Question

For atmospheric air, given: dry bulb temperature, 85℉; wet bulb temperature, 70℉; barometer, standard. The air cooled at constant pressure to 50℉. Using psychometric chart, calculate the heat rejected.
A. 12.5 Btu/lbda
B. 13.7 Btu/lbda
C. 10.5 Btu/lbda
D. 9.5 Btu/lbda
(could you include the usage of psychometric chart?)

Answer 1

1) Find point-1 in the chart at 85 F DBT and 70 F WBT.

2) At constant pressure, draw a horizontal line towards left, representing decrease in temeprature.

3) After reaching the saturation curve, follow the curve upto 50 F DBT. This curve means that the water will begin to condense before reaching 50 DBT.

4) Find point-2 at 50 F.

5) Calculate enthalpy at both points.

ZOOMED IMAGE:

NOTE: Blue line represent the process. Red lines are for reference.

Now, using the psychrometric chart, we can find the enthalpy at point-1 as;

Also, at point-2

Now, amount of heat rejected will be;

$\Rightarrow q =13.9\;\;Btu/lb\;\;of\;\;dry\;\;air\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;...(Answer)$

Hence, answer (approximate) is;

(B) 13.7 Btu/lbda