Psychrometrics — during a humidification process, which property of moist air must increase? In heating, ventilation, and air-conditioning (HVAC) practice, humidification means adding moisture to the air stream. Considering standard adiabatic or steam-injection humidifiers, identify the property that necessarily increases as humidification proceeds (other conditions may change depending on the device).

Introduction / Context:
Humidification is a core psychrometric process used in air-conditioning systems to ensure occupant comfort and protect sensitive materials. It involves introducing additional water vapor into an air stream. Understanding which air properties must increase (and which may increase or decrease) is essential for correct system design, control, and troubleshooting.

Given Data / Assumptions:

• Moist air is treated as a mixture of dry air and water vapor.
• Humidification implies adding moisture (mass of water vapor) to the air stream.
• Process may be adiabatic (e.g., evaporative pads) or involve steam injection; fan work and duct heat gains are small.

Concept / Approach:
The defining variable for moisture content is the humidity ratio (also termed specific humidity), usually denoted w = mass of water vapor per unit mass of dry air. By definition, any humidification process increases the mass of water vapor while the dry air mass remains practically unchanged, so w must rise. Other properties such as dry-bulb temperature, wet-bulb temperature, and relative humidity can move in different directions depending on whether latent heat comes from air (evaporation) or from injected steam, and on heat exchange with surroundings.

Step-by-Step Solution:
Recognize that humidification adds water vapor → m_v increases while m_da is constant.Compute humidity ratio: w = m_v / m_da. Since m_da is constant and m_v increases, w increases.For evaporative (adiabatic) humidification, dry-bulb temperature typically decreases while wet-bulb remains approximately constant; for steam injection, dry-bulb often increases.Relative humidity may increase or even decrease transiently depending on heat effects; it is not the defining must-increase variable.

Verification / Alternative check:
On a psychrometric chart, humidification traces move to the right (higher w). Evaporative cooling follows a line of nearly constant wet-bulb temperature, while steam injection trends upward and to the right. In all cases, the x-axis (humidity ratio) increases.

Why Other Options Are Wrong:
(b) Relative humidity does not necessarily increase; it depends on temperature changes. (c) Dry-bulb may either rise (steam) or fall (evaporation). (d) Wet-bulb does not always decrease; in adiabatic humidification it is approximately constant. (e) Static pressure change is a duct/ fan matter, not a defining psychrometric effect of humidification.

Common Pitfalls:
Equating relative humidity with moisture content; RH is temperature dependent. Always check humidity ratio when diagnosing humidification behavior.

Final Answer:
specific humidity (humidity ratio)