From notes on Rothemuhle and Ljungstrom information of many years ago:
Heating surface : 180-260, with an average of 230 m2/m3
Overall heat transfer coefficient: 14.5-20, average 16.5 kcal/(m2.h.K)
Flue gass mass flow based on total cross section: 1.85-3, average 2.25 kg/(s.m2)
Air mass flow varies from 1.5 to 2.2, average 1.7 kg/(s.m2)
Pressure drop on flue gas and air sides (Ljungstrom): ~20 mm WG, and ~ 40 mm WG, respectively.