The vacuum drying oven is a commonly used drying device, suitable for powdered materials or products that are difficult to dry. So, where does the moisture go after drying? Here’s some insight from a manufacturer of electric-heated vacuum drying ovens.

　　First, we need to understand the drying principle of a vacuum drying oven. The operating principle is to lower the boiling point of water under negative pressure, thereby enabling even materials that are difficult to dry to dry rapidly. So where exactly does the evaporated moisture go? Today, we’ll explain this to you.

　　During the drying process, a significant portion of the moisture vapor is removed by the vacuum pump. Depending on the moisture content of the product, auxiliary equipment can be added to protect the vacuum pump, such as a vacuum drying chamber or a condenser dryer; however, if the moisture content is low, such equipment may not be necessary.

　　A significant portion of the moisture will also remain inside the vacuum chamber, condensing into water. This is why you often notice substantial condensation on the chamber door when opening it. Any moisture that accumulates inside the chamber must be thoroughly wiped away to prevent long-term corrosion of the chamber’s inner liner.

　　The vacuum drying oven, also known as an electric heating constant-temperature reduced-pressure drying oven, is a standard laboratory instrument used for drying, degassing, and removing moisture from products. It is particularly well suited for materials that are difficult to dry using conventional drying methods and can also be employed for sealing tests on certain products, such as packaging integrity testing.

01. Why is there no temperature uniformity parameter specified for electric heating vacuum drying ovens?

Typical electric (forced-air) drying ovens are specified with a temperature uniformity parameter: for natural-convection models, it is 3% of the upper limit of the operating temperature; for forced-convection models, it is 2.5%. However, electric vacuum drying ovens do not have such a temperature-uniformity specification—why is that? Inside a vacuum drying oven, the possibility of achieving uniform chamber temperature through gas-molecule motion is virtually eliminated. Therefore, in principle, the temperature-uniformity definition used for conventional electric (forced-air) drying ovens cannot be applied to vacuum drying ovens. Moreover, specifying such a parameter under vacuum conditions would be meaningless. The amount of thermal radiation is inversely proportional to the square of the distance; for the same object, the radiative heat received at a point 20 cm from the heating wall is only one-quarter of that received at a point 10 cm from the heating wall, representing a substantial difference. This phenomenon is analogous to the experience of basking in the sun during winter: the side exposed to the sun feels warm, while the shaded side feels relatively cold. Given that the design of a vacuum drying oven makes it extremely difficult to ensure uniform and consistent radiative heat distribution at every point within the three-dimensional chamber volume, coupled with the lack of an authoritative evaluation method, this is likely the reason why the standard for electric vacuum drying ovens does not include a temperature-uniformity parameter.

02. Why is there a discrepancy between the instrument readings of the electric heating vacuum drying oven and the readings of the glass-rod thermometer inside the vacuum chamber?
Typical electric vacuum drying ovens employ a heating method in which the walls of the vacuum chamber are heated first, and heat is then transferred to the workpieces by radiation from the chamber walls. Under this configuration, the temperature sensor of the temperature-control instrument can be mounted on the outer wall of the vacuum chamber, where it is exposed to convective, conductive, and radiative heat simultaneously. In contrast, a glass-rod thermometer placed inside the vacuum chamber can only sense radiative heat; moreover, since the emissivity of the glass rod can never reach unity, a significant portion of the radiative heat is reflected rather than absorbed. Consequently, the temperature reading indicated by the glass-rod thermometer will invariably be lower than the temperature displayed by the control instrument.

In general, under 200°C operating conditions, a temperature difference of no more than 30°C between the instrument’s temperature reading and that of a glass-rod thermometer is considered normal. If the temperature sensor of the temperature-control instrument is installed inside the vacuum chamber, the discrepancy between the glass-rod thermometer reading and the instrument’s display can be reduced to some extent, but it cannot be completely eliminated; moreover, this arrangement introduces an additional potential point of failure in the vacuum chamber’s sealing integrity. If, from an operational standpoint, such a discrepancy is undesirable, it can be addressed by utilizing the temperature-control instrument’s built-in display-correction function.

03. Why a vacuum electric heating drying oven is first evacuated and then heated?
Proper operating procedure for an electric vacuum drying oven: first evacuate the chamber to create a vacuum, then raise the temperature; once the set temperature is reached, if the vacuum level drops, perform another brief evacuation. This practice helps extend the service life of the equipment. The specific reasons are as follows:

1) Placing the workpiece in a vacuum chamber and evacuating it serves to remove any gas constituents that can be extracted from the material. If the workpiece is heated first, the gases will expand upon heating. Given the vacuum chamber’s excellent sealing, the resulting high pressure from the expanding gases could cause the tempered glass of the observation window to shatter—a potential safety hazard. By following the procedure of evacuating the chamber before raising the temperature, this risk can be effectively avoided.

2) If the procedure of first heating and then evacuating is followed, the heated air will be drawn away by the vacuum pump, carrying heat with it and causing the pump’s temperature to rise excessively. This can reduce the pump’s efficiency.

3) The heated gas is directed to the vacuum pressure gauge, causing a temperature rise in the gauge. If this temperature rise exceeds the gauge’s specified operating temperature range, it may result in indication errors.