Basics Flashcards
1st year
What does the compressor do
Creates pressure difference needed to facilitate refrigerant flow through system heat
Define pressure
Pressure is the force applied to a specific unit of area. The atmosphere around the Earth has a weight and therefore exerts pressure
Capacitance
Capacitors store electric charge. Rated in microfarads. Run capacitors used to increase running efficiency, start capacitors used to increase starting torque
Teansformers
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)
What is designed superheat
Typically between 8°F and 12°F
Superheated vapor does not follow a pressure/temperature relationship
What does the compressor do
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
What is subcooling
The cooling of a liquid refrigerant below it’s saturation temp
Common compressor types
Scroll, reciprocating, and rotary
Four temperature scales are
Celcius, Fahrenheit , Kelvin, Rankin
BTU
British thermal units
Random defrost is accomplished
When the refrigeration system has enough reserve capacity to cool more then the load requirement
An accumulator at the outlet of the evaporator is there to
Provide for the peak amount of active liquid refrigerant in the evaporator
What prevents the oil pressure safety control from de-energizing a compressor immediately after start-up
Built-in 90 second time delay
King valve
The valve on the outlet of the receiver ; allows you to isolate line, or check pressure on the suction line.
Purpose of a TEV Sensing bulb with a limited, or gas charge
They produce a maximum operating pressure @ high operating superheat
Specific Heat of Air? , water? Ice? Steam?
Air = .24 btus/lb/°F Water = 1.00 btu/lb/°F Ice = .50 btu/lb/°F Steam = .50 btu/lb/°F
How many btu’s to change ice to a liquid
144 btu’s/lb
How many btu’s to change state of liquid to steam
970 btu’s/lb
How many btu’s to change from ice to a boil
180 btu’s/lb
Solenoid Valve
Used to start or stop refrigerant flow. Normally open or normally closed. Valves must be installed with arrow pointing in direction of flow.
TXV controls refrigerant flow to the evaporator based on…
Evaporator suction line superheat
Heat Relcaim
When heat is rejected back into building to be reused rather than rejected into the air.
De-Superheat
Removing sensible heat from a vapor to the point it is a saturated vapor.
typical low pressure gas piping system designed to operate @
7”- 14” WC with a pressure drop of 1” WC
Define Pressure in terms of molecular theory of gases
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
factors that determine the gas flow through an orifice are…
The pressure drop across the orifice
the area of the opening
the specific gravity of the gas
the coefficient of discharge of the orifice
calibration of a domestic hot water tank can be achieved by
Removing the dial and re-positioning the stop with respect to the shaft position
The correct operation of a flame sensor working in conjunction with a flame rectification circuit
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
Explosive Limits
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
The Correction to excessive downdrafts of an appliance vent would be to
Install a draft diverter
The purpose of venting a gas is to
Remove the products of Combustion
The orifice size is a precisely sized hole in the …
Spud
Lp Gas is normally supplied by a cylinder whose outlet pressure from the regulator is preset by the manufacturer at
11” WC
The burner Orifice
Meters the Correct amount of gas to the burner
If a burner designed to fire on natural gas is converted to burn propane then . . .
The manifold pressure will increase and orifice size will decrease.
What happens to a current in a circuit if the resistance is increased
The Current Decreases V 120v/60r = 2
c/r 120v/80r = 1.5
Describe a Thermistor
Resistor that changes resistance with temperature change
PTC
NTC
PTC (Positive Temperature Coefficient) = Resistance rises with rise in temperature
NTC (Negative Temperature Coefficient) = Resistance drops with rise in temperature
Thermopile
Multiple thermocouples 500-800 Milli-Volt
Low Pressure Controls
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.
High Pressure Controls
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
Pressure Transducers
Transducers convert the sensed pressure to an electronic signal. signal is processed by the micro processor, the processor adjusts the system operation.
Cooling thermostats
Close on a rise in temperature
heating thermostats
close on a temperature drop
Heat Anticipator
Reduces system overshoot
Cold Anticipator
Anticipates system Lag
Micro Amps
The Amount of current flow that is acceptable to prove the presence of a good flame
Flame Rectification
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.
Flame Current
Quantity of electrons flowing in one direction that have been sent through the sensing flame
Flame failure response time
The time it takes the control module to detect the presence or absence of a flame
To verify the operation of a pressure switch you would
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
The proper sequence of operation for a DSI Ignition system is. . .
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.
Blower door switch is designed to
Kill power to all exposed electrical functions to prevent unauthorized individuals from get a shock.
The function of the ECO
A safety device located in the controlled medium that will open the pilot circuit if an over temperature condition exists.
Medium Pressure gas piping system operates at . . .
2 Psig with a pressure drop of 1.5 psig
What does the evaporator do
Absorbs heat from area to be cooled, Low pressure side of system
Charles Law
Relates Volume + Temperature
General Law of Perfect Gases
Relates pressure, volume and Temperature
P1 x V1 P2 x V2
———- = ————
T1 T2
Simplified = T2 x P1 x V1 = T1 x P2 x V2
Dalton’s Law
Relates pressures of gases in a mixture
How is energy Purchased
Electrical = KWH
Fuel/Oil = Gallon
Natural Gas = Ft^3
What is the equation for work? What is work measured in?
