Condensers

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ObjectivesAfter studying this unit, you should be able to:Explain the purpose of the condenser.Describe differences between the operating characteristics of water-cooled and air-cooled systemsDescribe the basis of the heat exchange in a condenser

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Objectives (contd.)Explain the difference between a tube-within-a-tube coil-type condenser and a tube-within-a-tube serviceable condenserDescribe the difference between a shell-and-coil condenser and a shell-and-tube condenserDescribe a wastewater systemDescribe a recirculated water system

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Objectives (contd.)Describe a cooling towerExplain the relationship between the condensing refrigerant and the condensing medium for cooling tower systemsCompare an air-cooled, high-efficiency condenser with a standard condenserDescribe the operation of head pressure control values

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The CondenserHeat exchange surface that rejects system heatRejects sensible heatDesuperheats vapors from the compressorCondenses vapors to liquidSubcools the liquid refrigerantRejects latent heat during condensing process

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The Condenser (contd.)The greatest amount of heat is transferred during the change of stateCondenser is on the high pressure side of the system

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Water-Cooled CondensersMore efficient than air-cooled condensersWater temperature can be maintainedDirectly affects system pressuresThree types of water-cooled condensersTube within a tube condenser (double tube)Shell and coil condenserShell and tube condenser

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Tube-within-a-Tube CondensersHeat exchange takes place between the fluids in the inner and outer tubesRefrigerant flows in the outer tubeWater flows in the inner tubeRefrigerant and water flow in opposite directions to maximize the transfer rateDepending on the construction, the condenser can be cleaned mechanically or chemically

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Tube-within-a-Tube Condensers (contd.)Figure 22-3 (A) A tube-within-a-tube condenser constructed by sliding one tube through another tube. The tubes are sealed in such a manner that the inside tube is separated from the outside tubeCourtesy Noranda Metal Industries, Inc. Photos by John Tomczyk

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Mineral DepositsHeat from the discharge gas causes minerals to come out of solutionForm scale that adheres to the pipesThe scale acts as an insulator and reduces the rate of heat transferWater is chemically treated to reduce scale formation on the interior pipe surfacesDirty condensers lead to high head pressures

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Mineral Deposits (contd.) Figure 22-3 (D) Scale deposits shown on the inner water tube of a tube-within-a tube condenser Courtesy Ferris State University. Photos by John Tomczyk

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Cleanable Tube-within-a-Tube CondensersTube-within-a-tube condenser has end flanges, which are removed to access the water circuitThe refrigerant circuit remains sealed while the water circuit is openThe mechanically cleanable tube-in-tube condenser is more costly than the chemically cleanable version of the condenser

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Cleanable Tube-within-a-Tube Condensers (contd.)Figure 22-8 (B) Cutaway view of cleaning brush

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Shell-and-Coil CondensersCoil of tubing enclosed in a welded shellWater flows through the coilRefrigerant from the compressor is discharged into the shellThe shell also acts as the receiverWhen refrigerant comes in contact with the cool coil, it condenses & falls to the bottomThis condenser must be cleaned chemically

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Shell-and-Coil Condensers (contd.)Figure 22-9 The shell-and-coil condenser. The hot refrigerant gas is piped into the shell, and the water is contained inside the tubes

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Shell-and-Tube Condensers (contd.)Can be cleaned mechanicallyCompressor discharge gas is piped into the shellWater flows through the tubes in the condenserThe ends of the shell are removed for cleaningThe shell acts as a receiverRefrigerant circuit is not disturbed when the ends of the shell (water boxes) are openedMost expensive type of condenser

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Shell-and-Tube Condensers (contd.)Figure 22-10 (A) Water can be circulated back and forth through the condenser by using the end caps to give the water the proper direction. This would be a 4-pass shell and tube condenser. The number of water passes will vary with the end plate design

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Wastewater SystemsWater used once and then wasted down the drainEconomical if water is free or if the system is smallDrawback: the water temperature can vary greatlyTypical water temperature is about 75FWater typically leaves the condenser at 95F

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Wastewater Systems (contd.)Figure 2211 The wastewater system is used when water is plentiful at a low cost, such as from a well or lake

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Refrigerant-to-Water Temperature Relationship for Wastewater Systems

Water flow is controlled by a water regulating valveTwo pressures control the water regulating valveThe head pressure pushes to open the valveThe spring pressure pushes to close the valve

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Refrigerant-to-Water Temperature Relationship for Wastewater Systems (contd.)

The valve opens when the head pressure risesWater temperature is higher in the warmer monthsWater temperature is lower in the cooler months

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Recirculated Water SystemsThe water flowing through condenser pumped to a remote location, cooled and reusedDesign water temperature is 85FA water flow rate of 3.0 gpm per ton of refrigeration is required to absorb heatThere is a 10 degree split across the water circuit

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Recirculated Water Systems (contd.)Figure 2214 This water-cooled condenser absorbs heat from therefrigerant and pumps the water to a cooling tower at a remotelocation. The condenser is located close to the compressor, and the tower is on the roof outside the structure

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Cooling TowersDevice used to remove heat from the water used in recirculated water systemsTowers can cool the water to a temperature within 7F of the wet bulb temperature of the air surrounding the towerIf the wet bulb temperature is 90 degrees, water can be cooled to a temperature as low as 83FNatural draft, forced draft, or evaporative

