Refrigerant Piping One of the Fundamental Series
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Air Conditioning Clinic Refrigerant Piping One of the Fundamental Series June 2011 TRG-TRC006-EN Refrigerant Piping One of the Fundamental Series A publication of Trane Preface Refrigerant Piping A Trane Air Conditioning Clinic Figure 1 Trane 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. It is intended to acquaint a technical audience with various fundamental aspects of heating, ventilating, and air conditioning (HVAC). We have taken special care to make the clinic as uncommercial and straightforward as possible. Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text. This particular clinic introduces the reader to refrigerant piping. © 2011 Trane All rights reserved ii TRG-TRC006-EN Contents period one Refrigerant Piping Requirements .................... 1 period two Suction Line ......................................................... 10 Requirements for Sizing and Routing ..................... 10 Sizing Process ....................................................... 12 Other Considerations ............................................ 24 period three Discharge Line ..................................................... 31 Requirements for Sizing and Routing ..................... 31 Sizing Process ....................................................... 33 Other Considerations ............................................ 39 period four Liquid Line ............................................................ 41 Requirements for Sizing and Routing ..................... 41 Sizing Process ....................................................... 45 Other Considerations ............................................ 57 period five Hot-Gas Bypass Line ......................................... 60 Hot-Gas Bypass to Evaporator Inlet ........................ 62 Hot-Gas Bypass to the Suction Line ....................... 66 Other Considerations ............................................ 68 period six Review ................................................................... 70 Quiz ......................................................................... 76 Answers ................................................................ 78 Glossary ................................................................ 79 TRG-TRC006-EN iii iv TRG-TRC006-EN period one Refrigerant Piping Requirements notes Refrigerant Piping period one Refrigerant Piping Requirements Figure 2 The focus of this clinic is on the design and installation of the interconnecting piping for vapor-compression refrigeration systems. Reviewing the physical changes that the refrigerant undergoes within the refrigeration cycle will help demonstrate certain demands that the piping design must meet. This clinic focuses on systems that use Refrigerant-22 (R-22). While the general requirements are the same for systems that use other refrigerants, velocities and pressure drops will differ. Vapor-Compression Refrigeration condenser D C expansion device compressor B A evaporator Figure 3 Figure 3 illustrates a basic vapor-compression refrigeration cycle. Refrigerant enters the evaporator in the form of a cool, low-pressure mixture of liquid and vapor (A). Heat is transferred to the refrigerant from the relatively warm air that is being cooled, causing the liquid refrigerant to boil. The resulting refrigerant vapor (B) is then pumped from the evaporator by the compressor, which increases the pressure and temperature of the vapor. TRG-TRC006-EN 1 period one Refrigerant Piping Requirements The resulting hot, high-pressure refrigerant vapor (C) enters the condenser notes where heat is transferred to ambient air, which is at a lower temperature than the refrigerant. Inside the condenser, the refrigerant vapor condenses into a liquid and is subcooled. This liquid refrigerant (D) then flows from the condenser to the expansion device. This device creates a pressure drop that reduces the pressure of the refrigerant to that of the evaporator. At this low pressure, a small portion of the refrigerant boils (or flashes), cooling the remaining liquid refrigerant to the desired evaporator temperature. The cool mixture of liquid and vapor refrigerant (A) enters the evaporator to repeat the cycle. The vapor-compression refrigeration cycle, and the four major components of the refrigeration system (evaporator, compressor, condenser, and expansion device), are discussed in more detail in separate clinics. Refer to the list of references at the end of Period Six. Interconnecting Refrigerant Piping C discharge line condenser liquidliquid line expansion compressor line device B compressor A suction D line evaporator Figure 4 These individual components are connected by refrigerant piping. The suction line connects the evaporator to the compressor, the discharge line connects the compressor to the condenser, and the liquid line connects the condenser to the expansion device. The expansion device is typically located at the end of the liquid line, at the inlet to the evaporator. There is more to the design of refrigerant piping than moving refrigerant from one component to another. Regardless of the care exercised in selection and application of the components of the refrigeration system, operational problems may be encountered if the interconnecting piping is improperly designed or installed. 2 TRG-TRC006-EN period one Refrigerant Piping Requirements notes Refrigerant Piping Requirements V Return oil to compressor V Ensure that only liquid refrigerant enters the expansion device V Minimize system capacity loss V Minimize refrigerant charge Figure 5 When a refrigeration system includes field-assembled refrigerant piping to connect two or more of the components, the primary design goals are generally to maximize system reliability and minimize installed cost. To accomplish these two goals, the design of the interconnecting refrigerant piping must meet the following requirements: Return oil to the compressor at the proper rate, at all operating conditions Ensure that only liquid refrigerant (no vapor) enters the expansion device Minimize system capacity loss that is caused by pressure drop through the piping and accessories Minimize the total refrigerant charge in the system to improve reliability and minimize installed cost Scroll Compressor stationary scroll seal discharge discharge port intakeintake intake motor driven shaft scroll Figure 6 The first requirement is to ensure that oil is returned to the compressor at all TRG-TRC006-EN 3 period one Refrigerant Piping Requirements operating conditions. Oil is used to lubricate and seal the moving parts of a notes compressor. For example, the scroll compressor shown in Figure 6 on page 3 uses two scroll configurations, mated face-to-face, to compress the refrigerant vapor. The tips of these scrolls are fitted with seals that, along with a thin layer of oil, prevent the compressed refrigerant vapor from escaping through the mating surfaces. Similarly, other types of compressors also rely on oil for lubrication and for providing a seal when compressing the refrigerant vapor. Characteristically, some of this lubricating oil is pumped along with the refrigerant throughout the rest of the system. While this oil has no function anywhere else in the system, the refrigerant piping must be designed and installed so that this oil returns to the compressor at the proper rate, at all operating conditions. Return Oil to Compressors discharge line hot liquidliquid condenser vapor line warm expansion compressor warm device compressor liquidliquid suction line cool vapor evaporator Figure 7 Returning to the system schematic, droplets of oil are pumped out of the compressor along with the hot, high-pressure refrigerant vapor. The velocity of the refrigerant inside the discharge line must be high enough to carry the small oil droplets through the pipe to the condenser. Inside the condenser, the refrigerant vapor condenses into a liquid. Liquid refrigerant and oil have an affinity for each other, so the oil easily moves along with the liquid refrigerant. From the condenser, this mixture of liquid refrigerant and oil flows through the liquid line to the expansion device. Next, the refrigerant–oil mixture is metered through the expansion device into the evaporator, where the liquid refrigerant absorbs heat and vaporizes. Again, the velocity of the refrigerant vapor inside the suction line must be high enough to carry the droplets of oil through the pipe back to the compressor. Without adequate velocity and proper pipe installation, oil may be trapped out in the system. If this condition is severe enough, the reduced oil level in the compressor could cause lubrication problems and, potentially, mechanical failure. 4 TRG-TRC006-EN period one Refrigerant Piping Requirements notes Thermostatic Expansion Valve (TXV) distributor TXV evaporator remote evaporator bulb liquidliquid refrigerant refrigerant external equalizer vapor Figure 8 The second requirement of the refrigerant piping design is to ensure that only liquid refrigerant enters the expansion device. There are several types of expansion devices, including expansion valves (thermostatic or electronic), capillary tubes, and orifices. In addition to maintaining the pressure difference between the high-pressure (condenser) and low-pressure (evaporator)