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.

節流裝置是用閥門來控制流體與降低壓力，在工業界是普遍使用的裝置。由於節流過程是一種等焓過程(熱焓的上下流量是一樣的)，沒有能量損失。根據熱力學第一定律其效率是最佳的。可是，其具備一典型機械上不可逆的特點，使得流體壓力下降及熵無法用來做功，結果損失有用的能量，且壓降後的流體不太能再度做功以產生能量，如串連在後的渦輪膨脹製程。
所以，針對降低流體壓力目的而言，最好是用等熵膨脹藉由渦輪機提供有用功，如果不可行，工作壓力則應該維持低壓，避免閥件、量測設備等過大的壓力變動造成有用的能量損失，或必須使用壓縮機、泵浦提供額外能量。
工業界常用的做法是設計時在汽渦輪機的入口維持壓力，此說明利用進氣口閥門控制汽渦輪機。
根據熱力學第二定律，最好訂有一壓力變動範圍的規範，且可將控制閥門全開。
一般認為節流閥越大越好。在最大流率下，節流程序可使壓降5%~10%，可達到令人滿意的效果。依照過去的實務經驗，節流閥若太小，壓降達25%~50%。
所使用泵浦的規格也必須匹配，而且要考慮變速裝置。
有一較好的替代技術是採用背壓渦輪機，它可以維持等焓條件，且是完全可逆(熱力學名詞)的。背壓渦輪機轉動可用來發電。

Increases fuel consumption.

增加燃料耗用。

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.

根據其經濟性及下列各因素應用在新設或重新整修的工廠：
●渦輪機被用來發電或提供機械動力給馬達、壓縮機或風車。從能源效率的角度來看，背壓渦輪機頗具吸引力，通過背壓渦輪機的蒸汽量必須能達到整個工廠的蒸汽質能平衡。使用過多的背壓渦輪機往往會產出太多低壓蒸汽，而使工廠或生產單元無法去化，這些多出的蒸汽只好放空，是不節能的做法。
●流過背壓渦輪機的蒸汽必須預估妥當，且大部分時間必須可以使用。未預估妥當或斷續供應的情形是無法可靠使用。(除非是為了尖峰供應或使用量不一致的情況下)。當兩個蒸汽系統的壓力水準接近時，背壓渦輪機不能使用，因為渦輪機需要有較大的流量和壓差才有效果。在鋼鐵工廠的高爐製程使用壓降渦輪機，因為操作高爐時有大量的製程氣通過高爐。

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.

在成本上裝設渦輪機比控制閥貴數倍，在取代控制閥前可以考慮用最小規格，同時必須先考慮整廠蒸汽質能平衡。在經濟觀點上，小質能流量的使用渦輪機並不算合理，未能達到經濟性。能源回收必須充分可靠，生產期間必須能依需求充分供應。

Energy Efficiency (2009) 3.2.3