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Cooling Towers (contd.)Figure 2216 The relationship of a forced-draft cooling tower to the ambient air. Cooling tower performance depends on the wet-bulb temperature of the air. This relates to the humidity and the ability of the air to absorb moisture

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Natural-Draft Cooling TowersRedwood, fiberglass, or galvanized sheet metalThere are no blowers to move air throughNatural breezes move air through the towerWater enters the tower from the top and is cooled as the water falls to the bottomSome water evaporates in the process, helping to cool the remaining water in the towerAdditional water is added through a float valve

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Forced- or Induced-Draft TowersUse a fan or blower to move air throughAs water falls through the tower, air is moved across it to aid in cooling Can be located almost anywhereThe fan is cycled on and off to maintain the desired water temperatureForced draft: air is pushed through the towerInduced draft: air is pulled through the tower

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Evaporative CondensersDesigned to operate full of liquidA latent heat transfer takes place throughout the coilCoil efficiency is maximizedOther devices must be used to prevent liquid from entering the compressorNormally use a float-type metering device to keep the liquid level in the coil high

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Air-Cooled CondensersUses air to absorb heat rejected by the system in locations where water is difficult to useHorizontal, vertical, or side intake and top dischargeHot gas enters the condenser from the topFor standard efficiency systems, the refrigerant will condense at a temperature about 30F higher than the outside ambient temperature

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Air-Cooled Condensers (contd.)Figure 22-26 Fins designed to give the coil more surface area

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High-Efficiency CondensersHave larger surface areas than standard condensersAllow systems to operate at lower pressuresAllow systems to operate more efficientlyCan operate with head pressures as low as 10F higher than the outside ambient temperature

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The Condenser and Low-Ambient ControlsCondensing temperatures drop when the outside ambient temperature dropsThe condensing pressure must be at least 75 psig higher than the evaporator pressure for the metering device to operate properlyLow ambient controlsDesigned to maintain the desired head pressureNeeded on systems that operate year-round

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Head Pressure ControlMethod to maintain correct head pressureFan Cycling ControlsVariable Frequency Drive (VFD) Condenser Fan Motor ControlsAir Shutters or DampersCondenser FloodingCondenser Splitting

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Head Pressure Control (contd.)Fan cycling controlsUsed on air-cooled condensersAs the head pressure drops, fan cycles offAs the head pressure rises, fan cycles onSome condensers have more than one fanSome fans remain on all the timeOthers cycle on/off to maintain proper pressureCan be controlled by pressure or temperature

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Head Pressure Control (contd.)Variable frequency drive (VFD) condenser fan motorsMotor speed changes to maintain head pressureAs head pressure drops, fan slows downAs head pressure rises, fan speeds upMaintains a more constant head pressureCan be controlled by pressure or temperature

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Head Pressure Control (contd.)Air shutters or dampersLocated at the inlet or outletOperates by pressure-controlled pistonControls airflow through the condenser coilAs ambient temperature drops, the dampers close to reduce the amount of airflowAs ambient temperature rises, the dampers open to increase the amount of airflow

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Head Pressure Control (contd.) Condenser floodingValve installed in parallel with the condenserValve closed when ambient temperature is highValve opens as ambient temperature dropsAs the valve opens, refrigerant backs up in the condenser, reducing the heat transfer surfaceDuring very cold weather, the condenser will be almost completely filled with liquid refrigerantSystems must have an oversized receiver

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Head Pressure Control (contd.) Condenser splittingReduces extra refrigerant charge needed for condenser floodingSplits the condenser into two separate and identical condenser circuitsSummer/Winter condenser; Summer condenserSolenoid valve control energized/de-energized by controller sensing outside ambient

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Using the Condenser SuperheatAir-cooled condensers have high discharge temperaturesCan be captured and redistributed as heat for the structure in winter or to heat water

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Heat ReclaimFigure 2252 A system that can supply heat to the store

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Floating Head PressuresTerm used for attaining the lowest possible condensing temperatureAllows the head pressure to follow the ambient temperature without using head pressure controlsNewer expansion devices can operate properly with pressure differences as low as 30 psigSystems become more efficient since they operate at lower pressures

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SummaryThe condenser is the system component responsible for rejecting system heatCondensers reject both latent and sensible heatWater-cooled condensers are more efficient than air-cooled condensers

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Summary (contd.)Three types of water-cooled condensers are the tube within a tube, shell and coil, and the shell and tubeMineral deposits in the water circuit reduce the heat transfer rate between the water and the refrigerant

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Summary (contd.)Some condensers can be mechanically cleaned while others must be cleaned chemicallyWastewater systems use water once and then waste it down the drain

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Summary (contd.)Wastewater systems typically supply 75-degree water to the condenser and require 1.5 gpm/tonRecirculating water systems typically supply 85-degree water and require 3.0 gpm/ton

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Summary (contd.)Wastewater systems utilize a water-regulating valve while recirculated water systems do notEvaporative condensers use a combination of water and air to achieve the condensing process

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Summary (contd.)High efficiency condensers operate with lower head pressures than standard efficiency condensersLow ambient controls allow systems to operate properly when the ambient temperature is low

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