Throttling devices are very common in industry and are used to control and reduce pressure mainly through valves. Since the throttling process is isenthalpic ( where the enthalpy up and down flows are equal) no energy is lost and according to the first law of thermodynamics, its efficiency is optimal. However, this has an inherent typical mechanical irreversibility which reduces pressure and increases the entropy of the fluid without giving any additional benefit. Consequently, exergy is lost and the fluid (after the pressure drop) is less capable of producing energy, e.g. in a subsequent turbine expansion process.
Therefore, if the aim is to reduce the pressure of a fluid, it is desirable to use isentropic expansions and provide useful work in addition through turbines. If this is not possible, the working pressure should always be as low as possible, to a void large pressure changes, with associated exergy losses through valves, measuring devices (see Section 2.10.4) or by using compressors or pumps to input additional energy.
A regular practice in industrial installations is to keep the pressure at the inlet of a turbine at the design conditions. This usually implies the use (and abuse) of inlet valves to control the turbine.
According to the second law of thermodynamics, it is better to have variation of the pressure specifications (sliding pressure) and to keep the admission valves completely open.
As a general recommendation, valves should be sized as large as possible. A satisfactory throttling process can be achieved with a pressure drop of 5 -10 % at maximum flow, instead of 25 – 50 % as has been past practice with valves of too small a size. The pump driving the fluid must be also sized to take account of the variable conditions.
However, a better alternative is to use a backpressure turbine, which almost retains the isentropic conditions and is completely reversible ( in thermodynamic terms). The turbine is used to generate electricity.
Increases fuel consumption.
Applicable in new or significantly refurbished systems, according to t he economics and the following factors:
●the turbine is used to generate electricity or to provide mechanical power to a motor; compressor or fan. Whereas backpressure turbines are the most attractive from a point of view of energy efficiency, the quantity of steam passing through the backpressure turbines should fit with the overall steam balance of the whole site. Use of excessive numbers of backpressure turbines will result in more steam being generated at low pressure levels than can be consumed by the plant/site. This excess steam would then have to be vented, which is not energy efficient. The steam flow from the backpressure turbine also needs to be available for a large percentage of the time, and in a predicable way. A n unpredictable o r discontinuous source cannot be used reliably ( unless, r arely, peaks in supply and demand can be matched)
●backpressure turbines are not useful when the two pressure levels are close together, as the turbines need a high flow and pressure differential. In the steel industry in the blast furnace process, pressure drop turbines are used because of the huge number of gases which flow through the blast furnace.
Turbines are several orders of magnitude more expensive than control valves. The minimum size to be effective and to be considered before substituting therefore has to be considered with the steam balance. In the case of low mass flows, turbines are not reasonable from an economic point of view. To be economic, the recovered energy should be sufficiently reliable, available for a large percentage of production time and match demand.
Energy Efficiency (2009) 3.2.3