Subtopic: Psychrometry – properties of air-water vapour mixture
Question Statement:
When moist atmospheric air undergoes SENSIBLE HEATING across a heat exchanger at constant barometric pressure, which of the following psychrometric properties remain CONSTANT?
Options:
(A) Humidity ratio (Specific humidity, $W$)
(B) Dew point temperature ($T_{dp}$)
(C) Relative humidity ($\phi$)
(D) Partial pressure of water vapour ($p_v$)
Answer Key:
A, B, D
Step-by-Step Solution & Explanation
1. In sensible heating, no moisture is added to or removed from the air stream.
2. Therefore, humidity ratio $W = 0.622 \frac{p_v}{P_b - p_v}$ remains strictly constant. (Statement A is True)
3. Since $W$ and total barometric pressure $P_b$ are constant, the partial pressure of water vapour $p_v$ is constant. (Statement D is True)
4. Because dew point temperature is defined strictly by $p_v$, $T_{dp}$ remains constant. (Statement B is True)
5. As temperature increases, saturation vapour pressure $p_{vs}$ increases exponentially, causing relative humidity $\phi = p_v / p_{vs}$ to DECREASE. (Statement C is False)