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Hvac/R > Basics > Flashcards

Basics Flashcards

1st year

1
Q

What does the compressor do

A

Creates pressure difference needed to facilitate refrigerant flow through system heat

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2
Q

Define pressure

A

Pressure is the force applied to a specific unit of area. The atmosphere around the Earth has a weight and therefore exerts pressure

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3
Q

Capacitance

A

Capacitors store electric charge. Rated in microfarads. Run capacitors used to increase running efficiency, start capacitors used to increase starting torque

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4
Q

Teansformers

A

Produce an electric potential in a secondary circuit by electromagnetic induction. Often used to create 24 volt power source for control circuits. Rated in volt-amperes ( VA)

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5
Q

What is designed superheat

A

Typically between 8°F and 12°F

Superheated vapor does not follow a pressure/temperature relationship

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6
Q

What does the compressor do

A

Pumps heat laden vapor from the evaporator to the condenser by increasing the refrigerant pressure. Reduces pressure on the low side, increases pressure on the high side

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7
Q

What is subcooling

A

The cooling of a liquid refrigerant below it’s saturation temp

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8
Q

Common compressor types

A

Scroll, reciprocating, and rotary

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9
Q

Four temperature scales are

A

Celcius, Fahrenheit , Kelvin, Rankin

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10
Q

BTU

A

British thermal units

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11
Q

Random defrost is accomplished

A

When the refrigeration system has enough reserve capacity to cool more then the load requirement

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12
Q

An accumulator at the outlet of the evaporator is there to

A

Provide for the peak amount of active liquid refrigerant in the evaporator

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13
Q

What prevents the oil pressure safety control from de-energizing a compressor immediately after start-up

A

Built-in 90 second time delay

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14
Q

King valve

A

The valve on the outlet of the receiver ; allows you to isolate line, or check pressure on the suction line.

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15
Q

Purpose of a TEV Sensing bulb with a limited, or gas charge

A

They produce a maximum operating pressure @ high operating superheat

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16
Q

Specific Heat of Air? , water? Ice? Steam?

A 
Air = .24 btus/lb/°F
Water =  1.00 btu/lb/°F
Ice = .50 btu/lb/°F
Steam = .50 btu/lb/°F
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17
Q

How many btu’s to change ice to a liquid

A

144 btu’s/lb

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18
Q

How many btu’s to change state of liquid to steam

A

970 btu’s/lb

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19
Q

How many btu’s to change from ice to a boil

A

180 btu’s/lb

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20
Q

Solenoid Valve

A

Used to start or stop refrigerant flow. Normally open or normally closed. Valves must be installed with arrow pointing in direction of flow.

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21
Q

TXV controls refrigerant flow to the evaporator based on…

A

Evaporator suction line superheat

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22
Q

Heat Relcaim

A

When heat is rejected back into building to be reused rather than rejected into the air.

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23
Q

De-Superheat

A

Removing sensible heat from a vapor to the point it is a saturated vapor.

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24
Q

typical low pressure gas piping system designed to operate @

A

7”- 14” WC with a pressure drop of 1” WC

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25
Q

Define Pressure in terms of molecular theory of gases

A

Defined as the force being exerted on the walls of a container by the random and continuous action of molecules in motion. Expressed as force per unit of area

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26
Q

factors that determine the gas flow through an orifice are…

A

The pressure drop across the orifice
the area of the opening
the specific gravity of the gas
the coefficient of discharge of the orifice

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27
Q

calibration of a domestic hot water tank can be achieved by

A

Removing the dial and re-positioning the stop with respect to the shaft position

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28
Q

The correct operation of a flame sensor working in conjunction with a flame rectification circuit

A

An ac voltage is supplied to a flame rod, providing flame present, a very small amount of current will flow in one direction, this current is in the range of (micro-amps) will flow through the flame from the ground back to the control where it is sensed,if the dc micro-amperage is in an acceptable range the ignition process is allowed to continue

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29
Q

Explosive Limits

A

Refer to the upper and lower percent of fuel in an air/fuel mixture that will support combustion when the process operates outside the confines of a burner system

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30
Q

The Correction to excessive downdrafts of an appliance vent would be to

A

Install a draft diverter

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31
Q

The purpose of venting a gas is to

A

Remove the products of Combustion

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32
Q

The orifice size is a precisely sized hole in the …

A

Spud

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33
Q

Lp Gas is normally supplied by a cylinder whose outlet pressure from the regulator is preset by the manufacturer at

A

11” WC

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34
Q

The burner Orifice

A

Meters the Correct amount of gas to the burner

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35
Q

If a burner designed to fire on natural gas is converted to burn propane then . . .

