SYSIEM OVERVIEW
SYSTEM OVERVIEW
The Lab-Volt Refrigeration Training System consists ofrefrigeration components, as
wellas instrumentation and control components. These components are located on
the front paneland at the rear bottom of the trainer, as Figures 1-1 and 1-2 show. (ln
Figure 1-2,
the
protecting plate along
the
mountang
base of
the
compressor/condenser assembly is not shown).
.
The refrigeration components include a compressor, a condenser, expansion
devices (capillaries and a thermostatic expansion valve), an evaporator, and
refrigerant copper tubing.
.
The instrumentation and control components permit the electrical control,
monitoring, and protection of the refrigeration components. They include highand low-pressure gauges, a lhermostat, a solenoid valve, as well as pressure and
temperature lransmitters. An electronic pressure controller keepsthe compressor
operating at proper levels.
ln addition, an electrical control panel, located at the bottom of the trainer front
panel, provides electrical switches and control knobs to set the system under
various configurations. Silk-screened on the righlhand side of this panel is a
diagram that shows the electrical connections between the system components
requiring an AC voltage to operate. Banana.jacks permit the measurement of this
voltage at various points of the system for maintenance and troubleshooting
purposes.
The Refrigeration Training System is intended for use with the Heat, Ventilating, and
Air Conditioning (LVHVAC) software, which permits the real{ime monitoring of lhe
system variables. This requires that the computer used to run LVHVAC be
connected to the Refrigeration Training System, via a USB connection.
SYSTEM OVERVIEW
Figure 1-1. Front view of the Refrigeration Training System.
CIRGLED
NUMBER
CIRCLED
COMPONENT
NUMBER
COMPONENT
1
Solenoid valve
10
Low (LP)- and high (HP)-pressure gauqes
2
Filter/drier
11
Pressure controller
3
Liquid indicator
12
4
Water collectinq trav
Two-way hand-operated valves
13
Main POWER switch
COMPRESSOR switch
5
14
EVAPOMTOR-FAN SPEED control knob with
on/off switch
6
Expansion (metering) devices
15
CONDENSER-FAN SPEED control knob with on/off
switch
7
Evaporator and its fan
16
HEAT LOAD switch
8
Cooling chamber
17
I
Remote bulb thermostat
18
Electrical diagram with banana jacks
USB port
19
Faults access panel
Table 1-1 . Components located on the front panel (ref. to Figure 1-,1).
SYSTEM OVERVIEW
Figure1.2'ViewoftherearbottomoftheRefrigerationTrainingSystem.
CIRCLED NUMBER
COMPONENT
20
Suction line accumulator
21
High-pressure controller with manually-reset breaker
22
Refrigerant receiver
23
Compressor
24
Forced-air condenser
(ref' to Figure
Table .l -2. components located at the rear bottom of the trainer
REFRIGEr.',J7o/n{rF'// Gs
1
-2)'
SYSIEM OVERVIEW
Technical Data
Table 1-3 provides technical data on the Refrigeration Training System, at 120 VAC
and 2201240 VAC. This table is also provided as a reference in Appendix A of the
manual.
COMPONENT
TECHNICAL DATA
120 VAC
2201240 V AC
Compressor
Hermetic-type, 124 W
(0.'167 hp), start
capacitor, thermally
protected, 1 15 VAC,
60 Hz, 18-A lockedrotor current (LRA),
2.9-A rated load
current (RLA)
Hermetic-type, 186 W
(0.2s0 hp), sta(
capacitor, thermally
protected,
2001240 VAC, 50 Hz,
12.3-A locked-rotor
current (LRA), 2.3-A
rated load curent (RLA)
Refrigerant
R-134a
R-134a
Nominal charge
1.09 kg (2.4 lb)
1.09 kg (2.4 lb)
oit
Polyol esther
Polyol esther
Evaporator
Forced-air coil with
yariable-speed fan,
enclosed in a cooling
chamber, 120 VAC,
60 Hz, 0.58 A
Forced-air coil with
variable-speed fan,
enclosed in a cooling
chamber, 240 V,
50/60 Hz, 0.35 A
Condenser
Forced-air coil with
variable-speed fan,
120 VAC, 60 Hz,
0.41 A
Forced-air coil with
variable-speed fan,
230 VAC, 50/60 Hz,
0.2 A
Thermostat setpoint (typicat)
5"C (41'F)
conlroller Cut-in
(typicat) pressure
Pressure
settings
5'C (41'F)
(1)
2.07 barg (30 psig)
(1)
2.07 barg (30 psig)
(cr1)
Delay (ASd)
Cut-out
pressure (CO'1)
Operating
Lower point
pressures (typical)
Highest point
1 bar
gauge (barg) =
1OO
Null
Null
0.69 barg ('10 psig)
1.4 barg (20 psig)
(r)
7.6 barg (1'10 pstg;
kpa gauge = 14.S
psa
(1)
t1t
0.69 barg (10 psig)
1.4 barg (20 psig)
(r)
7.6 barg (110 psig)
gauge (psig)
Table 1-3. Technicaldata on the Ref.igeration Training System.
(rt
(1)
REF RI G ERATI
O N F U N D AM
ENTALS
Energy
Energy is the ability to do work. Energy exists in two forms, potential and kinetic:
.
Potential energy is energy a body possesses due to its position or particular
physical or chemical state.
.
Kinetic energy is energy a body possesses due to its motion.
Depending on the work
to be done, energy can be described as
thermal.
mechanical, electrical, etc.
Energy is measured in different units, such as
.
.
metric or S.l. units of joules (J). Since the joule is a relatively small unit, the
kilojoule (kJ) is used more often.
imperialunits of British thermal units (Btu)in thecontextof refrigeration systems.
The relationship between the above units is as follows:
1000J=1kJ=0.949Btu
Temperature
Temperature is a measure oflhe average kinetic energy ofthe particles that make
up a body. The greater the kinetic energy of the Particles is, the higher the
temperature of the body will be.
Thermal Energy and its Transfer
Whenever two bodies of different temperatures are broughl together, the particles
of the two bodies will collide due to their random motion. The particles of the hotter
body, which have greater kinetic energy, will be slowed down by the collisions, while
the slower particles of the cooler body will get faster.
As a result ofthese collisions, thermalenergy from the hotter body will be transferred
to the cooler body. This will cause the temperature of the hotter body to decrease
and the temperature of the cooler body to increase.
This phenomena is referred to as thermal energy transfer (or heat transfer).
Thermal energy transfer, if left to itself, will continue until the particles of the two
bodies have equal amounts of thermal energy. When this condition occurs, the tvvo
bodies have equal amounls ofthermal energy. The two bodies have attained equal
temperatures and are said to be in thermal equilibrium.
REF RI G E RATI O N F U N D AM ENTA LS
There are three mechanisms by which lhermal energy transfer occurs, which are
conduction, convection, and radiation.
.
Conduction: thermal energy is transferred by direct contact between the particles
ofa Single body, or between the particles of two (or more) bodies in good thermal
contact with each other.
.
Convection: thermal energy is transferred between the particles of a fluid.
Convection can be natural or forced. Natural convection causes the heated fluid
to become lighter and rise up into a cooler, denser region. Forced convection is
the forced circulation of a fluid by a mechanical device such as a fan or pump.
.
Radiation: thermal energy is transferred through the effects of electromagnetic
radiation. For example, the effect of radiation from lhe sun warming your face
while the surrounding air is relatively cool.
Basic Principles of Refrigeration Systems
The main function of a refrigeration system is to remove thermal energy (heat) from
a place and transfer it to another place. All refrigeration systems follow these basic
principles:
1. Thermal energy, when lost or gained by a fluid (a liquid or a gas), normally
causes the temperature of the fluid to change. The gained or lost energy that
causes a change in temperature is called sensible heat.
2. A change of state is said to occur when a fluid changes lo a gas or a liquid.
When it changes from liquid to gas, the fluid absorbs thermal energy. When it
changes from gas to liquid, the fluid loses thermal energy.
3.
During a change of state, the thermal energy gained or lost by the fluid does not
cause a change in the temperature ofthe fluid. This occurs because the gained
or lost energy goes into changing only the potential energy of the fluid, not its
kinetic energy. Consequently, the gained or lost energy is called latent heat.
During a change of state, the temperature of the fluid is proportional to the
saturation pressure of the fluid.
4.
Metallic parts used in a refrigeration system are selected as a function of their
thermal conductivity.
As figure 2-1 shows, a refrigeration system contains four basic devices:
-
a compressor;
a condenser;
an expansion (metering) device:
an evaporator.
REF RIG ERATI O N F U N DAM ENTALS
I
I
REFRIGERANT FLOW
<:-
/
7
LlaUlD LINE
H(mNStOl{ (mETER|NG)
DEVICE
CONDENSER
EVAPORATOR
4,
&I
THERMAL
ENERGY
SUCTION LINE
SUPERHEATED VAPOR
REFRIGERATED SMCE
I
COMPRESSOR
HIGH.PRESSURE
(GoNDENSTNG) SIDE
LOW+RESSURE
(cooLING) SIDE
DARK
I
RED
COLOR
KEY
LIGHT
RED
tt
ll
HIGH.
HIGH.
PRESSURE
GAS
PRESSURE
LIOUID
DARK
BLUE
I
LOW-
PRESSURE
LIQUID
LIGHT
BLUE
L__l
LOWPRESSURE
GAS
Figure 2-1. The four basic components of a refrigeration system'
The Refrigeration Cycle
ln Figure 2-1, the compressor is the heart of the refrigeration system: it makes the
refriglrant flow through the system. To do so, it creates, along with the expansion
device, a difference in pressure between the low-pressure (cooling) and highpressure (condensing) sides of the system'
As the refrigerant flows through the system, it gives up or absorbs thermal energy
by changing from gas to liquid, and then returning from liquid to gas'
-
The compressor produces a hightemperature, high-pressure superheated
vapor(') at its HP (discharge) side.
(1)Avaporissaidtobesuperheatedwhenitstemperatureishigherthanthalnormallycorrespondingtoitscurrentpressure. lnthiscondition,
the vapor is fully saturated.
REFRI G ERATI O N TRAI N I NG SYSTEM
REF RIGERATION F U N DAM ENTALS
-
This vapor enters the condenser and, as it flows through the tubing coils, gives
up thermal energy until it turns into a high-pressure saturated liquid. Once it has
lost all its latent heat(2), the liquid is said to be subcooled(3).
-
The subcooled liquid from the condenser then flows through the expansion
device, where it is forced through a restriction or a small orifice. Due to its high
resislance, the expansion device regulates the flow of refrigerant and causes a
drop in the pressure of the refrigerant, thereby decreasing its boiling point
accordingly. ThiS causes part ofthe liquid to turn into vapor, and the temperature
of the remaining liquid to decrease.
-
The mixture of liquid and vapor then flows through the evaporator, where it
absorbs thermal energy until it becomes a saturated vapor. This absorption of
thermal energy causes the temperature in the refrigerated space to decrease.
