Moist Air, Psychrometrics and Air Treatment
Comprehensive university textbook unit covering fundamental principles, governing relationships, and engineering applications of Moist Air, Psychrometrics and Air Treatment.
§4.1 Humidity ratio and vapor partial pressure
**Outcome:** Interpret humidity ratio and vapor partial pressure, perform a traceable estimate, diagnose a limitation, and explain how evidence could change the associated engineering decision.
§4.2 Relative humidity and saturation
**Outcome:** Interpret relative humidity and saturation, perform a traceable estimate, diagnose a limitation, and explain how evidence could change the associated engineering decision.
§4.3 Moist-air enthalpy per dry-air mass
**Outcome:** Interpret moist-air enthalpy per dry-air mass, perform a traceable estimate, diagnose a limitation, and explain how evidence could change the associated engineering decision.
§4.4 Air mixing and outdoor-air fraction
**Outcome:** Interpret air mixing and outdoor-air fraction, perform a traceable estimate, diagnose a limitation, and explain how evidence could change the associated engineering decision.
§4.5 Cooling-coil dehumidification
**Outcome:** Interpret cooling-coil dehumidification, perform a traceable estimate, diagnose a limitation, and explain how evidence could change the associated engineering decision.
Step-by-Step Refrigeration and Air Conditioning Solutions
Three original questions connect calculations, definitions and interpretation. Open each solution to follow the reasoning.
Problem Statement: An engineering practitioner analyzes a mechanical system governed by **Humidity ratio and vapor partial pressure** under steady operational loading.
Worked Engineering Derivation:
**Assumed inputs and computed answer:** With total pressure 101.3 kPa and water-vapor partial pressure 2.0 kPa, humidity ratio is approximately 0.0125 kg water/kg dry air. Reproduce the result using the equation rather than copying the displayed answer. Convert all values to compatible units before substitution. Distinguish kW from kWh, kPa from Pa, and a percentage from a dimensionless fraction. Then perform three small checks: does the output have the expected dimensions, is its sign physically meaningful for the chosen reference direction, and does it approach a sensible limiting value when the driving difference or mass flow tends toward zero? If a limiting case gives a contradiction, examine the equation and model boundary before trusting the decimals.
Problem Statement: An engineering practitioner analyzes a mechanical system governed by **Relative humidity and saturation** under steady operational loading.
Worked Engineering Derivation:
**Assumed inputs and computed answer:** At 25 °C and saturation vapor pressure about 3.17 kPa, partial pressure of 1.585 kPa corresponds to roughly 50% relative humidity. Reproduce the result using the equation rather than copying the displayed answer. Convert all values to compatible units before substitution. Distinguish kW from kWh, kPa from Pa, and a percentage from a dimensionless fraction. Then perform three small checks: does the output have the expected dimensions, is its sign physically meaningful for the chosen reference direction, and does it approach a sensible limiting value when the driving difference or mass flow tends toward zero? If a limiting case gives a contradiction, examine the equation and model boundary before trusting the decimals.
Problem Statement: An engineering practitioner analyzes a mechanical system governed by **Moist-air enthalpy per dry-air mass** under steady operational loading.
Worked Engineering Derivation:
**Assumed inputs and computed answer:** For T=25 °C and w=0.010 kg/kg dry air, the approximation gives 50.58 kJ/kg dry air. Reproduce the result using the equation rather than copying the displayed answer. Convert all values to compatible units before substitution. Distinguish kW from kWh, kPa from Pa, and a percentage from a dimensionless fraction. Then perform three small checks: does the output have the expected dimensions, is its sign physically meaningful for the chosen reference direction, and does it approach a sensible limiting value when the driving difference or mass flow tends toward zero? If a limiting case gives a contradiction, examine the equation and model boundary before trusting the decimals.
Problem Statement: An engineering practitioner analyzes a mechanical system governed by **Air mixing and outdoor-air fraction** under steady operational loading.
Worked Engineering Derivation:
**Assumed inputs and computed answer:** For x=0.30, outside enthalpy 70 and return enthalpy 50 kJ/kg dry air, mixed enthalpy is 56 kJ/kg. Reproduce the result using the equation rather than copying the displayed answer. Convert all values to compatible units before substitution. Distinguish kW from kWh, kPa from Pa, and a percentage from a dimensionless fraction. Then perform three small checks: does the output have the expected dimensions, is its sign physically meaningful for the chosen reference direction, and does it approach a sensible limiting value when the driving difference or mass flow tends toward zero? If a limiting case gives a contradiction, examine the equation and model boundary before trusting the decimals.
Problem Statement: An engineering practitioner analyzes a mechanical system governed by **Cooling-coil dehumidification** under steady operational loading.
Worked Engineering Derivation:
**Assumed inputs and computed answer:** With 0.9 kg/s dry air, w_in=0.014 and w_out=0.009 kg/kg, condensate rate is 0.0045 kg/s, about 16.2 kg/h. Reproduce the result using the equation rather than copying the displayed answer. Convert all values to compatible units before substitution. Distinguish kW from kWh, kPa from Pa, and a percentage from a dimensionless fraction. Then perform three small checks: does the output have the expected dimensions, is its sign physically meaningful for the chosen reference direction, and does it approach a sensible limiting value when the driving difference or mass flow tends toward zero? If a limiting case gives a contradiction, examine the equation and model boundary before trusting the decimals.