Pneumatics Midterm Flashcards
What are the parts of a commercial HVAC system?
Boiler, fan or pump, and heating units, ductwork or piping, and controls
What is Passive Dehumidification?
Dessicants placed in the air ways
What is Baseboard Heat and what is it used for?
Uses electrial resistance elements at the bottom of the outside walls and blows heated air back into the room. Warm rooms, heat envelope through natural convection
What is a VFD and how does it work?
Adjusts frequency to change voltage and current supply, intern adjusting speed of controlled device
What is a Controller?
A device that receives a signal from the sensor, compares it to a setpoint value, and sends an appropriate output signal to a controlled device
What are Differential Pressure Controls?
A specific pressure in the system is maintained based on a difference in pressure between two points in the system
What is a Closed Loop and Open Loop?
Closed Loop: control in which feedback occurs between the controller, sensor, and controlled device. Open Loop: control in which no feedback occurs between the controller, senor, and controlled device
What is a Self-contained Control System?
Control system that does not require external power supply. Powered by fluid-filled element with gas, liquid, or mixture
What is the purpose of a Bleed Port?
Oriface that allows small amount of air to be expelled from controller, changes via bimetallic element which covers or uncovers the port
How does a System Powered Control System work?
No electricity, air controlled system, controller operated by static pressure only from fan and ductwork
What is Spring Range?
Difference in pressure at which an actuator shaft moves and stops.
How do 2-way valves pertain to steam valves?
Steam valves are always two-way valves bexause steam does not require a return to the steam header or the boiler (goes to the DA tank)
How do Outside Air Dampers work on AHUs?
Adjust metal blade or set of blades to control air flow in response to controller signal
How do Winter/Summer T-stats work?
Pilot Bleed T-stat, changes setpoint and action between modes. 2 bimetallics, one DA, one RA.
How do Dual Duct AHUs work?
Has hot and cold air ducts connected to mixing boxes at each building space.
How do Controled Devices work?
Regulate flow of fluid in system in response to signal from controllers. Are dampers, valves, compressors, etc.
How do Open Systems work?
No feedback from controlled devices
How do Radiant Heat Panels work?
Ceiling panels with embedded electrical resistance elements. Heat is radiated to a local area, usu. 150 - 170 degrees, 120 or 240VAC
What is the common percentage for OA?
20 percent
What is the temperature for economizers?
Dry Bulb: OAT must be below 65 degrees to work. Enthalpy: OAT below 65, and humidity (mixed-air) below 60 but above 30 percent
What are the types of blades and how do they work?
Propeller: most common, large quantities of air, low static, usu. 24in. To 100 ft. Variable Pitch: 48 in. more common, propeller attached to moveable hub changes pitch. Centrifugal: similar to AHU fans, fan in housing, air enters from side of fan and expelled at outer edge of housing
What are the types of dampers?
Parallel Blade: blades are parallel. Opposed Blade: blades are adjacent and move in opposite directions. Round Blade: used in round ductwork, single damper
What are VAV systems and how do they work?
Use VFD to speed up and slow down fan speed to control static. Often with variable damper.
How do controllers work and operate?
Receive signla, compares to setpoint value, sends output to controlled device. Accuracy adjustments are Proportional Band, Gain, Throttling Range
How are IAQ issues resolved?
Adding more outside air to the space
What is Setpoint?
is the desired value to be maintained by a system
What is Control Point?
Actual value that a control system experiences at any given time
Describe ON/OFF Digital circuits?
Control in which a controller produces only a 0-100 percent output signal
What is the proper voltage for HVAC controls?
24VAC
What is a Pneumatic Control System?
Variable analog system that uses air to signal controllers and actuators
Why do we control environments in buildings?
Tenant comfort
What are Electromechanical Controls?
Uses electricity 24 VAC and up to operate devices (line voltage). Usu. Use mechanical devices which vary from system to system.
What are Electronic Control Systems?
(not automated) designed to replace pneumatics. Packaged unit for large commercial buildings, 24VAC and less, solid-state, uses resistive bridges, no intelligent controllers
What is an Automated System?
(BAS, EMS,etc) Digital solid state components that relys on a PC. Uses step-down transformers and rectifiers that change AC-DC. Intelligent local controllers
What is a Hybrid System?
Uses transducers to change electric signal to pneumatic output. Usu. 0-10VAC to 0-13 psi
How do Pneumatic Actuators work?
Accepts signal from controler and causes a proportional mechanical motion to occur
How does a 3-way valve work?
Controls the flow of water and provides constant system pressure in supply and return piping and through pumps, boilers, heat ex. And chillers. Mixing valve: 1 common outlet, 1 NO inlet, 1 NC inlet. Diverting (Bypass): 1 inlet, 2 outlets (1 to eqt and 1 to bypass loop)
What is Linear in reference to valves?
Flow through is equal to amount of stroke (actuator movement)
What is Turndown Ratio for valves?
Relationship between max flow and min flow which is controllable. Flow through valve
What is a Bimetallic Element?
Sensing device consisting of two metals joined together. Expands and contracts at different rates
What is a Bleed-type T-stat?
One-pipe, Simplest, changes air pressure to valve or damper actuator by changing amount of air expelled to atmosphere with bimetallic
What is a Master/Submaster T-stat?
Has a third line to a manual air regulator or an outside transmitter, changes setpiont based on OAT or allows operator to change. Conserves energy while maintaining control point. Maintains temp differential between OAT and Building Temp
What is a Pilot Bleed T-stat?
Two-pipe. Uses air volume amplified by relay to control temp. Relay (Pilot) increases amount of air bled through port
What does Hygroscopic mean?
Readily taking up or retaining moisture
What is a Diverting Valve?
Diverting (Bypass): 1 inlet, 2 outlets (1 to eqt and 1 to bypass loop)
What is a Butterfly Valve?
Round plate that rotates and controls flow
