As an alternative to mechanical pumps, liquid refrigerant may be circulated by means of gas pressure. There are several different concepts and methods of control in industrial use, but a common element is a pumping vessel held at low pressure during the portion of the cycle in which liquid drains from the low pressure receiver, and is then subjected to high-pressure vapor to deliver the liquid. Both the J.E.Watkins and H.A.Phillips organizations were influential in developing gas pumping systems in the United States. Two gas-pumping concepts used in industrial practice will first be explained—the two-pumper drum system and the controlled-pressure receiver system. The controlled pressure receiver is the most popular although the two-drum concept is also widely used. After the principles of these two systems are explained, an analysis of the pumping energy of gas pumping and mechanical pumping will be conducted. The advantages and disadvantages of gas pumping with respect to mechanical pumping will finally be explored.
In the two-pumper system of Figure 8.25, high-pressure vapor is imposed on one vessel while the other is filling. The functions of the two vessels then reverse. The switching operation is achieved through the use of two 3-way valves, Valve A and Valve B. The common ports of the valves are connected to their respective pumping vessel. While Valve A is positioned to connect the vent to the common port, Valve B allows high-pressure vapor to its pumping vessel. During this portion of the cycle, liquid from Vessel B is forced to the evaporators, and liquid drains from the low-pressure receiver to Vessel A.
To control the switching operation, it would be possible to sense the liquid levels in the pumping vessels, switching the positions of Valves A and B when the level drops to a low point in the vessel being emptied and/or rises to a high level in the vessel being filled. Instead, in gas pumping facilities such as this, a timer usually controls the switching operation. Time durations of two minutes for each of the modes of operation are typical. After deciding upon the pump/drain time, the pumping vessels can be sized to comfortably contain the maximum volume of liquid pumped from one vessel and drained into the other during the time increment. The maximum flow rate will generally occur when the evaporators are demanding the highest rate of refrigerant flow. The pressure of the pumping gas significantly affects the rate of liquid delivery, so for low temperature evaporators in a two-stage system, vapor at the intermediate pressure should be chosen for the pumping gas. The intermediate pressure is likely to hold quite constant. The pumping gas for a recirculation system operating at the intermediate pressure uses gas at the condensing pressure, and since this pressure may vary widely, one approach is to maintain a constant pressure using a downstream pressure regulator in the high-pressure vapor line.