18. Operating point of a fan remains unchanged as long as the operating conditions remain unchanged. True False 19. Fan curves and mine curves must be plotted and transposed to obtain an operating point intersection. True False 20. Diffusers on blower fans convert velocity head to static head. True False 21. Mine heads static, velocity, and total) are heads that the fan must develop to deliver a given quantity of airflow to the mine. True False 22. For fans in series, heads are cumulative for a given airflow quantity True False 23. For fans in parallel, quantities are cumulative for a given friction loss. False 24. Natural ventilation head is treated as a fan with a horizontal H-Q characteristic curve. False 25. Airway shape control head loss (H.). False True True True