真空泵是(shi)從負壓(ya)下抽氣(qi)、一般(ban)在大氣(qi)壓(ya)下排氣(qi)的輸送機械,其目的是(shi)造成(cheng)并(bing)維持工藝系統所要求的真空(kong)度(du)。煉油和石油化工生(sheng)產中常用的真空(kong)泵(beng)有往復(fu)真空(kong)泵(beng)、水環真空(kong)泵(beng)和噴射真空(kong)泵(beng)。
(一)往復真空泵
往復(fu)真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)的(de)構造和(he)作(zuo)用(yong)原理與(yu)往復(fu)壓(ya)縮(suo)機基本相同,氣(qi)(qi)(qi)缸內外壓(ya)差很小(xiao),但其壓(ya)縮(suo)比很大(例(li)如(ru),對于95%的(de)真(zhen)(zhen)(zhen)空(kong)(kong)度,壓(ya)縮(suo)比約(yue)為20左右)。因此,往復(fu)真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)的(de)吸(xi)入(ru)和(he)排(pai)出活門必(bi)(bi)須(xu)輕巧靈活、啟閉方便;其余隙(xi)(xi)(xi)容(rong)積必(bi)(bi)須(xu)很小(xiao),同時為了(le)減小(xiao)余隙(xi)(xi)(xi)中(zhong)殘余氣(qi)(qi)(qi)體(ti)對真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)抽(chou)氣(qi)(qi)(qi)速率的(de)不良(liang)影響,常在(zai)真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)氣(qi)(qi)(qi)缸兩頭的(de)內壁上設(she)有(you)連通活塞兩側的(de)平衡(heng)氣(qi)(qi)(qi)道,如(ru)圖2-58中(zhong)1所示。在(zai)排(pai)氣(qi)(qi)(qi)終了(le)時,平衡(heng)氣(qi)(qi)(qi)道能使(shi)余隙(xi)(xi)(xi)與(yu)活塞的(de)另(ling)(ling)一側短暫(zan)時間相通,以便使(shi)余隙(xi)(xi)(xi)中(zhong)的(de)殘存(cun)氣(qi)(qi)(qi)體(ti)通過(guo)平衡(heng)氣(qi)(qi)(qi)道流到(dao)活塞的(de)另(ling)(ling)一用(yong)側.以降低余隙(xi)(xi)(xi)中(zhong)氣(qi)(qi)(qi)體(ti)的(de)壓(ya)力,從而提高了(le)吸(xi)氣(qi)(qi)(qi)效(xiao)率,即提高了(le)容(rong)積系數。往復(fu)真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)所產(chan)生的(de)極限真(zhen)(zhen)(zhen)空(kong)(kong)可達(da)2kPa左右(絕壓(ya))。往復(fu)真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)所排(pai)送(song)的(de)氣(qi)(qi)(qi)體(ti)不應含有(you)液體(ti),如(ru)氣(qi)(qi)(qi)體(ti)中(zhong)含有(you)大量蒸氣(qi)(qi)(qi),必(bi)(bi)須(xu)把可凝性(xing)氣(qi)(qi)(qi)體(ti)設(she)法除(chu)去(qu)(一般采用(yong)冷凝)之后(hou)再進入(ru)真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)內,即往復(fu)真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)屬于干(gan)式(shi)真(zhen)(zhen)(zhen)空(kong)(kong)泵(beng)。
(二)水環式真空泵
水(shui)環真(zhen)空(kong)(kong)(kong)泵(beng)在吸氣中(zhong)可允許夾帶少量水(shui),屬于濕式真(zhen)空(kong)(kong)(kong)泵(beng)。水(shui)環真(zhen)空(kong)(kong)(kong)泵(beng)的結構簡單緊(jin)湊,沒有(you)活(huo)門,工(gong)作安全可靠。但它(ta)所能產(chan)生的真(zhen)空(kong)(kong)(kong)度不(bu)高,最(zui)高可達85%的真(zhen)空(kong)(kong)(kong)度,且因轉(zhuan)子在水(shui)中(zhong)旋轉(zhuan),將引起額外的摩擦阻力損失,因而效(xiao)率(lv)較低,約為30%~50%。水(shui)環真(zhen)空(kong)(kong)(kong)泵(beng)運(yun)轉(zhuan)時(shi),要(yao)不(bu)斷(duan)地補充清水(shui),以維持泵(beng)內液封,同時(shi)也起冷卻泵(beng)體的作用(yong)。
如將水環(huan)真空泵的(de)進口接大氣,出口接設備時,可作(zuo)鼓風機(ji)用,所產生的(de)風壓(ya)(ya)不超過(guo)98kPa(表(biao)壓(ya)(ya))。
(三)旋片真空泵
如果要獲(huo)得較高真(zhen)空度(du)時,宜用(yong)油封(feng)式旋片真(zhen)空泵,排氣(qi)(qi)(qi)閥(fa)浸(jin)在油箱中(zhong),可以防止(zhi)氣(qi)(qi)(qi)體回流到(dao)氣(qi)(qi)(qi)缸(gang)(gang)內。氣(qi)(qi)(qi)缸(gang)(gang)外(wai)面設有水套,對氣(qi)(qi)(qi)缸(gang)(gang)壁進行(xing)冷卻。
旋片真空泵可達較高的真空度(極限壓(ya)力可達0.06Pa),但抽氣(qi)(qi)速率比較小,適用(yong)于(yu)抽除干燥或含(han)(han)有少量可凝(ning)性蒸氣(qi)(qi)的氣(qi)(qi)體。不適用(yong)于(yu)抽除含(han)(han)塵和對潤滑油起(qi)化學作用(yong)的氣(qi)(qi)體。
(四)噴射真空泵
噴(pen)射真空泵屬于流體(ti)作(zuo)用(yong)(yong)式機械,它(ta)是利用(yong)(yong)高(gao)速流體(ti)射流時壓力(li)能(neng)向(xiang)動(dong)能(neng)轉換所造(zao)成的真空來吸送流體(ti)的,既可(ke)用(yong)(yong)來吸送氣體(ti),也可(ke)用(yong)(yong)來吸送液(ye)體(ti)。噴(pen)射泵的工(gong)作(zuo)流體(ti)如(ru)果(guo)是蒸汽(qi),則為蒸汽(qi)噴(pen)射泵;工(gong)作(zuo)流體(ti)如(ru)果(guo)是水或(huo)其他液(ye)體(ti),則稱為水或(huo)液(ye)體(ti)噴(pen)射泵。 |