Question 3.8: Conditioned air is supplied to a space at 54 F db and 90 per......
Conditioned air is supplied to a space at 54 F db and 90 percent RH at the rate of 1500 cfm. The sensible heat factor for the space is 0.80, and the space is to be maintained at 75 F db. Determine the sensible and latent cooling loads for the space.
Chart 1a can be used to solve this problem conveniently. A line is drawn on the protractor through a value of 0.8 on the SHF scale. A parallel line is then drawn from the initial state, 54 F db and 90 percent RH, to the intersection of the 75 F db line, which defines the final state. Figure 3-11 illustrates the procedure. The total heat transfer rate for the process is given by
\dot{q} = \dot{m}_{a} (i_{2} – i_{1})
and the sensible heat transfer rate is given by
\dot{q}_{s} = (SHF ) \dot{q}
and the mass flow rate of dry air is given by
\dot{m}_{a} = \frac{\dot{Q}}{v_{1}} = \frac{1500 (60)}{13.11} = 6865 lbma /hr
where v_{1} = 13.11 ft^{3}/lbma is read from Chart 1a. Also from Chart 1a, i_{1} = 21.6 Btu/lbm dry air and i_{2} = 27.8 Btu/lbm dry air. Then
\dot{q} = 6865(27.8 – 21.6) = 42,600 Btu/hr
\dot{q}_{s} = \dot{q} (SHF ) = 42,600 (0.8) = 34,100 Btu/hr
and
\dot{q}_{l} = \dot{q} – \dot{q}_{s} = 8500 Btu/hr
The process 1–2 with its extension to the left is called the condition line for the space. Assuming that state 2, the space condition, is fixed, air supplied at any state on the condition line will satisfy the load requirements. However, as that state is changed, different quantities of air must be supplied to the space. The closer point 1 is to point 2, the more air is required; the converse is also true.
