Refrigeration Piping: The Use of Double Risers

May 12, 2000
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TABLE 1: Double riser suction line sizes.
Final of three parts.

For systems equipped with capacity control compressors, or where tandem or multiple compressors are used with one or more compressors cycled off for capacity control, a single suction line riser may result in either unacceptably high or low gas velocities.

A line properly sized for light load conditions may have too high a pressure drop at maximum load, and if the line is sized based on full-load conditions, velocities may not be adequate at light load conditions to move oil through the tubing.

On air conditioning applications, where somewhat higher pressure drops at maximum-load conditions can be tolerated without any major penalty in overall system performance, it is usually preferable to accept the additional pressure drop imposed by a single vertical riser.

However, on medium- or low-temperature applications, where pressure drop is more critical and where separate risers from individual evaporators are not desirable or possible, a double riser may be necessary to avoid an excessive loss of capacity.

A typical double riser has a smaller and larger riser. The two lines should be sized so that the total cross-sectional area is equivalent to the cross-section area of a single riser that would have both satisfactory gas velocity and acceptable pressure drop at maximum load conditions.

The larger line is trapped. The smaller line must be sized to provide adequate velocities and acceptable pressure drop when the entire minimum load is carried in the smaller riser. Table 1 shows suction line sizes.

Defrost Gas Supply Lines

Sizing refrigeration lines to supply defrost gas to one or more evaporators has not been an exact science. The parameters associated with sizing the defrost gas lines are related to allowable pressure drop and refrigerant flow rate during defrost.

Engineers have used approximately two times the evaporator load for effective refrigerant flow rate to determine line-sizing requirements. The pressure drop is not as critical during the defrost cycle, and many engineers have used velocity as the criterion for determining line size.

The effective condensing temperature and average temperature of the gas must be determined. The velocity determined at saturated conditions will give a conservative line size.

It is recommended that initial sizing be based on twice the evaporator flow rate, and that velocities from 1,000 to 2,000 feet per minute (fpm) be used for determining the defrost gas supply line size.

Equivalent Lengths Of Valves, Fittings

Refrigerant line capacity tables are based on unit pressure drop per 100-ft length of straight pipe, or per combination of straight pipe, fitting, and valves, with friction drop equivalent to a 100-ft length of straight pipe.

Generally, pressure drop through valves and fittings is determined by establishing the equivalent straight length of pipe of the same size with the same friction drop. Line sizing tables can then be used directly.

Expansion and Contraction

Temperature change will expand and contract all refrigeration piping material. Therefore, techniques must allow for expansion and contraction changes; this will prevent stresses that may buckle, bend, or rupture the refrigerant piping.

The two common methods of taking care of expansion and contraction in copper piping used in the refrigeration industry are the “expansion loops” or “pipe offsets.” During the installation of the line, care must be taken so that the line maintains perfect alignment.

On average, copper’s coefficient of expansion is 0.0000104 in./1 in./1°F. Expansion of copper is 1.25 in./100 ft/100° change.

For example, a copper compressor discharge line 75-ft long at 225° could have a temperature change of 150° in a 70° room. Therefore, 1.25 x 1.55 (temperature change per 100°F) x 0.75 (length per 100 ft) will equal 1.453 in. of expansion.

Location, Arrangement

Refrigerant lines should be as short and direct as possible to minimize tubing and refrigerant requirements and pressure drops.

Plan piping for a minimum number of joints using as few elbows and other fittings as possible, but provide sufficient flexibility to absorb compressor vibration and stresses due to thermal expansion and contraction.

Arrange refrigerant piping so that normal inspection and servicing of the compressor and other equipment are not hindered. Do not obstruct the view of the oil level sight glass or run piping so that it interferes with the removal of the compressor cylinder head, end bells, access plates, or any internal parts.

Suction line piping to the compressor should be arranged so that it will not interfere with removal of the compressor for servicing.

Provide adequate clearance between piping and adjacent wall and hanger, or between pipes, for insulation installation.

Use sleeves that are sized to permit installation of both pipe and insulation through floor, walls, or ceilings. Set these sleeves prior to pouring of concrete or erection of brickwork.

Run piping so that it does not interfere with passages or obstruct headroom, windows, and door. Refer to ASHRAE Standard 15, Safety Code for Mechanical Refrigeration, and other governing local codes for restrictions that may apply.

Piping Insulation

All piping joints and fittings should be thoroughly leak tested before the insulation is sealed.

Suction lines should be insulated to prevent sweating and heat gain. Insulation covering lines on which moisture can condense, or lines subjected to outside conditions, must be vapor sealed to prevent any moisture from traveling through the insulation or condensation in the insulation.

Although the liquid line ordinarily does not require insulation, the suction and liquid lines can be insulated as a unit on installations where the two lines are clamped together.

When it passes through an area of higher temperature, the liquid line should be insulated to minimize heat gain.

Hot-gas discharge lines usually are not insulated; however, they should be insulated if the heat dissipated is objectionable, or to prevent injury from high-temperature surfaces.

Vibration and Noise in Piping

Vibration transmitted through or generated in refrigerant piping (and the resulting objectionable noise) can be eliminated or minimized by proper piping design and support.

Two undesirable effects of vibration of refrigerant piping are:

1. Physical damage to the piping, which may result in the breaking of brazed joints and consequently, loss of charge; and

2. Transmission of noise through the piping itself and through building construction with which the piping may come into direct physical contact.

And one final note: Always follow manufacturers’ recommendations.

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