What is IBH?

IBH is a way to decrease compressor efficiency and reduce mass flow through the compressor. Inlet bleed heat(IBH) takes compressor discharge air and routes it back to the inlet of the compressor. By taking compressor discharge air away from the combustion wrapper you reduce mass available to the combustor. Also introducing this very hot air into the turbine inlet raises the temperature of the air and decreases the efficiency of the compressor since warm dry air has less mass than cold damp air. All of this is typically done to reduce mass flow through the compressor at low load conditions to help control emissions. It also helps to decrease the formation of ice in cold climate areas, and lastly helps to provide compressor protection at low loads, again by reducing the efficiency of the compressor. Inlet bleed heat demand is typically a logrithmic function of IGV angle, although you would be advised to review application code for the particular unit you are familiar with.
This description is basically for unloading, when the IGVs are modulated in the closed direction to allow Premix Combustion Mode operation below approximately 80% of rated power output. When the unit is being loaded, the IGVs are held closed to minimize air flow through the unit while the IBH (Inlet Bleed Heat) system is in operation in order to cause the unit to transfer into Premix Combustion Mode sooner than would otherwise be possible--but as the unit is loaded and exhaust temperature increases the IGVs must be opened in order to prevent the exhaust temperature from exceeding the limit until the IBH system is de-activated, at which time the unit will be loaded on IGV Exh. Temp Control until it reaches Base Load and the IGVs are fully open.

To operate the unit in Premix Combustion Mode below approximately 80% of rated power output the IGVs must be closed to angles less than the usual minimum operating angle of 57 DGA--but doing so can cause abnormal axial compressor operation under certain conditions. IBH reduces the flow through the axial compressor by recirculating a portion of the axial compressor discharge air (which is hot!) to a manifold in the Inlet Air Duct and heating the axial compressor inlet air a few degrees making it less dense, thereby increasing the compressor operating limit, sometimes referred to as compressor operating margin. This is done in conjunction with lowering the IGV angles, which also reduces the air flow through the compressor. The IGV (Inlet Guide Vane) Temperature Control curve mirrors the CPD-biased Exhaust Temperature control curve which has a negative slope up to a horizontal limit which is sometimes referred to as the "isothermal limit", which is usually approximately 1100 deg F. As the unit is unloaded and CPD (Compressor Pressure-Discharge) decreases, the Exhaust Temperature Reference increases up to the maximum isothermal limit value. The negative slope of the Exhaust Temperature Limit Control Curve represents a constant first-stage turbine nozzle temperature which is also called "firing temperature"--which varies with CPD. As CPD decreases, the air flow through the unit decreases and one of the results is that for a constant firing temperature the exhaust temperature increases. To protect the exhaust diffuser components the exhaust temperature can never be allowed to exceed the isothermal limit, so as the unit is operated at Part Load the IGVs are modulated to maintain the Exhaust Temperature Limit--which has a maximum value equal to the isothermal limit at part load operation. There is usually approximately a 5 DGA deadband (overlap) for IBH operation, meaning that IBH can be active at IGV angles of approximately 62 DGA. It should be noted that without Inlet Bleed Heat units would not transfer into Premix Combustion Mode until approximately 80% of rated power output. So, units with IBH will transfer into Premix Combustion Mode sooner than units without IBH (at approximately 40-50% of rated power output).