The block or rectangles used represent a unit operation. The blocks are connected by straight lines which represent the process flow streams which flow between the units. These process flow streams may be mixtures of liquids* gases and solids flowing in pipes or ducts* or solids being carried on a conveyor belt.
In order to prepare clear* easy to understand and unambiguous block flow diagrams a number of rules should be followed:
  • unit operations such as mixers* separators* reactors* distillation columns and heat exchangers are usually denoted by a simple block or rectangle.
  • groups of unit operations may be noted by a single block or rectangle.
  • process flow streams flowing into and out of the blocks are represented by neatly drawn straight lines. These lines should either be horizontal or vertical.
  • the direction of flow of each of the process flow streams must be clearly indicated by arrows.
  • flow streams should be numbered sequentially in a logical order.

  • unit operations (i.e.* blocks) should be labeled.
  • where possible the diagram should be arranged so that the process material flows from left to right* with upstream units on the left and downstream units on the right.

This figure depict a very small and simplified BFD:
Example - Block Flow Diagram