Handbook of air conditioning and refrigeration / Shan K
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Handbook of air conditioning and refrigeration / Shan K

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21.52 CHAPTER TWENTY-ONE<br />

Zone Control <strong>and</strong> Sequence <strong>of</strong> Operations <strong>of</strong> a Fan-Powered VAV System<br />

with Parallel Fan-Powered Box<br />

Consider a control zone rx1 in the perimeter zone served by a parallel fan-powered box FBX1 that<br />

mixes low-temperature cold primary <strong>air</strong> with warm plenum <strong>air</strong>, <strong>and</strong> a control zone ri1 in the interior<br />

zone in which the cold primary <strong>air</strong> is directly supplied from the VAV box, as shown in Fig. 21.13.<br />

Because the temperature <strong>of</strong> cold primary <strong>air</strong> from the AHU that serves fan-powered box FBX1 <strong>and</strong><br />

VAV box VI1 T c,p � 44°F (6.7°C), the summer indoor space relative humidity drops to about<br />

40 percent. The summer indoor design temperature is still maintained at 75°F (23.9°C). At winter<br />

design conditions, the zone temperature is maintained at 72°F (22.2°C) with a relative humidity <strong>of</strong><br />

20 to 30 percent.<br />

1. When the time-<strong>of</strong>-day clock signals the AHU to “<strong>of</strong>f,” the AHU <strong>and</strong> the fan-powered box<br />

FBX1 <strong>and</strong> VAV box VI1, as shown in Fig. 21.13a, are deenergized.<br />

2. When the clock signals the AHU in the on position, if the zone temperature sensed by the<br />

temperature sensor T11, Trxn � 75°F (23.9°C), then the AHU <strong>and</strong> fan-powered box FBX1 <strong>and</strong> VAV<br />

box VI1 call for cooling. The DDC system controller then opens the two-way valve <strong>of</strong> the water<br />

cooling coil <strong>and</strong> resets the discharge <strong>air</strong> temperature from the AHU Tdis � 44°F (6.7°C). The DDC<br />

terminal controllers also open the single-blade volume dampers in the fan-powered boxes <strong>and</strong> VAV<br />

boxes to the maximum cooling position.<br />

If the outdoor temperature To � 75°F (set point <strong>of</strong> the temperature economizer cycle), the DDC<br />

system controller also positions the outdoor damper to a minimum opening position <strong>and</strong> fully opens<br />

the recirculating damper.<br />

3. In the perimeter zone, the control zone temperature Trx1 is sensed by sensor T11. The DDC<br />

terminal controller modulates the volume damper <strong>and</strong> thus the volume flow <strong>of</strong> cold primary <strong>air</strong> supply<br />

in the fan-powered box FBX1 to maintain a preset zone temperature <strong>of</strong> Trx1 � 75°F (23.9°C)<br />

with a PI control mode. The fan in the fan-powered box FBX1 is not energized.<br />

The control zone temperature in the interior zone Trin is sensed by sensor T21. Another DDC<br />

terminal controller modulates the volume damper in the VAV box VI1 to maintain a preset zone<br />

temperature Tri1 � 75°F (23.9°C).<br />

4. When 52°F � To � 75°F (11.1°C � To � 23.9°C), the DDC system controller fully opens<br />

the outdoor <strong>air</strong> damper <strong>and</strong> closes the recirculating damper to use 100 percent outdoor <strong>air</strong> free cooling<br />

as the first-stage cooling in the AHU. Based on the measured discharge <strong>air</strong> temperature by sensor<br />

T1, the DDC system controller modulates the two-way valve <strong>of</strong> the water cooling coil as the secondstage<br />

cooling simultaneously to maintain a preset discharge <strong>air</strong> temperature Tdis � 44°F (6.7°C).<br />

When the outdoor temperature To drops from 65 to 52°F (18.3 to 11.1°C), Tdis is reset inversely<br />

proportional by the DDC system controller from 44 to 52°F (6.7 to 11.1°C). At the same time, the<br />

DDC terminal controllers modulate the volume damper as well as the volume flow <strong>of</strong> cold primary<br />

<strong>air</strong> supply to maintain a preset zone temperature <strong>of</strong> 75°F (23.9°C) in the control zones rx1 <strong>and</strong> ri1.<br />

The fan in the fan-powered box FBX1 is not energized.<br />

5. When 52°F � To � 75°F (11.1°C � To � 23.9°C) <strong>and</strong> the supply volume flow rate <strong>of</strong> the<br />

cold primary <strong>air</strong> from the fan-powered box FBX1 in the perimeter zone has dropped to 0.40 V˙ sn,d,<br />

the fan in the fan-powered box FBX1 is energized in low-speed operation <strong>and</strong> extracts recirculating<br />

ceiling plenum <strong>air</strong> to the fan-powered box, <strong>and</strong> the supply volume flow rate <strong>of</strong> recirculating plenum<br />

<strong>air</strong> V˙ sn,r to the control zone rx1 is roughly increased to about 0.40V˙ sn,d.<br />

If the volume flow rate <strong>of</strong> the cold primary <strong>air</strong> supplied from the fan-powered box FBX1 has<br />

dropped to 0.35 V˙ sn,d,<br />

the fan in the fan-powered box is energized in high-speed operation, <strong>and</strong> its<br />

supply volume flow <strong>of</strong> recirculating plenum <strong>air</strong> V˙ sn,r is further increased to 0.45 V˙ sn,d.<br />

6. When 52°F � To � 75°F (11.1°C � To � 23.9°C) <strong>and</strong> the supply volume flow rate <strong>of</strong> the<br />

cold primary <strong>air</strong> from the fan-powered box FBX1 has dropped to the minimum setting, that is,<br />

V˙ sn,d<br />

0.3 , if the control zone temperature T rx1 sensed by sensor T11 drops below 75°F (23.9°C), that<br />

is, 72°F � T rx1 � 75°F (22.2°C � T rx1 � 23.9°C), then the fan-powered box FBX1 is in dead-b<strong>and</strong><br />

operation, as shown in Fig. 21.13d. In dead-b<strong>and</strong> operation

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