Question 19.4: Winter Peak Design Consider the CAV system analyzed under pe......

We will illustrate the use of the graphical procedure as previously.
1. Locate specified points 0 and 7 on the psychrometric chart (see Figure 19.5b),
which are specified from the problem statement.
2. Determine outdoor air mass flow rate.
The outdoor air mass flow is given by

Note that this mass flow rate is over 13% higher than the previous case and is attributed to the difference in the specific volumes of outdoor air between summer and winter conditions.
3. Locate mixed air condition point 1.
The inverse relation between mass flows and line segment lengths (Equation 13.32) is used to locate 1. The ratio of air mass flows is (4,743/17,140) = 0.277. The dry-bulb temperature and humidity ratios can be calculated from the chart or just as simply using the weighted average rule:

$\frac{\dot{m}_{a1}}{\dot{m}_{a2}} = \frac{h_{2}  –  h_{3}}{h_{3}  –  h_{1}} = \frac{W_{2}  –  W_{3}}{W_{3}  –  W_{1}}$               (13.32)
a. The mixed-air temperature: $T_{db,1} = [0.277 × 40°F + (1 − 0.277) × 72°F] = 63.15°F$
b. The humidity ratio: $W_{1} = [0.277 × 0.0021  lb_{w}/lb_{a} + (1  −  0.277) × 0.0084  lb_{w}/lb_{a}] = 0.00665  lb_{w}/lb_{a}$.
The corresponding moist-air enthalpy is read from the psychrometric chart, $h_{1} = 22.4  Btu/lb_{a}$.
4. Determine supply air condition point 5.
As previously discussed, the slope of line 5–7 is equal to the inner scale of the protractor corresponding to $SHR_{space} = 0.80$. We determine enthalpy $h_{5}$ of the specified supply air mass flow rate as follows:

$= 35.15  Btu/lb_{a}$
The intersection of the enthalpy line corresponding to $35.15  Btu/lb_{a}$ and the line 6–7 yields a point whose corresponding drybulb value is $T_{db,5} = 102°F$ (this is acceptable since it is lower than the stipulated maximum value of 105°F). The corresponding humidity ratio $W_{5} = 0.0096  lb_{w}/lb_{a}$.
5. Determine the amount of steam needed.
This is easily calculated since the entering and leaving humidity ratio of the airstreams are known:

6. Compute sensible heating needed.
We can use different combinations of preheat and reheat amounts to attain the desired effect. One option is to assume that all heating is provided by the preheater. This is then a simple heating and humidication problem as treated in Section 13.6.6. We draw a line from point 6 parallel to the outer scale of the protractor corresponding to 1146 (since enthalpy of steam $h_{steam} = 1146  Btu/lb_{w}$) that intersects the horizontal line from point 1 at point 2. This is determined to be $T_{db,2} = 100.8°F$ and $h_{2} = 31.6  Btu/lb_{a}$.
Finally, the sensible heat provided by the heating coil is found:

= 157,688  Btu/h