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