The system mixes hot and cold air to satisfy zone thermostat. Cold and hot air streams distributed in separate ducts. This is variation of CAV/RH system.
• High duct cost
• Large plenum space required
• Unlimited number of zones
+Q +P +Q
Design a dual-duct system for the classroom at PSU (Use the data from previous example). Assume cooling coil could reach a relative humidity of 90%.
Summer cooling processes:
O + R => M Hot duct: M => H
Cold duct: M=> C C + H => I
O: io =68.15 kJ/kg, Wc =14.4 g/kg
R: iR =50.72 kJ/kg, WR =10 g/kg
I: ii = 37.91 kJ/kg, Wi = 9 g/kg
M: iM = 54.95 kJ/kg
Total cooling load = 5000 W ma =0.396 kg/s Fresh air: 80 L/s
Find: Fan, cooling and heating coil capacities.
Total air supplied:
Ma = 0.396 kg/s
Fan capacity: O Fresh outdoor air:
Mo = 80 L/s = 0.080 m3/s x 1.2 kg/s = 0.096 kg/s Wi = 9 g/kga, WR = 10g/kga, Wo = 14.4 g/kga
Wh = WM = (mRWR+ mo W0)/ ma=[(ma — mo) WR + m0 W0 ]/ ma = [(0.396 — 0.096) x 10 + 0.096 x 14.4)]/0.396 = 11 g/kga
From the analysis in the psychrometric chart, no heating in the hot duct is needed. Then,
1h = 1m = 54.95 kJ/kg
Heating coil capacity: O
From psychrometric chart: ic = 36.5 kJ/kg
NtH + me = ma ntH + me = 0.396
NtH iH + me ic = ma ii mn 54.95 + me 36.5 =0.396 x 37.91
NtH = 0.03 kg/s
Me = 0.366 kg/s
Cooling coil capacity: O
Tc = 13.5 °C is lower than that in the previous example.
The design should be continued for winter condition as well. Then the equipment capacities can be determined.