计算综合6.如图所示为换热器采用蒸汽加热工艺介质的控制流程图,要求介质出口温度T达到规定的控制指标。其中介质流量GF比较稳定,但蒸汽压力PV波动较大。控制阀为气开式。要求:1）指出控制系统的类型，并画出它的方块图；（2）确定出主、副被控变量。

<p class="MsoNormal" style="line-height:150%;"><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;"><font face="宋体">（</font></span><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;">1</span><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;"><font face="宋体">）串级控制</font></span><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;"><o:p></o:p></span></p><p class="MsoNormal" style="line-height:150%;"><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;"><font face="宋体">方框图</font></span></p><p class="MsoNormal" style="line-height:150%;"><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;"><font face="宋体"><img src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/examination/imgUplod/14973385038484e6897428e9d47c4a8029b89c09e19a7.png" alt="上传图片"><br></font></span></p><p class="MsoNormal" style="line-height:150%;"><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;">（</span><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;">2</span><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;">）主变量：出口温度；副变量：蒸汽压力</span><span style="mso-spacerun:\'yes\';font-family:宋体;line-height:150%;font-size:12.0000pt;mso-font-kerning:1.0000pt;"><o:p></o:p></span></p>
（1）串级控制方框图（2）主变量：出口温度；副变量：蒸汽压力