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trgtrc010_book.book Page ii Wednesday, September 1, 1999 10:39 AM Preface Centrifugal Water Chillers A Trane Air Conditioning Clinic Figure 1 The Trane Company believes that it is incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs, and field experience. The Trane Air Conditioning Clinic series is one means of knowledge sharing.
trgtrc010_book.book Page iii Wednesday, September 1, 1999 10:39 AM Contents Introduction ........................................................... 1 period one Components .......................................................... 3 Compressor ............................................................ 4 Condenser .............................................................. 7 Expansion Device .................................................... 8 Economizer ............................................
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trgtrc010_book.book Page 1 Wednesday, September 1, 1999 10:39 AM Introduction notes Chilled Water System Figure 2 Water chillers are used in a variety of air conditioning and process cooling applications. They are used to make cold water that can be transported throughout a facility using pumps and pipes. This cold water can be passed through the tubes of coils to cool the air in an air conditioning application, or it can provide cooling for a manufacturing or industrial process.
trgtrc010_book.book Page 2 Wednesday, September 1, 1999 10:39 AM Introduction notes Water chillers using the vapor-compression refrigeration cycle vary by the type of compressor used. Reciprocating and scroll compressors are typically used in small chillers. Helical-rotary (or screw) compressors are typically used in medium-sized chillers. Centrifugal compressors are typically used in large chillers. As mentioned earlier, this particular clinic discusses centrifugal water chillers.
trgtrc010_book.book Page 3 Wednesday, September 1, 1999 10:39 AM period one Components notes Centrifugal Water Chillers period one Figure 5 Many of the components of the centrifugal water chiller are similar to those of other chiller types.
trgtrc010_book.book Page 4 Wednesday, September 1, 1999 10:39 AM period one Components notes Compressor Figure 7 Compressor The centrifugal compressor uses the principle of dynamic compression, which involves converting energy from one form to another, to increase the pressure and temperature of the refrigerant. It converts kinetic energy to static energy. Impeller blades Figure 8 The core component of a centrifugal compressor is the rotating impeller.
trgtrc010_book.book Page 5 Wednesday, September 1, 1999 10:39 AM period one Components notes Centrifugal Compressor volute diffuser passage impeller passages Figure 9 The rotation of the impeller causes the refrigerant vapor to accelerate within the impeller passages, increasing its velocity and kinetic energy. The accelerated refrigerant vapor leaves the impeller and enters the diffuser passages. These passages start out small and become larger as the refrigerant travels through them.
trgtrc010_book.book Page 6 Wednesday, September 1, 1999 10:39 AM period one Components notes centrifugal compressor Energy Conversion static pressure velocity refrigerant enters diffuser refrigerant enters impeller path through compressor refrigerant enters volute Figure 10 Again, in the passages of the rotating impeller, the refrigerant vapor accelerates, increasing its velocity and kinetic energy.
trgtrc010_book.book Page 7 Wednesday, September 1, 1999 10:39 AM period one Components notes the accelerated refrigerant vapor leaves the last impeller, it collects in the compressor volute and travels on to the condenser. Condenser refrigerant vapor baffle cooling tower water tube bundle liquid refrigerant Figure 12 Condenser The high-pressure refrigerant vapor is discharged from the compressor into a heat exchanger that acts as a condenser.
trgtrc010_book.book Page 8 Wednesday, September 1, 1999 10:39 AM period one Components notes expansion device Orifice Plates orifice plates to evaporator H1 Figure 13 Expansion Device An expansion device is used to maintain the pressure difference between the high-pressure (condenser) and low-pressure (evaporator) sides of the refrigeration system, as established by the compressor.
trgtrc010_book.book Page 9 Wednesday, September 1, 1999 10:39 AM period one Components notes expansion device Orifice Plates orifice plates to evaporator H2 Figure 14 As the load decreases, less refrigerant moves through the chiller and the level of the liquid column drops. Now, as the liquid refrigerant passes through the orifice plates, it only undergoes a pressure drop equal to the lower head (H2) before some of it flashes to vapor.
trgtrc010_book.book Page 10 Wednesday, September 1, 1999 10:39 AM period one Components notes Liquid refrigerant from the condenser enters the first expansion device, which reduces the pressure of the refrigerant to that of the second-stage impeller inlet. This pressure drop causes a portion of the liquid refrigerant to evaporate, or flash, and the resulting mixture of liquid and vapor enters the economizer chamber.
trgtrc010_book.book Page 11 Wednesday, September 1, 1999 10:39 AM period one Components notes Again, the final expansion device reduces the pressure and temperature of the refrigerant to evaporator conditions.
