Industrial treatments of surfaces can take advantage of process switching between high pressure and low pressure (or vacuum), both at high temperatures. For the optimization of the industrial throughput, as well as for surface quality and thermal efficiency, it may be practical to avoid a delay in cooling the gas before pumping. Now, safety requires precaution in order to avoid ignition of oil-sealed mechanical pumps when very hot gas must be pumped close to atmospheric pressure. This article searches for a mechanism to achieve fast cooling before gases enter the pumping system. The author suggests that heat can be trapped momentarily during pumping at high pressure, and that the stored heat can be dissipated over a longer period of time after pumping. This concept is simple and practical; it provides a low cost solution for many cases. It is relevant when gases are pumped transiently. Reliability of this solution depends on scaling the heat trap to the maximum temperature and volume of gases, as well as to the pumping speed to be achieved. Design considerations are of the utmost importance. In particular, mass and exchange area of the heat sink are essential parameters. This article proposes simple mathematical tools for safe dimensioning. Model exploitation is illustrated on an example considering pumping a 2m3 chamber filled to atmospheric pressure by a gas at 500 or 800°C. The author assumes the vessel is pumped in 210min by a “roots” type pump connected in series with an oil-sealed mechanical pump (60350m3h). Model predictions indicate that a spiral rolled stainless steel sheet is more efficient than a collection of tubes as a heat sink although a trap made of tubes provides a cheaper apparatus.

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