A constant pressure process is a thermodynamic process where pressure stays the same while volume, temperature, and energy transfer can change. In Thermodynamics II, you see it most clearly in Brayton cycle heat addition and gas turbine modeling.
A constant pressure process is a thermodynamic process in which the pressure stays fixed while other properties, especially temperature, volume, and internal energy, change. In Thermodynamics II, this usually shows up as an idealized step in gas turbine analysis, where you treat a heating or expansion stage as happening at one pressure so the cycle is easier to model.
The simplest way to picture it is a piston with a constant load on top. As heat enters the gas, the gas expands and pushes the piston up, but the pressure stays the same because the load does not change. That means the gas can do boundary work while its temperature rises. For an ideal gas, this gives you the common work relation W = PΔV, which is just pressure times the change in volume.
This process matters because it connects heat transfer, work, and enthalpy. At constant pressure, the heat added to a closed system often lines up with the change in enthalpy, not just the change in internal energy. That is why enthalpy gets used so much in steady-flow devices and in cycle analysis, where pressure changes are easier to track than internal energy alone.
In Brayton cycle problems, the constant pressure process is the heat addition step. Air leaves the compressor at a higher pressure, fuel is added, and the working fluid absorbs heat while the pressure is treated as roughly constant across the combustor. Real combustors are not perfectly constant-pressure devices, but the ideal model gets you close enough to calculate performance and compare cycle behavior.
A common mistake is to assume constant pressure means no work happens. It is the opposite here: the gas can still expand and do work because volume changes. Another mistake is mixing it up with isentropic process steps in the Brayton cycle. Isentropic compression and expansion are the turbine and compressor ideals, while constant pressure is the heating step in between.
This term shows up whenever you analyze the Brayton cycle by process instead of by just looking at the whole engine. If you know which step is constant pressure, you can write the right energy balance, estimate heat input, and connect that heat input to pressure ratio, specific work output, and thermal efficiency.
Thermodynamics II leans hard on process models like this because real gas turbines are complicated. You are not usually solving the exact combustion chemistry and fluid dynamics of a combustor in a first pass problem. Instead, you use the constant pressure idealization to get clean numbers for cycle work and efficiency, then compare that ideal result to real device behavior.
It also gives you a better reading of enthalpy changes. In many cycle problems, the enthalpy rise across the combustor is the quantity you care about, and constant pressure is the reason that setup works so smoothly. If you can identify the constant pressure leg, the rest of the cycle diagram becomes much easier to interpret.
That makes this term useful in problem sets, cycle sketches, and short-answer explanations about gas turbine systems. It is one of the places where the math, the diagram, and the physical machine line up almost perfectly.
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view galleryBrayton cycle
The Brayton cycle is the main place you see a constant pressure process in this course. The heat addition stage is modeled as occurring at constant pressure, so the turbine cycle can be analyzed with simple state changes instead of messy combustor details. If you identify the Brayton cycle steps correctly, the constant pressure leg is usually the one tied to combustion or external heat input.
Heat capacity
Heat capacity helps describe how much heat the working fluid needs to raise its temperature during a constant pressure process. For an ideal gas, constant-pressure heating is linked to the specific heat at constant pressure, often written as c_p. That is why you often see enthalpy changes and c_p show up together in cycle calculations.
isentropic process
Isentropic process is the process model used for the compressor and turbine in the ideal Brayton cycle, while constant pressure is used for the heat addition step. These two ideas are easy to mix up because they both appear in the same cycle diagram. The difference is that isentropic means no entropy change in the ideal model, while constant pressure means pressure stays fixed during the step.
Pressure Ratio
Pressure Ratio is usually defined across the compressor in a Brayton cycle, so it sets the starting and ending pressures around the constant pressure heat addition step. A higher pressure ratio changes the temperatures entering and leaving the constant pressure segment, which shifts the work output and thermal efficiency. In other words, the constant pressure process sits inside the cycle that pressure ratio helps define.
A problem set or quiz will usually ask you to spot the constant pressure segment on a Brayton cycle diagram, then use it to calculate heat added, work, or enthalpy change. You might be given pressure and volume data and asked to use W = PΔV, or given temperatures and asked to connect the step to c_p and Δh. In a longer cycle problem, the usual move is to separate the compressor, combustor, and turbine steps, then label only the heat addition stage as constant pressure. If the question is about real vs ideal behavior, explain that the model assumes pressure is fixed even though an actual combustor has small pressure losses. That distinction is a common place to earn points.
These get mixed up because both are idealized steps in the Brayton cycle, but they describe different things. A constant pressure process keeps pressure fixed, while an isentropic process keeps entropy fixed in the ideal model. In turbine and compressor problems, the process you choose changes the equations you use, so check whether the question is asking about heat addition or adiabatic compression/expansion.
A constant pressure process keeps pressure fixed while volume, temperature, and energy can change.
In Thermodynamics II, you usually meet it in the Brayton cycle during the heat addition step.
For an ideal gas, boundary work in a constant pressure process is often written as W = PΔV.
The enthalpy change across a constant pressure step is a big clue in cycle problems and heat transfer calculations.
Do not confuse constant pressure with no work or with an isentropic process, since those are different ideas.
It is a thermodynamic process where pressure stays the same while the system’s volume and temperature can change. In Thermodynamics II, you usually see it in Brayton cycle heat addition, especially when modeling a gas turbine combustor as an idealized constant-pressure stage.
Yes, isobaric process is the same idea as constant pressure process. Both mean pressure does not change during the process. In your class, the term you see will usually depend on the professor or textbook, but the calculation setup is the same.
For a simple constant pressure boundary process, the work is W = PΔV. That works well when pressure is fixed and the volume changes, like a gas pushing a piston. In cycle problems, you still need to check the sign convention your class uses for work done by the system versus on the system.
Enthalpy is useful because at constant pressure, the heat transfer for many idealized processes tracks the change in enthalpy closely. That is why Brayton cycle heat addition is often analyzed using Δh and c_p instead of trying to work only with internal energy. It makes the energy balance cleaner.