Questions: Psychrometric Analysis and Humid Air Properties

Question 1

A room contains air at 20°C with a relative humidity of 60%. A furnace heats the air to 30°C without adding any moisture. What happens to the relative humidity and the humidity ratio?

• A. Relative humidity increases; humidity ratio increases — warming air draws moisture from walls
• B. Relative humidity decreases; humidity ratio stays constant — warmer air can hold more vapor, but actual vapor content is unchanged
• C. Relative humidity stays constant; humidity ratio increases — heating proportionally raises both
• D. Both decrease — high temperature drives moisture out of the air stream

Answer: (B)

Humidity ratio ω (mass of water vapor per kg of dry air) is conserved when no moisture is added or removed — sensible heating only changes temperature. However, relative humidity φ = p_w / p_sat(T): since saturation pressure p_sat increases strongly with temperature (roughly doubles every ~10°C), the same partial pressure of water vapor now represents a smaller fraction of the new saturation pressure. So φ falls. On the psychrometric chart, sensible heating moves horizontally rightward — temperature increases while the horizontal humidity-ratio axis is unchanged. This is the fundamental reason why indoor winter air feels so dry: outdoor cold air with moderate RH is brought inside and heated, drastically dropping its relative humidity.

Question 2

An air conditioning unit cools warm humid air. The air first cools sensibly, then the coil surface temperature drops below the air's dew point. On the psychrometric chart, which sequence of paths describes this process?

• A. First moves horizontally leftward (T decreases, ω constant), then curves down along the saturation curve as condensation begins
• B. Moves vertically downward throughout — temperature and humidity ratio both drop proportionally during cooling
• C. Moves diagonally toward the origin — both temperature and relative humidity decrease simultaneously
• D. Moves horizontally leftward throughout — temperature decreases but humidity ratio is always conserved during cooling

Answer: (A)

Cooling below the dew point happens in two stages. Before the dew point is reached, cooling is sensible: temperature falls but no condensation occurs, so humidity ratio is constant — the process moves horizontally leftward on the chart. Once temperature hits the dew point (where φ = 100%), the air is saturated. Further cooling forces condensation: liquid water drips off the cooling coil, removing moisture from the air stream. The air state now follows the saturation curve (φ = 100%) downward as both temperature and humidity ratio decrease together. This two-stage process explains why air conditioners both cool AND dehumidify — the dehumidification only begins after the dew point is crossed.

Question 3

High relative humidity usually indicates that the air contains a large mass of water vapor per kilogram of dry air.

• T. True
• F. False

Answer: False

False. Relative humidity tells you how close the air is to saturation at its current temperature, not the absolute amount of moisture. Cold saturated air (100% RH at 0°C) contains only about 3.8 g of water vapor per kg of dry air. Warm air at 30°C with just 50% RH contains about 13.4 g/kg — more than three times as much moisture as the cold saturated air. This distinction matters enormously in HVAC and meteorology: cold winter air is often nearly saturated (high RH) yet carries very little moisture mass; tropical air at moderate RH carries enormous amounts. Humidity ratio, not relative humidity, is the quantity that tracks actual moisture content.

Question 4

When the dry-bulb temperature equals the wet-bulb temperature of an air sample, the relative humidity of that air is 100%.

• T. True
• F. False

Answer: True

True. The wet-bulb thermometer reading is suppressed below the dry-bulb temperature by evaporative cooling — the faster water evaporates from the wet wick, the greater the cooling effect. Evaporation rate depends on how far the ambient air is from saturation: dry air evaporates quickly (large wet-bulb depression); air already near saturation evaporates slowly. When the air is fully saturated (φ = 100%), no net evaporation from the wick can occur — the air cannot accept any more moisture — so the wick temperature equals the ambient dry-bulb temperature. The wet-bulb depression (dry-bulb minus wet-bulb) is therefore zero if and only if the air is saturated.

Question 5

Explain the difference between relative humidity and humidity ratio, and describe a situation where one is high while the other is low.

Model answer: Humidity ratio (ω) is the mass of water vapor per kilogram of dry air — it measures actual moisture content. Relative humidity (φ) is the ratio of actual water vapor pressure to saturation vapor pressure at the current temperature — it measures how close the air is to its moisture-holding limit. A clear example where RH is high but ω is low: cold outdoor air at −5°C at 90% RH contains only ~2.5 g/kg of water vapor. A clear example where ω is high but RH is low: tropical air at 35°C with 40% RH contains about 14 g/kg — far more moisture mass, yet well below saturation. Heating cold saturated air indoors takes it from high-RH/low-ω to low-RH/low-ω (unchanged ω, decreased RH).

This distinction is fundamental to HVAC design and human comfort. Comfort is related more to ω than to φ — what matters for evaporative cooling of the human body is the actual vapor pressure gradient between skin and air, which depends on absolute moisture content. But mold growth and condensation risks depend on φ relative to surface temperatures. An HVAC engineer must track both: the psychrometric chart encodes them simultaneously, which is why it is the central design tool for air conditioning and ventilation analysis.