Vacuum evaporators and utility selection: Low-pressure steam (saturation temperature 105–120°C) is commonly used with vacuum levels of about how many millimetres of mercury (referenced to a 760 mm Hg barometer)?

Introduction / Context:
Evaporators often operate under vacuum to reduce boiling temperature, protect heat-sensitive products, and improve energy efficiency. Selecting a suitable vacuum level aligns product properties, available steam temperature, and overall heat transfer coefficients.

Given Data / Assumptions:

• Low-pressure steam with saturation temperature about 105–120°C is available.
• Vacuum level is referenced to 760 mm Hg (i.e., barometric).
• Goal is to obtain boiling at substantially reduced temperatures.

Concept / Approach:
A vacuum noted as 650 mm Hg on a 760 mm Hg barometer indicates the absolute pressure is near 110 mm Hg. At such low absolute pressures, water and many aqueous solutions boil at significantly lower temperatures, enabling use of low-pressure steam with adequate temperature driving force across the heating surface.

Step-by-Step Solution:

Verification / Alternative check:
Typical single- or multi-effect vacuum evaporators report vacuums of 600–700 mm Hg for aqueous solutions when using low-pressure steam utilities.

Why Other Options Are Wrong:

• 250 or 450 mm Hg: higher absolute pressure, less temperature drop, less typical for low-pressure steam utilities.
• 750 mm Hg: extremely deep vacuum near barometric limit, uncommon for routine industrial operation.

Common Pitfalls:
Confusing absolute pressure with “vacuum” reading; failing to account for boiling point elevation in non-ideal solutions; ignoring non-condensable gas purging needs.

Final Answer:
650 mm Hg