What flow should a hospital suction outlet provide?

The 40 L/min figure is the flow each terminal unit should be capable of passing, not the plant capacity: the plant is sized from the diversified flow of the areas it serves (see how a vacuum plant is sized). Vacuum pump capacity is stated as free air aspirated in L/min (HTM 02-01, 10.27).

Design flows by area (HTM 02-01, Table 10)

Here n is the number of beds, treatment spaces or single rooms where the procedure is performed, not the number of terminal units. No extra flow is added for the anaesthetic room, because needing to clear an airway there while a patient in the operating room is still anaesthetised is considered extremely unlikely (4.118).

Dental suction is a different system

Dental high volume suction is based on 300 L/min at each chair cannula connector, against a typical 40 L/min for medical vacuum, and HTM 02-01 asks for a dedicated dental vacuum system for extracting bodily fluids during dental procedures (11.9, 11.39 and 10.14).

The outlet itself

Terminal units are covered by ISO 9170-1:2017, which deals with their gas-specific assembly, flow, leakage and pressure drop, among other properties. HTM 02-01 refers to BS EN ISO 9170-1 for the outlet vacuum requirement. On the supply side, ULTRAVAC plant capacity is given per pump, from 3 to 1,600 m³/h, in the ULTRAVAC datasheet.