两流体在一套管换热器中换热,热流体温度由<img src="/lqwvjeImg/6a/6a46c763e2a6449057b03993f7bc43bc.png"style="width:43px;height:25px;">降至<img src="/lqwvjeImg/15/1565bc95019159ccb06a512ba63a0c9b.png"style="width:39px;height:24px;">,冷流体温度由<img src="/lqwvjeImg/d5/d5e12c351c331731934a8fc7bd184acc.png"style="width:41px;height:22px;">升至<img src="/lqwvjeImg/00/008d16cf63fb42d7c8deb3764ec97668.png"style="width:44px;height:23px;">。若逆流操作,平均传热温差为( )。<br>·<img src="/lqwvjeImg/77/770bb8f362fdc9e025401411fef8f1b0.png"style="width:44px;height:23px;"><br>·<img src="/lqwvjeImg/92/92162d3630e2e06a88e0b031af2e91bd.png"style="width:46px;height:21px;"><br>·<img src="/lqwvjeImg/30/30e69238cd9bc6018c7f7daddd966685.png"style="width:42px;height:23px;"><br>·<img src="/lqwvjeImg/87/878633dc6a4ce7204749b8bbbede544c.png"style="width:50px;height:23px;">