In a variable volume system, where the supply air volume deliv-ered varies with the building load, it is necessary to provide means for fan capacity modulation and static pressure control in the duct system.
Static pressure control prevents excessive noise generation and energy wastage as variable air volume systems reduce the volume of air deliv-ered through the terminal units.
The principal methods used for fan capacity modulation and static pressure control in HVAC systems include:
• Riding the fan curve.
• Return air dumping.
• Discharge damper control.
• Variable inlet vane control.
• Fan speed control.
Of these methods, the first relies on inherent fan performance, the second controls air direction without reduction of volume, and the other three modify fan performance.
Riding the Fan Curve
Fans in HVAC systems are usually centrifugal blowers, and may have forward-curved, backward-curved, or airfoil-bladed wheels. Each wheel type has different pressure-volume-horsepower relationships but the performance of all types can be predicted using the “fan affinity laws” which relate the air volume handled, the static pressure devel-oped, and the horsepower required to the fan speed.
Basically, the fan affinity laws state that the air volume delivered by a centrifugal blower fan varies directly with changes in fan speed, while the pressure varies with the square of changes in fan speed, and the horsepower required varies with the cube of changes in fan speed.
The forward-curved wheel has a characteristic curve which shows that horsepower increases and decreases in direct relation to the volume of air delivered when operated at a constant speed.
The backward-curved and airfoil-bladed wheels have a limiting value at a given speed beyond which no further pressure will be devel-oped or horsepower will be required, but the power requirements do
not reduce in proportion to volume reductions as closely as forward-curved bladed wheels.
The characteristics of the forward-curved bladed wheel allow fan modulation by “riding the fan curve.” This means that no action is taken to modulate the fan speed. The fan is allowed to shift its operating point on its fan characteristic curve as the pressure and volume change. When the fan is operating at a constant speed, as a variable volume air damper acts to decrease the air volume delivered, the pressure developed by the fan will increase to a new operating point on the constant rpm curve and the power required will usually decrease.
“Riding the fan curve” with a forward curved wheel is the lowest first cost control method.
Although “riding the fan curve” will occur with other types of fans, the reduction in power required will not be as much as with a forward-curved bladed wheel. For this reason, other fan modulation systems are more economically attractive. When “riding the fan curve,”
the air volume delivered is varied by action of the terminal unit damp-ers alone with no control at the fan. Some terminal units do not have the ability to control air volume at high inlet pressures that result from
“riding the fan curve.” When supplying that type of terminal, some other static pressure control method must be used to prevent overpow-ering the terminal boxes and disturbing the temperature control func-tions of variable air volume systems.
Return Air Dumping
When system duct pressure is controlled by return air dumping, as the pressure sensed by the duct static pressure controller increases above the setpoint of the controller, a signal is generated to position an actuator to open a bypass damper installed between the supply and the return air duct and to allow airflow from the supply duct to the return air duct.
This method is illustrated in Figure 2-15. This is the second lowest first cost control method.
Discharge Damper Control
When a centrifugal blower fan is operating at a given speed, each point on a given fan speed curve represents a different combination of air volume delivered and pressure developed, so that when the volume of air delivered is reduced, the pressure developed increases.
When the air volume delivered by a fan is to be reduced, the static
pressure can be increased by dampering or “closing-down” the air sup-ply from the fan discharge. An automatic static pressure control damper in that location is called a “discharge damper.” A discharge damper is usually a standard opposed-blade volume control damper controlled from either a static pressure controller or a controller with a remote static pressure sensor.
As the static pressure at the duct static pressure monitoring point increases, the controller generates a signal to position the damper to-ward closed. As the damper is positioned toto-ward closed, the pressure loss through the partially closed damper increases, the air volume deliv-ered decreases, the static pressure at the fan discharge increases, and the duct static pressure decreases until an equilibrium point is reached.
The variable volume terminal box dampers are continually reposi-tioning, which causes continual changes in duct static pressure and re-sulting continual repositioning of discharge dampers. Discharge damp-ers may be the least expensive choice of control methods for application in retrofit projects. Fan power requirements for discharge dampers are higher than for some other methods. With forward-curved bladed fan wheels, discharge dampers give about the same power consumption as variable inlet vanes.
Variable Inlet Vane Control
The characteristic curve for each centrifugal blower wheel varies with type of wheel and wheel diameter. The performance characteristics
Figure 2-15. Fan Bypass or Return Air Dumping
of any fan can be altered by changing the speed, as described below, or by changing the airflow direction at the inlet to the wheel.
When a radial-blade damper is placed at the inlet to the fan, with a control actuator connected to change the angle at which the radial blades cut the air entering the wheel, the blades are called “variable inlet vanes.” The use of variable inlet vanes allows modulation for all types of centrifugal fans. The lowest power requirements are obtained with airfoil-bladed and backward curved fans.
