Candle type filter element is the filter element in the oil filter of BOLL host, which can effectively control the pollution level of the system. The working principle is that the unfiltered liquid enters the filter through the inlet of the filter, intercepts impurities through the filter element, and the filtered liquid flows out from the outlet into the clear liquid storage tank. As the filtration progresses, the cake layer thickens and the filtration resistance increases. When the set pressure difference is reached, the filtering element needs to be backwashed. First, close the inlet and outlet ports, introduce compressed gas into the tank, and filter out the residual liquid in the filter tank. When backwashing the filter column, compressed gas is introduced into the liquid outlet of the filter to blow and expand the filter cloth, causing impurities in the intercepted filter cake and filter cloth gaps to fall off and be discharged from the lower discharge port.