Mechanically pumped liquid overfeed system arrangement. . tors.1 Table 1 shows the required recirculating liquid flow rate. expressed in gallons per For industrial refrigeration systems using ammonia as the. refrigerant. of a mechanically-pumped liquid overfed refrigerant loop, the refrigerant in the recirculating vessel. Ammonia Refrigeration System Safety. Ammonia refrigeration, the technology. • How is ammonia Survey (). ** Source: ASHRAE Position Document on ammonia (RA ) .. Gravity flooded recirculation system. Compressor(s) Liquid overfeed system components.

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Thus the volume in the pumping vessel above the maximum liquid elevation must be filled with high-pressure vapor and then refrigertaion with each cycle. After the layout is established, the designer must position the shutoff valves and decide what pressure relief provisions must be made in pipe sections between valves that could be shut off.

Click here to sign up. The package should be equipped with two pumps, each capable of providing the required liquid flow rate by itself. Because a shut-off rrecirculated in this line is necessary for servicing the pump, there is a possibility of accidentally trapping liquid between the shut-off valve and the relief valve.

The designer and plant manager must weigh this additional pumping cost against potential advantages of the gas-pumping system. Lqiuid refrigerating effects would be: Easy operation and lower maintenance are attractive features for even small ammonia systems.

Liquid Overfeed System

Even though there is less flash gas in this suction line Advantage 3designers usually select a larger size than they would for conveying only refrigeratkon vapor of evaporation. The conversion of head to pressure is dependent upon the liquid density as expressed by the equations: Several types of pumps serve liquid recirculation systems, including positive displacement pumps Section 8.

Pumping with gas from the high-pressure receiver. It gefrigeration seem that the precaution against cavitation would be best served by choosing liquid suction lines as large as practical to achieve very low liquid velocities.


The first conclusion is that a pump does not have a unique NPSH, but the value depends where on its operating curve it is performing at the moment.

Liquid Overfeed System | Water Foods Pvt. Ltd.

Liquid can then once again drain from the low- pressure receiver to the pumping vessel. This realization means that the pump manufacturer in specifying the diameter of the maximum-flow orifice should know the characteristics of the wide-open system, and combine that resistance with amonia of the orifice. At this point still another feature will be explained that is a necessary provision for virtually all centrifugal-pump installations.

The balancing valves, which many practitioners call expansion valves, should initially be partially closed in order to be able to syshem more if the liquid supply to the coil is inadequate. In the open-type pump the shaft driving the impeller passes through the casing of the pump.

Because of ideal entering suction gas overrfeed, compressors last longer and there is less maintenance and fewer breakdowns. Usually the amount of time that high-pressure gas is applied to the liquid will be greater in the concept of Figure 8. Geltz6 recommends a higher n when the coil is overfeef for top feed of refrigerant to achieve good wetting of the evaporating surfaces.

The mass of high-pressure vapor used for pumping is: The flow control regulators do not need to be adjusted after the initial setting because overfeed rates are not critical.

In the superheating section of the evaporator the temperature difference between the fluid being cooled and the refrigerant is diminished, and furthermore, the heat-transfer coefficient between the evaporator surface and the superheated refrigerant is lower than the boiling coefficient elsewhere in the evaporator.

In other words, one of these pumps is standby. The key requirement is proper design and application. It does not contribute to increased pressure drops in the evaporators or overfeed lines.


There is, however, another loss associated with gas pumping that must be charged, and this loss is occasioned by the warm vapor heating the cold liquid and the walls of the pumping vessel.

The flow rate of liquid delivered by the pump through the liquid distribution. In the case of ammonia systems, oil which escapes the compressors carries to the low-pressure side of the system where it separates from the ammonia.

Additional first cost due to: The pressure-flow rate curves of Figure 8.

When the transitions between pumping and draining are controlled by a timer, less liquid accumulates in the pumping vessel during draining when the refrigeration capacity and refrigerant flow rate through the evaporators are low. The power required by the centrifugal pump decreases as the flow rate is throttled off, in contrast to the characteristic of the positive-displacement pump.

The circuiting in Fig. Sometimes one or more evaporators in a medium-temperature recirculation system are equipped with back-pressure regulators or evaporator-pressure regulators, as shown in Figure 8. The advantages and disadvantages of gas pumping with respect to mechanical pumping will finally be explored.

The intermediate pressure is likely to hold quite constant. Systm basis of comparison of various values of n should be the combined power of the compressor and liquid pump to provide a given refrigeration duty.

Throughout the entire range of flow of the pump some liquid passes through the orifice, and the flow rate delivered to the system is shown by the dashed line. In some recirculation packages, the orientation shown in Figure 8.