Question 27

The Correct Answer is B ### Why Option B (27%) is Correct The process described is sensible heating, which means heat is added to the air without adding or removing moisture. On a psychrometric chart, this process moves horizontally from left to right, maintaining a constant humidity ratio ($W$). 1. **Locate the Initial State (State 1):** * Dry Bulb Temperature ($T_{DB1}$) = $60^\circ\text{F}$ * Relative Humidity ($\text{RH}_1$) = $52\%$ * Find the intersection of the $60^\circ\text{F}$ vertical line and the $52\%$ relative humidity curve. 2. **Determine the Humidity Ratio ($W$):** * From State 1, read the horizontal line to the right axis (humidity ratio scale). * Reading the chart (Illustration RA-0022, which is assumed to be a standard psychrometric chart), the humidity ratio $W$ is approximately $0.006$ lb water/lb dry air (or grains/lb dry air, depending on the chart units, but the value itself is crucial). Since sensible heating occurs, this humidity ratio remains constant ($W_1 = W_2$). 3. **Locate the Final State (State 2):** * Dry Bulb Temperature ($T_{DB2}$) = $80^\circ\text{F}$ * Humidity Ratio ($W_2$) = $0.006$ lb water/lb dry air (constant from State 1). * Move horizontally from State 1 (at the $W=0.006$ level) until you intersect the vertical line corresponding to $T_{DB2} = 80^\circ\text{F}$. 4. **Determine the Final Relative Humidity ($\text{RH}_2$):** * State 2 lies on the intersection of the $80^\circ\text{F}$ dry bulb line and the constant humidity ratio line established in step 2. * Reading the relative humidity curve that passes closest to State 2, the value is approximately **$27\%$**. ### Why the Other Options are Incorrect * **A) 19%:** This value would correspond to a final state with a lower humidity ratio or a significantly higher off-coil temperature (e.g., $90^\circ\text{F}$), which does not match the conditions ($T_{DB2} = 80^\circ\text{F}$, $W = 0.006$). * **C) 55%:** Sensible heating (adding heat without moisture) always **decreases** the relative humidity, as the saturation capacity of the air increases significantly. A value of $55\%$ is higher than the initial relative humidity ($52\%$) and is therefore physically impossible for this process. * **D) 70%:** Like option C, this value represents an increase in relative humidity, which would only occur during a process like cooling (sensible cooling or cooling and dehumidification) or humidification, not sensible heating.