JT Units
JT Unit Skids | Joule-Thompson Effect | Operational Temperatures

Joule-Thompson Effect

"JT" is an abbreviation for Joule-Thompson effect. James Joule and William Thompson in 1854 proved that cooling occurs when a non-ideal gas expands from high pressure to low pressure. This cooling effect can be amplified by using the cooled gas to pre-cool the inlet gas in a gas heat exchanger. The efficiency of a JT unit is directly related to the efficiency of the gas heat exchange involved in the process.

DPC Model A JT Unit

DPC Model A JT Unit
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The JT unit in the picture above consists of a gas to gas exchanger (long item), a JT valve (control or motor valve) and a two phase separator. The additional instrumentation is associated with a hot gas bypass, the pneumatic methanol pump and the methanol distribution system.

The process of expanding gas to produce cooling is not considered to be an energy efficient cooling process but can be very cost effective when "free" pressure drop or excess pressure is available. "Free" pressure drop is associated with high pressure gas reservoirs or pressure let down stations, when the pressure drop associated with the cooling effect must be taken (such as taking a pressure drop across a choke) regardless of whether a JT unit or another process is utilized.

JT units become expensive to operate when the pressure reduction is no longer "free" and must be provided by mechanical compression. A JT unit can require anywhere from 100 psi to 800 psi differential pressure to operate. A well designed unit will minimize this pressure differential through increased use of heat exchangers and operate in the 100 to 300 psi range. The cost savings associated with the reduction of compression horsepower and compression fuel will dwarf the incremental costs to upgrade a JT unit with the extra heat exchange.

JT units have a limited application. The cooling generated from the expanded gas is limited and will only condition a gas stream that is fairly low in heavier hydrocarbon components. 


DPC Model C3-JT
DPC Model A JT Unit
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The C3-JT unit in the picture above uses three DPC "C" heat exchangers mounted on two skids to reduce the pressure differential across the unit. In this case, at 50 MMCFD, the savings of 200 psi in pressure differential can reduce the compression requirements by 1350 BHP or approximately $70,000/month in compression fuel ($6.50/MMBTU) and rental.