In HVAC chemical dehumidification (using hygroscopic absorbers or adsorbers), which of the following changes correctly describe the air-state outcomes as moisture is removed?
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ADew point temperature decreases
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BWet-bulb temperature decreases
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CDry-bulb temperature increases
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DAll of these
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EOnly dew point temperature decreases while others remain unchanged
Answer
Correct Answer: All of these
Explanation
Introduction / Context:Chemical dehumidification removes moisture from air by exposing it to hygroscopic materials (desiccants) such as silica gel, lithium chloride, or solid desiccant wheels. Unlike cooling-based dehumidification, this process is exothermic and usually warms the air while reducing its moisture content.
Given Data / Assumptions:
- Process is a chemical (desiccant) dehumidification step, not cooling coil dehumidification.
- Air loses moisture to the desiccant; heat of adsorption/absorption raises air temperature.
- Psychrometric properties of interest: dry-bulb temperature, wet-bulb temperature, and dew point temperature.
Concept / Approach:When moisture is removed at nearly constant enthalpy input from adsorption heat, the humidity ratio decreases. Dry-bulb temperature typically increases because the exothermic process adds sensible heat to the air stream. Lower moisture content shifts both dew point and wet-bulb temperatures downward, since both are functions of moisture content at a given pressure.
Step-by-Step Solution:
1) Moisture transfer: water vapor moves from air to desiccant → humidity ratio decreases.2) Exothermic release: heat of sorption warms the air → dry-bulb temperature increases.3) With less vapor, saturation tendency reduces → dew point temperature decreases.4) Wet-bulb temperature, linked to adiabatic saturation and moisture content, also decreases.Verification / Alternative check:Plot the initial and final states on a psychrometric chart: the final state shows lower humidity ratio, a lower dew point, a lower wet-bulb, and a higher dry-bulb temperature as commonly observed with desiccant wheels (before any post-cooling stage).
Why Other Options Are Wrong:
- (a), (b), (c) each capture only one aspect; chemical dehumidification causes all three listed changes.
- (e) Incorrect because wet-bulb also changes due to altered moisture content, and dry-bulb typically rises.
Common Pitfalls:Confusing chemical dehumidification with cooling coil dehumidification (cooling and dehumidifying), where dry-bulb temperature generally decreases. Also, assuming wet-bulb stays constant is a misconception from evaporative processes, not chemical drying.
Final Answer:All of these