Compensating for the Joule-Thomson effect

When choosing a direct electric heater, it is important to take many factors into consideration, including the Joule-Thomson (J-T) effect. Grimwood Heating can analyse your process conditions to accurately confirm the true J-T effect based on the gas composition.

What is the J-T effect?

Typically, the J-T effect refers to the drop in temperature that occurs when gas pressure is reduced.

The magnitude of the temperature drop depends on the starting pressure, the pressure change and the composition of the gas. The J-T coefficient, or the change in temperature per unit change in pressure, is not a constant and is highly variable for any particular gas mixture depending on the starting and ending conditions.


Under large pressure changes, the temperature of the gas or gas mixture can fall to a point where the gas or a component of the mixture starts to condense into a liquid, or in extreme cases to solidify (such as ice formation where traces of moisture exist). This can cause equipment to malfunction with catastrophic consequences.

The way to prevent this is to preheat the gas before the pressure reduction so that the temperature after expansion is above the saturation temperature of components of the gas mixture.

Gas heating options

There are three main methods of heating gas flows: heat tracing the pipeline, a water bath heater, and direct electric heating.

Heat tracing

Heat tracing involves wrapping a heating cable around a length of pipe to heat the pipe and its contents.

Heat tracing is simple to install, requires no ongoing maintenance, and can be retrofitted to an existing pipeline. Disadvantages of this method are that it can be difficult to get fine control with variable flow, and it requires a reliable power supply onsite.

Water bath heaters

A water bath heater is typically a horizontal, cylindrical, water-filled tank with a fire tube in the bottom and a gas pipe serpentine through the water to raise the gas temperature. Water bath heaters may also be heated with hot oil.

While fine control is also difficult with water bath heaters, they can operate in remote sites with no electricity supply and can use waste heat from other processes. On the other hand, they can be high maintenance, requiring repair and replacement parts. The water must be treated, and there can be issues of corrosion. All these lead to planned and unplanned downtime.

Direct electric heating

Finally, direct electric heating involves electric elements in the gas stream to heat the gas as it flows over the elements. The benefits of direct electric heating are its compact size, and therefore small footprint. While this method does require a reliable source of power onsite, fine and accurate control is possible, which maximises energy efficiency, especially under fluctuating flow rates.

This method also requires no ongoing maintenance, and therefore it has low levels of downtime. Minimal heat is lost during this process, and the method is flexible in location – the unit can be positioned at any angle, and can be indoors and immediately adjacent to the point of consumption to minimise ‘cold slug’ on start up.

Why choose a Grimwood direct electric heater?

Grimwood has the proven capability to design and build direct electric heaters to the highest and most stringent standards in the world. It was the first heater manufacturer in the world to obtain an International Electrotechnical Commission Explosive Certificate of Conformity for these products.

Grimwood can design and build the control system to heat gas to the temperature required depending on the flow rates, and inlet temperatures and pressures without unnecessary and wasteful overheating. The company can deliver the heater with a short lead time, reducing overall project lead time and working capital investment.

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