As the static pressure controller senses an increase in duct static pressure, the controller generates a signal to position the variable inlet vanes to close partially, which causes the air entering the round fan wheel inlet to start a spiral movement. The spiral movement directs the incom-ing air in a direction against the direction of rotation of the fan wheel and causes the fan to perform on a different characteristic curve from the full-open damper position. That results in lower air volume delivered, lower static pressure developed, and lower power required. This regulation of fan volume and static pressure is achieved at the expense of an added sys-tem pressure loss for the dampers, which amounts to about 3% to 5% base load horsepower penalty. Although the first cost for variable inlet vanes when factory installed is lower than for a variable speed motor, inlet vanes are not suitable for retrofit applications.
Fan Speed Control
Fan speed control is the most effective method for controlling fan performance, but it is also the most expensive. Each centrifugal blower wheel has a characteristic curve for each discrete fan speed. The perfor-mance “curve” for a fan running at different speeds is plotted as a fam-ily of curves, with a curve for each increment of fan speeds.
A change in fan speed will result in a corresponding change in the air volume delivered, static pressure developed, and power required.
Methods available for controlling fan speed include: mechanical speed changing mechanism which uses mechanical methods to vary the motor output shaft speed from a constant speed motor, and electric current in-verters which modify the electric current characteristics to cause stan-dard constant-speed motors to operate at variable speed. The latter method is more popular in HVAC systems.
Mechanical speed control. This system consists of a variable-speed belt drive plus a speed controller. A sheave with a spring-loaded
vari-able pitch diameter mechanism is mounted on the fan motor drive shaft and is connected with a specific duty v-belt drive to a fixed pitch sheave on a countershaft which connects to the fan shaft with standard v-belts and sheaves.
Upon receiving a signal from the controller, the variable drive base moves closer to or further from the fixed drive base. This allows the faces of the variable pitch diameter sheave to move against their com-pensating springs; they move closer together to increase diameter of the variable pitch diameter sheave when the drive bases are close together, and force the sheave faces further apart and decrease the pitch diameter as the drives move further apart. As the pitch diameter of the motor sheave increases, the speed of the fan sheave increases: as the pitch diameter decreases, the fan sheave speed decreases.
Electric current inversion. This system uses an inverter, which is a solid-state device used to control motor speed. The incoming electric current is processed through an inverter to change the electrical current characteristics so that a standard constant-speed induction motor will operate at any speed down to about 25% of the nominal motor RPM.
Inverters use one of three basic technologies—variable voltage in-put (VVI), current source inin-put (CSI), or pulse width modulation (PWM). Each inverter system uses three common components: 1) a rec-tifier to convert alternating current to direct current, 2) a regulator to receive the automatic control system speed input signal and establish the appropriate dc voltage, and 3) an inverter which processes the dc signal to generate a proportioned voltage-to-frequency waveform. Each of the three inverters uses different techniques to modify the electrical current for frequency and voltage.
Variable Voltage Input Inverter
This inverter, shown by schematic in Figure 2-16, has 3-phase line power as the primary input into the phase controlled rectifier which controls the phase angle. The rectifier receives a motor speed reference signal from the regulator, which causes the rectifier to vary its output by controlling the firing angle of the silicon controlled rectifier (SCR) or thyristor banks.
The pulsing dc is filtered and then inverted to an ac waveform by six power switching devices within the inverter. The frequency of the wave is generated on the basis of the speed referenced signal received
from the regulator section. The resulting voltage and current waveforms are as shown in Figure 2-17. The resulting voltage and current wave-forms are proportioned in volts and frequency to produce the required speed-to-torque relationship to suit the driven motor and load.
When it is desirable to produce minimum line noise and a power factor near unity, the phase controlled rectifier diagrammed in Figure 2-16 can be replaced with a diode rectifier/chopper module where voltage is controlled in a transistorized chopper section by converting the dc current to a truncated waveform of controlled amplitude or voltage, as diagrammed in Figure 2-18. The diode/chopper module may also be used on CSI and PWM type inverters.
Current Source Input Inverter
The current source input inverter (CSI), or adjustable current in-verter, controls current, not voltage. The adjustable current waveform is shown in Figure 2-19. The current waveform is similar to the 6-step waveform shown in Figure 2-17 with the voltage waveform being gen-erated by the back emf (electro-motive force) of the motor.
Pulse Width Modulation Inverter
The pulse width modulation (PWM) inverter uses an inverter sec-tion which simulates the ac sine wave by rapidly pulsing the dc voltage, positive and negative, as shown in Figure 2-20. The PWM inverter can use either transistors or fast turn-off SCRs for this service.
The proper voltage required to achieve the voltage-to-frequency ratio is controlled by the width of the pulse. Because the PWM inverter
Figure 2.16. Mechanical Speed Control (Courtesy Dynamatic)
must use microprocessor logic to control the switching sequence, the PWM inverter is considerably more complex than the 6-step inverters used in VVI and CSI systems. CSI or VSI is more efficient because pulses in the PWM sine wave are transformed into heat, which reduces both the efficiency and the service life of motors. Typical PWM voltage and current waveforms are shown in Figure 2-21.
Open Loop Fan Tracking
When a supply air fan is being controlled for volume and static pressure, it is necessary to control the interlocked return air fans in order to maintain building pressure balance. When the return air fan is
con-Figure 2-17.
Block Diagram for a Typical VVI Drive (Courtesy Dynamatic)
Figure 2-18.
Typical VVI