2.7 Mechanical Unloading
All Carlyle screw compressors are equipped
with one step of mechanical unloading.The
unloader valve is controlled by a solenoid
mounted on the compressor body.
The compressor is unloaded when the
solenoid is de-energized and loaded when
the solenoid is energized.
The compressor
should always be started unloaded (for a mini-
mum time determined by the control module)
which will provide a soft start by partially
relieving the compression chamber back to
suction. Unloaded operation reduces the effec-
tive capacity by 30% to 62%, depending on
the model and system condition
(see tables below).
ESTIMATED PERCENTAGE
UNLOADING BY MODEL
Models
ft
3
/
m
3
/
Low
Med.
@ 60 Hz
min
min
Temp. Temp.
06T**033
33
0.93
60%
50%
06T**039
39
1.10
59%
49%
06T**044
44
1.25
58%
48%
06T**048
48
1.36
56%
46%
06T**054
54
1.53
55%
45%
06T**065
65
1.84
50%
40%
06T**078
78
2.21
45%
35%
06T**088
88
2.49
40%
30%
Models
ft
3
/
m
3
/
Low
Med.
@ 50 Hz
min
min
Temp. Temp.
06T**033 27.5
0.78
62%
52%
06T**039 32.5
0.92
61%
51%
06T**044
36.7
1.04
60%
50%
06T**048
40.0
1.13
59%
49%
06T**054
45.0
1.28
58%
48%
06T**065
54.2
1.53
55%
45%
06T**078
65.0
1.84
50%
40%
06T**088
73.3
2.08
47%
37%
06T**108
90.0
2.56
46%
-
Unlike reciprocating compressors that should
not be run unloaded continuously, the Carlyle
screw compressor can be run unloaded con-
tinually without affecting the reliability of the
compressor. Other methods of reducing the
compressor capacity are available and must be
approved by Carlyle Application Engineering.
9
Vi SETTINGS CHART (PSIG)
350
300
250
200
150
100
50
0
0
5
10
20
25
30
35
40
45
50
Suction Pressure psig (bar)
Discharge Pressure psig (bar)
Set at High Vi in this Region (Energized)
15
Set at Low Vi in this Region (De-energized)
(24.1)
(20.7
(17.2)
(13.8
(10.3)
(6.9)
(3.4)
(0)
(0)
(.3)
(.7)
(1.0) (1.4) (1.7) (2.1) (2.4) (2.8) (3.1) (3.4)
2.8 High Discharge Pressure Control
A high pressure cut out must protect the com-
pressor from exceeding 350 psig (25.2 bar).
The compressor may be brought back on
line after the discharge pressure falls
below 300 psig (21.4 bar). The maximum pres-
sure differential (discharge-suction) is 350 psi.
The internal pressure relief valve will open if the
pressure differential exceeds 400 psi (+/-3%).
2.9 Low Suction Pressure Cut Out
A low suction pressure cut out must protect
the compressor from operating in a vacuum
(below 10" hg). Each compressor must be indi-
vidually protected with a low pressure switch
connected to the low side access port. The
compressor may be brought back on line after
a 3 minute delay.
2.10 Volume Index (Vi) Control
All low temperature models (05TR/06TR) are
supplied with a Vi control valve that allows for
two Vi settings (see chart below). This dual Vi
allows for optimum efficiency over a wide
range of head pressures. The Vi must be set to
low (solenoid de-energized) during start-up for
a minimum of 30 seconds. The Vi may then be
set as desired for optimum energy efficiency.
The following chart and graph reflect the oper-
ational specifications of the Vi control.
The current LonCEM® controller continuously
monitors the operating pressure ratio and con-
trols Vi output accordingly. The older CEM
requires external controls to accomplish the
same task.
System Pressure Ratio
Vi
Solenoid
Greater Than 5:1
High: 4.0
On
Less Than 5:1*
Low: 2.8
Off
*Medium/High temperature “TA” compressor models
have a fixed Vi of 2.8.
