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REDUCED CONDENSING PRESSURE OPERATION ININDUSTRIAL AMMONIA REFRIGERATION SYSTEMSINTRODUCTIONAmmonia refrigeration systems are often operated atcondensing pressures that are higher than necessary.Thisensures proper system performance, particularly duringReducing the condensingperiods of low ambient temperature.pressure will decrease compressor power and energy, increasesystem capacity, and may reduce wear on compressors andcondenser fans. Operating at reduced condensing pressurealso increases condenser fan energy. Problems with liquidcirculation, expansion valve performance, liquid injectioncooling, hot-gas defrost, and glycol freezer floor heatingmay appear at reduced condensing pressure.This study models refrigeration system performance topredict reductions in power and energy for two ammoniarefrigeration systems at reduced condensing pressures.TerminologyThroughout this study, standardindustry terminology is used. Although referring to theCondensing Pressure.same conditions, any of the following expressions may beappropriately used when discussing the condensing pressureof a refrigeration system:Condensing PressureCondensing TemperatureHead PressureCompressor Discharge Pressure

2Fixed and Floating-Head. As refrigeration load orambient temperature increase, the condensing pressure risesCondenser fans may be set toas necessary to reject heat.cycle, maintaining high condensing pressures.If thecondensing pressure is held high enough, the pressure maynot rise to reject heat during periods of high refrigerationload or ambient temperature. Maintaining the condensingpressure at high levels is termed "fixed-head" operation.Setting condenser fans to maintain a lower condensingpressure will operate the fans more, allowing the condensingpressure to rise and fall (or "float") with refrigerationloads or ambient temperatures. This type of operation iscalled "floating-head" operation.Study ProcedureThe study was broken into five sections: monitoring,model development, model validation, predictions, andapplications.Two ammonia refrigeration systems weremonitored during fixed and floating-head operation. TheMonitoring.data was used to:1. Provide short term information concerningsavings due to reduced condensing pressure.2. Identify problems resulting from reducedcondensing pressure.3. Provide field data to aid in the development andvalidation of a refrigeration system model.Model Development.A computer model was created usingcompressor and condenser performance data obtained from

3The model also uses refrigerationseveral manufacturers.loads, weather data, and system setpoints to predict systemperformance and operating cost.Model Validation. The model was used to simulatesystem operation during each of the fixed and floating-headperiods.The model predictions were compared with themonitored data to validate the model.Predictions.The model was used to make annualpredictions of system performance and operating costs atreduced condensing pressure.Although only used to predict systemperformance at reduced condensing pressures, the model mayApplications.be used to predict system operation with differentequipment, refrigeration loads, system setpoints, oroperating strategies.

4BACKGROUNDSingle-Stage Refrigeration SystemAmmonia refrigeration systems contain six maincomponents:ReceiversExpansion ValvesLiquid PumpsCompressorsCondensersEvaporatorsThe component arrangement for a single-stage ammoniarefrigeration system utilizing liquid overfeed evaporatorsFollowing compression, superheatedis shown in Figure 1.refrigerant is condensed in the condenser, releasing heat efrigeration LoadFigure 1: Component Layout of SingleStage Refrigeration Systemthe ambient air (if an evaporative or air-cooled condenseris used).The liquid refrigerant flows from the condenser

5The liquid is expandedto the high pressure receiver (HPR).to low pressure through an expansion valve and stored in thelow pressure receiver (LPR). The liquid is then pumped tothe evaporator (for liquid overfeed systems) where itevaporates while removing heat from the refrigeration load.The compressor receives vapor from the evaporator and thecycle is completed.Two-Stage Refrigeration SystemThe minimum practical evaporating temperature for aAsingle-stage ammonia system is approximately -20 F.second stage of compression can provide evaporatingtemperatures of -65 F12.The components for a two-stage refrigeration system isshown in Figure 2. Two-stage systems are similar singlestage systems, with the exception of compression andexpansion.The liquid may be expanded twice. The first expansionsupplies liquid at an intermediate pressure for use inhigher temperature evaporators, intercoolers, or storage inan intermediate pressure receiver. The intermediatepressure liquid is expanded again, producing the lowpressure liquid required by the system. The boostercompressors receive vapor from the low pressure evaporator,while the high-stage compressors receive vapor from thebooster compressor discharge and the intermediate pressureevaporator.

