Refrigeration piping: on the rise

May 5, 2000
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Hot gas lines should be designed to avoid trapping oil at part-load operation. The lines should also prevent condensed refrigerant and oil from draining back to the head of the compressor.

Such lines need carefully selected connections from a common line to multiple compressors to avoid developing excessive noise or vibration from hot gas pulsation, compressor vibration, or both.

When sizing discharge lines — lines that conduct refrigerant vapor from the compressor to the condenser — considerations similar to those applied to the suction line are observed.

A pressure loss in hot gas lines increases the required compressor power per unit of refrigeration and decreases compressor capacity by increasing the compression ratio.

While the discharge line pressure drop is not as critical as that of the suction line, the accepted maximum values are 4 psi for R-12 and 6 psi for R-22.

The same minimum gas velocities of 500 feet per minute (fpm) in horizontal runs and 1,000 fpm in vertical runs with upward gas flow are observed. The maximum acceptable gas velocity, based on noise considerations, is 4,000 fpm.

Condenser drain line

The line between a condenser (not providing liquid subcooling) and a liquid receiver, when such an arrangement is used, must be carefully sized. While it is almost impossible to oversize such a line, undersizing is to be avoided.

An undersized line can restrict the flow of refrigerant to the extent that some of it is held in the condenser. If some of the condenser surface is flooded, the capacity is reduced. This causes the head pressure to rise, decreasing overall system capacity. At the same time, the power to drive the compressor rises.

There are a few points that the piping designer should keep in mind:

  • Condenser drain line velocity is designed at 100 fpm or less.
  • The distance between the condenser and receiver should be kept as short as possible.
  • The condenser must be located above the receiver.

If the system is equipped with an air-cooled condenser and a liquid receiver, it is good practice to locate the receiver within the building. Some positive means should be provided to isolate the receiver from the condenser during the cold weather shutdown, such as a combination check and relief valve.

The receiver

Refrigerant receivers are vessels used to store excess refrigerant circulated throughout the system.

Receivers perform the following functions and are designed to:

  • Provide pumpdown storage capacity when another part of the system must be serviced or the system must be shut down for an extended time; in some water-cooled condenser systems, the condenser also serves as a receiver if the total refrigerant charge does not exceed its storage capacity.
  • Handle the excess refrigerant charge that occurs with air-cooled condensers using the flooding-type condensing pressure control;
  • Accommodate a fluctuating charge in the low side and drain the condenser of liquid;
  • Maintain an adequate, effective condensing surface on systems where the operating charge in the evaporator and/or condenser varies for different loading conditions; when an evaporator is fed with a thermal expansion valve, hand expansion valve, or low-pressure float, the operating charge in the evaporator varies considerably depending on the loading.


During low load, the evaporator requires a larger charge since the boiling is not as intense. When the load increases, the operating charge in the evaporator decreases, and the receiver must store excess refrigerant.
  • Hold the full charge of the idle circuit on systems with multi-circuit evaporators that shut off the liquid supply to one or more circuits during reduced load and pump out the idle circuit.

Additionally:

  • The receiver is kept close to the condenser. If there is any doubt about the line size, the larger of the two line sizes should be used.
  • The minimum vertical dim-ension required to overcome friction should always be adhered to.
  • And there needs to be a pressure-relief device on top of each receiver and on the condenser.

Remember, surge receiver pressure relief devices are piped together with condensers. Size to 40% to 125% of the refrigerant charge, depending on system load variance.

When a through-type receiver is used, the liquid must always flow from the condenser to the receiver. The receiver and its associated piping provide free flow of liquid from the condenser to the receiver by equalizing the pressure between the two, so that the receiver cannot build up a higher pressure than the condenser.

If a vent is not used, the piping between the condenser and the receiver is sized so that liquid flows in one direction and gas flows in the opposite direction. Sizing the condensate drain line for 100-fpm liquid velocity is usually adequate to attain this flow.

