Gas Pressure-Drop Experiment

Abstract

Most chemical engineering undergraduate laboratories have fluid mechanics experiments in which pressure drops through pipes are measured over a range of Reynolds numbers. The standard fluid is liquid water, which is essentially incompressible. Since density is constant, pressure drop does not depend on the pressure in the pipe. In addition, flow measurements using rotameters are independent of the pressure in the liquid system. However, if the fluid is gas, pressure drops and flow measurements do depend on the density of the gas, which is a function of pressure, molecular weight, and temperature. For the same mass flowrate, pressure drop increases as density decreases. Lower gas density results if pressure or molecular weight is lower or if temperature is higher. The importance of this effect is vital to the education of chemical engineers since we deal extensively with gas streams flowing through pipes. Another important concept is adjusting readings of flow meters in gas service for conditions (densities) that are different than those used to calibrate the flow meter. Typically flow meters are calibrated at standard conditions (temperature and pressure) with a standard gas (given molecular weight). The meter constants must be adjusted for the actual density of the gas at the conditions in the flow measurement device, and this correction factor is not just a simple ratio of densities. A gas pressure-drop experiment has been operated in the Lehigh Chemical Processing Laboratory for several years that demonstrates these important concepts associated with gas flow and effectively illustrates the important differences between liquid and gas systems. The equipment is simple, safe, and inexpensive.