The saturated vapor further absorbs thermal energy before it leaves the
evaporator, turning into a surperheated vapor. This prevents liquid refrigerant
from entering the compressor.
-
This low-temperature, low-pressure superheated vapor then goes to the
LP (suction) side of the compressor, where it is compressed and turned into a
high{emperature, high-pressure superheated vapor, to start a new refrigeration
cycle.
Note: Superheat does not necessaily mean that the refrigerant is hot. ln fact,
superheated vapor can bequitecold. Similarly, subcooted does not necessarily
mean that the refigerant is cold, as it can be quite warm.
Pressure Measurements
Pressure can be measured in different units. such as
.
.
.
metric units of bars (bar);
S.l. units of kilopascats (kPa):
imperial units of pounds-force per square inch (psi).
The relationship between the above unils is as follows:
1 bar = 101.3 kPa = 14.7 psi
To monitor a refrigeration cycle, pressure measurements must be performed along
the system. This can be performed by using pressure gauges, which provide a direcl
visual reading of the pressure on a dial.
r2)
Latent heat, es earlier menlioned, is lhe energy gained or lost during a change of slate that does not cause a change in the
lemperature
of the liquid or gas.
l:) A liquid
is said to be
subcooled when
its temperature is lower lhan that nofinally corresponding lo ils
curent pressure.
REF RIG ERATI O N F U N D AM ENTALS
There are several types of pressure gauges, including:
-
absolute pressure gauges;
gauge pressure gauges;
compound pressure gauges.
Ab sol ute Pressure Gauges
The air pressure in outer space is 0 bar (0 psi or 0 kPa), because there is no air
(perfect vacuum). When measured with respect to vacuum, pressure is called
absolute pressure.
Absolute pressure gauges, therefore, measure pressure with respecl to vacuum. At
atmospheric pressure, they provide a non-null reading of about 1 bar absolute.
(14.7 psia or 101.3 kPa absolute). Because of this, they are not commonly used in
refrigeration systems.
'Atmospheric pressure at sea level.
Gauge Pressure Gauges
When measured with respect to atmospheric pressure, pressure is called gauge
pressure.
Gauge pressure gauges, therefore, provide a null reading of 0 bar gauge (barg) at
atmospheric pressure (or 0 psig, 0 kPa gauge, etc., depending on how the gauge is
graduated). Thus, atmospheric pressure is not included in the gauge pressure
reading.
REFRIG ERATIO N FUN DAM ENTALS
Gauge pressure gauges are commonly used in the high-pressure section of
refrigeration systems. A gauge pressure gauge is shown in Figure 2-2.
,,V,,,*
AT
OSPHERIC PRESSURE
0 barg , 0 p.lg
- I bar abs. / 14.7 psla
Figure 2-2. Gauge pressure gauge (barg/psig readings).
As a summary, Table 2-1 indicates the relationship between absolute and gauge
pressure measurements, in the metric, imperial, and S.l. systems of units.
METRIC
IMPERIAL
s.t.
0 barg
= 0 psig
= 0 kPa gauge
= 1 bar abs.
= 14.7 psia
= 101 .3 kPa abs.
'1
= 14.7
barg
psig
= 101.3 kPa gauge
Table 2-1. Relationship between gauge and absolute pressure measurements.
REF RI G ERAN A N F U N DAM ENTALS
Compoun d Pressure Gauges
Compound pressure gauges can permit the measurement of either gauge or
absolute pressures, as Figure 2-3 shows.
The gauge can read pressures above atmospheric pressure, in barg, psig, or
kPa gauge, depending on how it is graduated.
-
It can also read pressures below atmospheric pressure, in bar abs., psia,
of mercury (mmHg), or inches of water (inHrO), for example,
millimeters
depending on how it is graduated.
bars or
/
pors
/
ATUOSPHERIC PRESSURE
(0barg/0pelg)
io
40
so
--_____-->
\
barabs.orpeia
\
Figure 2-3. Compound gauge.
Temperature Measurements
ln addition to pressure
measurements, temperature measurements must be
performed to monitor a refrigeration cycle.
Temperature is measured on a temperature scale. There are fourtemperature scales
whic'h are in use today: the Celsius scale, the Kelvin scale, the Fahrenheit scale, and
the Rankine scale. Appendix C shows a comparison of the four temperature scales.
ln refrigeration systems, the Celsius (S.1. and metric unit) or Fahrenheit (imperial
unit) scales are normallY used.
.
The water boiling point corresponds to 100 on the Celsius scale, or 212 on the
Fahrenheit scale.
REFRI G ERAT I O N TRAI N I NG SYS TEM
R EF RIG
ERATI O N F U N DAM ENTA LS
.
The ice melting point corresponds to O on the Celsius scale, or 32 on the
Fahrenheit scale.
Based on these two points, temperature measuremenls can be converted from
degrees Celsius to degrees Fahrenheit, and vice versa, as indicated inTable 2-2.
S.I. OR METRIC
FACTOR
Degrees Celsius ('C)
x
'1.8 + 32
IMPERIAL
= Degrees Fahrenheit ("F)
FACTOR
32 x 0.55
S.I. OR METRIC
Degrees Celsius
('C)
Table 2-2. Temperature conversion.
Refrigerants
A refrigerant is a fluid that can absorb or give up thermal energy (sensible and latent
heat). The amount of thermal energy gained or lost by a refrigerant under specific
conditions is delermined by the type of the refrigerant. A property of refrigerants is
that their boiling point at any given pressure is low, in comparison with other types
of fluids.
The refrigerant used in the Lab-Volt Refrigeration Training System is R-134a. This
chlorine-free refrigeranl was designed to replace the R-12 refrigerant. lt has no effect
on the ozone layer [Ozone Depletion Potential (OZp) = O], whiie its properties make
it suitable for a variety of air-conditioning and refrigeration applications.
REF RI G ERATI O N F U N D AM ENTALS
Table 2-3liststhe temperature-pressure relationship forthe R-134a refrigerant under
salurated condition.
REFRIGERANT TEMPERATURE
REFRIGERANT PRESSURE
-17.6"C (0"F)
0.43 barg (6.3 psig)
-14.9"C (5'F)
0.61 barg (8.8 psig)
-12.1'C (10'F)
0.80 barg (1'1.6 psig)
-6.6'C (20"F)
1
.24 barg (18.0 psig)
1.77 barg (25.6 psig)
1.1'C (30'F)
4.4"C (40'F)
2.38 barg (34.5 psig)
9.9'C (50'F)
3.10 barg (44.9 psig)
15.4'C (60'F)
3.92 barg (56.9 psig)
20.9"C (70'F)
4.88 barg (70.7 psig)
26.4'C (80'F)
5.96 barg (86.4 psig)
31.9"C (eo"F)
7.20 barg (104.2 psig)
37.4'C (100"F)
8 58 barg ('124.3 psig)
42.9"C (110'F)
10.13 barg (146.8 psig)
48.4'C (120"F)
1
53.9'C (130'F)
13.8 barg (13.8 psig)
'1
.86 barg (171 .9 psig)
"C (140"F)
15.91 barg (230.5 psig)
64.9"C (150" F)
18.24 bary (264.4 psig)
59.4
Table 2-3. R.1 34a temperature-pressure .elationship.
The figures in Table 2-3 come from the reference chart shown in Figure 2-4. This
chart, provided by the refrigerant's manufacturer, is usually provided on the form of
a practical pocket guide. lt shows the temperature-pressure relationship for different
types of refrigerants (e.g. R-22, R-123, R-134a, R-404A, etc.). As indicated on the
chart, the boiling point of the R-134a is -26.'l "C (-14.9'F) at atmospheric pressure.
Oil is used in refrigeration systems to lubricate and cool the compressor and
compressor motor. Because of this, some amount of oil is present in the circulating
refrigerant.
REF RIG ERATI O N F U N DAM ENTALS
@0D, Sur".."frigerants
R.l3ila BOILING POINT
AT
AT
OSPHERIC PRESSURE
:l
f1
i!
;
gl
g
Figure 2-4. Refrigerant manufacturer's chart.
REFRIGERATION COMPO,MMS FART 0
!ntroduction
As mentioned earlier, a refrigeration system consists of refrigeration components,
as well as instrumentation and control components.
are'. a compressor, a condenser, an
expansion device, and an evaporator. Additional refrigeration components are
necessary to ensure the safe and efficient operation of the system, as well as its
maintenance and troubleshooting, as Figure 3-1 shows:
The basic refrigeration components
-
a liquid receiver;
a filter/drier;
a moisture/liquid indicator;
a suction line accumulator;
service valves.
ln this job sheet, you will study the operation of the components highlighted in
Figure 3-1 . You will study the operation of the other components in the next exercise.
REFRTGERATTON COMPOTVEVTS (PART 0
EVAPORATOR
sPAl{stoN
(
ETERrl{c)
DEVICES
MANUAL
VALVES
MOISTURE
LIOUID
INDICATOR
SUCTION
FILTER
DRIER
<--
CO
/
LIOUID
RECEIVER
PRESSOR
DISCHARGE
CONDENSER
Figure 3-1. Retrigeration components of the system.
Compressor
The compressor draws low-pressure superheated vapor from the cooling (Lp) side
of the system at its suction inlet. lt compresses it to a high-pressure superheated
vapor and expels it, through its discharge outlet, to the condensing (Hp) side of the
system.
Figure 3-2 shows the compressor used on the Refrigeration Training System. This
compressor is of the reciprocating type. lt is said to be hermetic because it is
totally encased in a sealed housing. The housing contains an electric motor, a crank
shaft, a single cylinder, and lhe compressor. (Note that reciprocating compressors
can have more than one cylinder).
/
REFRTGERATION COMPONEMS FART t)
The rotation of the motor and crank shaft makes the cylinder piston move back
and forth (reciprocate). For every compression stroke ofthe piston, a new volume
of vapor is compressed and discharged at a high pressure.
The motor is cooled by the transfer of thermal energy from the stator to the case
(conduction), and by passing the returning vapor around the motor windings
(forced convection) before it is compressed.
A length of tubing (called process stub) connected to the case provides access
to the refrigerant inside the compressor for servicing purposes, and can be
resealed once lhe compressor is repaired. However, hermetic compressors are
often replaced instead of repaired, because lhe compressor case is welded
closed.
The compressor needs a certain amount of oil for lubrication of its moving parts.
sucnox
col{PRESSOR
TERIIINAL
(hrLET) UtlE
DISCHARGE (OUTLET} LINE
COVER
PROCESS
TUBE (STUB)
Figure 3.2. Reciprocating hermetic compressor of the Refrigeration Training System.
C o
m p re
ssor
Cha
racleristlcs
lmportant characteristics of a compressor are the volumetric flow rate, the
volumetric efficiency, and the compression ratio.
.
