HONEYWELL CP980C (01) PDF DOCUMENT

The HONEYWELL CP980C combines ultrasensitive air velocity sensing with a pneumatic controller to detect and control airflow in air terminal units, regardless of the system’s static pressure. It can accurately sense velocities lower than 100 ft/min (0.5 m/s) and can handle a maximum duct velocity of 3500 ft/min (17.7 m/s) with the use of a flow limiting orifice.

In a Type B controller, air from the main line supply passes through a primary jet restriction to a secondary jet restriction, as well as to the minimum and maximum flow limits. The pressure from a bleed type thermostat determines the pressure to the secondary restriction, which in turn determines the flow through the emitter tube and the jet velocity. The minimum flow limit prevents the thermostat from bleeding the pressure below the minimum setting, and the maximum flow limit prevents it from building pressure above the maximum setting.

A two-stage bleed type amplifier amplifies the jet collector pressure to provide the branchline pressure.

• For reverse acting models, the branchline pressure is inversely proportional to the velocity of the air flowing perpendicular to the jet in the sampling tube.

• For direct acting models, the branchline pressure is directly proportional to the velocity of the air flowing perpendicular to the jet in the sampling tube.

Velocity is reset between adjustable minimum and maximum settings by a bleed thermostat. An increase in pressure at the thermostat increases the velocity control point. The selected orifice and controller settings determine the velocity range. The maximum and the minimum limits set the range as a percentage of the rated velocity established by the orifice.

When using a Type C controller for sequencing, a pressure signal from the thermostat operates a ratio relay, which controls the pressure to the secondary jet restriction. The ratio relay is biased to a 9 psi (62 kPa) start point at zero velocity. This allows any thermostat signal below 9 psi to sequence other control devices. If the minimum adjustment is set above zero, the start point for the thermostat reset will be proportionately higher than 9 psi.

If the thermostat calls for airflow greater than the maximum setting or less than the minimum setting, these controller adjustments take precedence over the thermostat’s control to maintain the specified air volume.

The HONEYWELL CP980C is compatible with bleed type thermostats (TP973, TP975) and also operates with capacity type thermostats (TP970, TP971, TP972). The Type C controller, with its isolated pilot chamber, is suitable for use with all thermostats, whether they require sequencing or not.

When using a thermostat with its own main supply (two-pipe) with a Type B controller, you must always use an RP470B relay as a repeater.

Bleed thermostats or equivalent bleed devices other than Honeywell, when connected directly to Port 3 of a Type B controller, must be capable of bleeding down to 1.25 psi (9 kPa) at a compressed airflow rate equivalent to that of a 0.007-inch restriction with a mainline pressure of 20 psi (138 kPa).

Follow these steps for maximum flow calibration:

1. Check the main pressure to ensure it is at a minimum of 18 psi (124 kPa).

2. For a Type B controller, cap thermostat Port 3. For a Type C controller, connect main air to Port 3.

3. Install flow measuring test equipment.

4. Measure the actual flow in ft³/min (m³/s).

5. Adjust or calibrate the MAX dial to the desired flow level using the flow measuring equipment. Write the new dial setting and airflow on a tag or on the unit for future reference.

Follow these steps for minimum flow calibration:

1. Vent Port 3.

2. Turn the MAX dial above 10.

3. Install flow measuring test equipment.

4. Measure the actual flow in ft³/min (m³/s).

5. Adjust or calibrate the MIN dial to the desired flow level using the flow measuring equipment. For future reference, write the new dial setting and airflow on a tag or on the unit.

Follow these steps:

1. Divide the required minimum and maximum flow values by the duct area to determine the required velocity.

2. Refer to Table 1 to calculate the percent of velocity rating.

3. Select an orifice that is as close to 70 percent of the velocity rating for maximum as possible. This provides optimum adjustment of the maximum scale.

4. Check the dial settings. For a minimum set at shut off, select a setting of 0.

The available models are detailed in the following table:

The Velocitrol sensor consists of two small tubes, an emitter jet and a collector tube, separated by a fixed gap. It does not depend on the velocity pressure of the airstream. Instead, a strong signal of compressed air moves from the emitter tube to the collector tube across the gap, perpendicular to the airflow in the duct. The collector pressure is a direct function of supply air pressure and is inversely proportional to the velocity of the duct air. As duct airflow increases, it deflects the jet, and less supply air bridges the gap to the collector. This provides usable pneumatic output signals at duct air velocities as low as 100 ft/min (0.50 m/s).

The high and low limit regulators dictate the maximum and minimum flow control points. These adjustments operate by limiting the supply pressure to the emitter tube. For example, if the high limit is set at a thermostat pressure of 10 psi (69 kPa), it establishes a maximum flow control point. Any further increase in space temperature will not affect the emitter pressure or the airflow. Similarly, the low limit adjustment establishes a minimum control point. The system will operate at duct air velocities between the high and low limit settings, even if the thermostat calls for velocities outside this range.

The control range can be increased to higher velocities by using an orifice. The orifice proportionally reduces the velocity of the air entering the flow tube, allowing the more sensitive velocity jet to maintain control. Different orifices allow the sensor to accurately control duct velocities up to 3500 ft/min (17.7 m/s).

If the low limit adjustment is set at a pressure greater than that of the high limit adjustment, the high limit control point takes precedence and acts as both the high and low limit. This effectively establishes a fixed control point.

When a sequencing relay is added to the system, the thermostat pressure must be higher than 9 psi (62 kPa) before the volume controls begin to operate. For thermostat signals below 9 psi (62 kPa), the signal can control another device, such as a reheat coil, while the minimum limit adjustment on the HONEYWELL CP980C controls the duct air velocity.