30 November 1998
VAV boxes to function. The "Fan-On" relay contact disconnects PC from IP, causing DA to hold IV in the
closed position (unloaded) on fan shutdown; the purpose in unloading the fan is to allow it to start
unloaded. DPI is a low differential pressure gauge used as an indicator for DPT. Details of the action of
the rest of the control system devices connected to the IP's output are similar to comparable parts of
other loops previously described. For DDC applications, the DDC panel takes the place of the static
pressure controller PC.
Figure 3-11. Supply duct static pressure control loop.
8. RETURN FAN VOLUME CONTROL LOOP. The return fan volume control loop is shown in figure
3-12. Flow sensing elements and linearized transmitters FTs in the supply air and the return air get
signals from duct-mounted air flow measurement stations and sensing arrays, AFMA. Both FTs send
signals to controller FC. These signals are the information necessary to maintain a fixed flow difference
(in L/s (cfm)) between the supply air and return air ducts. The controller measures and controls the
return air flow through the PV input based on the supply air flow measured at the CPA input. More than
likely, the ranges of the air flow velocities in each duct will be different because of differences in design
velocity and in the cross-sectional areas of the ducts. The FTs in the supply air duct and return air duct
may or may not have the same span and range. This means that a given flow rate in the supply duct
may have a different signal level than the exact same flow rate in the return duct. In order for FC to
control the return air flow at a specific rate (L/s (cfm)), the CPA signal from the supply fan FT must have
the same value that will appear at PV when the setpoint is achieved. To achieve this, the CPA signal
from the supply duct must be converted by FC's ratio and bias feature to perform two functions. A ratio
factor must be applied to the signal from the supply air flow transmitter/air flow measurement station
(FT/AFMA) combination so that it will match the signal range of the return air FT/AFMA combination.
Also, the ratio is used to account for differences in the cross-sectional area of the ducts at the locations of
the measuring stations. The signal must then be biased to maintain the design fixed air flow difference.
For example, if the fixed difference is required to be 1415 L/s (3,000 cfm) for minimum outside air
requirements when the supply air flow is 9440 L/s (20,000 cfm), CPA tells FC to control PV at 8025 L/s
(17,000 cfm); when the supply air flow is 5900 L/s (12,500 cfm), CPA tells FC to control PV at 4485 L/s
(9,500 cfm). The 1415 L/s (3000-cfm) difference is the bias to be set in the controller in L/s (cfm) units.
The CPA signals at two such supply air flow points must match the PV signals at two corresponding
return air flow points. When this is achieved at two points, the required results will be achieved for any
supply fan flow and the appropriate return fan flows within the turndown capabilities of the return fan.
The ratio can be calculated according to equation 3-2. Equation 3-2 assumes that the low end of the
transmitter span is 0 m/s (0 fpm) at 4 milliamperes for each transmitter.
R = Ratio (dimensionless)
As = Area of supply air duct at the measuring station (sq m (sq ft)).
Ar = Area of return air duct at the measuring station (sq m (sq ft)).
Vs = Span of the flow transmitter in the supply duct (m/s (fpm)).
Vr = Span of the flow transmitter in the return duct (m/s (fpm)).