The volumetric flow rate is the volume of gaseous refrigerant discharged by the
compressor per unit of time. The formula for calculating the theoretical
volumetric flow rate is as follows:
Notei r/min stands for revolutions per minute.
Metric units:
volumetric flow rate(r"r.in) = Displacement(m3/r) . 5haft
speeduT,niny
REFRTGERATTON COMPONE TTS
?ART
t)
lmperial units:
Volumetric flow ratelr3rmin; = Displacemen\fl3/i .
speed,,rmin)
"n.L
When the compressor is of the reciprocating type, the volumetric flow rate is the
volume displaced by the cylinder(s) on each piston stroke (distance traveled from
bottom to top):
n2
v.'4 rTu L.N.n
where Vp
=
D=
L=
N=
n=
.
Volumetric flow rate [m3/min or ft3/min)]
Diameter of cylinder piston (m or ft)
Length of the piston stroke (m or ft)
Compressor speed (number of revolutions/min)
Number of cylinder(s)
The actual volumetric flow rate of the compressor is less than the theoretical
volumetric flow rate because of internal pressure losses across the cylinder
valves. The ratio of actual volumetric flow rate to theoretical volumetric flow rate
is called the volumetric efficiency. lt is expressed as:
Volumetric efficiency(%)
Actual volumetric flow
rate
.
100
Theor. volumetric llow rate
Small compressors used in domestic refrigeration syslems have volumetric
efficiencies between 40 and 75o/o, with 50% being an average value. Larger
commercial compressors have volumetric efficiencies between 50 and 80%, with
70% being an average value.
The compression ratio (a pure number) indicates the efficiency of the
compressor when using a particular refrigerant. lt is expressed as:
Compression ratio
=
Absolute pressure on suction side
Absolute pressure on discharge side
For a single stage reciprocating compressor, the maximum allowable
compressor ratio should be below 10. Table 3-1 lists typical compression
ratios for different refrigerants under normal conditions of 30"C (86"F)
condensing temperature and -15'C (5'F) evaporating lemperature. A value
of 4.7 for refrigerant R-134a means that the absolute discharge pressure
should be less than 4.7 times that of the absolute suction pressure. A higher
ratio can result in a loss of efficiency and in possible damage to the
compressor.
"The compression rulio oI 4.7 for the R-134a refrigerant is for a condensing
temperature of 54.4"C (130'F) and an evaporating temperature of 1.7'C
(35"F).
REFRTGERATTON COMPONENTS
REFRIGERANT
COMPRESSION RATIO
R-22
4.16
R-134a*
4.70
R-717
4.94
R-718
6.95
R-744
3.10
R-764
5.61
qART
t)
Table 3-1. Maximum typical compression ratios for different refrigerants'
Liquid Receiver
The liquid receiver is normally placed downstream of the condenser. lt stores liquid
refrigerant in excess and ensures a constant supply of liquid to the expansion device.
The liquid receiver is used in refrigeration systems that use an expansion valve as
the expansion (metering) device. The liquid receiver is unnecessary when the
metering device is a capillary tube, because in that case, all the liquid refrigerant
remains stored in the evaporator.
Typically, the liquid receiver should be large enough to hold all the refrigerant in the
system, because it will have to store all the refrigerant when the system is shut down
for repair or servicing.
Figure 3-3 shows a liquid receiver of the same type as the one installed on the
Refrigeration Training System. The receiver is designed for vertical mounting. The
receiver outlet is equipped with a Rotalock@valve used for initial refrigerant filling.
ROTALOCK
VALVE
INLET
(FROM CONDEI{SER OUTLET)
OUTLET
GO FTLTER DR|ER)
'
Figure 3-3. Liquid receiver of the Refrigeration System.
REFRTGERATTON COMPOTVEVTS
PART t)
FiltedDrier
The filter/drier is normally installed downstream of the liquid receiver. The filter
removes contaminanls such as dust, dirt, metal fillings, rust, from the flowing
refrigerant to prevent clogging of the small restriction of the expansion device and
damage to the compressor. The drier collects and holds moisture from the
refrigerant. The arrow points in the direction of refrigerant flow (from inlet to ouflet).
DIRECTION OF
REFRIGERANT FLOW
OUTLET
Figure 3.4. Fitter/drier.
Moisture/Liquid lndicator
The moisture/liquid indicator is normally installed just upstream of the expansion
(metering) device. lt consists of a sight glass and a sensing element that are used
to check the charge and condition of the flowing liquid refrigerant, as Figure 3-5
shows.
.
.
Bubbles seen through the sight glass can indicate a lack of refrigerant, a low
discharge pressure, or some form of restriction in the liquid line.
The color of the sensing element changes as the amount of moisture in the
refrigerant changes. By matching the color of the element with the colors on the
surrounding scale, the moisture content can be determined. With refrigerant
R-134a, for example, dark blue is often used to indicate a dry system and a
salmon color indicates a wet system. (However, the color selection is arbitrary
and varies by manufacturer).
REFRTGERATTON COMPONENTS
qART
t)
COLOR
SCALE
SENSING
ELEMENT
Figure 3-5. Moisture/liquid indicator.
Suction Line Accumulator
The suction line accumulator is placed just upstream of the compressor. lt is used
to prevent sudden surges of liquid refrigerant and oil from reaching the compressor.
Refrigerant flood back to the compressor is a frequent cause of broken compressor
valves, blown gaskets, and bent connecting rods.
OUTLET
FO COMPRESSOR)
INLET
(FROI EVAPORATOR)
I
I
V
LEAK+ROOF
FUSIBLE PLUG
METERING HOLE
Figure 3-6. Suction line accumulator.
REFRTGERATTON COMPOTVEVTS
PART 0
or horizontal. Figure 3-6 shows a vertical
accumulator. lt consists of a U-shaped tube with a small metering hole at the bottom,
where small amounts of liquid refrigerant and oil are temporarily collected. The
metering hole sends this mixture with the gaseous refrigerant, at a controlled rate,
to the compressor. Since the liquid boils as it flows through the suction line, only the
oil and refrigerant go to the compressor.
Accumulators are either vertical
It is
very important that the inlet and outletofthe accumulator be correcfly connected
an
to prevent refrigerant and oil from becoming trapped. The selection of
accumulator is based on the following factors:
.
.
.
.
the pressure drop created by the accumulator, which must not be too high;
the ability of the accumulalor lo return refrigerant at an adequate rate;
the refrigerant holding capacity ofthe accumulator, which should not be less than
50% of the total system refrigerant capacity;
the diameter, length, and orientation of the accumulator, which must suit your
application.
REFRIGERATION COMPOTVEMS qART tt)
AND ENTHALPY DIAGRAM
lntroduction
ln the previous job sheet, you familiarized yourself with part of the refrigeration
components that make up a basic refrigeration system. ln this job sheet, you will
study the remainder of these components: the condenser, the expansion
(metering) device, and the evaporator, as Figure 4-1 shows.
EVAPORATOR
EXPANSION (METERING)
L.-
AIUAL
VALVES
OISTURE
LIOUID
INDICATOR
^stsilil.?sFJ
fi_
COMPRESSOR
Figure 4-1. Refrigeration components of the system.
R E F R I G ER AT I O
N IRA'IV"VG SYSIEM
FILTER
DRIER
/
LIQUID
RECEIVER
'
REFRTGERATTON COMPOTVENTS
AND ENTHALPY DIAGRAM
?ART t0
You willthen familiarize yourself with the pressure/enthalpy diagram of a refrigerant.
You will learn how this diagram can be used to graphically represent the refrigeration
cycle of a system.
Condenser
The condenser removes thermal energy from the superheated vapor discharged
by the compressor, turning it into a saturated liquid. At the outlet of the condenser,
this liquid, because it has lost all its latent energy, is said to be subcooted.
The thermal energy removed by the condenser is transferred to the surrounding air,
as long as the temperature of the air stays lower than the condensing temperature
of the refrigerant.
-
As Figure 4-2 shows, a condenser consists of copper or aluminum tubing bent in
a serpentine shape through which the refrigerant is conducted.
-
As the refrigerant flows through the tubing, thermal energy from the refrigerant
transfers to the coils by forced convection and by conduction.
THERMAL
\lt
FROtf, COMPRESSOR
ENERGY
REJECTED
\t/
-*
HIGH+RESSURE SUPERHEATED VAPOR
&I**
HIGH+RESSURE LIQUIO REFRIGERANT
METER|NG
DEvlcE /
t\
+\
THERMAL
ENERGY
REJECTED
Figure 4-2. Removal of thermal energy through the condenser tubing.
Thermal energy from the tubing then transfers to the air flowing across the coils
by convection, causing the temperature of the refrigerant to decrease. The
convection is forced by a fan. The higher the rotation speed of the fan is, the
faster the circulation of fresh air across the tubing and, therefore, the higher the
rate of thermal energy transfer between the tubing and the air.
Attached to the tubing is a web of thin metallic fins that increase the surface of
thermal energy transfer between the tubing and the surrounding air, which further
increases the rate of thermal energy transfer by forced convection.
REFRIGERATTON COMPOTVEMS (PART tt)
AND ENTHALPY DIAGRAM
Figure 4-3 shows the condenser used on the Refrigeration Training System.
(b) FRONTVTEW
(A) REARVTEW
Figure 4-3. Condenser used on the Refrigeration Training System.
Expansion (Metering) Device
The expansion device is connected between the condenser and the evaporator. lt
makes the breakpoint between the high- and low-pressure sides of the system.
The expansion device consists of a restriction or small orifice, through which the
liquid refrigerant from the condenser is forced to flow. This creates a drop in the
pressure of the refrigerant, thereby decreasing its boiling point accordingly. This
in turn causes part of the liquid to turn into vapor, and the temperature of the
remaining liquid to decrease.
Due to its high resistance to the flow of refrigerant, the expansion device regulates
(meters)this flow to the evaporator.
The Refrigeration Training System has two types of expansion (metering) devices:
.
capillary tubes, which have a fixed restriction (orifice size); and
.
a thermostatic expansion valve, whose restriction can be adjusted.
Capillary Tube
A capillary tube is a simple length of tube having an inside diameter smaller than
that of the main refrigerant line, as Figure 4-4 shows. Because of this, the capillary
tube restricts and meters the liquid refrigerant.
R
EF RI
G
E RAT I O N TRAI N I NG S YSTEM
REFRTGERATTON COMPOTVEVTS
qART
tD
AND ENTHALPY DIAGRAM
The friction and evaporation of the refrigerant that takes place within the tube are
responsible for the pressure drop created across the capillary tube. The longer the
capillary tube, the greater the created pressure drop, for any given diameter of the
tube.
The Refrigeration Training System comes with two capillary tubes, mounted on the
front panel, just ahead ofthe evaporator. They both have the same inner diameter,
but they are of differing lengths, allowing you to test the effect that this difference
makes on the operation of the system. Since capillary tubes offer a flxed restriction
to refrigerant flow, they do not have the ability of thermostatic expansion valves
(TEV'S) to adapt to significant changes in the heat load.
