Discharge Air Control

Transcription

1 Issue Date June 17, 2002 APPLICATION NOTE Discharge Air Control Discharge Air Control...2 Introduction... 2 Key Concepts... 3 Discharge Air Control Overview... 3 Design Considerations Johnson Controls, Inc. Code No. LIT

2 2 Discharge Air Control Application Note Discharge Air Control Introduction Discharge Air Control Application Note describes how to use Direct Digital Control (DDC) to implement discharge air control.

3 Discharge Air Control Application Note 3 Key Concepts Discharge Air Control Overview Discharge air control of an air handling unit provides air at an appropriate temperature to either the space or terminal units. Discharge air control attempts to provide appropriate air by the most economical method. A controller uses a sensor in the discharge (supply) air duct to modulate the controlled devices to maintain discharge air setpoint when the fan is running. The controlled devices may be one or more of the following: mixing dampers (outside air and return air damper), heating coil valve, or cooling coil valve. If more than one controlled device is present, the devices modulate in sequence. On a call for full heating, the heating valve opens, the outside air damper positions at minimum, the return air damper opens, and the cooling coil valve closes. As the discharge air temperature increases, the heating coil valve modulates closed. With a continued increase in temperature, the outside air damper modulates open and the return air damper modulates closed (in unison). If the discharge air temperature continues to rise, the cooling valve modulates open. A mixed air sensor can provide Mixed Air Low Limit (MALL) control. The MALL overrides the control signal and modulates the outdoor air damper closed and the return air damper open (in unison), if mixed air temperature drops below its setpoint. Always provide MALL on units with a wetted coil. With the supply fan off, the outside air damper closes, and the return air damper opens. Figure 1 is an example of Discharge Air Control.

4 4 Discharge Air Control Application Note Exhaust Return Outside Air Discharge Mixing Damper Actuators Mixed Air Sensor Low Limit Thermostat (Safety) Heating and Cooling Valve Actuators Discharge Air Sensor Discharge Air Control Figure 1: Discharge Air Control Component Choices The following tables list the component choices for the Discharge Air Control. Table 1: Mixing Damper Actuators Mixing Damper Actuator DCM ASC M9206-GGA-2, minimum of 53 lb in (6 N m) torque, CCW or CW, spring return, proportional M9216-GGA-2, minimum of 140 lb in (16 N m) torque, CCW or CW, spring return, proportional M110GGA, minimum of 25 lb in (2.8 N m) torque, CCW, spring return, proportional M110CGA, minimum 25 lb in (2.8 N m) torque, CCW or CW, spring return proportional M130GGA, minimum 50 lb in (5.7 N m) torque, CCW, spring return, proportional M130CGA, minimum 50 lb in (5.7 N m) torque, CCW or CW, spring return, proportional Notes: 6 lb in per square foot of damper face is the recommended average. Use M100J Series actuator as a slave to M100G or C Series actuators when using multiple actuators.

5 Discharge Air Control Application Note 5 Table 2: Mixed Air Sensor Mixed Air Sensor DCM ASC TE , Nickel ±1%, 1000 ohms at 70 F (21 C), 8 feet TE , Nickel ±1%, 1000 ohms at 70 F (21 C), 17 feet TEA-L000000, Platinum, 1000 ohms at 32 F (0 C), 10 feet TEA-M000000, Platinum, 1000 ohms at 32 F (0 C), 20 feet TEA-J000000, Platinum, 100 ohms at 32 F (0 C), 10 feet * TEA-K000000, Platinum, 100 ohms at 32 F (0 C), 20 feet * * TQ Transmitter required with this application. Table 3: Low Limit Thermostat Low Limit Thermostat DCM ASC A70BA-18C (15/55 F) 20 foot element A11A-1C (15/55 F) 20 foot element Table 4: Valves and Valve Actuators Heating and Cooling Valves and Valve Actuators DCM ASC 1/2 in. and 3/4 in. VG7000 Series Bronze Globe Valve with VA or VA /2 in. and 3/4 in VG4000 or VG5000 Series Zone Valve with VA in. to 2 in. VG7000 Series Bronze Globe Valve with VA /2 in. to 1 1/4 in. VG7000 Series Bronze Globe with VA-4233-GGx, spring return 1-1/2 in. to 2 in. VG7000 Series Bronze Globe Valve with M9216-GGx, spring return 1/2 in. to 1-1/2 in. VG1000 Series Ball Valve with M92xx-GGx, spring return 1/2 in. to 1-1/2 in. VG7000 Series Ball Valve with M91xx-GGx 2-1/2 in. to 6 in. VG2000 Series Cast Iron Flange Valve with VA-3100 series 2 in. to 6 in. VF Series Butterfly Valve with M91xx-GGx 2 in. to 14 in. VF Series Butterfly Valve with VA-9070 series 16 in. to 20 in. VF Series Butterfly Valve with VA-9000 series

6 6 Discharge Air Control Application Note Table 5: Discharge Air Sensor Discharge Air Sensor DCM ASC TE , Nickel ±0.1%, 1000 ohms at 70 F (21 C) TE , Nickel ±1%, 1000 ohms at 70 F (21 C) A99BB-25C, Silicon ±1%, 1035 ohms at 77 F (25 C) TEP-L000000, Platinum, 1000 ohms at 32 F (0 C) TEP-J000000, Platinum, 100 ohms at 32 F (0 C) * Note: All elements require TE mounting kit. * TQ Transmitter required with this application. Design Considerations This section lists things to consider when choosing the DDC components for this application. Mixing Dampers Properly size the mixing dampers to provide good controllability. Sizing the outdoor air and return air dampers correctly allows proper mixing of the air streams to occur in the mixing plenum. If these dampers are incorrectly sized, the air streams will tend to stratify, and the risk of freezing coils in a cold climate increases. Opposed blade dampers provide a more turbulent air stream, allowing for improved mixing. Damper Actuator Use spring return for the mixing dampers to close the outdoor air damper if a power loss occurs. If the outdoor temperature in your area drops below freezing, spring return reduces the risk of freezing the cooling and heating coils. Do not cut power to the actuator to close the damper; always drive the actuator to the closed position. Depending on the choice of actuator and the damper size, the damper may require multiple actuators. Mixed Air Sensor Use a nickel 1000-ohm averaging element (TE ). If this sensor does not provide enough length to adequately cover the duct area, then use the TE These sensors provide a cost-effective installation. The mixed air sensor should always be an averaging sensor. The controller often uses this sensor to provide the low limit override should the mixed air temperature drop to near freezing temperatures. If neither of the sensors adequately covers the duct, use multiple sensors.

7 Discharge Air Control Application Note 7 Low Limit Thermostat When remote indication is required, use the A70BA-18C (manual reset) thermostat for low limit protection. If not using remote indication, then use the A11A-1C (manual reset). Wire these thermostats into the fan starter circuit to stop the fan when the temperature exceeds the setpoint. The element of the low limit thermostat must reside in a horizontally serpentine position. Failure to follow this rule results in a nonfunctioning thermostat. Heating and Cooling Valves and Valve Actuators For small coils, use the VG4000 or VG5000 Series Electric Zone Valve with the VA-7452 actuator, or the VG7000 Series Bronze Globe Valve with the VA-8122, VA-7152, or VA-4233 actuators. The VA-7452, VA-8122, VA-4233, or VA-7152 actuators provide proportional control from a 0 to 10 VDC signal. For larger coils, use the VG1000 Series Ball Valve with M9xxx Series Actuators, VG7000 Series Bronze Globe Valve with VA or M9xxx Series Actuators, or a VG2000 Series Cast Iron Valve with VA-3100 Series. For even larger coils, apply the VF Series Butterfly Valves. Discharge Air Sensor Use a nickel 1000-ohm sensor (TE with TE mounting hardware) wired in a 2-wire application. This sensor provides a cost-effective installation and if additional resistance due to wire length is considered when engineering the software, the reading is within one degree of the actual temperature. Place the element far enough down the duct so that proper mixing has taken place. Heating, Ventilating, and Air Conditioning Controller The AS-AHU controller or AS-AHU100-0 plus AS-AHU102-0 can accomplish the control sequence in most instances. If you have an extremely complex unit with humidity, static pressure, and building pressurization control, we recommend the use of a D controller or NU-DCM140-0 controller in conjunction with an NU-NCM Controls Group 507 E. Michigan Street P.O. Box 423 Milwaukee, WI Printed in U.S.A.