11. April 2017

Pipelines in refrigeration systems

Types, flow rates and pressure losses of refrigerant piping.

Pipelines in refrigeration systems

The piping network of a refrigeration system is hermetically sealed and connects all necessary components to one system. The refrigerant flowing in the pipes is in different states of aggregation depending on the pressure and temperature. The pipe material used for ammonia refrigeration plants is steel, and for all other refrigerants copper and steel.

The pipelines are classified according to their function in:

  1. suction
  2. pressure line
  3. liquid line
  4. Injection line
  5. Control and impulse line

 

Dimensioning of pipelines

 

Careful dimensioning and faultless installation is a prerequisite for the proper functioning of a refrigeration system and its economical operation. Malfunctions in a newly installed refrigeration system, in particular the failure of compressors or if the contractually agreed cooling capacity is not achieved, are often due to incorrectly dimensioned or improperly installed pipelines. The designer is responsible for the dimensioning and laying of the pipelines. Small pipe dimensions lead to a favorable material and assembly price, but also to higher speeds and thus to greater pressure losses . However, the necessary pipe size depends crucially on the refrigerant used and their volumetric cooling capacity.

The dimensioning of the pipelines is therefore always an optimization task

High pressure losses can be avoided if:

  • the speed is small
  • the pipeline is short
  • there are few bends or restrictors

 

For optimal flow velocities, it is recommended depending on the refrigerant:

  R717 R134a R290 R744 Suction gas line [m / s] 15-22  8 - 12 10 - 14  6 - 10 Liquid line [m / s] 0.3-0.6 0.3 - 0.5 0.4 - 0.6 0.3 - 0.5 Compressed gas line [m / s] 15 - 25  6 - 10  6 - 10  6 - 10

 

Suction

 

The pressure loss in the suction line leads to an equivalent temperature drop, which does not exceed 0.5K to max. Should be 1K. This temperature drop is dependent on the saturation temperature of the respective refrigerant. So z. B. a pressure drop in the suction line of an ammonia refrigeration system of 0,1bar, depending on the applied suction pressure, different Saugdrucktemperaturabsenkungen:

Pressure loss suction line bar 0.1         Pressure evaporation bar a 0.70 1.00 2.00 3.00 5.00 Temparatur evaporation ° C -40.51 -33.65 -18.91 - 9,28 4.10 Temperature drop suction steam TO -2.83 -2.07 -1.16 -0.84 -0.56 t 0 suction nozzle compressor ° C -43.33 -35.73 -20.07 -10.12 3.54

 

To compensate for this drop in temperature of the piping, the pressure at the inlet of the compressor must be set to 0.1 bar lower. Lowering the suction pressure also leads to a loss of cooling capacity. By lowering the suction pressure, each refrigerant compressor loses cooling capacity and lowers the COP of the refrigeration system!

 

Liquid line

 

For pressure drop in fluid lines, due to flow losses (dynamic pressure losses), the static pressure difference of the geodetic height differences (H in m) must be considered!

With a flow from top to bottom (downpipe), the dynamic pressure loss is reduced by the static pressure! Means: it comes practically to a pressure increase (depending on the height difference) which has a rather positive effect.

With a flow from bottom to top (riser), the pressure losses add up. The pressure is thus lower at the highest point in the riser than at the lowest point (depending on the height difference). This pressure reduction results in low subcooling of the refrigerant for pre-evaporation.

If, due to the pressure drop, the saturation pressure (point on the boiling line) is undershot, pre-evaporation of the refrigerant already occurs upstream of the throttle element. Bubbles (flash gas) would already be visible in a sight glass. To prevent this partial evaporation as a result of unavoidable pressure losses, a subcooling of the refrigerant liquid is essential.

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