Analysis of Irreversible Thermodynamic Processes from Control Valves

The thermodynamic analysis of irreversible processes has an important influence in energy efficiency growing of any thermodynamic processes or systems. All of thermodynamic processes are entropy generators but there are processes with such a high level of irreversibility so that couldn’t be ignored. That is why is so important to provide a feel for what causes entropy generation and to identify those system components that contribute the most to the overall irreversibility of the thermodynamic system. One of the processes with the highest level of entropy generation is flow with friction in various ducts and flow networks such as control valves. In order to see the direct connection between frictional pressure drop and thermodynamic irreversibility the paper will firstly analyze the steady and adiabatic flow of pure substance through a short segment of pipe with variable section (control valve). Because the entropy generation value during the throttling process is proportional to control valve pressure drop, the paper will do a flow thermodynamic analysis inside the control valve and the conclusions about drop pressure in different working conditions will be taken. Pressure drops along a valve are not constant, but rather vary in relation to the port left open by the plug. They normally increase as the valve narrowest section is reduced, although the upstream drop does increase at a slower rate than the downstream one. Actual increases and decreases in pressure drops and their effects are related to valve type and flow direction. It can be deduced that, for all types, pressure drops increase at flow tending to close, mainly as a result of the increased drop generated downstream. Also the paper will take into account the drop pressure variation during the substance flow and will be analyzed its influence on the process irreversibility. In the same time, the paper will analyze the change of state influence during the liquid throttling process on the entropy generation, in situations of low titer bipolar phase fluid or high titer bi-phase fluid and will identify this phenomenon effects and remedies. Key-Words: control valves, throttling, flow, irreversible process, pressure drop