Building System Controls Test 1
differentiate between a refrigerated air drier and desiccant drier
refrigerated air dryers use thermodynamics of reduce the temperature to remove the air, desiccant dryers use absorption of the liquid onto a porous material
describe a deadband pneumatic thermostat
deadband is a range between two temperatures in which there is no heating or cooling
define indoor air quality
designation of the contaminants present in the air, more details about this can be found in ashrae 62
what must detergent oil not be used in an air compressor
detergents contaminate the compressed air and damage the O-rings and other controller parts
explain the purpose of lead/lag switches
determining which compressor is the primary compressor and which compressor is the backup compressor
describe the power supply of a system powered control system
duct air pressure
solar energy
energy transmitted from the sun through radiation
explain control system safety concerns
if the system were to fail, it must pose no immediate threat to occupants
how are air conditioning systems rated?
in terms of capacity
factors considered when choosing a heating system energy source
install costs, energy cost per unit, and climate
describe spring range overlap
interference between two actuators that have a psi in the same range as each other
list and define three devices used in the air compressor system to correct the pressure and volume of air
pressure switches, pressure regulators, safety relief valves
explain the need for temperature safety controls
prevents extreme high and low temperatures from damaging equipment
what is emergency heat
provided by outside temp drops below a certain volume or if pump fails
name three value flow characteristics
quick opening, linear valve, equal percentage valve
list the two types of positive displacement compressors
reciprocating (piston) and screw
define changeover relay
relay that causes the operation of the thermostat to change between two or more modes such as day/night
describe the most common application of a hybrid control system
retrofitting an existing building
list and define three air compressor performance tests
run time test, pressure test, and starts per hour test
four main control system components
sensors, controllers, control devices,.... control agents
list and define the four common control system characteristics
set point, control point, offset, feedback
differentiate between valve shut off rating and valve turn down ratio
shut off rating is the maximum fluid pressure against which a valve can completely close. turndown ratio is the relationship between the maximum flow and the minimum controllable flow through the valve.
why must an air compressor provide oil free air to the hvac controls in a building
similar to the partical control, the oil filters are used to reduce any obstructions that could affect any of the small orifices in the system
six energy sources for commercial building heating systems
steam, electricity, natural gas, coal, fuel oil, solar, and mechanical heat transfer
differentiate between system pressure control and differential pressure control
system pressure control is a pressure sensor that is used to open and close valves/ dampers based upon system pressure differential pressure control is used to control the difference in pressure between the system supply and return
why is return air temp. control more accurate than building space temp. control?
the building return air is an average of the building space temperatures
why are line voltage control systems rarely used in commercial building spaces
the danger of electrical shock, fire, and explosion due to arcing of the circuit in flammable areas
what is the fail safe position of an actuator
the fail safe position for an actuator depends on the location of building, as well as the type of valve
why must an air compressor provide clean air to the hvac controls in a building
the inlets of many actuators and sensors within a pneumatic system are so small than any reduction or addition to pressure can throw off reading in the sensors
what is the spring range
the overall range of the spring
how does a centrifugal compressor work
the use of a centrifugal impeller sending all the inlet fluid through the housing to the outlet port
what does the spring and shaft assembly do
this is an assembly that converts the air pressure change in the diaphragm into mechanical movement
how do actuators function in a pneumatic control system
through the use of a spring and shaft assembly
how is oil contamination of the air supply measured
through the use of an oil entrainment graph in ppm
what causes the thermostats to change their setpoints in winter/summer thermostats
two bimetallic elements, one direct acting and one reverse acting.
