen (~79%vol). In the flue gases resulting from burning hydrocarbons nitrogen stays practically as is, and most of the oxygen (MW:32) is replaced by CO2 (a bit higher Cp than for air), water (Cp almost double as for air), and some SO2 (~ 70% the Cp of air). In combustion with xs air, one gets "unused" air in the flues, a fact that makes the Cp more similar to that of air itself. When dealing with flues on a "dry" basis, as when burning hydrogen-free coal, the Cp of the flues becomes even nearer to that of air.


Tabulated enthalpies, kcal/kg, of gases at 500 and 1000^oC are:

air 124/260.8/
water 235/509
nitrogen 127.6/267.3
oxygen 117.1/247.5
SO2 86.7/187.4
CO2 122.2/269.8
CO 128.4/270.4