Psychrometrics — during sensible heating of moist air, which property remains unchanged? In heating, ventilation, and air-conditioning (HVAC) analysis, “sensible heating” means raising the dry-bulb temperature without adding or removing moisture. State whether the specific humidity (also called humidity ratio) stays constant in this process.

Introduction / Context:
Sensible heating is a fundamental psychrometric process used when analyzing HVAC coils, reheat sections, and duct heat gains. The key idea is that the air’s temperature rises while its moisture content is intentionally left unchanged. Knowing which property must remain constant helps you plot the process correctly on a psychrometric chart and size coils accurately.

Given Data / Assumptions:

• The air stream is heated without deliberate moisture addition or removal.
• No condensation occurs inside the heater (air remains unsaturated during the process).
• Atmospheric (barometric) pressure is approximately constant.

Concept / Approach:
Specific humidity (humidity ratio), usually denoted w, is the mass of water vapor per unit mass of dry air. In a sensible-only process, the mass of water vapor in the air does not change, and the mass of dry air also stays the same. Therefore the ratio w remains constant. On a psychrometric chart this appears as a horizontal move to the right (higher dry-bulb temperature at fixed w) until the state approaches saturation.

Step-by-Step Solution:
Define humidity ratio: w = m_v / m_da.In sensible heating, m_v is constant (no humidification or dehumidification).m_da is constant (no loss or gain of dry air in a control volume view).Hence, w remains constant while dry-bulb temperature increases.Relative humidity typically decreases because warmer air at the same w is further from saturation.

Verification / Alternative check:
Plot a typical sensible reheat step on a psychrometric chart: the state point moves horizontally at constant w; lines of constant w are chart abscissae, confirming no change in moisture content.

Why Other Options Are Wrong:
(b) Disagree is incorrect because, by definition, sensible heating excludes moisture change. (c) and (d) impose unnecessary conditions; constancy of w does not require saturation or constant wet-bulb. (e) Altitude affects density and chart scaling but not the definition of sensible heating.

Common Pitfalls:
Confusing relative humidity with humidity ratio; RH will change significantly even when w is fixed. Also, if the air is initially near saturation, a small temperature drop could cause condensation, which would no longer be a purely sensible process.

Final Answer:
Agree