explain the use of bimetallic elements
two metals are placed together and both expand at different rates to allow for flexure, this is common in a pneumatic system to show temperatures changes
what are limit thermostats used for
used to keep temperature above or below a specific point. this is usually a safety factor to prevent harm to the system (freezing or excessive heat)
how is electricity used in air conditioning systems
used to power the motors of air side fans
Explain mechanical system heat transfer
uses evaporator to compressor to a heat exchanger with outside air
how are valves sized
valves are sized based on the medium (steam or water), the inlet temperature of the medium, the valve inlet and outlet pressure, the flow needed to meet the load,, and the heat exchanger or coil connection
describe vav air handling systems
variable air volume, constant temperature
name different ways to mount thermostats
wall mounted, aspirator installation (inside wall), on return grills
when is valve rebuilding performed
when a valve is mechanically damaged or the internal parts are worn by foreign material or wire drawing
when is valave repackaging performed
when the valve begins to leak excessively at the valve stem and packing gland
what is a heat pump
Direct expansion refrigeration system contains devices and controls that reverse the flow of refrigerant
name the two most common air line materials
copper and plastic
electrical heating elements
a coil that produces heat when energized
what is an automated control system
a control system that uses digital solid state components. completely controlled using on/off controls
describe pneumatic humidistats
a controller that uses compressed air to open or close a device which maintains a certain humidity level inside a duct or area
describe the most common method of humidity control
a humidity sensor or controller
differentiate between a normally open and normally closed valve
a normally open valve is a valve that does allow fluid to flow when the valve is in its normal position. a normally closed valve is a valve that does not allow fluid to flow when the valve is in its normal position
compare a pilot bleed thermostat to a bleed type thermostat
a pilot bleed thermostat has a relay while a bleed type does not. The pilot bleed type is a two pipe system. the relay in a pilot bleed type acts like an amplifier. see book page 165 for more info on relay
explain the use of a resistive bridge circuit in an electronic control system
a resistive bridge circuit contains four arms and four resistors. two are fixed and two are variable, the bridge circuit output is zero at setpoint and when setpoint is not met, the variable resistors allow for voltage to be sent to an actuator, changing the temperature
differentiate between a screw compressor and a vane compressor
a screw compressor uses two main shafts with a male and female gears to push air, a vain compressor is only one large shaft that rotates and pushes air using vanes and pistons
describe liquid chiller
a system that uses a liquid to cool building spaces
what is a bleed type thermostat and how does it work
a thermostat that utilizes a bimetallic element and bleed port. the thermostat changes the air pressure to a valve or damper actuator by changing the amount of air that is expelled to the atmosphere.
what is a pneumatic thermostat
a thermostat that utilizes a bimetallic element that bends proportionally to the change in temperature of the room
describe a diaphragm
acts as a barrier between the pneumatic supply and the piston cup
what are the advantages and disadvantages of self contained control systems
adv. - no external power supply is needed, and low installation cost disadv. - cannot be expanded to provide more control sequences, low accuracy
list four main groups of components that pneumatic control systems can be grouped into based on their function
air compressor station, transmitters and controllers, auxiliary devices, and controlled devices
describe how bimetallic elements work
an element that consists of two different types of metals that expand at different rates, the element facing out expands less compared to the inner material, allowing for proportional expansion to temperature change adjustments
what is a bleedport
an orifice that allows a small volume of air to be expelled to the atmosphere
how are overshooting and undershooting compensated for in on/off control systems
anticipators, a device that turns heating or cooling equipment on or off (on an on/off controller) before it normally would
advantages and disadvantages of using fuel oil
basis and grade of fuel oil is based on weight and viscocity of the oil, adv. - fuel oil and natural gas are interchangeable, lower cost, easier availability, purchase in large quantities when cost is low disadv. - increased environmental pollution, winter heating of the fuel oil also increases cost of the fuel oil
name four pressure switch models
bleed and pilot bleed, direct acting and reverse acting
three ways temperature control can be accomplished
building space temperature, return air, air volume
differentiate between closed control loop and open loop control
closed loop control is when feedback occurs between the three devices (if the temperature increases, the VAV will automatically open) open loop control is control in which on feedback occurs between the three devices (unit ventilator uses outside air to start controlled device, not wall temperature sensor)
how does natural gas and lp generate heat
combustion, gas reacts with the oxygen to create combustion
define comfort
comfort is a condition in which people cannot sense a difference between themselves and the surrounding air
explain proportional control
control in which the controlled device is positioned in direct response to the amount of offset in the system (10% in heat causes 10% increase in valve position)
what is an absorption refrigeration system
is a refrigeration system that uses the absorption of one chemical by another chemical and heat transfer to produce a refrigeration effect
define control loop
is the arrangement of the controller, sensor and controlled device
name applications for pneumatic control systems
large commercial buildings such a hospitals and schools
what are three ways to prevent and remove moisture in a receiver
manual drains, automatic drains, refrigerated air dryer, desiccant dryer (absorption), and Filters
identify and define the two types of three way valves
mixing valve diverting valve
what is a hybrid control system
most commonly a combination of pneumatic and low voltage automated controls that uses a PE device to convert pneumatic information to low voltage controls
why is automatic operation essential in a controls system?
not all buildings are staffed to enable continual adjustment of environmental controlled devices
list five energy sources for hvac cooling systems
outside air, electricity, cold water, steam, mechanical system heat transfer
differentiate between overshooting and undershooting
overshooting is the increase of a controlled variable above the controller setpoint undershooting is the decreasing of a controlled variable below the controller setpoint
list and define the three damper classifications
parallel blade, opposed blade, and round
identify and define the two types of air compressors
positive displacement and dynamic compressors
describe the power supply of a self contained control system
power is provided by a sealed liquid bulb and a liquid that allows for expansion
what three tools are required to troubleshoot any type of system
pressure gauge, gauge adapter, and an accurate thermometer