Parallel compounding with optimized suction header

General requirements and recommendations

• The concept is applicable to systems with
• ≤ 4 compressors of equal capacity and without capacity control
• identical lubrication system for the compressors and monitoring of the oil supply by an oil level switch (centrifugal lubrication) or oil pressure limiter (pump lubrication). Compressors with centrifugal and pump lubrication may only be combined when designed with oil level controllers (Parallel compounding with oil level controllers).
• Load sequence switching at a rate of max. 1 h is required to ensure that the temperature of shut-off compressors does not drop too much. Ideally, the cylinder heads should not be cooled below the condensing temperature, in order to avoid back condensation. A higher temperature also reduces the refrigerant concentration in oil, thus reducing oil carry over during the compressor start.
• System design / Pipe works: The prerequisite for safe operation is a uniform oil transport in the system during full load and part load. To ensure this, the pipe works must be carefully designed with respect to the minimum flow velocities.
• System control: An intelligent control logic should be used which guarantees an automatic sequence change and uniformly long compressor running times at minimum switching frequency.

In case of deviating specification or for widely branched pipe works with critical oil balance, systems with oil level controllers are recommended (Parallel compounding with oil level controllers). Exceptions are possible in case of tested concepts for the application in question.

Constructive design of suction headers

• Absolutely symmetrical structure (s. fig. below) - both in the header design and at the inlet of the suction gas line and the pipe sections connected to the compressors
• Suction gas inlet:
• 2 compressors: centrally
• 3 and 4 compressors: via symmetrical "Y-joint"

A side entry is very disadvantageous (even at low gas velocity) due to uneven flow and oil distribution in the accumulator. If there are compelling reasons for the side connection, the speed-controlled compressor should be connected there.

• Pipe sections connected to the compressors:
• Exit upwards, 30° bevelled
• Intake openings in parallel to the header axis
• Pipe layout symmetrical to the compressors and with identical diameter

• Dimension of the header tube for 2 compressors:
• Central inlet with a diameter in accordance with its overall capacity
• Pipe sections connected to the compressors in the same way as the brazed connection of suction gas shut-off valves
• Dimension of the header pipe for 3 and 4 compressors:
• Suction gas velocity in case of transverse flow in the header < 1.5 m/s
• Mount header exactly horizontally

Constructive design of pressure headers

• Transverse connection with outlet on one side, avoid 90° bends and use e.g. 45° bends (Design of the pressure header)
• Arranged below the level of the discharge gas shut-off valves
• Pipe cross-section (same diameter throughout) ideally as large as the total cross-section of the individual pipes. Alternatively: Dimensioning based on the max. flow velocity - for high-pressure refrigerants such as R744, a max. flow velocity of 10 m/s should not be exceeded.
• Connecting lines from the shut-off valve to the header with downward slope, in order to avoid accumulation of oil and / or refrigerant condensate on the high pressure side of shut-off compressors. Diameter in accordance with the customary design criteria.

Check valves

Check valves in the line sections to the header are required under the following conditions (Design of the pressure header):

• If using an individual oil separator for each compressor – for protection against back condensation of refrigerant in the separator with the compressor shut-off. Arranged in each case downstream of the oil separator.
• If the cylinder heads of the compressors can cool down below the condensing temperature at standstill (Parallel compounding with optimized suction header).
• Check valve for a common oil separator (installation position towards the condenser):
• If there is a danger of back condensation from the condenser or liquid receiver and
• Systems with long shut-off periods

Check valves must be suitable for use in discharge gas lines – planning and dimensioning in accordance with manufacturer’s specifications. As an alternative to installing a check valve, heating the oil separator during standstill is possible.

The design of check valves for R744 systems with FI operation is very challenging and not recommended, if necessary consult Bitzer.

Oil separator and oil return

At a sufficient flow velocity, oil separators are not absolutely required for parallel compounding. However, they are generally recommended for systems with large variations in load and for low temperature applications in which the oil circulation in the system can widely vary. Depending on requirements, individual separators (per compressor) or a common separator can be used.

For the design of the separators, the entire capacity range, including operating times at higher evaporation temperatures (pull down conditions) among other things, must be taken into account. Particular care during design is required for systems equipped with a common separator.

• Oil return from central oil separator: directly into the suction gas line coming from the system – at least 300 mm upstream of the header or "Y-joint".
• Oil return from individual separators: directly into the oil return connection at the crankcase or into the suction gas line of the compressor in question