Dual-duct systems

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

Example:

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

Given:

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.