Job Sheet
REFRTGERATION COMPOTVE TTS (PART l0
AND ENTHALPY DIAGRAM
OBJECTIVE
In this job, you will learn how to observe the system temperatures and pressures
over lime at various points of the system, using the Trend Recorder of the LVHVAC
software. You willthen use an enthalpytable to determine the values required to plot
the refrigeration cycle ofthe system. You will then observe the refrigeration cycle of
the system and measure the coefficient of performance, using a Pressure/Enthalpy
diagram.
EQUIPMENT REQUIRED
Lab-Volt Refrigeration Training System, Model 3431
PROCEDURE
Observing the System Temperatures and Pressures Over Time
!
of the Refrigeration Training System, make sure the
main POWER switch is set to OFF (O).
1. On the control panel
Connect the Refrigeration Training System to the computer used to run the
LVHVAC software, via a USB cable. Turn on the computer and then run the
LVHVAC software.
!
2.
Activate the Trend Recorder of the LVHVAC softlvare: in the menu bar of
this software, select the Iools menu, and then select the Trend Recorder
option. This will bring up the Trend Recorder, as Figure 4-1 '1 shows (without
the signals displayed in this figure, since the recorder has not been started
vet):
-
Locate the section General of the Settings panel in the right section of
the Trend Recorder. Set the Sca/e (min) field of this section to 30. This
sets the displaytime (range ofobservation ofthe variables)ofthe Trend
Recorder to 30 minutes.
REFRIGERATTON COMPOTVEMS PART I0
AND ENTHALPY DIAGRAM
Locate the section Data of lhe Settings panel in the right section of the
Trend Recorder. Select the following variables only (deselect the other
variables if they are selected):
.
.
.
.
.
Pressure 1;
Pressure 2;
Temperature 3;
Temperature 7;
Compressor Current.
ln the menu bar of the Trend Recorder, select Acquisition, and then
Start to start the display of the selected variables.
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NORTAL HEAT
LOAD
C0rient
Tim (min)
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Time
(nir)
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Figure 4-'t'1. Trend Recorder of the LVHVAC software displaying the selected variables as a
function of time.
tr 3. On the Refrigeration
Training System, make the following settings:
Note: Do not tum on the Refrigeration Training Sysfem for now.
COMPRESSORSwitch
...
OFF(O)
EVAPORATOR-FANSPEEDcontrolknob. ... HtcH
CONDENSER-FAN SPEEDcontrolknob. ..... Hatf of H|GH
HEAT LOAD switch
. OFF (O)
Thermostatsetpoint
.....5"C(41.F)
Manually-operated valves
V1 . . .
. . . . . Open (handleturnedfullycounterclockwise)
V2...
V3...
....Closed(handleturnedfullyclockwise)
....Closed(handleturnedfullyclockwise)
REFRTGERATTON COMPONENTS
PART t0
AND ENTHALPY DIAGRAM
tr4.
!5.
Turn on the Refrigeration Training System, then turn on the compressor. Let
the system run for about 20 minutes.
On the Trend Recorder, examine the signal of Temperature 3 (temperature
at the evaporator outlet), and fill in the following sentences.
lmmediately after power up, the temperature sensed by the thermal bulb of
the TEV at the evaporator outlet is
(higher/lower) than the
thermostat setpoint of 5"C (41'F). This causes the opening of the
thermostatic expansion valve (TEV) to be maximum to allow the full
refrigerant flow to enter the evaporator. As a result, Temperature 3
(increases/decreases).
As the evaporator cools down, the TEV opening is reduced to decrease the
flow of refrigerant entering the evaporator, causing Temperature 3 to
(increase/decrease). As more refrigerant is allowed into the
evaporator, the superheat sensed by the TEV sensing bulb
(increases/decreases). This causes the TEV opening to
(increase/decrease), causing the superheat to increase. This phenomena
can repeat, cyclically or not, untilthermal equilibrium is reached.
n 6. (Refer to the recorded signals). While Temperature 3 varies, does
Temperature 7 (temperature in the cooling chamber) slowly decrease in a
steady way until it becomes close to the thermostat setpoint of 5'C (41 'F),
causing the compressor to stop?
nYes
trNo
n 7. (Refer to the
recorded signals). When
Temperatures 3 and 7 both start to
the
compressor stops,
(increase/decrease) in a
a certain amount, the
steady way. Once Temperature 7 has risen by
compressor restarts to begin a new cycle.
tr 8. Leave the Trend Recorder open for the rest of the exercise,
but minimize
this window. You should now see the Refrigeration Diagram of the
LVHVAC software.
