以電路模擬局部排氣導管中氣流行為

Modeling Local Exhaust Device Piping Airflow by Electrical Circuit

工業通風所使用的局部排氣裝置即一典型的流體力學管流系統。本文以電路學類似Kirchhoff電流與電壓律所描述的守恆觀點探討局部排氣裝置導管中的氣流風量與壓力行為。根據氣流與電流的共通性，氣流風量、壓力與壓力損失可分別以電流、電壓與電位差模擬。而壓力損失係數則以類似電阻的阻尼表示，並仿照電路學中常用的等效電阻方法推導串聯與並聯導管的等效阻尼。等效阻尼可使用於複雜導管系統中風量與壓力的計算。配合ACGIH或ASHRAE所建議的局部排氣設計方法，等效阻尼可更進一步計算推估設計裝置在各種操作條件下的性能。本文並以計算範例說明如何以等效阻尼計算法計算在給定排氣機風量或壓力下的導管風量與壓力變化情形，並更進一步推估動力需求曲線，同時探討部分氣罩關閉時對其他操作中氣罩風量的影響。

The local exhaust device for the purpose of industrial ventilation is a typical pipe-flow system. This article studies the airflow behavior in a local exhaust device piping from the viewpoint of electrical circuit. While the electrical circuit follows the conservation law described by Kirchhoff's current and voltage laws, the airflow in a piping system satisfies the conservation of flow-rate and the pressure balance at the merging point. Due to the similarity between fluid flow and electrical current, the flow-rate, pressure and pressure loss can be modeled by the electrical current, voltage and potential difference. The pipe-flow pressure loss coefficient then can be related to resistance. By using equivalent resistance, which is a common practice in electrical circuit analysis, the flow-rate and pressure in a complex piping system can be calculated systematically. This approach can work along with the design method recommended by ACGIH and ASHRAE to evaluate the performance of a post-designed system. This article provides calculation examples to illustrate how to calculate pipe flow-rate and pressure for given fan flow-rate or pressure, and the power requirement curve of the system. This method can also be used to evaluate the system performance while certain hoods are closed.