Inlet Air Flow - Natural Gas Combustion Turbines 2

[jason1]

In the anaylsis of a natural gas fired combustion turbine--at varying inlet air temperatures--what is the correct assumption to make regarding the inlet air flow: constant mass flow rate or constant volumetric flow rate or do both vary greatly with inlet air temperature?

Jason Richards

 

[hacksaw]

Usually a mass basis

 

[ozyjo]

Thanks Hacksaw,
normally and will in school mass flow rates were used. Volume rate is a pretty much affected by temperature, so vary degree of flow properties is observed.

 

[yates]

At a given rotational speed, constant fuel (gas) flow - turbine mass flow will increase with decreasing inlet (ambient) temperature, volume flow will stay constant. This is not exactly true since pressure drop across filters/silencers will increase with increasing mass flow, but it's a second order correction.
The main reason for increased turbine power output at low ambiant temperatures (apart, of course, from the greater delta-T available with respect to a fixed exhaust temperature) is precisely the increase in inlet air mass flow, which, with stoichiometric control of fuel flow, allows a greater fuel flow.

 

[davefitz]

Yates is correct- constant volume flow. You can confirm this by reviewing the ambient temperature correction curves the compressor/turbine vendor provides- the mass flow will increase directly proportional to the density increase as the absolute temperature drops .

A common method to increase combustion gas turbine output on a hot day is to use inlet chillers or evaporative coolers or inlet foggers- by lowering the inlet temperature the output can be increased as much as 11%, 49 F vs 110 F inlet ( arrid desert air at 110 F cooled to 49 F with inlet chiller)