RTHD and RTAC - Trane

Compressor Capacity - RTHD
The CHHC compressor is a "screw" or helical rotor design with two rotors, one
directly driven by the motor called the male and the other driven by the gears of
the male called the female. Design improvements from the original RTHB
compressor family were made to accommodate refrigerant 134a.
The RTHD compressor varies its capacity with a slide valve. The slide valve is
located internal to the compressor and directly controls the compressor's
capacity (mass flow rate) through varying the compressor's intake volume. The
slide valve is attached to a piston, which is moved by controlling the gas pressure
in the piston's cylinder. The slide valve is aptly named, since it moves by sliding
axially along the outside surface of the two mating rotors. The pressure in the
slide valve's piston cylinder and thus its position of the slide valve, is controlled
by the slide valve load and unload solenoid valves. The slide valve allows
modulated capacity from about 25% to 100% of full load tonnage Since the
piston must overcome static friction and force balances, it can only be effectively
moved while the compressor is running creating differential pressure. To prevent
starting at a partially loaded point, the compressor must be allowed to perform a
run un-load just prior to shutdown, if at all possible.
% of Full Load
120
100
80
60
40
20
Figure 30. Compressor Capacity
Checkout Procedure for the Load-Unload Outputs
0
Slide Valve Position
A dual triac LLID (1A5) controls the load and unload solenoid valve on the
compressor with 115VAC triacs (solid state relays). Unlike mechanical relays
however, a triac has a rather high leakage current when off, comparatively
speaking. While the leakage is not nearly enough to actuate a solenoid valve, it
may under a no load condition (as would be experienced when a solenoid coil
44 RLC-SVD05A-EN