Determining Refrigerant Enthalpy at Different Pressures and Temperatures
tr 9. Figure 4-12 shows an example of the temperatures
and pressures that can
be read at the various test points of the Refrigeration Diagram, under the
current operating cond itions.
RE F RI G ERAT I O N TRAI N I NG S YSTEM
REFRTGERATION COMPONEVTS
AND ENTHALPY DIAGRAM
?ART t0
Using the temperatures and pressures indicated in Figure 4-12, fill in
Table 4-1-once complete, this table contains all the data required to plot
the refrigeration cycle of the system.
-
ln the first (leftmost) column, record the gauge pressures indicated at
PS1 and PS2 of Figure 4-12. Conved these pressures into absolute
pressures and record your results in the second column.
ln the third column, record the temperatures indicated at T1 through T5
of Figure 4-'12.
ln the fourth and fifth columns, record the enthalpy corresponding lo
each recorded temperature (T1 through T5), according to the enthalpy
table for the R134-a in Appendix D of this manual.
GAUGE
PRESSURE
ABSOLUTE
TEMPERATURE
PRESSURE
('c/'F)
(barg/psig)
(ba. abs./psia)
HP side
(PS1):
HP side:
ENTHALPY (kJ/kg or Btu/lbm)
Ltouto
T1:
T4:
T5:
LP side
(PS2):
LP side:
T2.
T3;
* To be completed later
Table 4-1. Pressu.e, temperature, and enthalpy data required to plot the refrigeration cycle.
VAPOR
REFRTGERATTON COMPOTVENTS
PART il)
AND ENTHALPY DIAGRAM
@
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ail
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78.0'F
Figure 4.12. Example ofthe temperatures and pressures that can be read atthe various test points
ot the system.
n 10. According
to the data recorded in Table 4-'1, the temperature of the highpressure liquid refrigerant is at a minimum at test point
(T1lT4lT5). At that point, the enthalpy of the refrigerant is
(minimum/maximum), the refrigerant being
(superheated/subcooled) when it reaches the inlet of the expansion device.
\
REFRIGERATTON COM?OTVE VTS
AND ENTHALPY DIAGRAM
?ART t0
Conversely, the temperature of the low-pressure vapor refrigerant is
maximum at test point
(T2lT3). At that point, the enthalpy of
the refrigerant is
(minimum/maximum), the refrigerant being
(superheated/subcooled) at the evaporator outlet.
Observing the Refrigeration Cycte and Measuring the Coefficient of
Performance
n
11.
ln the LVHVAC software, locate the
Refrigeration Training System panel
to the right of the Refrigeration Diagram. The System lnformation section
of this panelallows you to enterthe current settings of the system. Enterthe
current temperature controller setpoint, and the settings of the fan speeds
and heat load.
Now locate the Temperature Test Points section of the Refrigeration
Training System panel, as Figure 4-13 shows. This section indicates the
current temperature, absolute pressure, and enthalpy at test points Tl
through T5 of the Refrigeration Diagram. This data is continuously
refreshed.
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Figure 4-13. Sysfem lnformation and Temperature Test Poinfs sections of the Refrigeration
Training System panel.
REFRTGERATTON COMPONE VTS
qART il)
AND ENTHALPY DIAGRAM
tr
12. While the compressor is running, make the following observations in the
Temperature
lest Polnfs section of the Refrigeration Training System
panel.
.
.
tr13
The pressures at T1, T4, and T5 are equal. They correspond to the
absolute pressure on the HP side of the system.
