1、4.1CHAPTER 4LIQUID OVERFEED SYSTEMSOverfeed System Operation . 4.1Refrigerant Distribution. 4.2Oil in System 4.3Circulating Rate. 4.3Pump Selection and Installation . 4.4Controls 4.5Evaporator Design. 4.5Refrigerant Charge. 4.6Start-Up and Operation . 4.6Line Sizing 4.7Low-Pressure Receiver Sizing 4
2、.7VERFEED systems force excess liquid, either mechanically orOby gas pressure, through organized-flow evaporators, separateit from the vapor, and return it to the evaporators.TerminologyLow-pressure receiver. Sometimes referred to as an accumula-tor, this vessel acts as the separator for the mixture
3、 of vapor and liq-uid returning from the evaporators. A constant refrigerant level isusually maintained by conventional control devices.Pumping unit. One or more mechanical pumps or gas-operatedliquid circulators are arranged to pump overfeed liquid to the evap-orators. The pumping unit is located b
4、elow the low-pressure re-ceiver.Wet returns. These are connections between the evaporator out-lets and low-pressure receiver through which the mixture of vaporand overfeed liquid is drawn.Liquid feeds. These are connections between the pumping unitoutlet and evaporator inlets.Flow control regulators
5、. These devices regulate overfeed flowinto the evaporators. They may be needle valves, fixed orifices, cal-ibrated manual regulating valves, or automatic valves designed toprovide a fixed liquid rate.Advantages and DisadvantagesThe main advantages of liquid overfeed systems are high systemefficiency
6、 and reduced operating expenses. These systems havelower energy cost and fewer operating hours because The evaporator surface is used efficiently through good refriger-ant distribution and completely wetted internal tube surfaces.The compressors are protected. Liquid slugs resulting from fluc-tuatin
7、g loads or malfunctioning controls are separated from suc-tion gas in the low-pressure receiver.Low-suction superheats are achieved where suction lines betweenthe low-pressure receiver and the compressors are short. Thisminimizes discharge temperature, preventing lubrication break-down and minimizin
8、g condenser fouling.With simple controls, evaporators can be hot-gas defrosted withlittle disturbance to the system.Refrigerant feed to evaporators is unaffected by fluctuating ambi-ent and condensing conditions. Flow control regulators do notneed to be adjusted after initial setting because overfee
9、d rates arenot generally critical.Flash gas resulting from refrigerant throttling losses is removed atthe low-pressure receiver before entering the evaporators. Thisgas is drawn directly to the compressors and eliminated as a factorin system low-side design. It does not contribute to increasedpressu
10、re drops in the evaporators or overfeed lines.Refrigerant level controls, level indicators, refrigerant pumps, andoil drains are generally located in equipment rooms, which areunder operator surveillance or computer monitoring.Because of ideal entering suction gas conditions, compressors lastlonger.
11、 There is less maintenance and fewer breakdowns. The oilcirculation rate to the evaporators is reduced because of thelow compressor discharge superheat and separation at the low-pressure receiver (Scotland 1963).Automatic operation is convenient.The following are possible disadvantages:In some cases
12、, refrigerant charges are greater than those used inother systems.Higher refrigerant flow rates to and from evaporators cause liquidfeed and wet return lines to be larger in diameter than high-pressure liquid and suction lines for other systems.Piping insulation, which is costly, is generally requir
13、ed on all feedand return lines to prevent condensation, frosting, or heat gain.Installed cost may be greater, particularly for small systems orthose with fewer than three evaporators.Operation of the pumping unit requires added expenses that areoffset by the increased efficiency of the overall syste
14、m.Pumping units may require maintenance.Pumps sometimes have cavitation problems caused by low avail-able net positive suction head.Generally, the more evaporators used, the more favorable the ini-tial costs for liquid overfeed compared to a gravity recirculated orflooded system (Scotland 1970). Liq
15、uid overfeed systems comparefavorably with thermostatic valve feed systems for the same reason.For small systems, the initial cost for liquid overfeed may be higherthan for direct expansion.Ammonia Systems. Easy operation and lower maintenance areattractive features for even small ammonia systems. H
16、owever, forammonia systems operating below 0F evaporating temperature,some manufacturers do not supply direct-expansion evaporators be-cause of unsatisfactory refrigerant distribution and control problems.OVERFEED SYSTEM OPERATIONMechanical PumpFigure 1 shows a simplified pumped overfeed system in w
17、hich aconstant liquid level is maintained in a low-pressure receiver. Amechanical pump circulates liquid through the evaporator(s). Thetwo-phase return mixture is separated in the low-pressure receiver.Vapor is directed to the compressor(s). Makeup refrigerant entersthe low-pressure receiver by mean
