Factors which affect Industrial Gas Turbine Air Mass Flow Rate
The main factors we at Real Time Power have observed which affect industrial gas turbine air mass flow rate are:
- Shaft rotational speed
- Inlet guide vane (IGV) angle
- Air density at the compressor inlet
- Compressor extraction flow rates
The compressor is well approximated as a constant volumetric flow rate machine when at synchronous speed with fixed IGV angle. The volumetric flow rate is then most influenced by the IGV angle, and the relationship between IGV angle and volumetric flow rate is nonlinear, because the effect of the IGV opening involves either a cosine or sine function, depending on how the angle is defined. This document will use the convention of IGVs fully open as 0° and fully closed would be -90°. In this case, a cosine curve would be a good fit to the volumetric flow rate response, since cos(-90°) = 0 and cos(0°) = 1. It is often the case that some additional flow rate can be generated by going beyond 0° opening, up to 5-6° is typical. This extra flow is created due to aerodynamic interactions between the IGVs and the first stage of rotating compressor blades, and is not predicted from simple “area of opening” calculations.
With constant volumetric flow rate, mass flow rate will then vary as a function of compressor inlet air density. The main factor which affects air density is the temperature of the air, and this is why evaporative coolers, chillers and foggers are deployed in order to lower the ambient temperature and therefore increase the air density and the air mass flow rate, and ultimately the power output of the gas turbine (which is directly proportional to air mass flow rate). Air pressure and relative humidity also affect air density, and the disadvantage of evaporative coolers and foggers over closed cycle chillers is that the lower density water vapor introduced into the air flow reduces some of the gains of the cooling. The air pressure at the compressor inlet is affected by the ambient air pressure and also by pressure drops across air filters and air conditioning equipment. Fouling of air filters and evaporative cooler media can lead to increased pressure drops and lower gas turbine power output.
The fourth factor listed is compressor extraction flow rates, which typically vary based upon ambient conditions, as well as the condition of the gas turbine. Extracting more flow from the compressor stages will in general increase the mass flow rate into the compressor inlet. When compressor extraction flows are scheduled roughly in proportion to IGV opening, then normalized compressor ratio (compressor discharge pressure / compressor inlet pressure) x (ISO air density / air density) correlates extremely well with the volumetric flow rate measured by Real Time Power’s AirSonic system.
One thing not on the list of major influencers is compressor efficiency (or cleanliness). Only in extreme cases of fouling does the volumetric flow rate decrease. In most normal situations, the effect of compressor fouling is to increase compressor discharge temperature, which will in turn increase turbine exhaust gas temperature and limit the maximum power output of the engine. So the main effect of compressor fouling is on maximum achievable MW output from the gas turbine.