This vessel is one that performs both the role of liquid/vapor separation and liquid storage. The dashed lines in Fig. 10.13 show five distinct liquid levels of interest. The level controller, whether it is a capacitance level sensor as shown in Fig. 10.13, or a float switch, regulates the solenoid valve in the liquid line. If the level in the vessel drops below the control point, the solenoid valve in the liquid supply line opens. When the level in the vessel reaches the control point, the solenoid valve closes. Volumes above and below this controlled level accommodate surge volume and provide ballast volume. The surge volume serves the purpose of accommodating liquid that might be forced out of evaporators during defrost. Another source of liquid surge in some plants is from the liquid/vapor line when pitched downward to the low-pressure receiver. Should the electric power in the plant be interrupted, the liquid continues to drain from the liquid/vapor line.
The controlled level is not the lowest operating level, because some liquid supply should be available between the controlled level and level that actuates the low-level alarm. The reason for needing this ballast volume is that during startup or resumption of operation of one or more evaporators, the pump may withdraw refrigerant from the vessel at a greater rate than is supplied at that moment by the combination of the controlled liquid supply and from the return from the liquid/vapor line. If the vessel empties to the low-level alarm status, operation continues but an operator or the security system is informed of the fact. On a further drop in level the low-level cutout is reached, which is usually set to stop pump operation.
There are also two designated control levels above the controlled level: the high-level alarm and the high-level cutout. The alarm simply notifies the operator or the security system to summon an on-site investigation. Should the liquid level reach the point of the high-level cutout, the compressors are shut down for their protection. Figure 10.13 shows for controllers a popular combination of the capacitance level controller and float switches. Several electrical current values can be picked off the 4–20 mA output of the capacitance level sensor that correspond to the controlled level, the high-level alarm, and the low-level alarm. To provide an independent backup, the ultimate safety provisions of the high-and low-level cutouts are actuated by separate float switches.
Computing the ballast volume. The ballast volume is provided to permit the pumps to draw liquid from the low-pressure receiver for a short interval to bring the liquid content of evaporators up to the steady-state amount following a shutdown. Typically, a five-minute time period is assumed adequate for this purpose, so the ballast volume is the design pump flow rate in volume flow per minute multiplied by 5.
Computing the surge volume. Two major contributors to momentary excess flow into the low-pressure receiver are flooding of abnormal rates of liquid out of the evaporator due to defrost or to sudden increases of refrigeration load, and the liquid in the liquid/vapor return line that drains back to the low-pressure receiver in the event of pump or power failures. During a defrost it is assumed that the entering defrost gas pushes all liquid in the coil out to the return line. The fraction of liquid in the coil during operation depends on whether the coil is top or bottom fed. For a top-fed coil the percentage of liquid is often assumed to be about 30%. For bottom-fed coils the percentage is sometimes chosen as high as 80%. Another approach to the estimate that incorporates the circulation ratio n but does not distinguish between top and bottom feed is
Equation 10.11 gives with a circulation ratio of 3, for example, the percentage of volume occupied by liquid to be 33%.
The premise that the liquid in a coil is pushed out during a hot-gas defrost and sent to the low-pressure receiver is now being questioned by some engineers. In times past, it was certainly true that the defrost gas was sent to the coil immediately after interrupting the supply of liquid to the coil, and the liquid would flow rapidly through the pressure-regulating valve or through a float drainer. As was emphasized in the treatment of hot-gas defrost in Chapter 6, the coil should first be emptied of liquid by closing the liquid supply solenoid but continuing refrigeration until all or most of the liquid in the coil has been drawn off by the compressor. The liquid that was contained in the coil thus never leaves as liquid. Only experience will determine whether the modern recommended defrost sequence will permit reduction in the required surge volume.
To estimate the fraction of liquid FL in the liquid/vapor return line, Sec. 9.12 presented relations that should bracket the correct value. If the liquid is assumed to move at the same velocity as the vapor, thus with no slip,
On the other hand, the other extreme described in Sec. 9.12 is if the liquid flows along the bottom of a horizontal pipe, dragged by the faster-moving vapor. The expression is:
Equation 10.12 underestimates the fraction of liquid occupying the pipe, because slip will always occur. Equation 10.13 overestimates the value, if the vapor velocity is high enough to generate mist flow rather than stratified flow, and also if the pipe is sloped in the direction of flow.