They have put up the price of coak again .
他们把煤的价钱又提高了。
But distilling tower temperature control is a typic big time-lag system , and the control of it is always a difficult problem in coak producing industry .
但由于氨塔温度控制属于典型的大时滞系统，对这种系统的自动控制一直是焦化生产控制中的一个难题。
coak