REFRIGERAI{T FLOW
SiIALL.DIA*IETER
J--l
CaPTLLARY IUBTNG
HIGH+RESSURE
SUBCOOLED LIQUIO
LOW+RESSURE
FRO
IXTURE
TO EVAPORATOR
COT{DENSER
LARGEROIATETER
REFRIGERA T TUBIiIG
Figure 4-4. capillary tube op€ration.
Thermostatic Expansion Valve (TEV)
A thermostatic expansion valve (TEV) is a device that meters the flow rate of
refrigerant to the evaporator by adapting it to any changes in the heat load. lt allows
more refrigerant to flow when the evaporalor warms up and, conversely, it allows
less refrigerant to flow when lhe evaporator cools down.
A TEV consists of a diaphragm wlth a valve and seat, and a spring that is adjusted
to obtain the desired level of superheat. As Figure 4-5 (a) shows, an external
thermal bulb, partly filled with liquid refrigerant, senses the temperature of the
refrigerant at the outlet of the evaporator.
.
.
The difference between the temperature sensed by the bulb and that of the
refrigerant in the evaporator results in a pressure difference that acts on the
spring of the TEV to modify the TEV opening accordingly.
Thus, when the sensed temperature becomes lower than the TEv-superheat
setting, the TEV opening is decreased to reduce the flow of refrigerant entering
the evaporator. Conversely, when the sensed temperature becomes higher than
the TEV-superheat setting, the TEV opening is increased to increase the flow of
refrigerant entering the evaporator.
REFRTGERATTON COMPONEVTS (PART lt)
AND ENTHALPY DIAGRAM
.
When the compressor is stopped, the valve is automatically closed to stop the
flow of refrigerant to the evaporator.
On the Refrigeration Training System, the TEV is mounted on the front panel of the
trainer, just ahead of the cooling chamber. As Figure 4-5 (b)shows, the thermal bulb
of the TEV is attached to the suction line of the system.
REFRTGERATTON COMPONE TTS qART t0
AND ENTHALPY DIAGRAM
THERMAL
BULB
LIOUID
LINE
(a) Thermostatic expansion valve construction
_
-
EVAPORATOR
TO SUCTION
LINE
(b) Thermal bulb and capillary tube connected
to evaporator inlet (trainer interval view)
Figure 4-5. Thermostatic expansion valve.
Evaporator
The evaporator, located within the cooling chamber, turns the mixture of liquid and
gas refrigerant coming out of the expansion device into
a
Iow-pressure
superheated vapor. ln the process, the evaporator removes thermal energy from
the cooling chamber.
REFRTGERATTON COMPONETVTS PART il)
AND ENTHALPY DIAGRAM
An evaporator is basically
'Figure
constructed
the same as a condenser. As
4-6 shows, the evaporator consists of copper or aluminum tubing bent in a
serpentine shape through which the refrigerant is conducted.
COOLING CHAilIBER
THERTIAL
LOW+RESSURE
ENERGY
SUPERHEATED VAPOR
ABSORBED
\l
/
tl
/l\
/l\
""rt'rI
*
COMPRESSOR SUCTION
EXPANSION DEVICE
THERMAL
ENERGY
ABSORBED
LOW+RESSURE TTIXTURE OF
LIQUIDAND GAS
Figure 4-6. Absorption of thermal energy through the evaporator tubing.
As the refrigerant (mixture of gas and liquid) flows through the evaporator,
thermal energy from the warmer air in the cooling chamber transfers to the
refrigerant, through the evaporator tubing, by conduction, and through air
recirculation by the forced convection created by the evaporator fan.
The higher the rotation speed of the fan is, the faster the recirculation created by
the fan and, therefore, the higher the rate of thermal energy transfer between the
surrounding air and the refrigerant.
Attached to the tubing is a web of thin metallic fins that increase the surface of
thermal energy transfer between the tubing and the surrounding air, which further
increases the rate of thermal energy transfer by forced convection.
As it flows through the tubing of the evaporator, the refrigerant absorbs thermal
energy (sensible heat). This increases its temperature to the boiling point. As it
further absorbs thermal energy (latent heat), the refrigerant turns into dry vapor.
The vapor then flows from the evaporator outlet to the suction side of the
compressor.
REF RtG ERATT O N CO M pOrVErVrS (p ART I 0
AND ENTHALPY DIAGRAM
Figure 4-7 shows the evaporator of the Refrigeration Training System. This
evaporator is of the forced-circulation type, and is enclosed, along with an electric
fan, in a compact metal housing.
AIR FLOW OUT
ELECTRICAL
wlRII{G TO
HEAT LOAD
(LIGHT BULBS)
t-.o fl,.....*ro
FRO EXPANSPN DEvlcE
sucnoN Lll{E AccuItluLrAToR
ff
DRAII{
AIR FLOW II{
Figure 4-7. Evaporator used on the Refrigeration Training System.
Enthalpy Table
The amount ofthermalenergy per unit ofmass ofa refrigerant is called enthalpy (or
heat content).
Enthalpy can be measured in different units, such as
.
.
metric or S.l. units of kilojoules per kilogram (kJ/kg);
imperial units of British thermal units per pound-mass (Btu/lbm).
As it flows through the evaporator to refrigerate the space, the refrigerant absorbs
lhermal energy. Consequently, the enthalpy of the vapor at the ouflet of the
evaporator is greater than that of the liquid al the inlet of the expansion device. The
thermal energy absorbed per unit of mass of the refrigerant is called the
refrigeration effect.
Refrigerant manufacturers normally provide tables indicating the enthalpy of their
refrigerants at various temperatures. Appendix D, for example, is a table indicating
the enthalpy of saturated liquid and vapor for the R-134a refrigerant, at different
temperalures. Take a look at this table.
REFRIGERATION COMPOTVE TTS PART t0
AND ENTHALPY DIAGRAM
Pressure-Enthal py Diagrams
ln addition to tables, refrigerant manufacturers provide diagrams that show the
enthalpy of their refrigerant as a function of absolute pressure. The
pressure/enthalpy diagram of the R-134a, for example, is shown in
Figures 4-8 (S.1. units) and 4-9 (imperial units).
.
The horizontal axis of the diagram is graduated in enthalpy units, while the
vertical axis is graduated in absolute pressure units.
.
Two curves on the diagram indicate the changes in state between saturated
vapor (right-hand curve)and saturated liquid (left-hand curve).
.
Between these two curves are horizontal lines of constant temperature.
.
A point representing the current refrigerant properties (pressure, enthalpy, and
temperature) that is located on a horizontal line indicates that the refrigerant is a
mixture of tiquid and vapor. lf this point is located on or to the right of the
saturated vapor curve, the refrigerant is superheated. lf this point is located on
or to the left of the saturated liquid curve, the refrigerant is subcooled.
REFRIGERATTON COMPOTVEVTS
AND ENTHALPY DIAGRAM
?ART t0
ABSOLUTE
ABSOLUTE
PRESSURE
PRESSURE
abe.)
(b.r, ab..)
(liP.,
4n
I
10
)ul'rrnt l l uolochcnr ie.rls
HFC-134a
I
Pressure Enthalpy Diagram
{SlUnits)
6
4
100
80
CRINCAL
60
AREA
PRESSURE
40
20
SATUMTED
1
0.8
8
0.6
6
0.4
4
o.2
2
LIQUID
LINE
0.1
SATURATED
0.08
VAPOR
0.06
uitE
0.04
0.02
ET{THALPY
0.01
100
250
=2l0kJ/kg
I
300
=270kJrkg
Figure 4-8. Pressure/enthalpy diagram for the R-134a (S.1. units).
(kJ /ks)
REFRIGERATTON COMPOTVEVTS (PART tt)
AND ENTHALPY DIAGRAM
ABSOLUTE
PRESSURE
I)trl)rutl
1000
l
l^<
la'
sl
lrrotircltcttt icals
HFC-134a
800
,,1
i
t9
lt
o
,o
@
Pressure-Enthalpy Draqram
(ErElish Units)
600
i
CRMCAL
I
PRESSURE
V,
AREA
z
tcr
,\
400
ili li5l
El lr;l
is.f'o,K
I
li
ti
o
@
SATURATED
LIOUID
100
LIE
80
60
40
SATURATED
VAPOR
LII{E
10
I
6
4
t
I
q Qqea o ooo
o o 99oo o cioo
I
I
I
!'i
7P!
:ss
c5oci
ra,
c,
(o
.DO,
W
tttlti.t;
't I lUlll;
ESdee
o' o oro'
o o oo oo ddc'o
;-8ot
1,4 ):-tt
'll,'t
ENTHALPY
(Btu / lbm)
=25Bh!rlbm =53Btu/lbm
Figure 4-9. Pressure/enthalpy diagram for the R-134a (imperial units).
R EF RIG E
RATI O N IRA'^"/VG
SYSIE'}'
REFRTGERATTON COMPONEVTS
AND ENTHALPY DIAGRAM
FART il)
The pressure/enthalpy diagram ofa refrigerant can be used to determine the amount
of thermal energy absorbed or removed by the refrigerant between two points.
ln Figure 4-8, for example,
a point
indicates that at 50"C, saturated liquid refrigerant contains around
270 kJlkg of enthalpy, while another point indicates that at 7"C, saturated liquid
refrigerant contains 210 kJ/kg of enthalpy;
consequently, if a decrease in liquid temperature from 50"C to 7'C occurs
through an evaporator, each kilogram of liquid entering the evaporator
theoretically loses 60 kJ of thermal energy. However, the actual decrease in
temperature produced under these conditions will be lower, due to the fact that
latent thermal energy (latent heat of vaporization) is absorbed by the liquid
changing to vapor in the evaporator.
Graphical Representation of the Refrigeration Cycle
The refrigeration cycle of a syslem can be represented by a simplification of the
pressure-enthalpy diagram of the refrigerant it uses. The refrigerant cycle is a
quadrilateral representing the refrigerant properties (pressure, temperature,
enthalpy) at any point of the cycle.
Figure 4-10, for example, shows a refrigeration cycle ofthe Lab-Volt Refrigeration
Training System, displayed in the LVHVAC software. lcons indicate the typical
equipment associated with each phase of the refrigeration cycle.
.
.
The horizontalline O ts O corresponds to a change ofstate from vapor to liquid,
as the refrigerant flows through the condenser. The temperature and absolute
pressure of the refrigerant stay constant, but the enthalpy decreases from right
to left. The pressure is at lhe compressor discharge (HP) level.
At point O, high-pressure subcooled liquid reaches the inlet of the expansion
device.
.
Between points O and g, the refrigerant flows through the expansion device: its
absolute pressure drops, while its enthalpy stays conStant. Part ofthe liquid tums
into vapor.
.
The horizontalline g
O corresponds lo a change of state from liquid to vapor,
as the refrigerant flows through lhe evaporator. The lemperature and absolute
pressure of the refrigerant stay constant, but the enthalpy increases from left to
right. The pressure is at the compressor suction (LP) level.
