What are the Differences between Evaporator in Vacuum Drying and Evaporator in Ordinary Drying?

What are the Differences between Evaporator in Vacuum Drying and Evaporator in Ordinary Drying?

Vacuum drying and ordinary drying (such as atmospheric hot air drying, convection drying, etc.) is the core difference between the drying environment pressure (vacuum drying for low-pressure environment, ordinary drying is mostly atmospheric pressure or micro-positive pressure), this difference directly leads to the two evaporator used in the design objectives, working conditions, structural characteristics, etc. There are significant differences.

The core function of the evaporator is to deal with the drying process steam (water vapor or solvent vapor), and the characteristics of the steam (temperature, pressure, density, etc.) by the drying environment, this is the most fundamental difference between the two.

The design of evaporator needs to be centered on the core demand of drying system, both of them focus on different design objectives due to the difference of application scenarios:

Evaporator in vacuum drying:

The core objective is to condense low-temperature steam efficiently under low pressure, while strictly maintaining the system vacuum (to prevent air from penetrating to destroy the low-pressure environment), and to protect the heat-sensitive materials (to avoid secondary contamination of the steam or high-temperature degradation).

For example, in pharmaceutical vacuum drying, the evaporator needs to ensure that there is no residual contamination in the condensation process and that high temperatures are not introduced to cause material denaturation.

Evaporator in general drying:

The core objective is to deal with the heat recovery of high-temperature steam or standard discharge, with lower requirements for system sealing (because there is no need for strict pressure preservation at atmospheric pressure), and more emphasis on temperature resistance and anti-scaling ability (high-temperature steam is prone to carry impurities that lead to scaling).

For example, in the hot air drying of grain, the evaporator (often a waste heat recovery heat exchanger) needs to recover the heat in the high-temperature exhaust gas, which is used to preheat the fresh air and reduce energy consumption.

The evaporator needs to work with the other equipment of the drying system, both due to different environmental pressures, auxiliary equipment configuration differences are obvious:

Vacuum drying evaporator:

must be linked with the vacuum pump (e.g., Roots pump, water ring pump), the evaporator needs to be condensed first more than 90% of the steam (to avoid the steam to enter the vacuum pump leading to a decline in pump efficiency or damage), not condensed trace non-condensable gases (e.g., air) by the vacuum pump pumping; part of the system also needs to be equipped with vacuum traps; part of the system is also required to configure a vacuum trap. Uncondensed trace non-condensable gases (e.g. air) are extracted by the vacuum pump; some systems also need to be equipped with vacuum traps (to prevent the introduction of air when condensate is discharged).

Ordinary drying evaporator:

No vacuum pump is needed, and the steam can be pumped out by natural discharge or induced draft fan; if heat is recovered, it is often linked with fan and heat exchanger (e.g. condensation heat is used to preheat the drying medium), and no complicated vacuum control equipment is needed.