The pressures at T2 and T3 are approximately equal. They correspond
to the absolute pressure on the LP side of the system.
ln the menu bar of the LVHVAC software, select the lools menu, and then
click on Options..., which will bring up the Opfions dialog box. Make sure the
Display the Cycle /cons check box is selected, and then click OK to close
the Opfions dialog box.
!
14. Activate the Pressure/Enthalpy Diagram of the LVHVAC software. To do
this, click on this window's tab, as Figure 4-14 shows.
The Pressure/Enthalpy Diagram shows the refrigeration cycle of the
system, plotted by using the temperatures and absolute pressures at T'1
through T5 of the system (as displayed in the Temperature Iesf Pornfs
section of the Refrigeration Training System panel).
The display of the refrigeration cycle is continuously refreshed to reflect any
changes that may occur in the measured temperatures and pressures.
R E F RI G
ERAT I O N
T
RAI N I NG
S
YSIEM
REFRIGERATTON COM?O,wE VTS
AND ENTHALPY DIAGRAM
qART r0
TAB TO CUCK
r.*6rr|,ipru.9-l
Pressure/Enthalpy Diagram -
R-1 34a
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a.0:
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237.88
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sup€rhc.t('{)
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Refrige.ntion Er.(t
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VoL Hoe nate o[ Refrb, at thd Evap. &*lct (m,/ntn)
Rcfrig. trh(t tor. by wrk of thc EsF D.vke (k],/kq)
lm t25
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t?5 N n5 lu
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rr5 l(I} eJ 1fi
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I
05
I
I
I
HEAT OF
COHPRESS!ON
FI
Figure 4-14. Pressure/Enthalpy Diagram with normal heat load when the TEV is used.
!
15. Allow the compressor to stop, then wait until it restarts to begin a new cycle.
3 minutes later, print the Pressure/Enthalpy Diagram and Refrigeration
Training System panel.
REF RI G ERATI O N T RAI N I NG S YS TE'T'
6,J
4.Il
(kJ,/kg)
td.C wdk of Cnlprcr.lon (kllkg)
flo, Rate of Rehi{rd.ot (IO/t)
R.frigeratio, C.paity (krv)
(@lncic.* ofPdfo]m.ne
il.t
157.69
3
L
1
i
94 97
5.(159
3 0102
3.0 102
52,93
REFRTGERATTON COMPOTENrS PART il)
AND ENTHALPY DIAGRAM
n
16. Determine the net refrigeration effect (N.E.R.) graphically on your printed
diagram. To do so, draw vertical lines downward from points
I
and
@
until
they cross the enthalpy (horizontal) axis, as shown in Figure 4-14. The
difference on the enthalpy axis where the lines cross this axis is equal to
the N.R.E. Record your result below.
Compare your result to the value displayed in the field Net Refrigeration
Effect of the System Pertormance section on your printed Refrigeration
Training System panel. Are these values approximately equal?
EYes
tr
trNo
17. Determine the heat of compression graphically on your printed diagram.
To do so, draw vertical lines downward from points 0 and @ untilthey cross
the enthalpy (horizontal) axis, as shown in Figure 4-14. The difference on
the enthalpy axis where the lines cross this axis is equal to the heat of
compression. Record your result below.
Compare your result to the value displayed in the field ldeal Work of
Compression
of the Sysfem Performance section on your printed
Refrigeration Training System panel. Are these values approximately
equal?
E Yes
tr
trNo
18. Based on the N.E.R. and heat of compression obtained in the previous
steps, calculate the coefficient of performance of the system. Record your
result.
Coefficient of performance
Net refrigeration effect,*r/ks
=
Heat of compression
REF RI G ERATIO N TRAI N I NG
or Bru/tbm)
(kJ/ks or Bru/tbm)
S YS
TEM
R E F RtG ERATT O N COM POTVENTS
qART
t t)
AND ENTHALPY DIAGRAM
Compare your result
Peiormance
to the value displayed in the field Coefficient of
Performance section on your printed
of the System
Refrigeration Training System panel. Are these values approximately
equal?
!Yes
tr
nNo
19. Turn off the trainer and clos6the LVHVAC software.
Date:
lnstructor's approval: