Question: Isobaric pressure process occurs at what pressure
About the article "Thermodynamics", section "Commonly considered thermodynamic processes".

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An isochoric process is described by the equation formula_22.
It would be convenient to have a similar equation for isobaric processes.
Substituting the second equation into the first yields formula_23 The quantity "U + p V" is a state function so that it can be given a name.
It is called enthalpy, and is denoted as "H".
Therefore an isobaric process can be more succinctly described as formula_24.
Enthalpy and isobaric specific heat capacity are very useful mathematical constructs, since when analyzing a process in an open system, the situation of zero work occurs when the fluid flows at constant pressure.
In an open system, enthalpy is the quantity which is useful to use to keep track of energy content of the fluid. 
A system undergoes a process when one or more of its properties changes.
A process relates to the change of state.
An isothermal (same temperature) process occurs when temperature of the system remains constant.
An isobaric (same pressure) process occurs when the pressure of the system remains constant.
An adiabatic (no heat exchange) process occurs when no heat exchange occurs. 
In the cycle, an ideal gas undergoes 1-2: Constant volume (isochoric) heat addition; 2-3: Isentropic expansion; 3-1: Constant pressure (isobaric) heat rejection—compression to the volume at the start of the cycle.
The expansion process is isentropic and hence involves no heat interaction.
Energy is absorbed as heat during the isochoric heating and rejected as work during the isentropic expansion.
Waste heat is rejected during the isobaric cooling which consumes some work. 
The pressure-volume conjugate pair is concerned with the transfer of mechanical or dynamic energy as the result of work.
An isobaric process occurs at constant pressure.
An example would be to have a movable piston in a cylinder, so that the pressure inside the cylinder is always at atmospheric pressure, although it is isolated from the atmosphere.
In other words, the system is dynamically connected, by a movable boundary, to a constant-pressure reservoir.
An isochoric process is one in which the volume is held constant, meaning that the work done by the system will be zero.
It follows that, for the simple system of two dimensions, any heat energy transferred to the system externally will be absorbed as internal energy.
An isochoric process is also known as an isometric process or an isovolumetric process.
An example would be to place a closed tin can containing only air into a fire.
To a first approximation, the can will not expand, and the only change will be that the gas gains internal energy, as evidenced by its increase in temperature and pressure.
Mathematically, formula_1.
We may say that the system is dynamically insulated, by a rigid boundary, from the environment.  
A Brayton-type engine consists of three components:  a compressor  a mixing chamber  an expander In the original 19th-century Brayton engine, ambient air is drawn into a piston compressor, where it is compressed; ideally an isentropic process.
The compressed air then runs through a mixing chamber where fuel is added, an isobaric process.
The heated (by compression), pressurized air and fuel mixture is then ignited in an expansion cylinder and energy is released, causing the heated air and combustion products to expand through a piston/cylinder; another ideally isentropic process.
Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement.
The term Brayton cycle has more recently been given to the gas turbine engine.
This also has three components:  a gas compressor  a burner (or combustion chamber)  an expansion turbine Ideal Brayton cycle:  isentropic process - ambient air is drawn into the compressor, where it is pressurized.
isobaric process - the compressed air then runs through a combustion chamber, where fuel is burned, heating that air—a constant-pressure process, since the chamber is open to flow in and out.
isentropic process - the heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines).
Some of the work extracted by the turbine is used to drive the compressor.
isobaric process - heat rejection (in the atmosphere).
Actual Brayton cycle:  adiabatic process - compression.
isobaric process - heat addition.
adiabatic process - expansion.
isobaric process - heat rejection.
Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the expander represent sources of inescapable working inefficiencies.
In general, increasing the compression ratio is the most direct way to increase the overall power output of a Brayton system.
The efficiency of the ideal Brayton cycle is formula_1, where formula_2 is the heat capacity ratio.
Figure 1 indicates how the cycle efficiency changes with an increase in pressure ratio.
Figure 2 indicates how the specific power output changes with an increase in the gas turbine inlet temperature for two different pressure ratio values.
The highest temperature in the cycle occurs at the end of the combustion process, and it is limited by the maximum temperature that the turbine blades can withstand.
This also limits the pressure ratios that can be used in the cycle.
For a fixed turbine inlet temperature, the net work output per cycle increases with the pressure ratio (thus the thermal efficiency) and the net work output.
With less work output per cycle, a larger mass flow rate (thus a larger system) is needed to maintain the same power output, which may not be economical.
In most common designs, the pressure ratio of a gas turbine ranges from about 11 to 16.