Technical Aspects of LNG-CompressionThermodynamic design

© NEA GROUP2

Thermodynamic design basics for

reciprocating compressors

Specialities of low suction temperature

Technical Aspects of LNG-CompressionThermodynamic design

3

Power and flow determination

Work per cycle

Volumetric flow per cycle

𝑃 = 𝑊𝑉 ∙ 𝑛𝑠𝑝𝑒𝑒𝑑

𝑉1𝑒 = 𝜆𝑒 ∙ 𝑉ℎ

ሶ𝑉1𝑒 = 𝑉1𝑒 ∙ 𝑛𝑠𝑝𝑒𝑒𝑑

© NEA GROUP

−𝑝 ∙ 𝑑𝑉

−ර𝑝 ∙ 𝑑𝑉

Technical Aspects of LNG-CompressionThermodynamic design

4

Flow determination

Degree of back expansion:

• Relative clearance:

• Polytropic change:

»

𝜆𝑅 =𝑉′

𝑉ℎ=𝑉ℎ − 𝑉𝑅𝑉ℎ

= 1 −𝑉𝑅𝑉ℎ

= 1 −𝑉𝑅𝑉0

∙𝑉0𝑉ℎ

𝑉0 + 𝑉𝑅𝑉0

= 1 +𝑉𝑅𝑉0

=𝑝2′

𝑝1

1𝑛𝑝𝑜𝑙

© NEA GROUP

𝜆𝑅 = 1 − 𝜇 ∙𝑝2′

𝑝1

1𝑛𝑝𝑜𝑙

− 1

𝜇 =𝑉0𝑉ℎ

𝑉𝑅𝑉0

=𝑝2′

𝑝1

1𝑛𝑝𝑜𝑙

− 1

Technical Aspects of LNG-CompressionThermodynamic design

5

Flow determination

Degree of back expansion:

© NEA GROUP

𝜆𝑅 = 1 − 𝜇 ∙𝑝2′

𝑝1

1𝑛𝑝𝑜𝑙

− 1

lR

VS = V0

Technical Aspects of LNG-CompressionThermodynamic design

6

Flow determination

Degree of heating up

© NEA GROUP

𝜏𝑇=20°𝐶 = 𝑓𝑝2′

𝑝1, 𝜅

𝜏 = 𝑓 𝜏𝑇=20°𝐶 , 𝑇𝑠

Technical Aspects of LNG-CompressionThermodynamic design

7

Flow determination

Degree of leakage

• An in-house leakage flow calculation tool based on conservation of energy and mass was developed to calculate the transient leakage flow of pistons/packings

© NEA GROUP

𝛿 = 𝑓 𝑝2′ , 𝑇2

′,𝑝2′

𝑝1, 𝜅, 𝑅, 𝑁, 𝐴

Technical Aspects of LNG-CompressionExample of a horizontal LNG BOG compressor

8

• Two stages

• Suction valve unloading control +

• Clearance pocket control

© NEA GROUP

Technical Aspects of LNG-Compression

© NEA GROUP9

Example of a horizontal BOG LNG compressor

Technical Aspects of LNG-CompressionSummary

© NEA GROUP10

• Thermodynamic design basics for power and flow determination of

reciprocating compressors were discussed

• Influence of low suction temperature for the flow calculation were

presented

• A complete design process of an LNF BOG compressor by means of NEA’s

design program KO³ were described