18、s of a refrigerant metering device.Figure 2 shows a horizontal low-pressure receiver with a mini-mum pump pressure, two service valves in place, and a strainer onthe suction side of the pump. Valves from the low-pressure receiverto the pump should be selected for minimal pressure drop. Thestrainer p
19、rotects hermetic pumps when oil is miscible with theThe preparation of this chapter is assigned to TC 10.1, Custom EngineeredRefrigeration Systems.4.2 2010 ASHRAE HandbookRefrigerationrefrigerant. It should have a free area twice the transverse cross-sectional area of the line in which it is install
20、ed. With ammonia, con-sider using a suction strainer. Open-drive pumps do not requirestrainers. If no strainer is used, a dirt leg should be used to reduce therisk of solids getting into the pump.Generally, minimum pump pressure should be at least double thenet positive suction pressure to avoid cav
21、itation. Liquid velocity tothe pump should not exceed 3 fps. Net positive suction head andflow requirements vary with pump type and design; consult thepump manufacturer for specific requirements. The pump should beevaluated over the full range of operation at low and high flow. Cen-trifugal pumps ha
22、ve a flat curve and have difficulty with systems inwhich discharge pressure fluctuates.Gas PumpFigure 3 shows a basic gas-pumped liquid overfeed system, withpumping power supplied by gas at condenser pressure. In this sys-tem, a level control maintains the liquid level in the low-pressurereceiver. T
23、here are two pumper drums; one is filled by the low-pres-sure receiver, and the other is drained as hot gas pushes liquid fromthe pumper drum to the evaporator. Pumper drum B drains when hotgas enters the drum through valve B. To function properly, thepumper drums must be correctly vented so they ca
24、n fill during thefill cycle.Another common arrangement is shown in Figure 4. In thissystem, high-pressure liquid is flashed into a controlled-pressurereceiver that maintains constant liquid pressure at the evaporatorinlets, resulting in continuous liquid feed at constant pressure. Flashgas is drawn
25、into the low-pressure receiver through a receiver pres-sure regulator. Excess liquid drains into a liquid dump trap fromthe low-pressure receiver. Check valves and a three-way equalizingvalve transfer liquid into the controlled-pressure receiver during thedump cycle. Refined versions of this arrange
26、ment are used for mul-tistage systems.REFRIGERANT DISTRIBUTIONTo prevent underfeeding and excessive overfeeding of refriger-ants, metering devices regulate the liquid feed to each evaporatorand/or evaporator circuit. An automatic regulating device continu-ously controls refrigerant feed to the desig
27、n value. Other commondevices are hand expansion valves, calibrated regulating valves, ori-fices, and distributors.It is time-consuming to adjust hand expansion valves to achieveideal flow conditions. However, they have been used with some suc-cess in many installations before more sophisticated cont
28、rols wereFig. 1 Liquid Overfeed with Mechanical PumpFig. 1 Liquid Overfeed with Mechanical PumpFig. 2 Pump Circulation, Horizontal SeparatorFig. 2 Pump Circulation, Horizontal SeparatorFig. 3 Double Pumper Drum SystemFig. 3 Double-Pumper-Drum SystemFig. 4 Constant-Pressure Liquid Overfeed SystemFig.
29、 4 Constant-Pressure Liquid Overfeed SystemLiquid Overfeed Systems 4.3available. One factor to consider is that standard hand expansionvalves are designed to regulate flows caused by the relatively highpressure differences between condensing and evaporating pressure.In overfeed systems, large differ
30、ences do not exist, so valves withlarger orifices may be needed to cope with the combination ofincreased refrigerant quantity and relatively small pressure differ-ences. Caution is necessary when using larger orifices because con-trollability decreases as orifice size increases.Calibrated, manually
31、operated regulating valves reduce some ofthe uncertainties involved in using conventional hand expansionvalves. To be effective, the valves should be adjusted to the manu-facturers recommendations. Because refrigerant in the liquid feedlines is above saturation pressure, the lines should not contain
32、 flashgas. However, liquid flashing can occur if excessive heat gains bythe refrigerant and/or high pressure drops build up in feed lines.Orifices should be carefully designed and selected; once in-stalled, they cannot be adjusted. They are generally used only fortop- and horizontal-feed multicircui
33、t evaporators. Foreign matterand congealed oil globules can restrict flow; a minimum orifice of0.1 in. is recommended. With ammonia, the circulation rate mayhave to be increased beyond that needed for the minimum orificesize because of the small liquid volume normally circulated. Pumpsand feed and r
