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 individual factors12 that would be included in a calculation are the following: This value of Frliq is noted on Fig.
High friction losses in these pipes only detract from the NPSH that has been provided. The objective becomes one, then, of passing the liquid out of the suction pipe in a reasonable amount of time. In nonliquid recirculation systems, some of the surface is used to superheat the leaving refrigerant with resultant low heat-transfer coefficients and reduced temperature differences between the refrigerant and the fluid being cooled.
Not shown in Fig. Geltz6 recommends a higher n when the coil is circuited for top feed of refrigerant to achieve good wetting of the evaporating surfaces. The evaporator and suction line contain a greater fraction of liquid than is the case with direct expansion. The magnitude of this loss is difficult to estimate, but some experimental results15 suggest an order of magnitude. The key requirement is proper design and application. This vent may be opened when the pump starts to clear the pump of vapor and thus permit initiation of liquid pumping.
Systej heat-transfer coefficients in the evaporator and the positive feeding of multiple evaporators are advantages of liquid recirculation in these cases too. 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.
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. In the past few years, however, attention has been directed toward low-charge systems.
Properly applied flooded evaporator coils operate with the same effectiveness of coils served by liquid recirculation. For the pump shown in Figure 8. The configuration in Figure 8.
Liquid Overfeed System | Water Foods Pvt. Ltd.
This is not to suggest that the positive-displacement pump has no role in industrial refrigeration systems. 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. Check valve Ckv A prevents higher-pressure gas from the CPR from flowing into the pumping vessel while draining of liquid is taking place. A favorable characteristic of the centrifugal pump is the non-overloading characteristic.
As valves in the system progressively open, the flow rate increases and eventually the pressure differential that the pump can develop drops. 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 discharge syztem of each of the pumps should be equipped with a check valve to prevent backflow through the idle pump. Thus the mass of high-pressure vapor required is: 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. Unless the pump has been chosen to provide this excess pressure, the evaporator with the pressure regulator will not be supplied with refrigerant.
The CPR recirculation system shown in Figure 8. Additional first cost due to: In systrm to ammonia, the latent heat of the halocarbon refrigerants is lower, but the liquid density is higher. The liquid line to the evaporators conveys the liquid destined for recirculation as well as ammoniaa for evaporation. 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.
Liquid Overfeed System
The basic diagram13 of the installation of the hermetic pump assembly is shown in Figure 8. Flash gas overceed removed in the machine room rather than having it appear after the expansion valve at each evaporator. In some recirculation packages, the orientation shown in Figure 8.
At this point the three-way valve switches, allowing high-pressure vapor to begin forcing liquid out of the pumping vessel. Flooded evaporators are supplied with high-pressure liquid like DX coils. The value of n affects the performance of the evaporator in several ways. The CPR concept is adaptable to two-stage systems as well, and a typical flow diagram is shown in Figure 8. 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.
The pump is small with an impeller diameter of mm 6 inwhich ovegfeed appealing, but in order to develop the necessary pressure difference it must operate at rpm. If there is desire to reirculated between liquid overfeed and liquid recirculation, it might be that liquid overfeed applies to the coil, while a liquid recirculation system incorporates the additional equipment to accommodate the overfeeding of the coil.
The intermediate pressure is likely to hold quite constant. When drawing from the liquid leg as in Figures 8. The early liquid recirculation systems used positive-displacement pumps, but in recent years the centrifugal type has almost universally replaced them. When these vapor bubbles move into a region of high pressure they collapse violently. Failure of this liquid flow could result in overheating of the motor and shutdown of the pump.
The temperature of refrigerant thus drops in the direction of flow as the pressure drops. It may seem that the precaution against cavitation would be recirdulated served by choosing liquid suction lines as large as practical to achieve very low liquid velocities.
The increased refrigerant pressure drop results in a higher evaporating temperature for a given pressure leaving the coil.