page_title: Enthalpy
section_title: Compressors
hierarchical_section_title: Enthalpy / Diagrams / Compressors
caption_attribution_description: Schematic diagrams of compressor
context_page_description: Enthalpy is a property of a thermodynamic system, that is a convenient state function preferred in many measurements in chemical, biological, and physical systems at a constant pressure. It is defined as the sum of the system's internal energy and the product of its pressure and volume. The pressure-volume term expresses the work required to establish the system's physical dimensions, i.e. to make room for it by displacing its environment. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, and not on the path taken to achieve it.
  The unit of measurement for enthalpy in the International System of Units is the joule. Other historical conventional units still in use include the British thermal unit and the calorie.
  The total enthalpy of a system cannot be measured directly, because the internal energy contains components that are unknown, not easily accessible, or are not of interest in thermodynamics. In practice, a change in enthalpy is the preferred expression for measurements at constant pressure, because it simplifies the description of energy transfer.
context_section_description: A power P is applied e.g. as electrical power. If the compression is adiabatic, the gas temperature goes up. In the reversible case it would be at constant entropy, which corresponds with a vertical line in the T–s diagram. For example, compressing nitrogen from 1 bar (point a) to 2 bar (point b) would result in a temperature increase from 300 K to 380 K. In order to let the compressed gas exit at ambient temperature Tₐ, heat exchange, e.g. by cooling water, is necessary. In the ideal case the compression is isothermal. The average heat flow to the surroundings is Q̇. Since the system is in the steady state the first law gives
  The minimal power needed for the compression is realized if the compression is reversible. In that case the second law of thermodynamics for open systems gives
  Eliminating Q̇ gives for the minimal power
  For example, compressing 1 kg of nitrogen from 1 bar to 200 bar costs at least (hc − hₐ) − Tₐ(sc − sₐ). With the data, obtained with the T–s diagram, we find a value of (430 − 461) − 300 × (5.16 − 6.85) = 476 kJ/kg.
  The relation for the power can be further simplified by writing it as
  With dh = T ds + v dp, this results in the final relation
caption: Schematic diagram of a compressor in the steady state. Fluid enters the system (dotted rectangle) at point 1 and leaves it at point 2. The mass flow is ṁ. A power P is applied and a heat flow Q̇ is released to the surroundings at ambient temperature Ta.
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