Two moist air streams with 85 % relative humidity, both...

Two moist air streams with $85 \%$ relative humidity, both flowing at a rate of $0.2 \mathrm{lbm} / \mathrm{s}$ of dry air are mixed in a steady flow setup. One inlet fiowstream is at $90 \mathrm{F}$, and the other at 61 F. Find the exit relative humidity.

Key Concepts

Dry Air Basis

The dry air basis refers to performing calculations per unit mass of dry air. This approach is used in psychrometrics to simplify the analysis, as the mass of dry air remains constant while only the water vapor content changes. This method is particularly useful when analyzing processes such as mixing, where the relative amounts of dry air and water vapor are key factors.

Psychrometrics

Psychrometrics is the study of the thermodynamic properties of moist air, including the relationships between air temperature, moisture content, and humidity. It provides the framework for analyzing how air properties change under processes like heating, cooling, and mixing, which is essential for HVAC and environmental control applications.

Relative Humidity

Relative humidity is defined as the ratio of the partial pressure of water vapor present in the air to the saturation pressure of water vapor at a given temperature, expressed as a percentage. It is a key parameter in determining how close the air is to being saturated, affecting both comfort levels and the potential for condensation.

Mixing of Air Streams

Mixing of air streams involves combining air masses that have different temperatures and moisture contents. The process requires the conservation of mass and energy, particularly of dry air and water vapor, leading to a new state where properties such as temperature, humidity ratio, and relative humidity are determined based on the contributions of each stream.

Mass Balance on Moisture

A mass balance on moisture involves ensuring that the total water vapor content remains consistent before and after the mixing of air streams. By considering the humidity ratios and flow rates (typically presented on a dry air basis), one can calculate the resultant moisture content in the mixed air.

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