The question of whether gaseous propane is endothermic
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State A (gaseous propane at the top of a propane tank at 1.44 MPa and 35°C) becomes State B (0.08 MPa, 20°C) after being throttled by a control valve. How can this process be explained? Is heat absorbed, and if so, how much?
This post was last edited by ylb913 on 2018-8-20 at 21:06. The temperature reduction wasn’t very significant after a substantial drop in pressure; this is likely due to the low flow rate. The amount of heat absorbed for pressure reduction is small; no additional heater is required, as the pipeline itself can absorb heat from the ambient temperature to maintain a sufficiently high temperature.
As a supplementary note: the outer surface behind the control valve is completely frozen
This post was last edited by ylb913 on 2018-8-20 at 21:09. Frost formation behind the throttle valve is normal; the temperature there gradually rises due to the heating effect of the ambient temperature.
It is an endothermic process, only the amount of heat absorbed is not large
Firstly, from a thermodynamic perspective, the so-called notion of heat absorption is not very accurate； The process described by the original poster is an adiabatic expansion process: the system expands and does work on its surroundings. Due to the adiabatic environment, the thermal energy of the fluid decreases, resulting in a drop in temperature. At an operating pressure of 1.44 MPaG, if it is a pure propane mixture, the corresponding temperature in the gas phase should be around 45°C; there is no value of 35℃ ； Throttle down to 0.08 MPaG, and the temperature drops to 22°C. Freezing occurs because of the large temperature difference between the stream after throttling and the ambient temperature, as well as poor insulation of the pipes; it is not the throttling process that absorbs heat.
It is equivalent to a J-T valve; it involves adiabatic throttling expansion, which is approximately an isenthalpic process. The volume of the fluid increases and its temperature decreases
When there is a difference between the medium temperature and the ambient temperature, frost forms on the surface, but freezing generally does not occur