Operating Principles Mechanical
RTHD-SVX02H-EN 77
The compressor used by the Series R chiller consists
of three distinct sections: the motor, the rotors and the
bearing housing. Refer to Figure 38.
Compressor Motor
A two-pole, hermetic, squirrel-cage induction motor di-
rectly drives the compressor rotors. The motor is cooled
by suction vapor drawn from the evaporator and enter-
ing the end of the motor housing (Figure 38) .
Compressor Rotors
Each Series R chiller uses a semi-hermetic, direct-drive
helical rotary type compressor. Excluding the bearings,
each compressor has only 3 moving parts: 2 rotors -
“male” and “female” - provide compression, and a slide
valve that controls capacity. See Figure 38. The male
rotor is attached to, and driven by the motor, and the
female rotor is, in turn, driven by the male rotor. Sepa-
rately housed bearing sets are provided at each end of
both rotors on the RTHD units. The slide valve is located
below (and moves along) the rotors.
The helical rotary compressor is a positive displacement
device. Refrigerant from the evaporator is drawn into the
suction opening at the end of the motor section. The gas
is drawn across the motor, cooling it, and then into the
rotor section. It is then compressed and released directly
into the discharge plenum.
There is no physical contact between the rotors and
compressor housing. Oil is injected into the bottom of
the compressor rotor section, coating both rotors and
the compressor housing interior. Although this oil does
provide rotor lubrication, its primary purpose is to seal
the clearance spaces between the rotors and compres-
sor housing. A positive seal between these internal parts
enhances compressor efficiency by limiting leakage
between the high pressure and low pressure cavities.
Capacity control is accomplished by means of a slide
valve assembly located in the rotor/bearing housing
sections of the compressor. Positioned along the bottom
of the rotors, the slide valve is driven by a piston/cylin-
der along an axis that parallels those of the rotors (Figure
38).
Compressor load condition is dictated by the coverage
of the rotors by the slide valve. When the slide valve
fully covers the rotors, the compressor is fully loaded.
Unloading occurs as the slide valve moves away from
the suction end of the rotors. Slide valve unloading low-
ers refrigeration capacity by reducing the compression
surface of the rotors.
Slide Valve Movement
Movement of the slide valve piston (Figure 38) deter-
mines slide valve position which, in turn, regulates
compressor capacity. Compressed vapor flowing in to
and out of the cylinder governs piston movement, and is
controlled by the load and unload solenoid valves.
The solenoid valves (both normally closed) receive
“load” and “unload” signals from the UC800, based on
system cooling requirements. To load the compressor,
the UC800 opens the load solenoid valve. The pressur-
ized vapor flow then enters the cylinder and, with the
help of the lower suction pressure acting on the face of
the unloader valve, moves the slide valve over the rotors
toward the suction end of the compressor.
The compressor is unloaded when the unload solenoid
valve is open. Vapor “trapped” within the cylinder is
drawn out into the lower-pressure suction area of the
compressor. As the pressurized vapor leaves the cylin-
der, the slide valve slowly moves away from the rotors
toward the discharge end of the rotors.
When both solenoid valves are closed, the present level
of compressor loading is maintained.
On compressor shutdown, the unload solenoid valve is
energized. Springs assist in moving the slide valve to the
fully-unloaded position, so the unit always starts fully
unloaded.
Oil Management System
Oil Separator
The oil separator consists of a vertical cylinder surround-
ing an exit passageway. Once oil is injected into the
compressor rotors, it mixes with compressed refrigerant
vapor and is discharged directly into the oil separator. As
the refrigerant-and-oil mixture is discharged into the oil
separator, the oil is forced outward by centrifugal force,
collects on the walls of the cylinder and drains to the
bottom of the oil separator cylinder. The accumulated
oil then drains out of the cylinder and collects in the oil
sump located near the top and in-between the evapora-
tor and condenser shells.
Oil that collects in the oil tank sump is at condensing
pressure during compressor operation; therefore, oil is
constantly moving to lower pressure areas.
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