Mechanical Unloading; High Discharge Pressure Control; Low Suction Pressure Cut Out; Volume Index (Vi) Control - Carlyle 06T 033 Series Application Manual

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
@ 60 Hz min
06T**033 33
06T**039 39
06T**044 44
06T**048 48
06T**054 54
06T**065 65
06T**078 78
06T**088 88
Models ft /
3
@ 50 Hz min
06T**033 27.5
06T**039 32.5
06T**044 36.7
06T**048 40.0
06T**054 45.0
06T**065 54.2
06T**078 65.0
06T**088 73.3
06T**108 90.0
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.
m / Low Med.
3
min Temp. Temp.
0.93 60% 50%
1.10 59% 49%
1.25 58% 48%
1.36 56% 46%
1.53 55% 45%
1.84 50% 40%
2.21 45% 35%
2.49 40% 30%
m / Low Med.
3
min Temp. Temp.
0.78 62% 52%
0.92 61% 51%
1.04 60% 50%
1.13 59% 49%
1.28 58% 48%
1.53 55% 45%
1.84 50% 40%
2.08 47% 37%
2.56 46% -
9

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
Greater Than 5:1
Less Than 5:1*
*Medium/High temperature "TA" compressor models
have a fixed Vi of 2.8.
Vi SETTINGS CHART (PSIG)
(24.1) 350
(20.7 300
Set at High Vi in this Region (Energized)
(17.2) 250
(13.8 200
(10.3) 150
100
(6.9)
Set at Low Vi in this Region (De-energized)
(3.4) 50
(0) 0
0 5 10 15
(0) (.3) (.7) (1.0) (1.4) (1.7) (2.1) (2.4) (2.8) (3.1) (3.4)
Vi Solenoid
High: 4.0 On
Low: 2.8 Off
20 25 30 35 40 45
Suction Pressure psig (bar)
50