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Thermodynamics: Turbines & Compressors

A turbine is a device that uses shaft work to drive an electric generator. Turbines can be found in steam, gas, and hydrostatic power plants. The heat transfer in and out of a turbine can typically be considered negligible as well as the potential energy unless otherwise stated. Though there is a change in kinetic energy of the gas going through the turbine, this can normally be removed out of the equation because the energy resulting in the change in enthalpy is much greater than the change in kinetic energy. Finally, it is possible that as superheated steam is pushed through a turbine it could become a saturated gas as it leaves. This will have to be taken in consideration when calculating the exit enthalpy. As a mechanical note, liquid can damage the blades of a turbine, so it is better to keep the fluid in a gas state as much as possible.

To show the basic energy balance of a turbine; refer to equation 1. This equation can be modified to fit other terms as needed.

energy balance of a turbine equation (1)

Turbine Diagram

A compressor, on the other hand, does the opposite of what a turbine does. Instead of using shaft work to create power, a required amount of shaft work is required to run the compressor. However, like a turbine the heat transfer and the potential energy can be considered negligible unless otherwise stated. Also, fluid velocity is negligible for a compressor which means the kinetic energy can also be considered negligible. Taking these into consideration the resulting energy balance equation can be seen in equation 2. This equation can be modified to fit other terms as needed.

energy balance of a compressor equation (2)

Compressor Diagram

Other devices that are similar to compressors are fans and pumps, and equation 5 can be used to describe the energy balance for a fan or pump. There are however some application difference between these devices. For instance, a fan does not raise the pressure of a fluid to a high pressure, instead a fan is mainly used to mobilize gas. Pumps on the other hand, are used only for liquids just like compressors are used only for gases. If a pump tried to pump air, it would most likely damage the pump. The same goes for a compressor if it were to try and compress a liquid it would most likely ruin the compressor.

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