Fridgewatch 2100 Controller Mk4
Publication 3-58
Section 3
Page 20 of 152
Issue 1.1 : 01/08
5. Principles
of
Control
The primary requirement of any refrigeration control system is to match
the measured variable (temperature or pressure) as closely as possible
to the desired control value (set point), while at the same time optimising
the use of available compressor capacity.
In this part of the manual we will look first at the basic capacity control
system fitted to HallScrew compressors, then examine the fundamentals
of proportional, integral and derivative control before going on to describe
how Fridgewatch 2100 starts and stops the compressor in single
compressor and master/slave applications. Having grasped the
underlying principles, it is possible to make a reappraisal of the control
system as a whole, and more readily understand the enhancements
possible to the basic control elements by using Fridgewatch 2100.
5.1. Compressor
Capacity
Control
The capacity control system, common to all HallScrew compressors,
permits compressor capacity to be altered to match loading conditions as
closely as possible.
5.1.1.
HallScrew 2000 Series Compressors
The compressor capacity control slide valves modulate capacity between
minimum and maximum (10 % and 100 % of full load). Oil pressure,
generated by the system suction/discharge pressure differential, moves
the capacity control piston attached to one end of the slide valves to load
or unload the compressor. Solenoid valves in the pipe lines supplying
and venting oil to either side of the capacity control piston are energised
(opened) or de-energised (closed) by Fridgewatch 2100 for a timed
period.
Three different methods of slide valve position indication have been fitted
to HallScrew HS 18 to HS 235 and 2000 series compressors; refer to
Table 13 and the descriptions in the HallScrew 2000 compressor
instruction manual, publication 2-49.
The LVDT and NLVDT is provided with three slide valve position
switches: minimum load (10 %), intermediate load (user defined) and
maximum load (100 %). The compressor loading range is divided into
four discrete bands as illustrated in Fig 6.
5.1.2.
HallScrew 2100 and 3100 Old Series Compressors
The compressor capacity control slide valve modulates capacity between
minimum and maximum (25 % and 100 % of full load). Oil pressure,
generated by the system suction/discharge pressure differential, moves
the capacity control piston attached to one end of the slide valve in the
load direction, while a compressed spring on the unload side of the piston
moves the piston in the unload direction when the oil pressure is
released. Solenoid valves in the pipe lines supplying and venting oil to
either side of the capacity slide valve piston are energised or de-
energised (opened or closed) by Fridgewatch 2100 for a timed period.
HallScrew 2100 and 3100 old series compressors are not fitted with an
LVDT to supply a 4 to 20 mA signal; three slide valve position reed
switches are provided, one for minimum load (25 %), one for intermediate
load (50 %) and one for maximum (100 %) load. The loading bands are
illustrated in Fig 6.
