Final Exam Flashcards
Two different control approaches
Pneumatic
Electronic
Pneumatic
Used compressed air as the operating medium
Almost never used today in modern construction
Stand alone, can’t be networked
Electric and electronic
Typically powered by a 24 VAC power source (usually a transformer mounted on the terminal unit)
Are being replaced by DDC
stand alone, can’t be networked
DDC (direct digital control)
Most popular and flexible option for most modern buildings
Digital refers to the use of digit computers in these systems
They use analog sensors along with digital computer programs to control HVAC systems
Temp measurements for DDC applications are made by 3 principle methods
Thermocouples
Resistance temperature detectors (RTDs)
Thermistors
Thermocouples
Consists of two dissimilar metals chosen to produce a measurable voltage at the temp of interest
RTDs
Use small, responsive sensing sections constructed from metals whose resistance-temp characteristic is well established and reproducible
First order resistance equation
R = Ro (1 + kT)
R - resistance in ohms
Ro - resistance at reference temp (0 degrees C) in ohms
k - temp coef of resistance
T - RTD temp
Two common materials for RTDs
Platinum
Balco (40% nickel, 60% iron alloy)
Thermistors
Semiconductors with the property that resistance is a strong but nonlinear function of temperature
VAV CONTROL SYSTEM DIAGRAM
See diagram in notes
14 actions
1
When the fan is energized, the control system is activated. Prior to activation, during unoccupied periods, the perimeter zone baseboard heating is under the control of room thermostats.
2
Return and supply fan interlocks are used to prevent pressure imbalances in the supply air ductwork.
3
The mixed-air sensor controls the outdoor air dampers to provide the proper coil air inlet temp. The dampers will be at their minimum position at about 40 degrees Fahrenheit typically.
4
The dampers minimum position controls the minimum outdoor airflow
5
As the upper limit for economizer operation is reached, the OA dampers are returned to their minimum position.
6
The return air temp is used to control the morning warm up cycle after night setback.
7
The OA damper is not permitted to open during morning warm up by action of the relay shown
8
Likewise, the cooling coil valve is de energized (NC) during morning warm up
9
All VAV box dampers are moved to fully open during morning warm up action of the relay override. This minimizes warm up time because the boxes will almost always be in the closed position. Perimeter zone coils and baseboard units are under the control of the local thermostat.
10
During operating periods, the PI static pressure controller controls both supply and return fan speeds (or inlet vane positions) to maintain approximately 1.0 inWG of static pressure at the pressure sensor location.
11
The low temp sensor initiates freeze protection measures
12
At each zone, room thermostats control VAV boxes (and fans, of present); as the zone temperature rises, the boxes open more.
13
At each perimeter zone, room thermostats close VAV dampers to their minimum settings and activate zone heat as the zone temp falls.
14
The controller, using temp information for all zones, modulates OA dampers and the cooling control valve to provide air sufficiently cooled to meet the load of the warmest zone.
BACnet
Building automation and control network
BACnet definition
A standard communication protocol developed by the American society of heating, refrigeration and air conditioning engineers (ASHRAE), which has been adopted by the American national standards institute (ANSI) and also by the international standards organization (ISO)
CAV CONTROL SYSTEM DIAGRAM
9 Actions
1
When the fan is energized, the control system is activated
2
The minimum air quality is set (usually only once during commissioning, as described earlier)
3
The OA temperature sensor supplied a signal to the damper controller
4
The RA temperature sensor supplied a signal to the damper controller
5
The damper controller positions the 4 dampers to use outside or return air, depending on which is cooler
6
The mixed-air low temperature controller controls the OA dampers to keep excessively low temperature air from entering the coils. If a preheat system we’re included, this sensor would control it.
7
Optionally, the space sensor could reset the coil discharge air PI controller
8
The discharge air controller controls the
- heating air damper
- outdoor air damper
- exhaust air damper
- return air damper
- cooling coil valve after economizer cycle upper limit is reached
9
The low temperature sensor initiates freeze protection measures, as described previously
