VACUUM EFFICIENCY

The lowest pressure which can exist in a condenser is the saturation pressure of steam corresponding to the temperature of water entering the condenser. However, the actual pressure is always greater than the ideal pressure by an amount equal to the partial pressure of air present in the condenser.

Let   p_s = saturation pressure of steam corresponding to the temperature of water entering the condenser.

p_a = partial pressure of air in the condenser.

p_b = barometric pressure

p_t = p_a + p_s

= total pressure of air and steam in the condenser

Then ideal vacuum possible without air leakage = p_b − p_s

Actual vacuum existing in condenser due to air leakage

= p_b − p_t = p_b − (p_a + p_s)

The “vacuum efficiency” is defined as the ratio of actual vacuum to ideal vacuum.

The factors affecting the vacuum efficiency are as follows:

1. Air leakage increases p_a, and hence, it decreases vacuum efficiency.
2. The vacuum efficiency decreases with increase in barometric pressure, keeping p_a and p_s the same.
3. With insufficient cooling water, the pressure in the condenser increases and subsequently, reduces the vacuum efficiency of the condenser.