Bilateral symmetry The arrangement of parts in fan organism in such a way that the structure can only be divided into similar halves (mirror images) along one plane.
左右对称：通过中央轴，只有一个对称面将生物体分成左右相等的两部分（镜像）。
2
Analyses the lowest rating hailer efficiency, the area of heat - supply service and the arrangement of forced draft fan and induced draft fan. The suggestions are advanced.