Brayton Cycle

The Brayton cycle is one of the many thermodynamic cycles used to generate cooling at cryogenic temperatures. Strictly speaking, when referring to cooling we should call this the reverse Brayton cycle as the original Brayton cycle describes the process of power generation or propulsion via a gas turbine. In many publications in cryogenics, however, the terms “reverse Brayton cycle” and “Brayton cycle” are used interchangeably to describe the use of the cycle to provide cooling. For convenience, I’ll use the term “Brayton cycle” to refer to the refrigeration application as well.

In a classic Brayton cycle, the working gas is compressed to high pressure and then the heat of compression is removed at room temperature. The 300K highpressure gas is then passed through a heat exchanger where it is cooled by a colder, lower pressure flow. The colder high-pressure gas is expanded in a turbo expander (see “Defining Cryogenics” in Cold Facts Vol. 31 No. 4 – and is thus cooled further via an approximation of isentropic expansion. The cold fluid then provides refrigeration and is returned to the inlet of the room temperature compressor via the heat exchanger, precooling the incoming highpressure gas. A Brayton cycle, then, is one in which the cooling is produced by isentropic expansion.

The room temperature compressor in many Brayton cycle machines is an axial compressor that is on the same drive shaft as the turboexpander. The work done by the fluid expanding against the turbo expander is thus recovered to partially drive the compressor.

Brayton cycle machines can be built to operate at many cryogenic temperature ranges and can range in size from tens of kilowatts in cooling power to small cryocoolers producing just a few watts of cooling. A wide variety of working fluids (e.g. helium, nitrogen, neon) can be used in this cycle. The use of mixed refrigerants (see “Defining Cryogenics” in Cold Facts Vol. 32 No. 1– is also possible. There are many modified Brayton cycles that contain variations such as multiple expansion turbines, parallel expansion turbines that expand to different pressures and expansion turbines that are situated after the system to be cooled. Figure 1 shows the conceptual design of a modified Brayton cycle machine proposed for the ESS Target Moderator Cryoplant. Such a plant would produce roughly 30 kW of cooling nominally at 15K.

Most large cryogenic refrigeration plants combine aspects of a variety of thermodynamic cycles (Brayton, Claude, Linde-Hampson) to provide optimum performance for a given set of requirements.

“Cryogenic Systems” by R. Barron and “Cryogenic Engineering” by T.M. Flynn both provide good surveys of thermodynamic cycles used in cryogenics.

Examples of Brayton cycle cryocoolers can be found in “High-capacity Turbo-Brayton Cryocoolers for Space Applications,” by M.V. Zagarola and John A. McCormick, Cryogenics 46 (2006) and “The Design and Fabrication of a Reverse Brayton Cycle Cryocooler System for the High Temperature Superconductivity Cable Cooling,” by J.H. Park, Y.H. Kwon and Y.S. Kim, Cryogenics 45 (2005).

An example of a proposed mixed refrigerant Brayton cycle machine is presented in “Nelium, a Refrigerant with High Potential for the Temperature Range between 27 and 70K,” by H. Quack, C. Haberstroh, I. Seemann and M.Klaus, Physics Procedia 67 (2015).

The usefulness of Brayton Cycle machines in natural gas liquefaction is discussed in “Thermodynamic Cycle Selection in Distributed Natural Gas Liquefaction,” by M.A. Barclay, D.F. Gongaware, K. Dalton and M.P. Skrzypkowski, Adv. Cryo. Engr. Vol. 49 (2004).

A description of the ESS Target Moderator cryoplant can be found in “Spallation Target Cryogenic Cooling Design Challenges at the European Spallation Source”, by J. Jurns et al. Adv. Cryo. Engr. Vol. 61 (2016).