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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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.

Furthermore, the low-pressure receiver can be smaller and contain less charge. Somewhere in the evaporator, perhaps near the outlet, the rising saturation pressure meets the falling actual pressure and vaporization begins.

The discharge line of each of the pumps should be equipped with a check valve to prevent backflow through the idle pump. Some other recommendations of the circulation ratio are as follows. The pump is small with an impeller diameter of mm 6 inwhich is appealing, but in order to develop the necessary pressure difference it must operate at rpm.

The oil circulation rate to the evaporators is reduced as a result of the low compressor discharge superheat and separation at the surge drum- accumulator low-pressure receiver. Liquid recirculation competes against flooded-coil evaporators and direct expansion. 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.

Skip to main content. What is distinctive about the pump is that there are no moving metal parts, such as the shaft, that must be sealed to prevent refrigerant leaks to the outside.

This liqquid must be periodically drained and with individual feeding of evaporators with high-pressure liquid, oil must be drained periodically from each evaporator. Each type of feed has its inherent advantages. 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.


The mean-temperature difference for the high n is less than for the optimum n, because the heat-transfer coefficient is higher with the high n. Lorentzen, G and O. One value of n is the optimal one that was found by some means, and the other n is higher than optimum.

Liquid Overfeed System

This requirement is to equip the system with continuous bypass of liquid, as Figure 8. The CPR feeds the intermediate-temperature evaporators and the intercooler supplies the low-temperature evaporators. The flooded evaporator, discussed in Chapter 6 and arbitrarily not included in the category of liquid recirculation, also operates in a liquid-overfeed manner, resulting in no superheat at the outlet of ooverfeed evaporator.

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. It may 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 overall heat-transfer coefficient is expressed as a ratio refriteration that experienced with the coil operating under control of an expansion valve that maintains a small amount of superheat.

The balancing valves, which many practitioners call expansion valves, should initially be partially closed in order to be able to open more if the liquid supply to the coil is inadequate. For the pump shown in Figure 8. Click here to sign up. The reason for choosing a lower value of n for halocarbons is also to avoid excessive pumping power. For multiple cold rooms and freezers it is supposed to be the best choice to save electric energy as well getting the highest efficiency of the refrigeration systems.

There is no need of individual accumulator for every ammonia air cooling unit because one large accumulator oveffeed drum of liquid circulation system. The mean-temperature difference between the air and refrigerant is represented by the area between the curves of the air and refrigerant temperatures.

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 recirculation is more advantageous when serving tube-type evapo rators in contrast to evaporators where boiling takes place outside tubes in the shell of the evaporator. The flow rate of liquid delivered by the pump through the liquid distribution.


The next section, Section 8. For halocarbon systems, new CFC replacements are expensive, so the cost of the refrigerant charge becomes a factor. This value of Frliq is systdm on Fig.

A strong preference for angle valves in the discharge line does not prevail because the pressure of the liquid is high enough that vaporization is not a problem. The arrangement in Fig. The mass of vapor used to pump liquid volume VB is: Volume VA should thus be kept to a minimum that still provides a vapor cushion in the pumping vessel.

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. Choosing the pipe diameters and selecting the pump becomes an iterative process, because the arbitrary choice of pipe diameters may result in a system pressure drop so high that a multistage pump is needed, or conversely the pressure drop is low enough that larger than necessary pipe is being used.

The intermediate pressure is likely to hold quite constant.

Liquid Overfeed System | Water Foods Pvt. Ltd.

While circulation ratios from 2 to 4 are standard, the possibility of choosing a circulation ratio much higher between 20 and 40 also has been explored8 on Downloaded from Digital Engineering Library McGraw-Hill www.

The value of n affects the performance of the evaporator in several ways. This information is usually not available to the manufacturer, so the specification of the orifice normally results in a conservative Downloaded from Digital Engineering Library McGraw-Hill www.

The key requirement is proper design and application. To enhance the desuperheating of the discharge gas, the bypass liquid can be atomized with a spray nozzle The basic diagram13 ssystem the installation of the hermetic pump assembly is shown in Figure 8.

Those configurations classified in this chapter as liquid recirculation have a central liquid-vapor separator capable of serving multiple evaporators. Furthermore, achieving more than about 1. In both cases the drain is from a region of stagnant motion where oil, being heavier than liquid ammonia, will separate.