.
{
At point O, low-pressure superheated vapor reaches the compressor suction
inlet.
.
.
Between points O and O, the compressor compresses the vapor, causing the
absolute pressure of the refrigerant lo rise, and its enthalpy to also rise.
At point O, high-pressure superheated vapor is expelled from the compressor
discharge outlet.
REFRTGERATTON COMPOTVENTS
FART lt)
AND ENTHALPY DIAGRAM
.
At point 6, the discharged vapor reaches the condenser inlet, and a new
refrigeration cycle begins.
PBs.,/Enth.b, tx.tqrd
trr
I
Pressure/Enthalpy Diagram - R134a
10.0
,:
o
It
6
o6
c,
f
o
o
c
o
,
o
l)
{to
HEAT OF
couPREssloN
250
I
I
n5
3S
hthalpv ftrke)
I
Figure 4-'10. Pressure/enthalpy diagram for the R-134a (metric units).
NRE, Heat of Compression, and Coefficient of Performance
lmportantcharacteristics of a refrigeration system are:the net refrigeration effect, the
heat of compression, and the coefficient of performance. These properties can be
determined graphically
by using the system's refrigeration cycle (refer to
Figure 4-10.)
.
The net refrigeration effect (N.R.E.) is the enthalpy removed by evaporation' lt is
determined by drawing vertical lines downward from points @ and O. The
REFRTGERATTON COMPOTVENTS
AND ENTHALPY DIAGRAM
d
?ART
tD
ifference on the enthalpy (horizontal) axis where the lines cross this axis is equal
to the N.R.E.
.
.
The heat of compression is the enthalpy added to the vapor, mainly by the work
done by the compressor. lt is determined by drawing vertical lines downward from
points O and 19. The difference on the enthalpy (horizontal) axis where the lines
cross this axis is equal to the heat of compression.
The coefficient of performance (COP) is a pure number indicating the efficiency
ofthe refrigeration cycle. lt corresponds to the ratio ofthe enthalpy removed by
evaporation (N.E.R.) to the enthalpy added to the vapor during the compressing
phase. ln equalion form:
Coefficient of performance
Net refrigeration effect
=
Heat of compression
(kJ,ks orBtu/bm)
{kJ,rs or Bru/tbm)
The higher the coefficient of performance, the better the efficiency of the system.
The COP of refrigeration syslems varies according to the conditions under which
they operate and the components constituting them. Therefore, this figure should
not be used to compare the performance of two refrigeration systems that operate
under differing conditions and/or that are not entirely made up of the same
componenls.
lnformation
Job Sheet
,{
ELECTRICAL CONTROL OF REFRIGERATION SYSTEI'S
lntroduction
Refrigeration systems require electricity to operate. Consequently, it is important to
know the basic principles of electricity and electrical control when working on these
systems.
Basic Principles
Electricity is a form of energy used for lighting, heating, or providing control and
power to do work. lt is produced by the flow of tiny particles of matter called
electrons through a conducting material. Examples of conducting materials are iron,
copper, and aluminum.
Electrical components such as wires, lamps, solenoids, fan motors, thermostats, and
electronic transmitters all use conducting material, and so allow electrons to pass
through them. To produce a flow of electrons, the electrical components must be
connected to a source of electromotive force that pushes the electrons through the
components. This source may be either a generator or a battery. For example,
Figure 5-1 shows a battery pushing electrons through electrical wires to energize the
solenoid of a solenoid valve. As a result, a magnetic field is created around the
solenoid.
<- OOO
'T;""_
Figure 5-1. Simple electrical circuit.
The electromotive force exerted by a source is called voltage. The magnitude of the
voltage is measured in volts (V). The instrument used to measure voltage is called
a voltmeter.
There is always an opposition to the flow of electrons through an electrical
component. This opposition to electron flow is called resistance. Resistance is
measured in ohms (a). The instrument used to measure resistance is called an
ohmmeter.
ELECTRICAL CONTROL OF REFRIGERAT'ON SYSIEMS
The result of electrons flowing through an electrical component is called current.
The magnitude of the currenl is measured in amperes (A). One ampere is equal to
the motion of6.24 x 1018 electrons past a cross section in 1 second. The instrument
used to measure current is called an ammeter.
Types of Electric Current
Current flow through an electrical circuit may be one of two types: direct current or
alternating cunent.
.
Direct current (DC) is the type of current produced by batteries and dc power
supplies: the direction of the electric charges remains constant. Figure 5-2 (a)
shows the symbol used to represent a DC power source in electrical diagrams.
The current, l, flows by convention from the positive, or hot (+) terminal of the
source lowards the negative, or common (-) terminal.
I ^r
T)
l+
|
'
\
')
\--l'
t______*
GBOUNDED
WIRE
=
(CONNECTED
TO EARTH)
OV
(a) DC power source
(b) AC power sourco
Figure 5-2. Symbols used to represent Dc and Ac power sources in electrical diagrams.
.
Alternating current (AC) is the type of current supplied to most houses and
businesses: the electric charges varies in a cyclicai way, as opposed to
DC current. Although DC current was the first type developed by Thomas Edison,
AC cu rrent was proved to be much less expensive lo transmit over long distances
by Serbian scientist Nikola Tesla, after what was called the "War of Currents",
and is therefore more commonly used.
Figure 5-2 (b) shows the symbols used to represent an AC power source in
electrical diagrams. The flow of cunent in an AC circuit continuously reverses
itself, flowing from the line (L, or hot side) terminal towards the neutral (N, or
common) side, and vice-versa. However, the convention used is the same as for
DC circuits: the current flows from the L side to the N side.
ELECTRICAL CONTROL OF REFRIGERAT'O'V SYSTEMS
For safety purposes, it is a common practice to connect the low-voltage side of
an AC circuit to the earth. The voltage ofthe earth is 0 V. This practice is called
grounding. As Figure 5-2 (b) shows, lhe wire connecting the N (low-voltage side)
of the power source to the earth is called the grounded wire. The grounded wire
is the green conductor in the US (or green-yellow conductor in Europe and Asia)
of the line power cord.
Ohm's Law
Ohm's law states that the magnitude of the current flowing through an electrical
component is equal to the voltage drop across lhe component divided by the
resistance of the component:
(^,
Current...
Voltaoe dropM
Resistance
(o)
Reformulated to calculate the voltage drop:
VoltagedropM = Resistance(o) x Currentlel
Or to calculate the resistance:
Resistance
Voltage dropu)
(o)
Current
11y
ln Figure 5-1 , for example, if the resistance of the solenoid is 10 O, and the current
flowing through the solenoid is 2 A, the voltage drop across the solenoid will be 20 V.
Electrical Power
The capability of an electrical source lo move electrons through a circuit is called
electrical power.
DC circuits
ln DC voltage circuits, electrical power is measured in watts (W). The amount of
power generated by the electrical source is equal to the voltage supplied by the
source multiplied by the current drawn by the circuit. ln equation form:
Power*
= Voltage M
x
Current 6y
Part of the electrical power generated by the source is dissipated as heat by each
component in the circuit, due to the resistance, or opposition to the current flow, of
the components. Usually, most of the power is consumed by an electrical device
called a load to perform a useful work such as producing light (lamp), providing
rotary motion (motor), moving a plunger (solenoid), etc.
The amount of power consumed by a load is equal to the voltage drop across this
load, multiplied by the current flowing through it. (lt is also equal to the square of the
REF RI G E RAT I O N T RA I N I N G
S
YSTE'U
ELECTRICAL CONTROL OF REFRIGERAT'O'V SYSTEMS
current flowing through the load, multiplied by the resistance ofthe load.) ln equation
form:
Consumed power@ =
Voltagedrop, x
Current
1n1
lf, for example, the voltage drop across the solenoid in Figure 5-1 is 10 V, and the
current flowing through the solenoid is 2 A, then the power consumed by the
solenoid will be 20 W.
AC circuits
ln AC voltage circuits, not all the power generated by the AC source goes to the
load. Part of this power returns to the source, due to the inductive and capacitive
components of the circuit. The power returning to the source is called reactive
power. Reactive power is measured in volts-amperes (VAR).
The power actually consumed by the load is equal to the product ofthe voltage and
the current produced by the source, multiplied by a power factor. That portion of
power is called true, or real power, and is measured in watts (W).
The power factor is usually expressed as a pure number comprised between 0
and 1. lf, for example, the power factor is 0.7, then 70% of the power golng to the
load will be consumed by the load.
True power, in watts, can be measured by using a wattmeter.
Closed and Open Circuits
Figure 5-3 shows a simple AC circuit used to power a compressor motor. This circuit
includes a 120-(2201240-\ V AC source, a POWER switch with normally open (NO)
contacl, a pressure switch with normally closed (NC), contact and a compressor. The
POWER switch allows an operator to make lhe cunent flow through the circuit or to
stop it. The pressure swilch is used for safety: its NC contact allows the current to
flow to the compressor as long as the circuit pressure stays within the pressure
range allowed by the culin and cut-out settings of the pressure controller.
.
When the operator actuates the POWER switch [Figure 5-3 (a)1, this switch
contact goes from open to closed. Since the pressure-switch contact is also
closed, a complete conducting path is established, starting at the line (L) terminal
of the source, through the switches and lhe compressor motor, back to the
neutral (N) terminal of the source. As a result, the circuit is in the closed
condition, and a currenl flows through the circuit. Consequently, the compressor
motor is running.
.
When the operator returns the POWER switch to the normal (deactuated) state
lFigure 5-3 (b)1, this switch contact goes from closed to open. This breaks the
continuity of the conducting path at the POWER SWITCH. As a result, the circuit
is in an open condition, and the current can no longer flow through the circuit.
Consequently, the compressor motor is off. The same thing would occur if, for
example, the pressure-switch contact were to go closed to open due to a circuit
pressure reaching the cut-in or cut-out setting of lhe pressure controller.
ELECTRICAL CONTROL OF REFRIG ERATION SYSTEMS
POWER SWITCH
IN NORMAL STATE
PRESSURE
SWITCH
.^.
120
(230) VAC
SOURCE
')
'----/
(a) Closed
\
COMPRESSOH
MOTOR
RUNNING
circuit
(b) Open circuit
Figure 5-3. A simple AC circuit used to power a compressor motor.
Measuring voltage, resistance, and current
As previously mentioned, voltage is measured with a voltmeter, resistance is
measured with an ohmmeter, and current is measured with an ammeter. These
meters are available as separate units, but they are usually found combined in a
single enclosure called a multimeter.
Figure 5-4 shows how to measure voltage drop, current, and resistance in an
AC circuit.
.
To measure the voltage drop across a component, connect a voltmeter or
multimeter set to measure AC volts across the component terminals, as
Figure 5-a (a) shows. Then turn on the source.
.
