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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But the penalites of increased n are twofold: 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.

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. To make the seal more effective, some pumps are equipped with a double seal, and the cavity between the two seals is filled with oil under pressure, as illustrated in Fig. Thus, the mass of refrigerant venting to the low-pressure receiver and compressed back to high-pressure is reduced.

Log In Sign Up. This leads to the practice of normally choosing an operating speed of rpm, which necessitates a pump of larger impeller diameter. The same advantage applies to the suction line from the evaporator also refer to Disadvantage la.

In a two-stage system, the low-temperature evaporators might be served by a liquid recirculation system, and the intemediate-temperature evaporators may operate in direct expansion. Compare the energy requirements when pumping by the following methods: The temperature of the liquid in the lower section of VB was essentially unaffected. The next section, Section 8. Since oil return provisions must be made for each coil in systems with flooded evaporators, when the number of coils exceeds three to five, it is usually more effective to choose liquid recirculation which has only one point of oil removal—the low- pressure receiver.

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. This is another reason for top-feed coils to be more adaptable to defrosting from external heat. The balance points or operating points are the intersections of the piping curve with the pump curve and are represented by points A and B.


The heat transfer capability of the liquid recirculation system is often compared to the direct-expansion evaporator with liquid feed controlled by a superheat-control valve which is also called a thermostatic expansion valve and is described in more detail in Chapter 11 on refrigerant control valves.

Valves controlling the liquid feed to the evaporator receive liquid at a uniform pressure, regardless of the condensing pressure. In the controlled-pressure receiver systems of Figures 8.

Each type of feed has its inherent advantages. Systems with low inventories of ammonia are attractive for safety reasons.

Indeed, the two-stage system of Fig. The CPR recirculation system shown in Figure 8. If the liquid supply lines were restricted, the positive-displacement pump reacts by building recirrculated more pressure, so a pressure relief valve opens when the pressure rises above the pressure setting.

None of the energy of the high-pressure vapor is recovered, and the pumping energy refdigeration the cycle is that required to elevate the low-pressure vapor back to that of the high-pressure pumping gas.

Some other recommendations of the circulation ratio are as follows. In the worst case the pump can become vapor bound and not pump any liquid. Lorentzen, G and O.

A pressure difference of kPa 29 psi was specified for the mechanical pump, so if intermediate-pressure vapor is available and its pressure is high enough, the efficiency of the pumping process could be essentially the same as mechanical pumping, except for the burden of VA.

Computing the system pressure drop. The density of vapor is less than liquid, so the mass will be less. 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. Unless the pump has been chosen to provide this excess pressure, the evaporator with the pressure regulator will not be supplied with refrigerant.

Improved heat-transfer coefficients in the evaporator and the positive feeding of multiple evaporators are advantages of liquid recirculation in these cases too.

One manufacturer of coils5 recommends: During the draining process the three-way valve connects the pumping vessel to the low-pressure receiver, allowing gas to vent from the pumping vessel. The temperature of refrigerant thus drops in the direction of flow as the pressure drops. Wile1 measured the influence of the circulation ratio in laboratory tests of a steel, finned tube, ammonia evaporator cooling air, and the results are shown in Fig. The reason for choosing a lower value of n for halocarbons is also to avoid excessive pumping power.


It would appear, however, that the pump is responsible for the refrigerant from the liquid level in the low-pressure receiver and back to that point.

Liquid Overfeed System | Water Foods Pvt. Ltd.

Both of these influences are embodied in the pump performance curves. The mean-temperature difference between the air and refrigerant is represented by the area between the curves of the air and refrigerant temperatures.

The purpose of the bypass flow is to avoid trapping liquid in the pump which would be churned by the impeller and vaporized from the heat of friction.

Several types of pumps serve liquid recirculation systems, including positive displacement pumps Section 8. During this portion of the cycle, liquid from Vessel B is forced to the evaporators, and liquid drains xystem the low-pressure receiver to Vessel A.

Liquid Overfeed System

The positive-displacement pump can easily develop the high differential pressures that might be needed for this assignment, and normally this application does not require throttling to low flow rates which could overload the pump motor. Cavitation is the conversion of liquid into bubbles of vapor because of an abrupt drop in pressure as the liquid flows into the pump housing or impeller.

Experienced design engineers can usually choose an appropriate size, perform a pressure-drop calculation, and if the pressure drop is unreasonable, can adjust the size. If the pressure falls rapidly, bubbles of vapor form almost immediately to absorb heat, which brings the liquid temperature back in conformity to the saturation temperature.

Liquid Recirculation Systems are most useful for multiple number of cooling units as well as having multi temperature cold rooms or freezers. The basic diagram13 of the installation of the hermetic pump assembly is shown in Figure 8.

This would normally be the case, except in instances where the pressure in the low-pressure receiver fluctuates. The magnitude of this loss is difficult to estimate, but some experimental results15 suggest an order of magnitude.