What are the structural features of pneumatic conveying devices?

氣力輸送裝置的結(jié)構(gòu)簡單，操作方便，可作水平的、垂直的或傾斜方向的輸送，在輸送過程中還可同時進(jìn)行物料的加熱、冷卻、干燥和氣流分級等物理操作或某些化學(xué)操作。與機(jī)械輸送相比，此法能量消耗較大，顆粒易受破損，設(shè)備也易受磨蝕。含水量多、有粘附性或在高速運(yùn)動時易產(chǎn)生靜電的物料，不宜于進(jìn)力輸送。

The pneumatic conveying device has simple structure and convenient operation. It can be used for horizontal, vertical or inclined conveying. During the conveying process, it can also carry out physical operations such as material heating, cooling, drying and airflow classification or some chemical operations. Compared with mechanical conveying, this method consumes more energy, the particles are easy to be damaged, and the equipment is also easy to be abraded. Materials with high water content, adhesion or easy to generate static electricity during high-speed movement are not suitable for pneumatic conveying.

在水平管道中進(jìn)行稀相輸送時，氣速應(yīng)較高，使顆粒分散懸浮于氣流中。氣速減小到某一臨界值時，顆粒將開始在管壁下部沉積。此臨界氣速稱為沉積速度。這是稀相水平輸送時氣速的下限。操作氣速低于此值時，管內(nèi)出現(xiàn)沉積層，流道截面減少，在沉積層上方氣流仍按沉積速度運(yùn)行。

When dilute phase transportation is carried out in a horizontal pipeline, the gas velocity should be high so that particles can be dispersed and suspended in the gas flow. When the gas velocity decreases to a certain critical value, the particles will begin to deposit in the lower part of the pipe wall. This critical gas velocity is called deposition velocity. This is the lower limit of gas velocity for dilute phase horizontal transportation. When the operating gas velocity is lower than this value, a deposition layer appears in the pipe, the cross-section of the channel decreases, and the gas flow above the deposition layer still operates at the deposition velocity.

在垂直管道中作向上氣力輸送，氣速較高時顆粒分散懸浮于氣流中。在顆粒輸送量恒定時,降低氣速,管道中固體含量隨之增高。當(dāng)氣速降低到某一臨界值時，氣流已不能使密集的顆粒均勻分散,顆粒匯合成柱塞狀,出現(xiàn)騰涌現(xiàn)象(見流態(tài)化)，壓力降急劇升高。此臨界速度稱噎塞速度，這是稀相垂直向上輸送時氣速的下限。對于粒徑均勻的顆粒，沉積速度與噎塞速度大致相等。但對粒徑有一定分布的物料,沉積速度將是噎塞速度的2～6倍。

Upward pneumatic conveying is carried out in a vertical pipe. When the gas velocity is high, the particles are dispersed and suspended in the air flow. When the particle conveying capacity is constant, the solid content in the pipeline increases with the decrease of gas velocity. When the gas velocity decreases to a certain critical value, the gas flow can no longer make the dense particles disperse uniformly, and the particles converge into a plunger shape, resulting in a surge phenomenon (see fluidization) and a sharp rise in pressure drop. This critical velocity is called choking velocity, which is the lower limit of gas velocity when the dilute phase is vertically transported upward. For particles with uniform particle size, the deposition velocity is approximately equal to the choking velocity. However, for materials with certain particle size distribution, the deposition rate will be 2 ~ 6 times of choking rate.

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