
522 Fuel Cells
relays that perform those operations. Each block is therefore a black box. The
rules require the logical sequence to go from left to right and top to bottom.
For various engineering processes, a process ow diagram (PFD) is used to
indicate the general ow of plant process streams and equipment (Thomas,
2007). The PFD indicates the relationship between major equipment of a plant
facility and does not show details such as minor equipment items, piping
materials of construction, and piping sizes. Another commonly used term
for a PFD is a process ow sheet. Typically, PFDs of a single unit process will
include process piping, uid ...