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
Define Power
What unit is power measured by?
How is it rated?
The Rate in which work gets done
Work per unit time = ft-lbs/min
Rated in Horse power ( 1 HP = 33000 ft-lbs/min)
What is Electrical power Measured in?
How Many Btu’s is in a watt? Kw?
How many watts in 1 HP?
Measured in watts
1 watt = 3.413 btu
1 KW = 3413 btu
1 HP = 746 watts
Refrigeration process temperature ranges
High Temp = Air Conditioning
Medium Temp = Fresh Food preservation (35°F - 40°F)
Low Temp = Frozen food preservation (0°F - -10°F)
Define Refrigeration
The process of transferring heat from a place that it is not wanted to where it makes little to no difference
What do you call heat when heating beyond the saturation Temperature
Superheat
What is Atmospheric pressure
@ Sea level 14.696 Psi, Usually rounded up to 14.7 psi
What happens when pressure is added to bourden Tube
The tube straightens out and needle shows the pressure changes
Define Matter
Described as any substance that occupies space and has mass, exists as a solid, liquid, or a gas.
How does liquid exert pressure
Downward and outward
How do gases exert pressure
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.
Define Mass
Property of matter that responds to gravitational attraction.
Define Weight
Force that matter (solid, liquid, or gas) applies to a supporting surface when it is at rest.
Define Density
Mass to volume relationship, Water has a density of 62.4 lb/ft3
Define Specific Gravity
Density of a substance divided by the density of water
specific gravity is unitless
Define Specific Volume
Volume of 1 lb of gas measured in ft3/lb
What do you always do when working with gas laws?
Always use absolute Pressures (PSIA) and temperatures (R or K)
PSIA = PSIG + 14.7 psi
Boyles Law
Relates Pressure + Volume
If a 100lb Block is on a surface of 100”^2 what is it’s PSI
1 PSI
How does heat travel
From a warm substance to a cold substance
What is absolute zero?
The temp @ which all moleculer movement stops.
-460°F 0°R
What is the equation to convert °C to °F and convert °F to °R
F = (1.8 x °C) + 32 R = °F + 460
What is the equation to convert °F to °C?
and Convert °C to K
C = (°F - 32) / 1.8 K = °C + 273
What are 2 Common Gauges
Compound Gauge ; Reads pressures both above and below Atmospheric pressure (used on low side)
High Pressure Gauge (used on high side)
Define Barometer
Measures atmospheric pressure in inches of mercury. two of the barometers used are the mercury and the aneroid.
Define Sensible Heat
Sensible heat causes a change in temperature that can be measured by a thermometer
Define Latent Heat
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.
What state does liquid typically enter the evaporator?
75% liquid 25% vapor
What does the condenser do?
Rejects system heat.
High pressure side
Parallel Circuits
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.
Series Circuit
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.
Ohms Law
Relationship between voltage, current, and resistance.
Voltage = Current x Resistance
Current = Voltage / Resistance
Resistance = Voltage / Current
Load
Device that uses electrical power can be resistive or inductive
Current
the amount of electron flow per unit time. Measured in amperes, indicated By A or I
What is Voltage
Electrical Pressure or Electromotive force (EMF) The difference in potential between two points
AC
Alternating Current. Continually reverses direction as power source is changing. Electron flow changes directions.
DC
Direct Current. Current travels in one direction. Negatively charged electrons flow to atoms with positive charges. typically found in circuits powered by batteries.
What is a magnets line of force called?
Magnetic Flux. When the lines of flux are cut with a conductor, electrical current is generated.
Insulators
Atoms with several electrons in the outer orbit are poor conductors, considered to be insulators.
Conductors
good conductors are those with few electrons in the outer orbit.
Neutrons
Neutrally charged Particles
Electrons
Negatively charged particles
Protons
Positively charges particles
Impedance
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.
Sine Waves
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.
Wire Sizes
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.
GFCI
Ground fault circuit interrupters.
senses small current leaks to ground
Trial for ignition
A maximum period of time set by the control in which to establish and successfully prove the presence of an adequate flame.
Approximate flame temp of natural gas
3500°F
What is the approximate percentage of natural gas required at the burner to obtain max efficiency
10%
Why does refrigerant get superheated after the evaporator
Ensures that no liquid gets into the compressor
What does a metering device do
Regulates refrigerant flow to the evaporator
Electronic Expansion Valves
Use either heat motors or step motors for valve movement use proportional, integral, and derivative (PID) controllers for their operations
Solid state controlled expansion valves
Use a thermistor to monitor the evaporator outlet temperature to control refrigerant flow to the evaporator
Pressure limiting TXV
Allows evaporator pressure to only reach a predetermined pressure. Valve closes when pressure exceeded desirable on low-temp applications.
Dual Port TXV’s
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.
Balanced - port TXV’s
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.
Primary function of an expansion valve is
Regulate flow of refrigerant into the evaporator.
Pressure limiting txv
Limits the refrigerant flow rate to the compressor capacity.
External equalizer-type TXV
Used when there is a pressure drop greater than 2.5 psig in the evaporator
Thermocouple
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
Gas pressure switches
Detect the presence of gas pressure in gas-burning equipment before burners are allowed to ignite. A safety control that should never be bypassed
Air pressure controls
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.