To measure the current flowing through a component, make sure the power
source is turned off, then connect an ammeter or multimeter placed in ammeter
mode in series with the component, as Figure 5-4 (b) shows. Then, turn on the
power source.
REF RIG ERATI O N TRAI N I NG S YSTEM
E LECT RI CA
L
C O N T RO
L O F REF RI G E RAT I O,il S YS TEI'S
POWER SWITCH
ACTUATED
3
VOLTMETER
(a) Measuring voltage across the compressor
3
(b) Measuring current drawn by the compressor
(c) Measuring compressor resistance
Figure 54. Measuring voltage, resistance, and current in an AC circuit.
ELECTRICAL CONTROL OF REFRIGERAT'O'V SYSTEMS
Note: Serles means that all the current will llow through the component
and the rest of the circuit when the power source is turned on.
.
To measure the resistance of a component, make sure the power source lS
TURNED OFF, then disconnect the component from the circuit. This may require
you to open one or more circuit connections. Connect an ohmmeter or multimeter
placed in ohmmeter mode across the component terminals, as Figure 5-4 (c)
shows. The ohmmeter has its own inlemal power source (battery) that supplies
a current used to test the resistance of the component.
Resistance measurements are often used to test the electrical continuity of a
component. When the resistance of a component is very high or infinite (- Q), the
component is said to be open. When the resistance is null, the component is said
to be shorted (0 O).
CAUTION!
Never measure resistance in a circuitrYhile the power source is turned on.
Failure to observe this rule may cause permanent damage to the meter'
Series and Parallel Circuits
A series circuit consists of electrical components that are all connected in series,
as Figure 5-5 (a) shows:
.
.
.
The total resistance of the circuit, Rr, seen by the source, is equal to the sum of
the resistances of each circuit component.
The current flowing through the circuit is equal to the source voltage divided by
the total resistance.
Whenever a circuit component is open (of infinite resistance), no current will flow
through the circuit. The source voltage will be present across the open
component.
A parallel circuit consists of electrical components that are parallel, or branch
connected. Figure 5-5 (b) shows a simple parallel circuit:
.
.
.
The voltage across each branch is equal to the source voltage.
Each branch has its own resistance. The total resistance, Rr, seen bythe source
is equal to or lower than that of the branch of lowest resistance.
The current flowing through the circuit is equal to the source voltage divided by
the total, or equivalent circuit resistance.
.
Whenever a component in a branch is open, the source voltage will be present
across the open component.
ELECTRICAL CONTROL OF REFRI GERATIO N SYSTEMS
v
=
120 (230) VAC
v
=
120 (230) VAC
r20 (230) vAc
RI = 7.4A
(a) Series circuit
v
=
120 (230) VAC
v
=
120 (230) VAC
o2a
"3[?EB.
120 (230) vAC
a
^
CoMPRESSOR
MOTOR
soo ,31["
a3=75O
Rr=5'8O
(b) Parallel circuit
Figure 5-5. Series and parallel circuits.
Electrical control circuits are usually a combination of parallel and series branches,
like the circuit used to control the Refrigeration Training system, shown
in
Figure 5-6:
'
Each separate branch is used to control the turning on or turning off of a specific
device independently.
'
When the HEAT LOAD switch is actuated, the heat source becomes energized.
However, this does not affect the solenoid valve, the evaporator fan, or the
compressor motor in the other branches.
ELECTRICAL CONTROL OF REFRIGERATION SYSTEMS
.
When the speed of the evaporator or condenser fan is varied, using the
associated SPEED CONTROL knob, the voltage across the fan changes.
However, the voltage across the heat source, solenoid valve, or compressor
motor does not change.
ELECTRONIC
PRESSURE
CONTROLLEB
CoMPBESSOR
Y
--a/o--------------s4
THERMOSTAT
COMPRESSOB
SOLENOID
VALVE
EVAPOBATOR
FAN
CONDENSER
FAN
ll
t-o/o--l
COMPRESSOFI
(22O
t 24OVAC
MOOELS ONLY)
Figure 5-6. Electrical control panel of the Refrigeration Training System.
Safety Rules
Working on electrical equipment requires that you observe the necessary
precautions, due to the possibility of electric shocks and/or damage to the
equipment. Motor circuits, for example, may carry very high currents that can cause
severe electric shocks. Even small currents may be dangerous under certain
conditions.
ln fact, the higher the circuit voltage, the higher the current that can flow through
the human body and, therefore, the greater the possibility for a severe or fatal
shock. The risk is increased when working in a wet room, or on components that are
damp and not insulated.
a.
Never connect or disconnect electrical leads or components while the electrical
power source is ON.
b. When performing voltage or current measurements, the test leads or probes of
the multimeter, voltmeter, or ammeter should have a protective covering over
their ends to avoid the risk of short circuits and electric shocks, since these
measurements require that the electrical power source be on.
REF RI G ERAT I O N TRAI N I NG SYSTEM
ELECTRICAL CONTROL OF REFRIGERATION SYSTEMS
c. To increase your protection against electric shocks, always use screwdrivers,
wrenches, or pliers that have insulated handles. Moreover, use rubber gloves,
rubber-sole shoes, or rubber boots.
d. Never leave any electrical lead unconnected. This may cause you to receive an
electric shock when you touch the unconnected end of a lead while the electrical
power source is on. This may also cause a short circuit to occur when the
unconnected end of a lead touches a metal surface.
e. All parts of a refrigeration system that can cause electric shocks must be properly
grounded. For example, the ground (green wire of the electrical power cord) is
normally connected to the ground wire of the motor compressor.
/
i
PRESSURE AND TEMPERATURE CONTROL
I N REF RI G E RAT'OA' S YS TE'}'S
Pressure Controllers
Pressure controllers are used in refrigeration systems for safety and pressure
control. They sense pressure and switch the compressor on or off when the desired
pressure (setpoint) is reached by breaking a conducting path in the electrical control
circuitry.
Pressure controllers come in two different types:the low-pressure type, and the highpressure type.
.
The low-pressure controller is found on the low-pressure (suction or LP) side
ofthe refrigeration system. lt maintains the pressure on the LP side ofthe system
below the level required to ensure efficient vaporization of the liquid refrigerant
within the evaporator.
At the same time, the low-pressure controller provides safety control against
pulling vacuums, thereby preventing damage to the compressor. Moreover, by
controlling the LP-side pressure, the low-pressure controller can also be used
to control the temperature in the cooling chamber. This occurs bemuse changes
in the evaporator temperature result in changes in lhe suction pressure. A
thermostat maytherefore be unnecessarywhen the low-pressure controller starts
and stops the compressor at the desired temperatures and pressures.
Finally, the low-pressure controller can also prevenl the formation of ice on the
evaporator.
.
The high-pressure controller is found on the high-pressure (discharge or HP)
side of the refrigeration system. lt is used mainly for safety purposes: it prevenls
the pressure on the HP side from becoming excessively high, thereby preventing
rupture of components in the system.
At the same time, the high-pressure controller maintains the pressure on lhe
HP side to the level required to ensure efflcient condensation of the gaseous
refrigerant within the condenser.
a Bourdon tube as the pressure
as the sensed pressure varies.
expand
or
contracl
These
elements
element.
sensing
When the pressure reaches the setpoint, an electrical contacl is mechanically closed
or open to make or break a conducting path in the electrical control circuitry.
Pressure controllers often use a bellows or
PRESSURE AND TEMPERATURE CONTROL
I N R E F R I G E RAI'O'V S YS TE'US
The Electronic Pressure Controller of the Refrigeration Training System
This controller, shown in Figure 6-1, can acl as either a low- or high-pressure
controller, depending on lhe pressure range (user adjustable) and on the type of
transducer used. As the figure shows, the controller mainly consists of a pressure
transducer, and an LCD display with touchpad.
-
The pressure transducer produces a DC voltage proportional to the sensed
pressure. The sensed pressure appears on the LCD display, in psig, and is
refreshed every two seconds.
-
When the sensed pressure reaches the cut-in or cut-out setpoints, an internal
relay with SPDT contacts becomes actuated, shifting its NO and NC contacts to
their opposite state. A stalus LED on the front face of the controller indicates
whether the relay is energized (LED is on) or deenergized (LED is off).
-
The 3-button touchpad allows the user to adjust the cut-in and cut-out
setpoints.
.
.
The Cut-ln setpoint (cil) sets the pressure (in psig)atwhich the intemalrelay
of the controller is energized.
The Cut-Out setpoint (col)sets the pressure (in psig) at which the internal
relay of lhe controller is de-energized.
lsecondary setpoints (ci2 and co2) are also available, but they are unused for the
control of the Refrigeration Training System.l
-
Once cil and col have been adjusted, the conlroller automatically selects the
necessary mode of control. The selected mode is indicated on the LCD display:
ooen-Hioh
ffi
or
ooen-towfl
PRI ARY
SETPOINTS
TRANSDUCER
SENSED
(GAUGE)
PRESSURE
oPE .t-ow
CONTROL
ODE
HARNESS
Figu.e 6-'1. The elect.onic pressure controller of the Refrigeration Training System.
PRESSURE AND TEMPERATURE CONTROL
IN REFRIGERAT'O'V SYSTEMS
The pressure controller has three selectable ranges for the cut-in and cut-out
setpoints. This range is factory setto 0-100 psig (0-6.9 barg). Given this range and
the type of pressure transducer used on the system, the controller will automatically
select the Open-low mode and therefore acl as a low-pressure controller.
Operation
Figure 6-2 shows the operation of the controller when cil and
30 psig (2.'l barg) and 10 psig (0.69 barg), respectively.
.
.
col
are adjusted to
The controller stops the compressor if the pressure on the LP side reaches the
cut-out point of 10 psig (0.69 barg).
The LP-side pressure is then allowed to rise to the cut-in point of 30 psig
(2.1 barg): at that point, the controller restarts the compressor.
.
The LP-side pressure is then allowed to decrease lo the cutout point, and so on.
100 psig
(6.9 barg)
LED IS ON
I
30 p6ig
(2.1 ba€)
I
CUT-IN (cil)
COMPRESSOH RESTARTS
I
SETPOINT
DIFFEHENTIAL
I
LED ON OB OFF
'10 psig
(0.69 barg)
CUT-OUT (col )
COMPRESSOR STOPS
I
LEO IS OFF
0 psis
(0 bars)
Figure 6.2. Operation ofthe electronic pressure controller in the open-low mode, with
set to 30 psig (2.1 barg) and 10 psig (0.69 barg), respectively.
cil
and
col
Thermostal Controllers
Temperature controllers, orthermostats, are used in refrigeration systems for safety
and temperature control. They sense temperalure and switch a device, such as a
compressor or a solenoid valve, on or off when the desired temperature (setpoint)
is reached.
PRESSURE AND TEMPERATURE CONTROL
I N REF RI G E RAT'O'V SYS IEMS
Figure 6-3, for example, shows a lhermostat ofthe fluid pressure type similar to that
used on the Refrigeration Training System. The thermostat mainly consists of a
thermal bulb, an SPDT switch, and a setpoint adjustmenl knob with dial.
