Refrigerant Piping Design Basics

Refrigerant Piping Design Basics. Refrigerant piping design is an important aspect of any air conditioning or refrigeration system. Proper design of the refrigerant piping system ensures that the system operates efficiently and reliably. Here are eight key factors to consider when designing refrigerant piping.

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1. Refrigerant System Layout

The layout of the refrigerant system should be designed to minimize the length of the piping and the number of fittings and inline components required. This reduces pressure drop in the system and helps improve efficiency. The total length of the refrigerant piping must not exceed the manufacturers requirements as this could result in a loss of capacity.

2. Refrigerant Pipe Sizing

The diameter of the piping should be chosen based on the required refrigerant flow rate and pressure drop. The wrong size piping can cause excessive pressure drops, leading to reduced system efficiency and capacity, while increasing power consumption. 

Liquid lines that are installed larger than required will increase the amount of refrigerant in the system, which could create additional problems. While under sizing liquid lines can cause the refrigerant to flash before it reaches the expansion valve, which will starve the evaporator and cause a loss in capacity, and the possible frosting up of the coil.

If the suction line is oversized then there could be problems with the return of oil to the compressor. And, if they are undersized there can be a loss of capacity and an increase in superheat.

3. Refrigerant Type

Different refrigerants have different properties, such as pressure, temperature, and viscosity. The refrigerant type should be considered when designing the piping system, and the system should be designed to accommodate the specific characteristics of the refrigerant used.

4. Refrigerant Piping Materials

The materials used for the piping should be compatible with the refrigerant and should be able to withstand the pressure and temperature of the system. ACR type Copper tubing is commonly used for refrigerant piping in the HVCAR industry.

5. Refrigerant Piping Insulation

Proper insulation is necessary to prevent refrigerant lines from losing their cooling capacity. The thickness of the insulation should be chosen based on the temperature difference between the refrigerant and the surrounding environment. Insulation thickness requirements can be found in the various codes that regulate the installation of the refrigerant piping. See our video on the proper methods for insulating refrigerant piping.

6. Refrigerant Piping Support

Refrigerant piping should be supported at regular intervals to prevent sagging and vibration, which can cause leaks and reduce system efficiency.

7. Expansion and Contraction

The refrigerant piping should be designed to accommodate the expansion and contraction of the piping due to temperature changes. Long lengths of piping can cause problems when temperature changes with the piping vary. The piping length will grow when heated up and contract when cooled down. Some method of compensating for the variable of expansion and contraction must be considered. 

Copper Piping Expansion = Delta-Temperature in piping x Piping Length x Coefficient of Expansion

8. Refrigerant Oil Management

Oil will be circulated around the system with the refrigerant and must be returned to the compressor where it’s needed to provide lubrication of bearings and moving parts. For this to happen it’s important that the refrigerant piping is sized correctly including the refrigerant velocity.

As refrigerant changes from a liquid to a vapor in the evaporator, the oil is separated out, which requires the correct velocity to ensure that the oil returns to the compressor. It’s important that refrigerant oil return to the compressor at the same rate at which it leaves.

Refrigerant Carrying Capacity of Piping

Refrigerant pipe sizing will also dictate the quantity of refrigerant required, as the larger the liquid line pipe size, the greater the volume of refrigerant required. We’ll look at the liquid line because it holds more refrigerant per linear foot than that of the same size suction line. We’ll compare the difference between 100 feet of pipe for various sizes using standard pressure in a R22 and R410A system.

R22 (100 feet of Liquid Line)

1/2” Pipe = 7 Lbs.

5/8” Pipe = 11.3 Lbs.

Difference in 4.3 Lbs.

So, by upsizing your liquid line from a 1/2” to a 5/8” line, the system would require approximately 4.3 Lbs. more of R22 refrigerant.

R410A (100 feet of Liquid Line)

1/2” Pipe = 5.8 Lbs.

5/8” Pipe = 9.2 Lbs.

Difference in 3.4 Lbs.

So, by upsizing your liquid line from a 1/2” to a 5/8” line, the system would require approximately 3.4 Lbs. more of R410A refrigerant.

When to Use Soft Copper

This is bound to create some controversy, as the ease by which soft copper can be installed is compelling from a labor standpoint, but practical engineering guidelines should be considered. Keeping soft copper installation to a maximum of a 50-foot roll is a prudent engineering request. Long lengths of soft copper tend to sag, and oil could be trapped where sags occur in the suction line.

Purging

When brazing refrigerant piping it’s important that a constant nitrogen purge be used to keep the system clean from the formation of copper oxides.

Refrigerant Pressure Drop Guidelines

The compressor will need to work harder for added pressure drop in the refrigerant piping design which considers pipe size, equivalent piping length which includes inline fittings, and components. Components may need to be oversized to compensate for excessive pressure drop in the system. By installing piping that’s too small there will be an increase in pressure drop and velocity, and a reduction in system capacity. It’s important that the overall equivalent pipe length be considered when selecting refrigerant pipe sizes.

Total pressure drop in the refrigerant piping system is determined by many factors including the pressure, velocity, and friction through pipe, valves, and fittings. And as previously stated, there is a loss in capacity of the system if the suction line is undersized. Smaller pipes have greater pressure losses, so ensuring the correct size is important for meeting design capacity.

Refrigerant Volume

It’s important to have the correct mass of refrigerant to achieve the design capacity of the system.

Sizing Refrigerant Piping

The process of sizing refrigerant piping begins with measuring the distance between the outdoor condensing unit and indoor fan coil while counting all the inline fittings and components. The routing should minimize the length of piping and number of fittings required, as each fitting or valve increases the overall pressure drop of the system. Upsizing the liquid line one size will increase the refrigerant carrying capacity by about 50% more, for example a 1/2” liquid line carries approximately 5.8 lbs. of R410A per 100 feet, while a 5/8” liquid line carries about 9.2 Lbs./100 feet.

If the pressure drop is too great in the liquid line, then it’s possible that the pressure drops below the saturation temperature of the refrigerant causing it to flash into vapor. This cause a loss in capacity and explains why the correct sizing of the piping is important, and why you should avoid additional fittings or too small of a liquid line. 

When the condenser is below the air handler than the Liquid Line requires “Vertical Lift”, and when the condenser is above the air handler than the suction line requires “Vertical Lift”. This is easy to determine if you think about the work the compressor must do, and where the compressor is located when running. If it’s on the bottom then it must push up, and if it’s on the top then it must pull up. Depending on where the compressor is in relationship to the air handler it either must push the liquid up or pull the suction gas up.

Summary

Overall, the refrigerant piping design should be carefully considered to ensure that the system operates efficiently and reliably. A well designed system will ensure that the suction, liquid and discharge piping is large enough to prevent excessive pressure drop, yet small enough to ensure that the velocity will carry the oil back to the compressor crankcase. It’s recommended to consult with a professional HVAC engineer to ensure proper design and installation.