Question 3.11: A VAV system operates as shown in Fig. 3-23. The solid lines......

The solution is carried out using Chart 1a, as shown in Fig. 3-23. Because the outdoor air is off during the minimum-load condition, the space condition and coil process lines will coincide as shown by line 3–2′–d. This line is constructed by using the protractor of Chart 1a with a SHF of 0.9. The apparatus dew point is seen to be 55 F, as compared with 50 F for the design condition. The air-flow rate for the design condition is given by

\dot{m}_{2}  =  \dot{q} (i_{3}  –  i_{2})

\dot{m}_{2}  =  \frac{100(12,000)}{29.4  –  23.2}  =  193,550  lbma/hr

or

\dot{Q}_{2}  =  \dot{m}_{2}  v_{2}/60  =  193,550(13.25)/60  =  42,700  cfm

Then the minimum volume flow rate is

\dot{Q}_{m}  =  0.2(42,700)  =  8500  cfm

and the minimum mass flow rate may be estimated by assuming a value for v_{2^{′}} :

\dot{m}_{m}  =  8500(60) /13.28  =  38,400  lbma/hr

State point 2′ may then be determined by computing i_{2^{′}} :

i_{2^{′}}  =  i_{3}  –  \frac{\dot{q}_{m}}{\dot{m}_{m}}  =  29.4  –  15(12,000)/38,400  =  24.7  Btu/lbma

Then, from Chart 1a, the air condition leaving the coil is 60.5 F db and 57.5 F wb. Calculation of the coil water temperature is beyond the scope of this analysis; however, the mean water temperature would be increased by about 7 degrees from the design to the minimum load condition due to decreased flow rate. The use of outdoor air during part load is discussed below.