A special situation is encountered frequently in the return line from a liquid overfeed evaporator, as shown in Figs. 9.12a and 9.12b. An example of the situation is a spiral freezer chamber where the liquid and vapor leaving the evaporators must be elevated to the liquid/vapor return line. Typically several evaporators are stacked vertically such that their combined height might be 3 m (10 ft) and then the riser extends another 3 m (10 ft) or more to reach the return line. Liquid accumulates in the riser and this column of liquid exerts a static head on the evaporators, raising the pressure and thus the evaporating temperature. Field measurements indicate that evaporators operating in the -30 to -40°C (-22 to -40° F) temperature may lose 15 to 20% of their capacity because of the column of liquid.
It is appropriate at this point to indicate that there is an effective means of avoiding the static head problem by the installation shown in Fig. 9.13. The liquid flowing out of the evaporators separates from the vapor in a vessel. When the liquid level rises to the setpoint in the vessel, the pump turns on and delivers the liquid into the liquid/vapor return line. Now the relatively dry vapor flows up the riser without the impediment of the accumulated liquid. The process of the separate transfer of liquid is not without its own problems. Some plant managers prefer not to have ammonia vessels in their processing area for safety reasons. Also, as was stressed in the discussion of pumps in liquid recirculation systems, Chapter 8, some suction head is required to avoid pump cavitation. The evaporators may be close to floor level, so the vessel and pump must be placed in a pit. Servicing a pump in a pit is not a welcome task for a mechanic. There is an alternative, however, and this is to use gas pumping and thus avoid the, net-positive-suction-head requirement. This choice is often made, but that arrangement has the disadvantage of blocking the flow from the evaporator during the pumping phase, which causes some system transients. In summary, if the physical situation of the plant permits and the plant manager approves, the liquid head problem can be addressed by the process of liquid removal and separate pumping.
In many situations it becomes necessary for the vapor to carry the liquid to a higher elevation in the riser, and the selection of the size of the riser significantly influences the pressure drop. A point to stress is that lowering the velocity does not necessarily lower the pressure drop. As Fig. 9.12a illustrates, a low velocity may permit liquid to accumulate in the riser and in the extreme case the bubbles of vapor rise due to their own bouyancy. With a riser of length h m (ft), 80% of which is occupied by liquid, for example, the pressure drop would be
Thus, a riser that is 6 m (20 ft) tall could contribute a pressure drop 32 kPa (4.6 psi).
On the other end of the scale, as Fig. 9.14 shows, with a very small riser the pressure drop due to friction is high. Richards13 translated experimental results into recommendations for riser sizes that will provide the minimum pressure drop. Table 9.6 shows these recommendations for various circulation ratios with ammonia evaporating at -40°C (-40°F). If the size of the riser is selected for full or design flow, then at part load with lower velocities, the vapor will fail to adequately blow out liquid from the riser. For this reason the assembly recommended for conveying oil shown in Fig. 9.10 is often used.
The next assignment after the pipe size has been determined for the design refrigeration capacity is to compute the pressure drop in the riser. For the case of -40°C (-40°F) ammonia when lifting liquid in a 3-m (10-ft) riser with a recirculation ratio of 5, Richards estimates the drop in saturation temperature to be of the order of 1.6°C (2.9°F) when operating at the vapor velocity for minimum pressure drop.