The comparison of mechanical pumping and gas pumping that will be presented in Section 8.16 will contend that greater mechanical energy must be expended in a gas-pumping system than in a system with mechanical pumps. While the overall merits of gas pumping and mechanical pumping are debated endlessly, there seems to be general agreement that gas pumping requires more power than mechanical pumping. A brief analysis follows to demonstrate not only the comparative power, but illustrates several of the factors that influence the power when gas pumping is employed.
The analysis will focus on the delivery of a given quantity of liquid at low pressure to a higher pressure in an arrangement as shown in Figure 8.28. The assignment is to elevate the pressure of a liquid volume VB from that of the low pressure receiver to that needed to deliver the liquid to the evaporators. In Figure 8.28, liquid drains from the low-pressure receiver to the pumping vessel until only a vapor volume of VA remains in the pumping vessel. At this point the three-way valve switches, allowing high-pressure vapor to begin forcing liquid out of the pumping vessel. When the liquid level has been reduced to the lower dashed line, the three-way valve reverses and the vapor at high pressure vents into the upper vessel. Liquid can then once again drain from the low pressure receiver to the pumping vessel. None of the energy of the high-pressure vapor is recovered, and the pumping energy for the cycle is that required to elevate the low-pressure vapor back to that of the high-pressure pumping gas. The analysis that follows will determine the energy required to pump a liquid volume VB.The mass of vapor used to pump liquid volume VB is: