Saturday, October 10, 2015

How ddc works: HVAC Furnace direct digital control


CENTRAL WORKSTATION (control, monitor, manage)
-         talks to global controllers/building controllers
-         send, receive, process, store, print the data
-         check component and system status
-         check trends and monitor performance of equipment
-         change set points, schedules and settings
-         ASCENT COMPASS - Alerton

GLOBAL CONTROLLERS (integrate devices)
-         talks to field controllers/zone controllers
-         connects/provides gateway to allow communication
with devices made by different manufacturers
-          integrates different devices
-         ACM, BCM, VLX - Alerton

FIELD CONTROLLERS (control terminal units)
-         operates actuators, fans, blowers, pumps
-         field controller for VAV, AHU, Fan coil unit, chiller
cooling tower, boiler, terminal unit
-         VLC, VAV - Alerton

-         provide input, send signal to controllers
-         display room setpoints and fan status
-         display room temperature, outside air temperature, relative humidity
-         MICROSET - Alerton

Receive Input Information From Sensors
Process Input Information
Respond with an Output Action
Receive Feedback when set point is met
Respond with another Output Action

Example:  How a Furnace DDC works?

1.      Thermostat (Sensor) calls for heat
2.      Thermostat switch is closed, sends voltage signal to control board
3.      Control board (“Brain”) receives and processes input signal from thermostat
4.      Control board closes contactor, inducer fan motor starts
5.      Air flow switch/Sail switch closes to confirm air flow supplied by fan
6.      Control board will activate the Spark Igniter
7.      Control board activates Gas Valve
8.      Burner ignites
9.      Flame Sensor detects flame
10.   Spark Igniter deactivates
11.   Control board energizes Indoor Fan Blower motor
12.   Furnace supplies heat to the structure (room, home, building, etc.)
13.   Thermostat temperature set point is met
14.   Thermostat opens switch, sends feedback signal voltage to control board
15.  Control board shuts off the Gas Valve and Induced draft fan
BAS uses an integrated network of Workstations, Global Controllers, Field Controllers, Sensors, Gateways, Routers, Repeaters, Cables and other associated components that are designed and programmed to provide the following building services:

  1. Networking
  2. Control, Monitoring and Alarms
  3. Data collection
  4. HVAC – Heating, Ventilation, Air-Conditioning
  5. Real-time Adjustments of Setpoints, Percent valve openings, etc
  6. Scheduling, Remote Operation and Remote Access
  7. Changing Modes, e.g. Economizer mode (to save on energy costs)
  8. Load and Demand Control
  9. Fire Protection
  10. Security
  11. Lighting management
  12. Energy Management



Saturday, October 3, 2015

Refrigeration System Sequence of Operation


Power is supplied to Timer Motor

Timer Motor controls time for Cooling and Defrost cycles

Room Thermostat Closes when the temperature rises above setpoint

Control Board will activate the Refrigeration/Cooling Mode

Timer Motor is on Refrigeration/Cooling Mode

Defrost Heater is Off

Defrost Limit Switch is Off

Defrost Termination Solenoid is Off

Defrost Termination Thermostat/Fan Delay Switch is On

Evaporator Fan Motor is On

Liquid Line Solenoid is energized

Liquid Line Solenoid Valve Opens to allow liquid refrigerant to Evaporator

Low Pressure Control/Switch is Closed

High Pressure Control/Switch is Closed

Overload Protection is Closed

Compressor Contactor Coil is Energized

Compressor Runs

Condenser Fan Motor Runs

When Desired Room Temperature is reached

Room Thermostat Opens

Liquid Line Solenoid is De-energized

Liquid Line Solenoid Valve Closes

Compressor continues to run to allow Pump Down of refrigerant

Low Pressure Control/Switch will Open

Compressor Contactor Coil will De-energized

Compressor will turn Off

Condenser Fan Motor will turn Off

When defrost time is reached by the Timer Motor

Defrost Mode/Cycle begins



Compressor Pump Down process

Timer Motor will be on Defrost Mode/Cycle

Defrost Heater is On

Defrost Limit Switch is On

Defrost Termination Solenoid is On

Defrost Termination/Fan Delay Switch is Off

Evaporator Fan Motor is Off

When defrost time is completed and Evaporator warms up

Defrost Termination Thermostat Closes

Low Pressure Control Closes

Compressor starts

Condenser Fan Motor Starts

Refrigeration Cycle Starts

When Evaporator coil temperature cools down

Fan Delay Switch will Close

Evaporator Fan Motor will turn On

Refrigeration cycle continues until the next Defrost Cycle is set by Timer Motor