Question

A sample of moist air is at a DBT of 25°C, WBT of 18°C and DPT of 13°C and corresponding saturation pressures of water vapor are 3.0 kPa, 2.0 kPa and 1.5 kPa respectively. What will be the relative humidity of air?
1. 47%
2. 50%
3. 65%
4. None of these

Answer 1

Correct Answer - Option 2 : 50%

Concept:

Let ‘P’ be the atmospheric pressure in (kPa)

‘ϕ’ be the relative humidity, ‘Pv’ be the partial pressure of the air in (kPa), ‘Pvs’ be the saturation pressure of water in (kPa), ‘ω’ be the specific humidity of air in (kg/kg of dry air).

\(ϕ = \frac{{{P_v}}}{{{P_{vs}}}} \)

\(ω = 0.622\;\times\frac{{{P_v}}}{{\left( {P\ - \ {P_v}} \right)}} \)

Given:

Calculation:

P_v = 1.5 kPa, Pvs = 3 kPa

\(ϕ = \frac{{{P_{v}}}}{{{P_{vs}}}} \)

\(\Rightarrow ϕ = \frac{{1.5}}{{{3}}}\)

ϕ = 50%