This guide 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.
Reviewing the physical changes that the refrigerant undergoes within the refrigeration cycle will help demonstrate certain demands that the piping design must meet.
Vapor-Compression Refrigeration
Figure below 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.
The resulting hot, high-pressure refrigerant vapor (C) enters the condenser 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.
Interconnecting Refrigerant Piping
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.
Refrigerant Piping Requirements
- Return oil to compressor
- Ensure that only liquid refrigerant enters the expansion device
- Minimize system capacity loss
- Minimize refrigerant charge
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
The first requirement is to ensure that oil is returned to the compressor at all operating conditions. Oil is used to lubricate and seal the moving parts of a compressor. For example, the scroll compressor shown in Figure above 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
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.
Thermostatic Expansion Valve (TXV)
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) sides of the system, a thermostatic expansion valve (TXV) also controls the quantity of liquid refrigerant that enters the evaporator. This ensures that the refrigerant will be completely vaporized within the evaporator, and maintains the proper amount of superheat in the system.
Subcooling
Inside the condenser, after all of the refrigerant vapor has condensed into liquid, the refrigerant is subcooled to further lower its temperature. This subcooled liquid refrigerant leaves the condenser (A) and experiences a pressure drop as it flows through the liquid line and accessories, such as a filter drier and solenoid valve, installed upstream of the TXV. On the pressure-enthalpy chart, Figure below on page 5, this moves the condition of the refrigerant toward the saturated liquid curve (B). If this pressure drop is high enough, or if the refrigerant has not been subcooled enough by the condenser, a small portion of the refrigerant may boil (or flash), resulting in a mixture of liquid and vapor (C) entering the expansion device.
The presence of refrigerant vapor upstream of the expansion device is very undesirable. Bubbles of vapor displace liquid in the port of the TXV, reducing the flow rate of liquid through the valve, therefore substantially reducing the capacity of the evaporator. This results in erratic valve operation.
The design of the piping system must ensure that only liquid refrigerant (no vapor) enters the expansion device. This requires that the condenser provide adequate subcooling at all system operating conditions, and that the pressure drop through the liquid line and accessories not be high enough to cause flashing. Subcooling allows the liquid refrigerant to experience some pressure drop as it flows through the liquid line, without the risk of flashing.
Pressure Drop in a Suction Line
The third requirement of the refrigerant piping design is to minimize system capacity loss. To achieve the maximum capacity from the system, the refrigerant must circulate through the system as efficiently as possible. This involves minimizing any pressure drop through the piping and other system components.
Whenever a fluid flows inside a pipe, a characteristic pressure drop is experienced. Pressure drop is caused by friction between the moving liquid (or vapor) and the inner walls of the pipe. The total pressure drop depends on the pipe diameter and length, the number and type of fittings and accessories installed in the line, and the mass flow rate, density, and viscosity of the refrigerant.
As an example, the chart in Figure above demonstrates the impact of pressure drop, through the suction line, on the capacity and efficiency of the system. For this example system operating with Refrigerant-22, increasing the total pressure drop in the suction line from 3 psi (20.7 kPa) to 6 psi (41.4 kPa) decreases system capacity by about 2.5 percent and decreases system efficiency by about 2 percent.
This reveals a compromise that the system designer must deal with. The diameter of the suction line must be small enough that the resulting refrigerant velocity is sufficiently high to carry oil droplets through the pipe. However, the pipe diameter must not be so small that it creates an excessive pressure drop, reducing system capacity too much.
Minimize Refrigerant Charge
The first three requirements have remained unchanged for many years. However, years of observation and troubleshooting has revealed that the lower the system refrigerant charge, the more reliably the system performs. Therefore, a fourth requirement has been added for the design of refrigerant piping: minimize the total amount of refrigerant in the system. To begin with, this involves laying out the shortest, simplest, and most-direct pipe routing. It also involves using the smallest pipe diameter possible, particularly for the liquid line because, of the three lines, it impacts refrigerant charge the most. The chart in Figure below shows that the liquid line is second only to the condenser in the amount of refrigerant it contains.
This reveals another compromise for the system designer. The diameter of the liquid line must be as small as possible to minimize the total refrigerant charge. However, the pipe diameter cannot be small enough to create an excessive pressure drop that results in flashing before the liquid refrigerant reaches the expansion device.
Involve the Manufacturer
If provided, use refrigerant line sizes recommended by manufacturer
This guide discusses the processes for sizing the interconnecting piping in an air-conditioning system. Some of the information required for selecting the optimal line sizes is best known by the manufacturer. Therefore, if the manufacturer of the refrigeration equipment provides recommended line sizes, or tools for selecting the optimal line sizes, we recommend that you use those line sizes.
If, however, line sizes are not provided by the manufacturer, the processes outlined within this guide could be used for selecting the sizes.
General Piping Requirements
- Use clean Type L copper tubing
- Copper-to-copper joints: BCuP-6 without flux
- Copper-to-steel (or brass) joints: BAg-28, non-acid flux
- Properly support piping to account for expansion, vibration, and weight
- Avoid installing piping underground
- Test entire refrigerant circuit for leaks
Before discussing the design and installation of the suction, discharge, and liquid lines, there are some general requirements that apply to all of these lines.
First, copper tubing is typically used for refrigerant piping in air-conditioning systems. This tubing is available in various standard diameters and wall thicknesses. The nominal diameter of the tubing is expressed in terms of its outside diameter. This tubing must be completely free from dirt, scale, and oxide. New Type L or Type ACR tubing that has been cleaned by the manufacturer and capped at both ends is recommended for air-conditioning applications.
The piping system is constructed by brazing copper tubes and fittings together. When brazing copper-to-copper joints, use BCuP-6* without flux. For copper-to-steel or copper-to-brass joints, use BAg-28* with a non-acid flux.
Based on the American Welding Society’s (AWS) Specification for Filler Metals for Brazing and Braze Welding, publication A5.8–1992
The refrigerant piping must be properly supported to account for expansion, vibration, and the total weight of the piping. When a pipe experiences a temperature change, it is subject to a certain amount of expansion and contraction. Because the refrigerant piping is connected to the compressor, vibration forces are transmitted to the piping itself. Finally, the weight of the refrigerant-filled pipe and fittings must be supported to prevent the pipes from sagging, bending, or breaking.
Avoid installing refrigerant piping underground. It is very difficult to maintain cleanliness during installation or to test for leaks. If underground installation is unavoidable, each line must be insulated separately, and then the lines must be waterproofed and protected with a hard casing (such as PVC).
After the piping has been installed, the entire refrigeration circuit must be tested for leaks before it can be charged with refrigerant. This process typically involves pressurizing the entire piping system with dry nitrogen to examine each brazed joint for leaks.
Each of these issues is discussed in greater detail in the Trane Reciprocating Refrigeration Manual.