Piping should slope at least 0.25 in./ft and eliminate any natural liquid traps. The condensate drain line should be sized so that the velocity does not exceed 100 fpm.

There are alternate refrigerant storage capacities (in pounds) for R-22 rated receivers. For example, a receiver that has a rated capacity of 299 lb of R-22 is used with R-407C. This receiver will have a rated capacity of 283 lb of R-407C. That is 299 lb x 0.9473 = 283 lb (of R-407C).

Liquid lines

Pressure drop should not be so large as to cause gas formation in the liquid line, insufficient liquid pressure at the liquid feed device, or both.

Systems are normally designed so that the pressure drop in the liquid line, due to friction, is not greater than that corresponding to about a 1° to 2°F differential change in saturation temperature. An HFC refrigerant is designed at 1° differential change.

Liquid subcooling is the only method of overcoming the liquid line pressure losses to guarantee liquid at the expansion device in the evaporator. If the subcooling is insufficient, flashing will occur within the liquid line and degrade the efficiency of the system.

Friction pressure drops in the liquid line are caused by accessories such as solenoid valves, filter-driers, and hand valves, as well as by the actual piping and fittings between the receiver outlet and the refrigerant feed device at the evaporator.

Liquid line risers are a source of pressure loss and add to the loss of the liquid line. The loss due to a riser is approximately 0.556 psi/ft of liquid lift, or 1 lb will lift refrigerant 1.8 ft. In 18 ft it takes 10 lb, in 36 ft it takes 20 lb, and in 54 ft it takes 30 lb. The total loss is the sum of all friction losses, plus the pressure loss from liquid risers.

Refrigeration systems that have no liquid risers and have the evaporator below the condenser-receiver benefit from a gain in pressure due to liquid weight and can tolerate larger friction losses without flashing.

Regardless of the routing of the liquid line when flashing takes place, the overall efficiency is reduced and the system may malfunction. The only way to reduce the effect of pressure losses and friction is by subcooling the refrigerant.

Suction lines

Suction lines are more critical than liquid and discharge lines from a design and construction standpoint.

Refrigerant lines should be sized to:

  • Provide a minimum pressure drop at full load;
  • Return oil from the evaporator to the compressor under minimum load conditions; and
  • Prevent oil from draining from an active evaporator into an idle one.


A pressure drop in the suction line reduces a system’s capacity because it forces the compressor to operate at a lower suction pressure to maintain a desired evaporating temperature in the coil. As the suction pressure is decreased, each pound of refrigerant returning to the compressor occupies a greater volume, and the weight of the refrigerant pumped by the compressor decreases.

For example, a typical low-temperature R-502 compressor at -40° evaporating temperature will lose almost 6% of its rated capacity for each 1-psi suction line pressure drop. Normally accepted design practice is to use as a design criterion a suction line pressure drop equivalent to a 2° differential change in saturation temperature.

Oil return

Of equal importance in sizing a suction line is the necessity of maintaining adequate velocities to properly return oil to the compressor.

Studies have shown that oil is most viscous in a system after the suction vapor has warmed up a few degrees from the evaporating temperature. In this case, the oil is no longer saturated with the refrigerant. This condition occurs in the suction line after the refrigerant vapor has left the evaporator.

Movement of the oil through suction lines is dependent on both the mass and velocity of the suction vapor. As the mass or density decreases, higher velocities are required to force the oil along.

Nominal minimum velocities of 700 fpm in horizontal suction lines and 1,500 fpm in vertical suction lines have been recommended and used successfully for many years as suction line-sizing design standards. Use of the one nominal velocity has provided a simple and convenient means of checking velocities.

However, tests have shown that in vertical risers, the oil tends to crawl up the inner surface of the tubing, and the larger the tubing, the greater velocity required in the center of the tubing to maintain tube surface velocities that will carry the oil.

The exact velocity required in the vertical line is dependent on both the evaporating temperature and the line size. Under varying conditions, the specific velocity required might be either greater or less than 1,500 fpm. An HFC refrigerant is design at 1,500 fpm or greater.

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