A

The manifold pressure will increase and orifice size will decrease.

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36
Q

What happens to a current in a circuit if the resistance is increased

A

The Current Decreases V 120v/60r = 2

c/r 120v/80r = 1.5

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37
Q

Describe a Thermistor

A

Resistor that changes resistance with temperature change

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38
Q

PTC

NTC

A

PTC (Positive Temperature Coefficient) = Resistance rises with rise in temperature
NTC (Negative Temperature Coefficient) = Resistance drops with rise in temperature

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39
Q

Thermopile

A

Multiple thermocouples 500-800 Milli-Volt

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40
Q

Low Pressure Controls

A

Normally Closed device that opens on a drop in pressure. Connected to the low pressure side, can be used for temperature control or for low charge protection. Usually an automatic reset device, can be manual or automatic.

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41
Q

High Pressure Controls

A

Normally Closed device that opens on the rise in pressure. Connected to the high pressure side of the system. Used to disable a compressor if the head pressure exceeds safe limit. can be automatic or manual reset. Can be factory set or field adjusted

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42
Q

Pressure Transducers

A

Transducers convert the sensed pressure to an electronic signal. signal is processed by the micro processor, the processor adjusts the system operation.

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43
Q

Cooling thermostats

A

Close on a rise in temperature

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44
Q

heating thermostats

A

close on a temperature drop

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45
Q

Heat Anticipator

A

Reduces system overshoot

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46
Q

Cold Anticipator

A

Anticipates system Lag

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47
Q

Micro Amps

A

The Amount of current flow that is acceptable to prove the presence of a good flame

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48
Q

Flame Rectification

A

The process where the control sends out an AC voltage to the flame rod, and the presence of a good flame would provide a path for DC current flow through the flame back to the control.

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49
Q

Flame Current

A

Quantity of electrons flowing in one direction that have been sent through the sensing flame

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50
Q

Flame failure response time

A

The time it takes the control module to detect the presence or absence of a flame

51
Q

To verify the operation of a pressure switch you would

A

Install a tee in the line and place a manometer in the line and blow or create a negative pressure to read the value that the switch makes and breaks the switch and verify the value you read on the manometer against the manufacturer spec

52
Q

The proper sequence of operation for a DSI Ignition system is. . .

A

Call for heat, High voltage spark on, and Gas valve opens, flame proven, spark stops, gas on until call for heat is satisfied as long as flame proves.

53
Q

Blower door switch is designed to

A

Kill power to all exposed electrical functions to prevent unauthorized individuals from get a shock.

54
Q

The function of the ECO

A

A safety device located in the controlled medium that will open the pilot circuit if an over temperature condition exists.

55
Q

Medium Pressure gas piping system operates at . . .

A

2 Psig with a pressure drop of 1.5 psig

56
Q

What does the evaporator do

A

Absorbs heat from area to be cooled, Low pressure side of system

57
Q

Charles Law

A

Relates Volume + Temperature

58
Q

General Law of Perfect Gases

A

Relates pressure, volume and Temperature

P1 x V1 P2 x V2
———- = ————
T1 T2

Simplified = T2 x P1 x V1 = T1 x P2 x V2

59
Q

Dalton’s Law

A

Relates pressures of gases in a mixture

60
Q

How is energy Purchased

A

Electrical = KWH
Fuel/Oil = Gallon
Natural Gas = Ft^3

61
Q

What is the equation for work? What is work measured in?

A

Work = force x distance
Measured in FT-Lbs
Ex. How much work is done to move a 150 lb object 100 ft
A= 150 lb x 100 ft = 15000 ft-lbs

62
Q

Define Power
What unit is power measured by?
How is it rated?