trgtrc010_book.book Page 12 Wednesday, September 1, 1999 10:39 AM period one Components notes Motor motor gear-drive impeller motor impellers direct-drive gears Figure 18 Motor A motor is used to rotate the impeller(s). A direct-drive motor is connected directly to the impeller shaft and the impeller rotates at the same speed as the motor. A gear-drive motor transfers its energy to the impeller shaft using a set of gears. This allows the impeller to rotate at a higher speed than the motor.
trgtrc010_book.book Page 13 Wednesday, September 1, 1999 10:39 AM period one Components notes chiller’s refrigeration system—and uses a coupling to connect the motor and compressor shafts. The heat generated by the hermetic motor is absorbed by liquid refrigerant that flows around, through, and over the motor. The heat must be rejected by the chiller’s condenser. The heat generated by the open motor is rejected to the air drawn in from the equipment room.
trgtrc010_book.book Page 14 Wednesday, September 1, 1999 10:39 AM period one Components notes performance is a system-wide issue, not just a matter of chiller design and control. A starter links the chiller motor and the electrical distribution system. Its primary function is to connect (start) and disconnect (stop) the chiller from line power—similar to what a switch does for a light bulb.
trgtrc010_book.book Page 15 Wednesday, September 1, 1999 10:39 AM period two Refrigeration Cycle notes Centrifugal Water Chillers period two Figure 22 A pressure–enthalpy (p-h) chart illustrates the refrigeration cycle of the centrifugal water chiller. 2-stage centrifugal chiller Refrigeration Cycle 2-stage compressor economizer condenser evaporator expansion devices Figure 23 First, let’s review the components of a 2-stage centrifugal chiller in the context of the refrigeration cycle.
trgtrc010_book.book Page 16 Wednesday, September 1, 1999 10:39 AM period two Refrigeration Cycle notes The pressure drop created by the second expansion device lowers the pressure and temperature of the refrigerant to evaporator conditions, causing a portion of the liquid refrigerant to evaporate. The resulting mixture of liquid and vapor enters the evaporator.
trgtrc010_book.book Page 17 Wednesday, September 1, 1999 10:39 AM period two Refrigeration Cycle notes 2-stage centrifugal chiller pressure Refrigeration Cycle Pc P1 Pe condenser 6 expansion devices 8 9 4 economizer 5 7 evaporator enthalpy 2-stage 2 compressor 3 1 Figure 25 Refrigerant leaves the evaporator as saturated vapor ➀ and flows to the firststage impeller of the compressor. There, the refrigerant vapor is compressed to a higher pressure (P1) and temperature ➁.
trgtrc010_book.book Page 18 Wednesday, September 1, 1999 10:39 AM period two Refrigeration Cycle notes 2-stage centrifugal chiller pressure Refrigeration Cycle Pc P1 Pe condenser 6 expansion devices 8 4 economizer 5 7 2-stage 2 compressor 3 10 9 1 evaporator & % enthalpy $ Figure 26 The change in enthalpy from C to A that occurs during the refrigeration cycle is called the refrigeration effect.
trgtrc010_book.book Page 19 Wednesday, September 1, 1999 10:39 AM period two Refrigeration Cycle notes Refrigerant Operating Pressures HCFC-22 pressure HFC-134a HCFC-123 atmospheric pressure temperature Figure 27 Refrigerants When selecting which refrigerant to use in a centrifugal water chiller, the manufacturer considers efficiency, operating pressures, compatibility with materials, heat transfer properties, stability, toxicity, flammability, cost, availability, and environmental impact.
period two Refrigeration Cycle notes 20 TRG-TRC010-EN
trgtrc010_book.book Page 23 Wednesday, September 1, 1999 10:39 AM period three Compressor Capacity Control notes Multistage Compressor impellers inlet vanes Figure 33 In a multistage centrifugal compressor, the operating characteristics of each impeller are modulated by the impeller’s own set of inlet vanes. This example shows 2 impellers in series. These impellers share the task of compressing the refrigerant. Centrifugal water chillers are generally available with 1, 2, or 3 impellers.
trgtrc010_book.book Page 24 Wednesday, September 1, 1999 10:39 AM period three Compressor Capacity Control notes refrigerant. Recall that kinetic energy is converted to static energy, or static pressure. Impeller Dynamics Vr ∝ refrigerant flow rate R Vr Vt ∝ rotational speed × diameter Vt diameter rotational speed refrigerant flow rate Figure 35 The radial velocity (Vr) for a given compressor is directly proportional (∝) to the flow rate of refrigerant vapor through the compressor.