ExpansionDeviceInter.PressureReceiver0Inter. Press.EvaporatorLiquidPumpEcSide eiverLow Pressure0- EvaporatorLiquidPumpBooster LoadFigure 2: Component Layout of TwoStage Refrigeration SystemCondenser OperationTo condense the high pressure refrigerant vapor, heatis removed by the condenser. A typical evaporativecondenser is shown in Figure 3.The refrigerant vaporpasses through tubes that are sprayed with water,transferring heat from the refrigerant to the water. Air is

7blown over the water to remove the heat through sensible(cooling the water) and latent (evaporating some water) heattransfer. Thus, to condense the refrigerant the refrigeranttemperature must be greater than the ambient temperature.More specifically, (since the process is governed by theevaporation of water) in an evaporative condenser therefrigerant temperature must be greater than the wet-bulbtemperature.Although the wet-bulb temperature may dropdramatically during the cooler portions of the year, somerefrigeration systems maintain artificially high condensingpressures, rather than letting the condensing temperaturevary or "float" with ambient conditions.AIR DISCHARGEELIMINATORS( REFRIG.VAPOR IN(( ( (( AA A n A /\WATER DIST. SYSTEMfCONOEN INGCOILREFRIG.UOUID OUT @ 95FW.B.275 CFM/TONPUMP(APPROX. 1 GPM/TON@ LOW PUMPING HEAD)Figure 3: Typical Evaporative Condenser(Evapco Inc.)Methods of Head Pressure ControlAmmonia refrigeration systems commonly use the fans onevaporative condensers to control head pressure. Decreasingthe airflow through the condenser reduces condensing

8capacity, causing the head pressure to rise. Similarly,increasing airflow increases condensing capacity, causingthe head pressure to drop. In this way, the minimumcondensing pressure can be held within a desired rangeduring cooler weather to ensure proper system operation.There are several methods used to control the capacity ofevaporative condensers:Refrigerant Pressure SwitchesManual Fan CyclingFan Air DampersVariable Speed Fan ControlMicro-Processor ControlsRefrigerant Pressure Switches. The most common methodof condenser capacity control is pressure switches to cyclethe fans.point.Each pressure switch has a cut-in and cut-outWhen the condensing pressure rises to the cut-inpoint, the fan cycles on.When the pressure drops to thecut-out point, the fan cycles off.The fan cyclingmaintains a minimum condensing pressure.A simpleadjustment of the pressure switches can reduce the minimumsystem condensing pressure.Manual Cycling.Some condenser fans are controlledmanually. Refrigeration system operators observe therefrigeration loads, system pressures, and ambientconditions to determine which condenser fans are required.The fan is turned on with a manual switch, and operatesuntil it is no longer required.In this case, leaving thecondenser fans on to reduce condensing pressure is a simplestrategy to implement.Dampers.Dampers are installed on some condensers,allowing more precise adjustment of condenser capacity thanthat supplied by fan cycling only. The dampers are commonly

9controlled by pressure switches which can be easilyadjusted.Variable Speed.Although not yet common, variablespeed condenser fans allow finer adjustment of condensercapacity, with higher efficiencies than fan dampers.Micro-Processor Controls. Computer controls arebecoming more common as a method of refrigeration systemAlgorithms may control the compressors, evaporatorfans, and condenser fans. Although reducing the condenserfan setpoints is relatively easy, a computer may takecontrol.condenser fan operation into consideration to keepSome controlrefrigeration system energy to a minimum.systems may optimize several system parameters, such assuction pressures, compressor sequencing, and evaporator fanDepending on the sophistication of the controlcycling.algorithms, savings may be realized due to reducedcondensing pressure.Advantages of Reduced Head PressureThere are three chief advantages of reducing condensingpressure:Reduced compressor powerIncreased refrigerating capacityReduced equipment wearCompressor Power and Capacity.For most applications,the compressor horsepower increases while the refrigeratingcapacity decreases with increasing condensing pressure.Toillustrate this point, an ideal thermodynamic refrigerationcycle is shown in Figure 4 with a higher condensingpressure, and in Figure 5 with a lower condensing pressure.

