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.

Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning