Considerations for Proper Sizing of Refrigerant Lines


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Refrigeration is the movement of heat from one location where it is not wanted, to a place where it is less objectionable.  This heat is moved through a mechanical system by utilizing refrigerant as a working fluid.  Properly sized interconnected piping enables the refrigerant to move through the system.  The piping needs to be sized to also transport the lubricant (carry-over from the compressor) through the system; regardless of whether the refrigerant is in a liquid, gas, or saturated state (i.e. both liquid and gas).

Cost considerations favor keeping the line size as small as possible.  However, the smaller the line size, the larger the pressure drop and the higher the refrigerant velocity moving through the line.

Accurate line sizing is important to achieve several important requirements:

  1. Obtain proper oil return while complying with refrigerant velocity guidelines.
  2. Ensure correct mass flow of the refrigerant to match the equipment design capacity,
  3. Minimize refrigerant pressure drop within the refrigerant circuit.

Balancing the needs of these elements – oil return, refrigerant mass flow, and refrigerant pressure drop –yields peak energy efficiency and minimal equipment wear.

Proper line sizing takes into consideration a number of different variables; these include:

  • Refrigerant type used,
  • Type of line that is being sized (see below),
  • Equivalent length of run (equivalent length includes fittings, valves, etc.),
  • Saturated suction temperature of the circuit or system,
  • Saturated condensing temperature of the system,
  • Liquid temperature of the system, and
  • BTU load or BTU equipment capacity.

There are eight refrigerant line types in a refrigeration system:

Line Type Position Velocity (fpm) Components in these lines and Notes
Compressor Discharge Line

 

From the compressor outlet to the condenser inlet 1,500 to 3,000 Oil separator, compressor check valves, discharge line pressure differential valve, heat reclaim valve and heat reclaim coil, split condenser valve, and isolation ball valves.
Liquid Line From the outlet of the condenser to the metering device 100 to 350 Receiver pressure regulator, winter control valve, receiver, liquid line dryer and bypass, sight glass, mechanical sub-cooler, liquid line solenoids, liquid header, thermostatic expansion valve(s) (or capillary tubes), and isolation ball valves.
Suction Line From the evaporator outlet to the compressor inlet 800 to 4,000 Evaporator pressure regulator(s), suction stop solenoid(s), defrost solenoid(s), suction riser, suction accumulator, suction filter(s) and isolation ball valves.
Suction Riser

 

Part of the suction line that runs from the evaporator to a horizontal run, usually at or just below ceiling level 1,500 to 4,000 Any suction riser greater than six feet should have a “P” trap in the line.  Additionally, a “P” trap should be added every 12 to 15 ft. of height.
Suction Header Placed between multiple suction lines and the compressor(s) N/A The suction header can be oversized with little negative performance effect.  However, under-sizing will restrict refrigerant flow to the compressors, especially under full load.
Liquid Header

 

Placed between multiple liquid lines and the evaporators. N/A The liquid header can be oversized with little negative performance effect, however, under-sizing will restrict refrigerant flow and will increase velocity maybe to the point of “water hammer”.
Defrost Lines

 

These lines carry the hot refrigerant gas from the defrost header to the circuit suction line. N/A Provided by the OEM, usually under 2ft.  in length and, are usually 5/8”. There will be a gas defrost solenoid and isolation ball valve on each line.
Oil lines Run from the outlet of the oil separator to each compressor on the system. N/A Oil filter, oil pressure reduction valve, oil reservoir, check valve(s), oil equalization lines, oil equalization valves, compressor oil floats, and isolation ball valves.


Related Issues

OEM-provided Refrigerant Stubs: 

Most manufacturers provide stubs for connecting refrigeration lines.  Merely matching those line sizes in not a good practice.  Due to field specific line lengths required, height of risers, loads, etc., each line should be sized.

Refrigerant Pressure Drop 

A key parameter impacting proper line sizing is the pressure drop within the piping.  Refrigerant pressure drop is important because every pound of pressure drop adds work to the compressors, which requires compressors to run harder and longer. Additionally, a high pressure drop in the suction line requires that the saturated suction pressure be set to a lower setpoint to maintain product temperature requirements.  Additionally, high pressure drops may require successively larger sized components.

Equivalent Line Length  

The technician can directly measure onsite, or by look at construction prints, to determine the needed distance of a piping run.  However, there is more than horizontal and vertical distances to be concerned with.  Any location where the refrigerant makes a turn, (“elbow” or “Tee”), or any fittings, valves (solenoids, check valves, etc.) will increase the Equivalent Line Length as each has an assocaited pressure drop that increases with increasing refrigerant velocity.

Pipe Expansion and Contraction  

Copper tube expands and contracts with temperature changes.  Therefore, in a refrigeration system where temperatures can have excessine swings, a long line length tends to buckle or bend the copper tube.  Severe stresses to the joints may also occur.  These can be prevented by the use of  expansion joints or offsets.  The equation to determine expansion and contraction of copper tube, is:

Expansion (inches) = Temperature rise in the tube (°F) X Line length in feet X 12 (inches per foot)  *Expansion Coefficient (inches/foot/°F)

[For copper, use an Exp. Coef. of 0.0001128 inch per linear foot per degree F.

Final Thought 

For unitary equipment offerings, as typically used in comfort cooling for the residential and commercial sectors, the OEMs provide very specific guidance as to what size refrigerant tubing needs to be used.  However, due to their built-on-site nature, and that store layouts and case line-ups can change dramatically between supermarkets and convenience stores, very little information is provided for sizing refrigerant piping used in commercial refrigeration systems.  For guidance and sizing instruction on same, go to the ACCA website and download the Technical Bulletin on Commercial Refrigeration Line Sizing

Danny Halel

Posted In: ACCA Now, Technical Tips

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