10Figure 4 shows the compression of 1 lb/min of vaporThe compressorfrom 33.5 psig (20 F) to 165.9 psig (90 F).requires 82 Btu/min of power to supply 474 Btu/min ofevaporating capacity. The system has a Coefficient ofPerformance (COP) of 5.8 (474/82).IDEAL AMMONIA REFRIGERATON CYCLE165.9 psig Cond and 33.5 psig Evap220200Vapor LineLiquid Line180h 144160h-700Condenser: 556 Btu/min140120100ExpansionCompressor: 82 Btu/minlb/minat8016040h 1 4420h 618Evaporator: 474 Btu/min000.40.2(Thousands)0.60.8Enthalpy (h, in units of Btu/lb)Figure 4: Example of Refrigeration Cycleat 165.9 psig Condensing. PressureIn Figure 5, the condensing pressure has been reducedThe compressor now requires only 57to 114.1 psig (70 F).Btu/min of power to supply an increased 497 Btu/min ofevaporating capacity. The system COP increases to 8.7(497/57).The power reduction results from the lower dischargepressure of the compressor.The system refrigeratingcapacity increase is due to the cooler liquid from thecondenser.A comparison of the two examples is summarizedin Table 1.Compressor power decreased 31%, or 1.55% perdegree reduction in condensing temperature.System

11refrigerating capacity increased 5%, or 0.25% per degreeSystem COP increasedreduction in condensing temperature.50%, or 2.5% per degree reduction in condensing temperature.IDEAL AMMONIA REFRIGERATON CYCLE114.1 psig Cond and 33.5 psig Evap220200Vapor LineLiquid Line180160140120h 12110080h-675Condenser: 554 Btu/minCompressor: 57atExpansionB tu/min1lb/min6040h-618Evaporator: 497 Btu/minh 12120000.40.2(Thousands)0.60.8Enthalpy (h, in units of Btu/lb)Figure 5: Example of Refrigeration Cycleat 114.1 psig Condensing PressureTable 1: Reduced Condensing Pressure forIdeal Ammonia Refrigeration Cycle.Pressure (psig):165.9Temperature ( F)Power (Btu/min):Capacity (Btu/min):Coefficient of Performance90824745.8114.170574978.7Change-20-31% 5% 50%%/ F-1.55% 0.25% 2.50%As discussed further in Appendix A, the compressorpower per unit of refrigerating capacity (BHP/TON) alwaysincreases with increasing condensing pressure.The resultof reduced condensing pressure operation is a decrease in

12compressor power requirements and energy consumption, and anincrease in system refrigerating capacity.Reduced Wear. An additional advantage of reducedcondensing pressure may be decreased equipment wear. Lowerpressure ratios across the compressor and reduced operatingtime can reduce the wear on the compressor.Disadvantages of Reduced Head PressureSix potential problems of reduced condensing pressureare associated with:System Liquid CirculationLiquid Injection CoolingOil SeparationDefrostFreezer Floor HeatingCondenser Fan Energy and LifeLiquid Circulation. Some refrigeration systems use thepressure of the condensed high-pressure liquid to move therefrigerant to the evaporators. This is common in Freonsystems that incorporate thermal expansion valves betweenthe condenser and evaporator. Thermal expansion valveperformance can be limited by the pressure drop across thevalve. Ammonia systems commonly incorporate needle valves,solenoid valves, and float switches to control liquidflowrates and expand the high-pressure liquid refrigerantinto a low-pressure liquid receiver (LPR). The needlevalves are generally less sensitive to fluctuations insystem pressures, and can be adjusted, modified, or replacedif necessary.Liquid Injection Cooling. Some rotary screwcompressors use liquid injection to provide cooling for the