A

The Rate in which work gets done
Work per unit time = ft-lbs/min
Rated in Horse power ( 1 HP = 33000 ft-lbs/min)

63
Q

What is Electrical power Measured in?
How Many Btu’s is in a watt? Kw?
How many watts in 1 HP?

A

Measured in watts
1 watt = 3.413 btu
1 KW = 3413 btu
1 HP = 746 watts

64
Q

Refrigeration process temperature ranges

A

High Temp = Air Conditioning
Medium Temp = Fresh Food preservation (35°F - 40°F)
Low Temp = Frozen food preservation (0°F - -10°F)

65
Q

Define Refrigeration

A

The process of transferring heat from a place that it is not wanted to where it makes little to no difference

66
Q

What do you call heat when heating beyond the saturation Temperature

A

Superheat

67
Q

What is Atmospheric pressure

A

@ Sea level 14.696 Psi, Usually rounded up to 14.7 psi

68
Q

What happens when pressure is added to bourden Tube

A

The tube straightens out and needle shows the pressure changes

69
Q

Define Matter

A

Described as any substance that occupies space and has mass, exists as a solid, liquid, or a gas.

70
Q

How does liquid exert pressure

A

Downward and outward

71
Q

How do gases exert pressure

A

Molecules travel at random when container with small amount of gas pressure is opened the gas molecules seem to repel each other and fly out.

72
Q

Define Mass

A

Property of matter that responds to gravitational attraction.

73
Q

Define Weight

A

Force that matter (solid, liquid, or gas) applies to a supporting surface when it is at rest.

74
Q

Define Density

A

Mass to volume relationship, Water has a density of 62.4 lb/ft3

75
Q

Define Specific Gravity

A

Density of a substance divided by the density of water

specific gravity is unitless

76
Q

Define Specific Volume

A

Volume of 1 lb of gas measured in ft3/lb

77
Q

What do you always do when working with gas laws?

A

Always use absolute Pressures (PSIA) and temperatures (R or K)
PSIA = PSIG + 14.7 psi

78
Q

Boyles Law

A

Relates Pressure + Volume

79
Q

If a 100lb Block is on a surface of 100”^2 what is it’s PSI

A

1 PSI

80
Q

How does heat travel

A

From a warm substance to a cold substance

81
Q

What is absolute zero?

A

The temp @ which all moleculer movement stops.

-460°F 0°R

82
Q

What is the equation to convert °C to °F and convert °F to °R

A 
F = (1.8 x °C) + 32
R = °F + 460
83
Q

What is the equation to convert °F to °C?

and Convert °C to K

A 
C = (°F - 32) / 1.8
K = °C + 273
84
Q

What are 2 Common Gauges

A

Compound Gauge ; Reads pressures both above and below Atmospheric pressure (used on low side)
High Pressure Gauge (used on high side)

85
Q

Define Barometer

A

Measures atmospheric pressure in inches of mercury. two of the barometers used are the mercury and the aneroid.

86
Q

Define Sensible Heat

A

Sensible heat causes a change in temperature that can be measured by a thermometer

87
Q

Define Latent Heat

A 
Latent heat (AKA Hidden heat) is heat added to a substance that causes a change of state. Can not be measured by a thermometer. 
Ex. Heat added to melting ice, causes it to melt but does not increase the temperature.
88
Q

What state does liquid typically enter the evaporator?

A

75% liquid 25% vapor

89
Q

What does the condenser do?

A

Rejects system heat.

High pressure side

90
Q

Parallel Circuits

A

The total voltage is applied across each resistance. the current is divided between the different loads according to their individual resistances, and the total current is equal to the sum of the currents in each branch. the total resistance is less than the value of the smallest resistance.

91
Q

Series Circuit

A

The voltage is divided across the different resistances. the total current flows through each resistance or load. the resistances are added together to obtain the total resistance.