34、eturn lines larger than minimum may be needed. Thisdoes not apply to halocarbons because of the greater liquid volumecirculated as a result of fluid characteristics.Conventional multiple-outlet distributors with capillary tubes ofthe type usually paired with thermostatic expansion valves havebeen us
35、ed successfully in liquid overfeed systems. Capillary tubesmay be installed downstream of a distributor with oversized orificesto achieve the required pressure reduction and efficient distribution.Existing gravity-flooded evaporators with accumulators can beconnected to liquid overfeed systems. Chan
36、ges may be needed onlyfor the feed to the accumulator, with suction lines from the accumu-lator connected to the system wet return lines. An acceptable arrange-ment is shown in Figure 5. Generally, gravity-flooded evaporatorshave different circuiting arrangements from overfeed evaporators. Inmany ca
37、ses, the circulating rates developed by thermosiphon actionare greater than those used in conventional overfeed systems.Example 1. Find the orifice diameter of an ammonia overfeed system witha refrigeration load per circuit of 1.27 tons and a circulating rate of 7.Evaporating temperature is 30F, pre
38、ssure drop across the orifice is8 psi, and the coefficient of discharge for the orifice is 0.61. The circu-lation per circuit is 0.528 gpm.Solution: Orifice diameter may be calculated as follows:d = (1)whered = orifice diameter, in.a = units conversion, 29.81Q = discharge through orifice, gpmp = pre
39、ssure drop through orifice, psiS = specific gravity of fluid relative to water at 30F= 5.701/8.336 = 0.6839Cd= coefficient of discharge for orificed = = 0.092 in.Note: As noted in the text, use a 0.1 in. diameter orifice to avoidclogging.OIL IN SYSTEMDespite reasonably efficient compressor discharge
40、 oil separators,oil finds its way into the system low-pressure sides. In ammoniaoverfeed systems, most of this oil can be drained from low-pressurereceivers with suitable oil drainage facilities. In low-temperaturesystems, a separate valved and pressure-protected, noninsulated oildrain pot can be pl
41、aced in a warm space at the accumulator (Figure6). The oil/ammonia mixture flows into the pot, and the refrigerantevaporates. At subatmospheric pressures, high-pressure vapor mustbe piped into the oil pot to force oil out. Because of oils low solu-bility in liquid ammonia, thick oil globules circula
42、te with the liquidand can restrict flow through strainers, orifices, and regulators. Tomaintain high efficiency, oil should be removed from the system byregular draining.Except at low temperatures, halocarbons are miscible with oil.Therefore, positive oil return to the compressor must be ensured.The
43、re are many methods, including oil stills using both electric heatand heat exchange from high-pressure liquid or vapor. Somearrangements are discussed in Chapter 1. At low temperatures, oilskimmers must be used because oil migrates to the top of the low-pressure receiver.Build-up of excessive oil in
44、 evaporators must not be allowedbecause it rapidly decreases efficiency. This is particularly critical inevaporators with high heat transfer rates associated with low vol-umes, such as flake ice makers, ice cream freezers, and scraped-surface heat exchangers. Because refrigerant flow rate is high,ex
45、cessive oil can accumulate and rapidly reduce efficiency.CIRCULATING RATEIn a liquid overfeed system, the circulating number or rate isthe mass ratio of liquid pumped to amount of vaporized liquid. Theamount of liquid vaporized is based on the latent heat for the refrig-erant at the evaporator tempe
46、rature. The overfeed rate is the ratio ofliquid to vapor returning to the low-pressure receiver. When vaporleaves an evaporator at saturated vapor conditions with no excessFig. 5 Liquid Overfeed System Connected on Common Sys-tem with Gravity-Flooded EvaporatorsFig. 5 Liquid Overfeed System Connecte
47、d on Common System with Gravity-Flooded EvaporatorsQaCdpS-0.50.52829.81 0.61 8/0.6839-0.5Fig. 6 Oil Drain Pot Connected to Low-Pressure ReceiverFig. 6 Oil Drain Pot Connected to Low-Pressure Receiver4.4 2010 ASHRAE HandbookRefrigerationliquid, the circulating rate is 1 and the overfeed rate is 0. Wi
48、th acirculating rate of 4, the overfeed rate at full load is 3; at no load, itis 4. Most systems are designed for steady flow. With few excep-tions, load conditions may vary, causing fluctuating temperaturesoutside and within the evaporator. Evaporator capacities vary con-siderably; with constant re
49、frigerant flow to the evaporator, the over-feed rate fluctuates.For each evaporator, there is an ideal circulating rate for everyloading condition that gives the minimum temperature differenceand best evaporator efficiency (Lorentzen 1968; Lorentzen andGronnerud 1967). With few exceptions, it is impossible to predictideal circulating rates or to design a plant for automatic adjustmentof the rates to suit fluctuating loads. The optimum rate can vary withheat load, pipe diameter, circuit length, and num
