How to Determine Temperature in an Evaporator Operated at Vacuum

What would be the temperature of the product inside the evaporator operated at 15 in. Hg vacuum?

a) The temperature cannot be determined.
b) The temperature is 100°C.
c) The temperature is below 100°C.
d) The temperature is above 100°C.

Answer:

The temperature of the product inside an evaporator operated at a 15 in. Hg vacuum is below 100°C because the boiling point of water decreases with reduced pressure.

When an evaporator is operated at a 15 in. Hg vacuum, it means the pressure inside is reduced from the standard atmospheric pressure. At sea level, standard atmospheric pressure is 29.92 in. Hg. If an evaporator is at 15 in. Hg vacuum, the pressure inside the evaporator is 29.92 - 15 = 14.92 in. Hg.

The boiling point of water decreases as the pressure decreases; hence, under a vacuum, water boils at temperatures lower than its normal boiling point of 100°C at 1 atm.

The exact temperature of the product inside the evaporator cannot be determined without additional data, such as the specific vapor pressure of water at that reduced pressure. However, based on the principle that boiling point decreases with reduced pressure, we can infer that the temperature inside the evaporator is below 100°C. Therefore, the correct answer to the question would be (c) the temperature is below 100°C.