Steam with high- & low pressure

In horticulture or agriculture the question is often raised whether low or high pressure steam boilers are most suitable to treat soil with hot steam respectively whether the higher investments for high pressure steam boilers are worth it.

Regarding the energy content there is only a small difference. Saturated high pressure steam at 10 at (gauge) only contains 30 kcal/kg (<5%) more energy than low pressure steam at max 0.5 at (gauge).

However high pressure steam can be better transported in pipes. This advantage becomes clear when considering expansion processes.
High pressure steam gets relieved to almost atmospheric pressure on the way from the steam boiler to the point of induction into the soil. It loses density and gains volume. This means that a minimum temperature of 150°C can be kept despite the expansion if transportation distance is not too long. Hereby superheated steam is generated which easily penetrates into the soil since steam can’t absorb more water during the process of pressure release.

Saturated low pressure steam at max 100°C also releases pressure on the way from the steam boiler into the soil. Due to its low pressure of up to 0.5 at (gauge), low pressure steam cannot further be heated up by the steam atmosphere and condensates right after exiting the boiler in the pipes, arriving in the soil as wet steam unsuitable for soil steaming.

Modern low pressure steam boilers which possess super heaters completely compensate this disadvantage. Already in the boiler steam is heated up to more than 200°C, so only dry hot steam arrives in the soil. The hotter and dryer steam the longer it can be transported without condensation.

Steam with high pressure penetrates soil better? No!

The steam can not be forced by pressure to penetrate the soil.  Steam needs time to enter the pores and to condensate.

If you would use high pressure you would pack the soil with the result, that the pores are closed and the steam can not enter soil anymore. The pressure would lift up the covering (sheet or even hood).

That is why low pressure is really the best way to steam soil and get a perfect result.