trgtrc010_book.book Page 25 Wednesday, September 1, 1999 10:39 AM period three Compressor Capacity Control notes Compressor Unloading R Vr R Vt Vt full load part load Vr Figure 36 Consider a given-diameter compressor impeller that rotates at a constant speed. As the load on the chiller decreases, the inlet vanes partially close and the flow rate of refrigerant through the compressor drops. Radial velocity (Vr), which is proportional to refrigerant flow, decreases as well.
trgtrc010_book.book Page 26 Wednesday, September 1, 1999 10:39 AM period three Compressor Capacity Control notes Surge Vr R Vr < static pressure Vt Figure 37 As the load and the corresponding refrigerant flow rate continue to fall, the radial velocity (force) drops, too. At some point, the radial force becomes smaller than the generated static pressure, letting the pressurized refrigerant vapor flow backward from the diffuser passages into the impeller.
trgtrc010_book.book Page 27 Wednesday, September 1, 1999 10:39 AM period three Compressor Capacity Control notes conditions that cause the compressor to surge. Any operating point that falls to the right of this line is satisfactory for stable operation.
trgtrc010_book.book Page 28 Wednesday, September 1, 1999 10:39 AM period three Compressor Capacity Control notes compressor map for a 2-Stage Compressor pressure difference $ r ge su 90 % & 75 51 10 14 25 63 36 unloading line refrigerant flow rate Figure 41 The starting point (A) is the full-load operating point.
trgtrc010_book.book Page 29 Wednesday, September 1, 1999 10:39 AM period three Compressor Capacity Control notes power consumption Adjustable-Frequency Drives inlet vanes only AFD + inlet vanes load Figure 42 An adjustable-frequency drive (AFD), or variable-speed drive, is another device used to vary the capacity of a centrifugal compressor.
trgtrc010_book.book Page 30 Wednesday, September 1, 1999 10:42 AM period four Maintenance Considerations notes Centrifugal Water Chillers period four Figure 43 This period discusses general maintenance requirements of centrifugal water chillers. Although some of the information applies specifically to the design presented in this clinic, requirements for other centrifugal chiller designs are also included.
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trgtrc010_book.book Page 34 Wednesday, September 1, 1999 10:42 AM period four Maintenance Considerations notes Scale deposits are best removed by chemical means. During this process, the water-cooled condenser is commonly isolated from the rest of the coolingtower-water circuit by valves, while a pump circulates cleaning solution through the condenser tubes. Sludge is removed mechanically.
trgtrc010_book.book Page 35 Wednesday, September 1, 1999 10:42 AM period four Maintenance Considerations notes Regularly logging oil and refrigerant charges, and examining the trends of this data, can help identify potential problems before they occur.
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trgtrc010_book.book Page 37 Wednesday, September 1, 1999 10:42 AM period five Application Considerations notes Condensing Temperature Control $ pressure difference & r ge su 90 75 % 51 10 14 25 63 36 vane position (degrees) unloading line refrigerant flow rate Figure 52 Condensing Temperature Control To achieve stable compressor unloading over a wide range of conditions, a reduction in condensing pressure (condenser relief) must accompany a reduction in load.
trgtrc010_book.book Page 38 Wednesday, September 1, 1999 10:42 AM period five Application Considerations notes condensing temperature control Cooling Tower Bypass 40°F [4°C] cooling tower bypass diverting valve 65°F [18°C] 55°F [13°C] condenser Figure 53 Controlling the refrigerant pressure difference between the evaporator and condenser of a water-cooled chiller is accomplished by varying the temperature or flow rate of the water flowing through the condenser.
trgtrc010_book.book Page 39 Wednesday, September 1, 1999 10:42 AM period five Application Considerations notes evaporator water flow Constant or Variable Flow chilled water pump evaporator variablespeed drive Figure 54 Constant or Variable Evaporator Water Flow Previous chiller designs required that a constant flow rate of water be maintained through the evaporator. This requirement has changed due to advances in chiller controls.
trgtrc010_book.book Page 40 Wednesday, September 1, 1999 10:42 AM period five Application Considerations notes 1) The water flow rate through the chiller evaporator must stay between the minimum and maximum flow rates for the evaporator bundle, as specified by the chiller manufacturer. These limits depend on the specific design variables of the actual evaporator bundle such as the number of tubes, number of passes, and geometry.
trgtrc010_book.book Page 41 Wednesday, September 1, 1999 10:42 AM period five Application Considerations notes Short Evaporator-Water Loops load tank chilled water pump evaporator Figure 57 Short water loops may be unavoidable in close-coupled or very small applications, particularly in systems where the load consists of only a few air handlers or processes.
trgtrc010_book.book Page 42 Wednesday, September 1, 1999 10:42 AM period five Application Considerations notes Heat Recovery heat-recovery condenser cooling tower water standard condenser Figure 58 Heat Recovery Salvaging usable heat from the refrigeration cycle—heat that would normally be rejected to the atmosphere—can significantly reduce the operating costs of many buildings.
