Controlling Duct Pressure

The UCM-420A is one of the most versatile products sold by Kele . Although this product is capable of performing many control functions, it was primarily designed as a Setpoint Controller. Since the introduction of the UCM-420A, response from customers indicates that pressure control is a popular application for this product. This article will discuss a typical application – controlling static pressure in a duct by varying fan speed with a Variable Frequency Drive (VFD). This system could also be used to control the pressure with inlet vanes, discharge dampers, bypass dampers, etc. The same principles involved in this application could be used in other control applications, such as controlling water pressure with variable speed pumping, VAV zone control or humidifier control.

 

 

Figure 1 shows a simple Variable Air Volume (VAV) system. As the spaces served by the VAV terminals are satisfied, the zone dampers will modulate closed and, without a pressure control system, the static pressure in the duct will rise. For this example, assume that a static pressure of 2″ W.C. is required. To monitor the static pressure in the duct, we will use a Model T30-030 Differential Pressure Transducer with a range of 0-3″ W.C. The 4-20 mA signal from this device will be fed into a UCM-420A Setpoint Controller, which in turn will control the speed of the fan by sending a 4-20 mA control signal to the fan VFD. The manual potentiometer on the UCM-420A will be used to set the static pressure at the desired level. See the wiring diagram in Figure 3 on next page for system wiring. This wiring diagram shows a Model DT13 Signal Isolator. The DT13 will not be needed unless the VFD requires an isolated control signal.

All programming of the UCM-420A is done by dip switches and jumpers. A list of the functions which are programmed by DIP switches as well as those programmed by jumpers is shown below. The proper settings for this example are also shown.

Dip switch settings:

  • Remote Setpoint Enable/Disable (A1) – Disabled
  • Local Setpoint Enable/Disable (A2) – Enabled
  • Direct/Reverse Acting (A3) – Reverse Acting
  • Integral Reset Rate – As Required
  • Throttling Range – As Required
  • Single Unit PWM Enable/Disable (B1) – Disable
  • Multiplex PWM Enable/Disable (B2) Disable
  • Multiplex Unit Address Off PWM Time base Off

Jumper functions:

  • Remote Setpoint Signal – None
  • Local Setpoint Signal 3-wire Potentiometer Input Signal 4-20 mA

In this example, we are using the setpoint potentiometer on the UCM-420A to adjust the pressure, so the local setpoint should be enabled and the remote setpoint disabled. As the pressure in the duct increases, the control signal from the UCM-420A must decrease to slow down the fan speed (assuming the VFD is direct acting-speed increases with an increase in control signal) therefore, reverse acting control should be selected for the UCM-420A. The integral reset rate and throttling range required will vary from system to system and must be determined in the field. This will be discussed later in this article. Since pulse-width modulation is not to be used in this example, all PWM functions are disabled. The jumpers will be set for three-wire potentiometer local setpoint and 4-20 mA input from the pressure transducer. The instructions furnished with the UCM-420A will explain in detail how to set the DIP switches and jumpers to accomplish the above setting.

The input to the UCM-420A from the Static Pressure Transducer has a range of 0-3″ W.C. Therefore, when there is no pressure in the duct, the input will be 4 mA and when the static pressure in the duct is 3″ W.C., the input will be 20 mA. The setpoint potentiometer scale on the UCM-420A is labeled 0-100%. In our example, we require 2″ W.C. in the duct so the setpoint potentiometer should be set for 2/3 x 100 = 67%.

As stated earlier, the throttling range and integral reset must be fine tuned in the field and cannot be calculated. The throttling range is defined as the amount of signal change at the input required to cause the output to go through its entire range (4-20 mA). For example, a 50% throttling range means that for every 1 mA change in the input, there will be a 2 mA change in the output. To properly adjust the throttling range, first disable the integral reset and set the throttling range at 100%. Then reduce the throttling range in increments until the controller begins to hunt. When the controller begins to hunt, the throttling range is too narrow, so widen it one step. The integral reset rate may now be adjusted. The integral reset corrects the output of the UCM-420A to compensate for the offset inherent in proportional-only controllers. The integral reset ramps the output in the direction that forces the input signal to exactly match the setpoint. The number of times per minute this offset is overcome by the output signal is the integral reset rate. The quicker the system responds, the higher (more resets per minute) the reset rate may be set.

This control system will provide very smooth, precise static pressure control in the discharge duct. It will also save energy since the fan will use only as much horsepower as required to maintain the setpoint pressure. In this example the pressure is adjusted with the setpoint potentiometer on the UCM-420A. This pressure could also be controlled by a remote potentiometer or an analog or pulse width signal from a BAS controller.

Another application would be controlling multiple zones (such as VAV boxes) from one BAS controller output by using the multiplex PWM mode. In this application a PreCon Type 3 Thermistor could be used as the temperature input to each UCM-420A and the BAS controller could send eight different setpoint signals to the zones. Multiple UCM-420As could receive the same setpoint signal.