Mass Balances Check

1. For a steady-state open system, the mass balance states that:
Mass in is always greater than mass outMass in equals mass outMass is destroyed in proportion to energy releasedMass balances only apply to liquids, not gases
2. A flow meter reads 2 m³/s of air at a density of 1.2 kg/m³. The mass flow rate is:
1.2 kg/s0.6 kg/s2.4 kg/s2.0 kg/s
3. Why does a combustion mass balance need the mass fraction of oxygen in air (~23.2%), not the volume/mole fraction (~21%)?
They're the same number and it doesn't matter which is usedMass and volume/mole percentages differ because oxygen and nitrogen molecules have different masses, and a mass balance must be done in mass unitsThe volume fraction is only used for liquid fuels23.2% is simply a more accurate measurement of the same 21% figure
4. A boiler burns 0.01 kg/s of methane, needing 0.206 kg/s of actual (excess) air. By mass conservation alone, the flue gas mass flow must be:
0.01 kg/s0.196 kg/s0.216 kg/sIt cannot be found without measuring it directly
5. An air handling unit has 24 g/s of water vapour entering and 16 g/s leaving in the supply air. By mass conservation, the condensate draining from the coil must be:
40 g/s8 g/s24 g/sIt depends on the air's temperature, not its mass balance

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