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trgtrc010_book.book Page 45 Wednesday, September 1, 1999 10:42 AM period five Application Considerations notes free cooling Refrigerant Migration from compressor condenser to compressor evaporator Figure 61 This leads to the discussion of a free cooling principle called refrigerant migration. It involves adapting a water chiller so that it functions as a simple heat exchanger.
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trgtrc010_book.book Page 47 Wednesday, September 1, 1999 10:42 AM period five Application Considerations notes given system—they simply define a common rating point to aid comparisons. Trends toward improved humidity control and energy efficiency have changed some of the actual conditions selected for specific applications. Impurities in the chilled- and condenser-water systems eventually deposit on evaporator and condenser tube surfaces, impeding heat transfer.
trgtrc010_book.book Page 48 Wednesday, September 1, 1999 10:42 AM period six Review notes Centrifugal Water Chillers period six Figure 64 Let’s review the main concepts that were covered in this clinic on centrifugal water chillers. Review—Period 1 compressor motor condenser control panel evaporator starter Figure 65 Period 1 introduced the following components of a centrifugal water chiller: compressor, condenser, expansion device, economizer, evaporator, motor, control panel, and starter.
trgtrc010_book.book Page 49 Wednesday, September 1, 1999 10:42 AM period six Review notes pressure Review—Period 2 Pc P1 Pe condenser 6 expansion devices 8 2-stage 2 compressor 3 7 9 4 economizer 5 evaporator 1 Figure 66 enthalpy Period 2 described the refrigeration cycle of 2-stage centrifugal water chillers with a pressure-enthalpy chart. The operation of the purge system for lowpressure chillers was also presented.
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trgtrc010_book.book Page 52 Wednesday, September 1, 1999 10:42 AM Quiz Questions for Period 1 1) A centrifugal compressor converts ___________ energy to ____________ energy.
trgtrc010_book.book Page 53 Wednesday, September 1, 1999 10:42 AM Quiz 8) What velocity component is directly proportional to the impeller’s rotational speed times its diameter? 9) What are the 2 most common devices used to vary the capacity of a centrifugal compressor? Questions for Period 4 10)Explain why proper water treatment is important.
trgtrc010_book.book Page 54 Wednesday, September 1, 1999 10:42 AM Answers 1) kinetic energy to static energy 2) impeller 3) By flashing a portion of the refrigerant prior to entering the evaporator, the economizer reduces the compressor power required since the refrigerant vapor generated in the economizer only needs to be compressed by the higher-stage impeller. 4) A hermetic motor is totally enclosed within the chiller’s refrigeration system and the heat it generates is absorbed by liquid refrigerant.
trgtrc010_book.book Page 55 Wednesday, September 1, 1999 10:42 AM Glossary adjustable-frequency drive (AFD) A device used to vary the capacity of a centrifugal compressor by varying the speed of the motor that rotates the impeller(s). air-side economizer A method of free cooling that involves using cooler outdoor air for cooling instead of recirculating warmer indoor air.
trgtrc010_book.book Page 56 Wednesday, September 1, 1999 10:42 AM Glossary economizer The component of a multistage centrifugal chiller used to remove vapor from the refrigerant mixture after it passes through an expansion device. This reduces the compressor power required since the refrigerant vapor generated in the economizer only needs to be compressed by the higher-stage impeller(s). enthalpy The property of a refrigerant indicating its heat content, both sensible and latent.
trgtrc010_book.book Page 57 Wednesday, September 1, 1999 10:42 AM Glossary notes pressure-enthalpy chart A graphical representation of the saturated properties of a refrigerant, plotting refrigerant pressure versus enthalpy. purge A device used to remove air, moisture, and other noncondensable gases that may leak into a low-pressure chiller. refrigerant migration A method of free cooling that allows the chiller to be used as a heat exchanger, without operation of the compressor.
85577_Cover.fm Page 2 Wednesday, September 1, 1999 10:08 AM The Trane Company Worldwide Applied Systems Group 3600 Pammel Creek Road La Crosse, WI 54601-7599 www.trane.com An American Standard Company Literature Order Number TRG-TRC010-EN File Number E/AV-FND-TRG-TRC010-999-EN Supersedes 2803-10-983 Stocking Location Inland-La Crosse Since The Trane Company has a policy of continuous product improvement, it reserves the right to change design and specifications without notice.
