We employ a facility that recycles fluidization gases to investigate the effects of gas density, scale and operating conditions on circulating fluidized bed risers. By matching five dimensionless parameters, experiments employing plastic and glass powders fluidized with mixtures of sulfur hexafluoride, carbon dioxide, helium and air near ambient temperature and pressure achieve hydrodynamic similarity with generic high-temperature risers of variable scale operating at pressures of 1 and 8 atm. Results in the upper riser are interpreted using steady, fully developed momentum balances for the gas and solid phases. The analysis shows that, for a wide range of experiments, two parameters capture the dependence of the pressure gradients upon the ratio of the mean gas and solid mass flow rates. The first is the ratio of the mean particle slip and superficial gas velocities. The second represents the spatial correlation between the radial profiles of interstitial gas velocity and voidage. Variations of the first with dimensionless parameters indicate that the "atmospheric" and "pressurized" experiments conform to distinct viscous and inertial regimes.