Trane RTWA 70-125 TON - Manual (Page 34)

RLC-SVD03A-EN 34
Compressor Capacity
The 35 to 60 ton helical rotary screw compressors are loaded and unloaded
by means of an internal slide valve and a female unloader valve. In simple
terms, these valves can regulate the amount of “bite” of the compressor
rotors as they turn at relatively constant speeds.
The slide valve is moved by a hydraulic cylinder and piston internal to the
compressor (the hydraulic fluid is oil from the refrigerant system). The
movement of the cylinder is controlled by the load and unload solenoid
valves, which either add oil at compressor discharge pressures, or withdraw
oil to suction pressures.
The female unloader valve is moved to either the On or Off position by using
internal suction and discharge gas pressure. The movement of this valve is
controlled by the female unloader valve solenoid. These solenoid valves are
electrically controlled by the MCSP module to handle compressor startup and
shutdown, maintain chilled water temperature setpoints and limit current,
condenser pressures, and evaporator temperatures etc.
Although the solenoids are an “on - off” device, effective modulation and high
resolution of the slide valve (under steady state conditions) is possible by
pulsing on and off the solenoid valves and varying the displaced volume of
the cylinder/piston. When a given compressor is operating, the MCSP will
energize (apply 115 VAC) either the load or the unload solenoid, if necessary,
for a period of between 40 and 400 milliseconds, once every 10 seconds to
control water temperature or limit conditions.
The female unloader valve solenoid receives a constant signal from the UCM
as the first step in compressor loading and the last step in compressor
unloading. Just prior to and just after a compressor start, and just before a
compressor stop, the MCSP will continuously energize the unload solenoid
for 20 to 30 seconds to assure unloaded starts. After a compressor stop, the
unload solenoid valve will remain energized for approximately one hour to
prevent slide valve movement due to changing cylinder/compressor
pressures.
The first procedure below will allow the checkout of the MCSP load and
unload outputs. The next procedure will allow the checkout of the Load and
Unload Solenoid valves located on the compressor as well as the operation of
the Slide Valve modulating unloader. Lastly, the female unloader valve and
solenoid will be discussed.