.
The thermal bulb is used as the temperature sensing element. lt is linked to the
thermostat via a capillary tube. The bulb is usually mounled on the evaporator
side or in the cooling chamber.
The bulb is filled with refrigerant, and is therefore sensitive to temperature
changes. As the temperature of the refrigerant changes at the bulb, a bellows
expands or contracts accordingly, acting on the SPDT switch, through a
mechanical link.
.
The setpoint adjustment knob permits adjustment of the setpoint (desired
temperalure in the chamber) on a calibrated dial.
BULB
^DIFFERENTIAL
RANGE }
ADJUSTiIENT
DI,AL
AIUUST ENT
SCALE
-'z
ELECTRICAL
CONNECTIONS
TO SPDT
SIVTICH
Figure 6-3. lnternal construction of a thermostat.
Operation
Assume that the NO contact ofthe thermostat SPDT switch is used. As long as the
temperature sensed by the bulb is above the setpoint, the switch is actuated, causing
its NO contact to be closed. This makes a conducting path in the electrical control
circuit.
Once the temperature has decreased lo the adjusted setpoint, the NO contact opens
to break the conducting path in the electrical control circuit. The temperature in the
cooling chamber then increases by a certain amount before lhe NO contacl closes
again to re-make the eleclrical circuit.
PRESSURE AND TEMPERATURE CONTROL
I N REF RI G ERAT'O'V S YS TEMS
The difference between the adjusted setpoint and the temperature at which the
NO contact closes to re-make the conducting path is called the differential. A
differential is required to prevent the NO contact from oscillating between the closed
and open conditions when the temperature is around the setpoint.
The differential is fixed on some models, while adjustable on others. The thermostat
shown in Figure 6-3, for example, has an adjustable differential which can be set by
sliding a lever along a scale plate.
Solenoid Valves
Solenoid valves are used in refrigeration systems to control the flow of liquid
refrigerant to the evaporator. The solenoid valve is installed in the liquid line
upstream of the expansion (metering) device. The intended purpose is to prevent
liquid refrigerant from flowing to the evaporator when the desired temperature is
reached.
Solenoid valves are also used in multi-chamber systems to permit independent
control of the temperature in each chamber. ln large refrigeration systems, they
permit the control of several evaporator sections, as the heat load varies.
Figure 6-4 shows a cross-section view of a direct-operated, two-way solenoid valve
similar to that used on the Refrigeration Training System. The valve is fully closed
(non-passing)when the solenoid is de-energized, and fully open (passing) when the
solenoid is energized.
.
When a current flows through the solenoid, a plunger is attracted toward the
center ofthe valve solenoid. This shifts the valve to the open condition, allowing
the refrigerant to flow to the evaporator. A minimum pressure drop is required
across the valve lo keep the valve in the open condition.
.
When the current flow to the solenoid is intenupted, the solenoid becomes deenergized. This causes a spring and the weight of the plunger to force lhe valve
stem against the valve seat, shifting the valve in the closed condition. This stops
the flow of refrigerant to the evaporator.
PRESSURE AND TEMPERATURE CONTROL
I N REF RI G E RAT'O'V S YSTEMS
--'r--
INLET
RE
OVABLE
SOLENOIO
PLUNGER
VALVE SEAT
CUTdWAY VIEW
Figure 6-4. cross-section view of a direct.operated, two.way solenoid valve.
A thermostat normally controls the flow of current to the valve solenoid to keep the
temperature in the cooling chamber around the setpoint, as Figure 6-5 shows.
.
.
When the adlusted setpoint is lower than the temperature sensed in the cooling
chamber, the thermostat NO contacl is closed to energize the solenoid of the
valve. The valve is in the open condition, allowing the refrigerant to flow to the
evaporator in order to produce a decrease in the temperature of the cooling
chamber.
When the temperature in the cooling chamber has decreased to the thermostat
setpoint, the thermostat NO contact opens to de-energize the solenoid valve. This
shifts the valve to the closed condition, thereby stopping the flow of refrigerant to
lhe evaporalor. The compressor will continue to run until the low-pressure
setpoint of the pressure controller is reached, causing the compressor to slop.
PRESSURE AND TEMPERATURE CONTROL
IN REFRIGERAT'ON SYSIEMS
{
THERTIIAL
BULB
/'---------\ _ _ _
rHERirosrAr
LOW+RESSURE
CONTROLLER
6)l
I
I
,
I
1#)
V
I
Figure 6-5. Schematic diagram of a simple refrigeration system.
THERMOSTATIC EXPANSION VALVE ADJUSTMENT
lntroduction
Figure 7-'l shows a refrigeration cycle for the R134-a. Each point used to plot this
quadrilateral (in red) represents the refrigerant properties (pressure, temperature,
enthalpy) at a speciflc point of the cycle.
.
.
Points of the quadrilateral that are located on horizontal lines (lines of constant
temperature) indicate that the refrigerant is a mixture of liquid and vapor.
Points ofthe quadrilateral that are located on the saturated vapor curve indicate
that the refrigerant is at the boiling point, and that it has completely turned into
vapor.
Points ofthe quadrilateralthat are located to the right ofthe saturated vaporcurve
indicate that thermal energy has been added to the vapor: the vapor is said to be
superheated.
.
Points of the quadrilateral that are located on the saturated liquid curve indicate
that the refrigerant is at the condensing point, and that it has completely turned
into liquid.
Points of the quadrilateral that are located to the left of the saturated liquid curve
indicate that thermal energy has been removed from the liquid: the liquid is said
to be subcooled.
THERMOSTATIC EXPANSION VALVE ADJUSTMENT
AASOLUTE
PRESSURE
(
Pa,.b..)
I
10
)ul\rnt
urrrrthtntic.rls
4
i
I
2
i
li
I
o,
,.
r<) o
I
o'
I
t
1r
I
o:.
o
gF
><)\
U,
?.,'?e
1R
rl
fi \i/.
,I
fl i\
it t\
i, 4cn,
cn
LIOUID
SUBCOOLING
-:8o
70',--+
VAPOR
SUPERHEATING
lit
ii
li
1l'l,l
iil,u
I q ,!I (ol (o ,'.\ iij i; t'
-o'
o
l--o ,' 6'
rl 1t+ rfi-l,*t:t-t-lrrtt 7r'f"- 1i
/l
&o' .
I
I'
,o
,.o
r-$
a: '-r4Ll:lil'L^ili tll I
0.06
v
J- /l
ll
0.04
il 3: +ilti[!lJ]i
: l:: :::3:/: ffi
tlt
l.
o
q3 q
oooo
ci
--l-L
r,Q ;- trrooal.,olJ.rok)ro
o ci; ,:
?-rt:,
;
rr,
ot.
1t! rrt
0.01
t.
ti('Ir:
.-{l"'c
J!
o.2
o.o2
ilfli I :li
o oro o
c, I
t;
k
Ilsi
E.
ii,
I
?
I
II
0.8
0.08
I I
o I
t":
0.6
c)
ll
ti Ol
li li
(or0
i
1
",1'
R
I
0.
,i
s.
Pressure-Enthalpy Diagram
(Sl Units)
6
0.4
.l' /
l
HFC-134a
I
1
I
jl
t8
& o
ffTfi-fifi
400
450
Figure 7-1. Vapor superheating and liquid subcooling.
THERMOSTATIC EXPANSION VALVE ADJUSTMENT
Superheat
The saturated vapor normally becomes superheated nearthe end ofthe evaporator.
Before it leaves the evaporator, the superheated vapor continues to absorb thermal
energy: this ensures that liquid refrigerantwill not enterthe compressor suction inlet.
The superheat of the superheated vapor at a given point is equal to the difference
between the temperature of the vapor and the temperature at which the vapor
becomes saturated, for a given pressure.
The superheat of the vapor is usually measured by using the steps below (refer to
Figure 7 -2\:
.
.
.
Firsl, measure the pressure of the superheated vapor at the evaporator outlet
(that is, where the sensing bulb ofthe thermostatic expansion valve is located);
Then, find the saturation temperature corresponding to the measured pressure
on a pressure-temperature (P/T) chart;
Finally, calculate the difference between the saluration temperature and the
temperature of the superheated vapor at the evaporator outlet. The result
corresponds to the superheat.
€.6rc (22.D
1.6
lztt
OUILET
0'c (32'R
1.6 barg
(22.4 Fls)
b.rg
D,,tgl
-5.6'C (22"F)
1.6 barg
122.1
rigl
5.6.C (ro.F)
SUPERHEAT
Figure 7.2. The pressure/temperature (PT) method of measuring sup€rheat.
Since there is usually no pressure port at the evaporator outlet, the pressure is often
measured at the compressor service valve instead. Therefore, in commercial units,
an additional value of 0.14 to 0.2barg (2or 3 psig) is usually added to the pressure
measured at lhe service valve, to take account of the pressure drop that occurs in
the refrigerant line between the evaporator outlet and the compressor suction inlet.
REFRIG ERATIO N TRAIN ING SYSTEM
THERMOSTATIC EXPANSION VALVE ADJUSTMENT
The measured superheat gives an indication of the efficiency of the evaporator coil.
For example, a high superheat can indicate thal the evaporator is operating very
inefficiently, because the refrigerant is vaporizing too quickly in the evaporator coil.
A high superheat can result in poor heat transfer in the cooling chamber. On the
other hand, a negative superheat value indicates that the refrigerant is not
completely vaporized at the evaporator outlet, which can cause liquid refrigerant to
reach the compressor suction inlet, and in turn can cause premature failure of the
compressor.
Table 7-1 indicates the recommended superheat for high-, medium-, and lowtemperalure refrigeration systems.
REFRIGERATION SYSTEM
RECOMMENDED SUPERHEAT
High-temperature [-1.1 "C (30'F) and
above in the evaporatorl
Between 5.5 and 6.6" ('10 and 12'F)
approximately
-1.1 "C
Medium{emperature [between -1 6'C and
(0'F and 30'F in the evaporator)l
Between 2.8 and 5.5'C (5 and 10 F)
approximately
Low-temperature [below -16"C (0"F) in
the evaporalor)l
Between 1.1 and 2.8'C (2 and 5"F)
approximately
Table 7-1. Recommended superheat for high-, medium-, and low-temperature refrigeration
systems.
ln many refrigeration systems, the superheat can be adiusted by turning a screw on
the thermostatic expansion valve (TEV). Turning the screw in the clockwise direction
will increase the superheat. Turning the screw in the counterclockwise direction will
decrease the superheat.
Before measuring the superheat, the system should be allowed to run for around
15 minutes.
Subcooling
Subcooling is the removal of thermal energy from the refrigerant, after
it
has
completely turned into a saturated liquid.
Subcooling takes place in the condenser, and is dependent upon proper airflow in
lhe condenser, room temperature, and refrigerant pressure in the condenser.