13lubricating oil and desuperheating for the refrigerant.High-pressure liquid refrigerant is injected into thecompressor rotor housing, where it expands and cools therefrigerant and oil mixture. This method of coolingrequires a minimum condensing pressure to ensure adequateliquid refrigerant flow into the compressor.The lubricating oil in a screwOil Separation.compressor must be separated from the discharge refrigerantbefore entering the condenser. In an oil separator, thesuperheated refrigerant and oil mixture passes throughcoalescing filters which trap the oil. As the condensingpressure is reduced, the refrigerant density decreases.This results in higher refrigerant velocities through thefilter elements, and the possibility of inadequateIn addition, at extremely low condensingseparation.pressures, the lubricating oil can "foam" at the compressordischarge as some to the oil vaporizes, making separationmore difficult.This results from the over-compressioninherent in screw compressors at reduced pressure ratios.Hot-gas is commonly used to remove frost fromthe evaporators. High-pressure vapor from the HPR orcompressor discharge is passed through the evaporator,melting the ice. A pressure regulation valve maintains aDefrost.specified pressure in the evaporator during defrost, usuallyThis ensures ammoniatemperatures that are 10 F to 15 F above freezing. Aminimum compressor discharge pressure of 90 to 110 psig may65 psig (44 F) to 70 psig (47 F)1.be necessary to overcome pressure drops in the defrostcontrols, valves, piping, and evaporator coils.Freezer Floor Heat.The superheated refrigerantdischarged from the compressors is commonly passed through aheat-exchanger to heat a glycol water solution.The warmglycol (65 F to 75 F) is passed through piping loops in the

14floor to prevent concrete from heaving or buckling. Theglycol heat-exchanger usually acts as a refrigerantdesuperheater, with little refrigerant condensation takingplace.Systems with high compressor discharge temperatures(i.e. reciprocating at 250 F to 275 F) allow sufficientheating of the glycol under most conditions. Screwcompressors with liquid injection have low dischargetemperatures (110 F to 140 F) with little refrigerantEnsuring proper glycol temperatures may besuperheat.difficult during periods of reduced condensing temperatures.In general, reducing condensingCondenser Fans.pressure will increase condenser fan energy. This will betrue on any system that uses the condenser fans to controlcondensing pressure.Although the condenser fans typicallyonly account for 5 to 10% of the system horsepower, theremay be instances when the increased fan power will outweighthe compressor savings.In systems that employ fan cycling to control headpressure, reducing the condensing pressure will increase theThis will reduce fanoperating time of the condenser fansOn the other hand,motor (also belt and bearing) lifetime.the fans will cycle on and off less, tending to increaseequipment lifetime. It may be necessary to refer toinformation from the motor manufacturers to determinewhether motor lifetime increases or decreases as a result ofreduced head pressure operation.Modifications Allowing Reduced Head PressureThe potential problems of reduced condensing pressuresshould not discourage anyone from trying to reduce

15compressor power and energy consumption.problem has a solution:PROBLEMSystem Liquid CirculationEach potentialSOLUTIONLiquid PumpsValve AdjustmentNew Valves and OrificesInjection Liquid PumpsNew Injection TX ValvesAdditional SeparatorsTime Clocks and SwitchesWater DefrostLiquid Injection CoolingOil SeparationDefrostIncreased SuperheatAlternate Heat SourceMonitoring and ModelingFreezer Floor HeatingCondenser Fan EnergyProblems associated with liquidcirculation and injection cooling can be solved with liquidLiquid Pumps.pumps.A liquid pump can be installed on the high-pressureTheliquid line from the high-pressure receiver (HPR).liquid circulation system will then have sufficient pressureto ensure proper flowrates. The compressors will thenbenefit from the power reductions and capacity increasesassociated with decreased condensing pressures.Valve Adjustment. Needle expansion valves may requireseasonal hand-adjustment to ensure proper operation atreduced condensing pressure.A regular pattern ofadjustment can be developed, based on experimentation andexperience.Valves and Orifices.The design of some valves andorifices may limit reduced condensing pressure operation.Replacing components will allow a wider range of condensingpressures.Possible candidates include hand-adjustedexpansion valves and thermal expansion (TX) valvescontrolling liquid injection cooling on screw compressors.

16Additional Separators. Problems associated with oilseparation can be solved with additional separators.Ratings f

Oregon State University in partial fulfillment of the requirements for the degree of Master of Science . (Masters Candidates in Mechanical Engineering at Oregon State University), for their support . January Weather Data for Portland Oregon 116 78. Febuary Weather Data for Portland Oregon 116 79. March Weather Data for Portland Oregon 116