92
Q

Ohms Law

A

Relationship between voltage, current, and resistance.
Voltage = Current x Resistance
Current = Voltage / Resistance
Resistance = Voltage / Current

93
Q

Load

A

Device that uses electrical power can be resistive or inductive

94
Q

Current

A

the amount of electron flow per unit time. Measured in amperes, indicated By A or I

95
Q

What is Voltage

A

Electrical Pressure or Electromotive force (EMF) The difference in potential between two points

96
Q

AC

A

Alternating Current. Continually reverses direction as power source is changing. Electron flow changes directions.

97
Q

DC

A

Direct Current. Current travels in one direction. Negatively charged electrons flow to atoms with positive charges. typically found in circuits powered by batteries.

98
Q

What is a magnets line of force called?

A

Magnetic Flux. When the lines of flux are cut with a conductor, electrical current is generated.

99
Q

Insulators

A

Atoms with several electrons in the outer orbit are poor conductors, considered to be insulators.

100
Q

Conductors

A

good conductors are those with few electrons in the outer orbit.

101
Q

Neutrons

A

Neutrally charged Particles

102
Q

Electrons

A

Negatively charged particles

103
Q

Protons

A

Positively charges particles

104
Q

Impedance

A

Total effect of resistance, capacitive reactance, and inductive reactance. The voltage leads the current in an inductive circuit. The current leads the voltage in a capacitive circuit.

105
Q

Sine Waves

A

Graphically represents alternating current through 360 electrical degrees. Shows peak to peak voltage values. effective voltage is RMS Value (root mean square) RMS value equal to 0.707 times the peak voltage.

106
Q

Wire Sizes

A

Resistance is affected by the material, cross sectional area, and length of the conductor. Lower resistance permits higher current flow. Larger diameter wire has more current carrying capability than smaller diameter wire.

107
Q

GFCI

A

Ground fault circuit interrupters.

senses small current leaks to ground

108
Q

Trial for ignition

A

A maximum period of time set by the control in which to establish and successfully prove the presence of an adequate flame.

109
Q

Approximate flame temp of natural gas

A

3500°F

110
Q

What is the approximate percentage of natural gas required at the burner to obtain max efficiency

A

10%

111
Q

Why does refrigerant get superheated after the evaporator

A

Ensures that no liquid gets into the compressor

112
Q

What does a metering device do

A

Regulates refrigerant flow to the evaporator

113
Q

Electronic Expansion Valves

A

Use either heat motors or step motors for valve movement use proportional, integral, and derivative (PID) controllers for their operations

114
Q

Solid state controlled expansion valves

A

Use a thermistor to monitor the evaporator outlet temperature to control refrigerant flow to the evaporator

115
Q

Pressure limiting TXV

A

Allows evaporator pressure to only reach a predetermined pressure. Valve closes when pressure exceeded desirable on low-temp applications.

116
Q

Dual Port TXV’s

A

Used with a system that has to go into pulldown mode. uses larger port for hot pulldowns, and smaller port for holding loads. Capacity is doubled when larger port is open all the way.

117
Q

Balanced - port TXV’s

A

Used where low ambient temperatures exist, head pressures vary widely, evaporator loads vary widely, large pressure drops exist across the txv, or low liquid line temps exist.

118
Q

Primary function of an expansion valve is

A

Regulate flow of refrigerant into the evaporator.

119
Q

Pressure limiting txv

A

Limits the refrigerant flow rate to the compressor capacity.

120
Q

External equalizer-type TXV

A

Used when there is a pressure drop greater than 2.5 psig in the evaporator

121
Q

Thermocouple

A

Consists of two dissimilar metals joined end to end; has a hot and a cold junction. Heat to the hot junction causes current flow generates about 20 milli-volts when heated. used in gas appliances to detect pilot light. the signal permits main gas valve to open. Thermopiles = multiple thermocouples
generate 500-800 milli-volts

122
Q

Gas pressure switches

A

Detect the presence of gas pressure in gas-burning equipment before burners are allowed to ignite. A safety control that should never be bypassed

123
Q

Air pressure controls

A

Can verify airflow through duct systems. Can be used to determine pressure differentials. Can be used as an interlock between the system blower and electric heater. Can be used to initiate the defrost cycle on heat pump systems.

Hvac/R (1 decks)

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