Subcooling increases the efficiency of some refrigeration systems, the liquid
refrigeranl being cooled before it passes the metering device.
One method of subcooling a refrigerant is to run the liquid line and suction line
together. This method also serves to superheat the cool refrigerant in the suction line
to ensure that no liquid refrigerant enters the compressor suction inlet.
TH ERM OSTA
TIC EXPA'VS'ON VALVE ADJ
U STM
ENT
To measure subcooling, use the steps below:
.
.
.
.
.
Measure the pressure of the liquid refrigerant as it leaves the condenser.
Once the pressure of the liquid refrigerant has been measured at the condenser,
add 0.14 to 0.2 barg (2 or 3 psig) to the measured pressure to take account of the
friction in the liquid line and of the vertical rise.
Find the saturation temperature corresponding to the resulting pressure on a
pressure-temperature chart.
Convert the resulting pressure into the corresponding saturation temperature by
using a pressure-temperature (PT) chart.
Finally, find the difference (in absolute value) between the saturation temperature
and the temperature at the evaporator inlet. The result corresponds to the
subcooling value.
RE F RI G ERATI O N TRAI N I NG SYSTEM
I
*rr*,ur*n ,o* r*o,*,*n
"rrrr*
|
6
TROUBLESHOOTING
lntroduction
A fault in any part of a refrigeration system will usually show up as an unsatisfactory
temperature or operating condition.
Each system has its own characteristics. Consequently, lhe more familiar you are
with the system, the quicker and easier it is to find faults and correct the problem.
To keep track ofthe system conditions, ensure maximum performance, and detect
suspect or faulty operation, maintenance technicians perform regular inspections of
the equipment, where they mark down operation data in an inspection report.
lmportant conditions to know about a refrigeration system are
.
.
the temperature of the evaporator and condenser during the operation cycle;
the discharge pressure and suction pressure during the operation cycle.
The operating conditions are lhen compared to the designed conditions in the
system when a fault is suspected. A drastic variation in a system's temperature or
pressure from the designed conditions can indicate system malfunction.
The two basic principles employed as a guide for troubleshooting a fault are
.
.
to observe the symptoms of the fault;
asking questions concerning the malfunction.
These two principles are used by system manufaclurers to build troubleshooting
charts. These charts, which take the form of tables or diagrams, consist of step-bystep observations and questions used to approach the problem and locate the fault
in a logical and systematic way.
Electrical Faults
Electrical faults are often the cause of refrigeration syslem breakdown. Many
electrical faults occur as a result of poor or corroded connections on components.
Some electrical faults are more obvious than others. These faults can usually be
found and repaired before any damage is caused, either to the system or chamber
being refrigerated. Examples of electrical faults are listed below.
R EF RIG E
RAT I O N IRA'IV,,VG SYSIEM
TROUBLESHOOTING
Evaporator Fan and Associated Circuit Failure
A fault occurring in the evaporator fan and associaled circuit is usually indicated by
the fan blade not turning and decreased cooling in the cooling chamber. Since less
heat is absorbed in the evaporator, the suction pressure decreases from its normal
pressure. This drastically reduces the efficiency of the system.
Condenser Fan and Associated Cicuit Failure
A fault occurring in the condenser fan and associaled circuit can also be indicated
by the fan blade not turning. This creates a higher head pressure since less heat is
being removed from the condenser coil. This will cause a greater pressure
differential, resulting in a warm cooling area.
Compressor Motor and Associated Circuit Failure
A fault occurring in the compressor motor can appear in the following ways: the
compressor will not run, which will result in no cooling and equal pressures on bolh
sides of the system. The fault may be the result of an open winding on the
compressor motor.
Before replacing the compressor, you should first check for external troubles,
including the power connections, lhe internal thermostat, the wire terminals, the
compressor relay, and lhe compressor capacitor (if any).
Once these components have been found operational, the compressor motor is
probably at fault.
Pressure Controller and Associated Circuit Failure
A typical fault in a high- or low-pressure controller can be a broken or loose
connection. This type offault can be indicated in several ways. The compressorwill
not run, and the system will therefore not operate.
Troubleshooting Electrical Control Circuits
Troubleshooting the electrical control section of a refrigeration system requires that
a systematic troubleshooting procedure be used, in orderto Iimit the numberoftests
to be performed.
The best way to start is to observe the symptoms in order to relate the problem to a
specific section of the circuitry. The choice of which circuit section to analyze should
not be done on a random basis, as industrial control circuits can be quite complex.
The two most often used methods of troubleshooting are the voltmeter method and
the ohmmeter method.
TROUBLESHOOTING
Voltmeter Method of Troubleshooting
The voltmeter method consists in tracing the voltage along a circuit branch or path
suspected to be defective, using a voltmeter. Basically, this method requires that the
voltage supplied to each component in the branch or path be checked to detect an
abnormal voltage level.
Figure B-1 shows this method for the electrical control circuit of the training system,
when a problem is suspected in the branch of the solenoid valve.
ELECTRONIC
PRESSURE
CONTROLLER
POWER
swrrcH
(cLosED)
:'
CoMPRESSOH
Y
----<v--o----------<: o
^z 120 (230) VAC
THERMOSTAT
SPEED
CONTROL
=
120 (230) VAC_
SPEED
CONTBOL
120 (230) VAC
< 120 (230) VAC
TP4
TP3
<
120 (230)
vAc
TP6
120
(230) vAC
,
,,
,
HEAT
SOURCE
EVAPOBATOR
FAN
VALVE
CONOENSER
FAN
I
I
I
I
|
1l
tll
rl
L-Ol-O--l1r
''utttt
COMPHESSOR
(22O
tt -
tt
-tttt"'
-t--t'ttt
I 24OVAC
MODELS ONLY)
VOLTMETER
Figure 8-1. The voltmeter method of troubleshooting a circuit branch.
The AC source voltage is checked first. With the POWER switch set to ON
(closed), the positive (+) terminal of the voltmeter is connected to TP1 , while the
common (COM) terminal of the voltmeter is connected to TP2 (circuit COMMON).
The voltmeter should indicate the source voltage. lf not, the source itself, the
POWER switch, or the leads connecting these components may be damaged or
open.
lf the supply voltage is correct, the + terminal of the voltmeter is moved to
measure the voltage applied to the solenoid of the valve (TP3), while the COM
terminal is left connected to the common side of the solenoid valve (TP2).
Assuming that the sensed temperature is above the thermostat setpoint, the
thermostat switch should be closed, so that the voltmeter should indicate the
REF RIG ERATI O N TRAI N I NG SYSTEM
TROUBLESHOOTING
source voltage approximately. lf not, the thermostat, the solenoid valve, or the
leads connecting these components (between TP1 and TP2 ) may be damaged
or open (infinite resistance).
It is important to understand, here, that voltage tracing along a branch or conducting
path requires that allthe components in that path be in the closed condition to permit
the flow of current through the path.
The same method could be used, for example, to test the branch ofthe evaporator
fan. ln that case, a voltage divider is created at the SPEED CONTROL knob.
Consequently, if there is no fault in this branch, the voltage between TP4 and TP2
will be less than the source vollage, and will be 0 V if the knob switch is set to OFF.
Ohmmeter Method of Troubleshooting
The ohmmeter method, also called continuity test method, consists in testing the
completeness of a path for the purpose of detecting a broken conneclion leading to
a component or a fault in the component. lt requires that the resistance of each
componenl and lead in the path be measured with an ohmmeter to detect an open,
or infinite-resistance condition.
When the path to test has two or more branches in parallel, the ohmmeter method
requires that each branch be tested separately by disconnecting the branches from
each other.
Figure 8-2, for example, shows ohmmeter lesting of the heat source of the training
system. The test leads ofthe ohmmeter are connected across the heat source. Since
the branch of the heat source has other branches in parallel, this branch must be
opened otherwise the measured resistance will correspond to the equivalent
resistance of all the other branches, not that of the heat source only. This condition
can easily be fulfilled just by opening the HEAT LOAD switch.
To lest the solenoid valve, the same principle applies: the thermostat NO contact
must be open, that is, lhe temperature in the cooling chamber must be lowerthan the
thermostat setpoint, in order for the branch of the valve to be isolated from lhe
others. Otherwise, you will have to open the branch leading to the valve (at TP3), as
the figure shows.
The same principle applies to test the continuity of the evaporator fan, compressor,
or condenser fan.
TROUBLESHOOTING
ELECTRONIC
PRESSURE
CONTROLLER
POWER
SWITCH
COMPBESSOR
(oPEN)
THERMOSTAT
120
(230) VAC
,
,
I
!
I
HEAT
SOURCE
EVAPOFATOR
FAN
tP2
LEGEND:
I
or=*
cncur
1z- BRANCH
I
OHMMETER
Figure 8-2. The ohmmeter method of testing the continuity of a branch or component.
RE F RI G ERATI O N TRAI N I NG SYSTE'U
Appendix
ff
TechnicalData on the Refrigeration Training System
COMPONENT
TECHNICAL OATA
120 VAC
220t240
Compressor
Hermetic-type, 124 W
(0.167 hp), start
capacitor, thermally
protected, 115 VAC,
60 Hz, 18-A lockedrotor cunent (LRA),
2.9-A rated load
current (RLA)
Hermetic-type, 186 W
(0.250 hp), start
capacitor, thermally
protected,
200i240 VAC, 50 Hz,
12.3-A locked-rotor
current (LRA), 2.3-A
rated load current (RLA)
Refrigerant
R-134a
R-134a
Nominal charge
1.09 ks (2.4 lb)
1.09 ks (2.4 lb)
oil
Polyol esther
Polyol esther
Evaporator
Forced-air coil with
variable-speed fan,
enclosed in a cooling
chamber, 120 VAC,
60 Hz, 0.58 A
Forced-air coil with
variable-speed fan,
enclosed in a cooling
chamber, 240 VAC,
50/60 Hz, 0.35 A
Condenser
Forced-air coilwith
variable-speed fan,
120 VAC, 60 HZ,
0.41 A
Forced-air coil with
variable-speed fan,
230 VAC, 50i60 Hz,
0.2 A
Thermostat setpoint (typical)
5"C (41'F)
5'C (41"F)
controller Cut-in
(typical) pressure
Pressure
settings
2.07 barg (30 psig)
(1)
V AC
2.07 barg (30 psig)
(1)
(cl1)
Delay (ASd)
Cut-out
Null
Null
0.69 barg (10 psig)
1r)
0.69 barg (10 psig)
(1)
pressure (CO'l )
Operating
Lower point
1.4 barg (20 psig)
1r)
1.4 barg (20 psig)
(1r
pressures (typical)
Highest point
ri)
1 bargauge
7.6 barg (1'10 psig)
(1r
7.6 barg ('110 psig)
(barg)= 100 kPa gauge = 14.5 psagauge (psig)
Table A-'1. Technical data on the Refrigeration Training System.
11)