40
ENG
pRack +0300025EN rel. 1.3 - 17.12.2015
Important:
pulsating valves must be associated with an SSR relay
output, to avoid damage.
V1, V2, V3 and V4 can be managed to obtain stage or continuous
compressor control.
Stage control
For the control in stages, normally involving four load stages, 25, 50, 75,
100 %, a table needs to be created that shows the behaviour of each
valve in the diff erent conditions (starting, 25 %, 50 %, 75 %, 100 %). The
fi gure shows one possible example (see the documents supplied by the
manufacturer of the compressor for how to complete the table):
V1
V2
V3
V4
Start
{
{
{
{
25%
{
z
z
{
50%
z
{
z
z
75%
{
{
z
{
100%
{
{
{
{
Tab. 6.e
If intermittent valves are used, the cycle time also needs to be set.
Note
: normally operation at minimum capacity (25 %) is only
possible for a limited time, after which the compressor must switch
to the next stage. This function can be enabled and the corresponding
time can be set.
Continuous control
For continuous control, a table needs to be created that shows the
behaviour of each valve in the diff erent conditions (start/stop, increase,
decrease, standby). The fi gure shows one possible example:
V1
V2
Start/Stop
{
{
Increase (25 to 100%)
z
;
Decrease (25 to 100%)
~
z
Standby
z
{
Tab. 6.f
If intermittent/pulsating valves are used, the cycle time also needs to be
set. Intermittent valves are opened/closed for 50 % of the set time, while
for pulsating valves the opening and closing time in theory depend on
the diff erence between the position of the slide and the capacity request.
As the position of the slide is generally not measurable, the variation in
the request is used to calculate the times for pulsating valves.
Note
: in continuous control, operation is normally only allowed for
an undetermined time when the capacity exceeds 50 %.
Starting procedure
pRack pR300 can manage the starting procedure for the screw
compressors by considering, following the star/delta or part-winding
starting selected, a further time of operation at minimum capacity,
established by the manufacturer or set to 60 s for generic compressors.
Once the starting procedure has ended, the compressor starts varying the
capacity according to the control request and if necessary considering
the duration at minimum capacity.
Series of compressors supported
pRack pR300 can manage several series of screw compressors made by
the main manufacturers, (Bitzer, Refcomp, Hanbell, …) which come with
the parameters described above already set.
The models managed by pRack pR300 are shown in the table:
Manufacturer Model
Bitzer
CSH65…95, HS.53-4/64, HS.74, HS85
Hanbell
RC2-100/140/180, RC2-170/200…1520
RefComp
134-S, 134-XS L1, 134-XS L2, SRS-S1XX…755, SRC-S785…985,
SRC-XS L1, SRC-XS L2
Tab. 6.g
For manufacturers or series of compressors that are not supported, the
generic type can be used and the corresponding parameters set as
described previously..
Note:
for further details on the series of compressors supported
and the related pre-confi gured parameters, contact Carel.
Envelope
For screw compressors, pRack pR300 can manage control of the envelope,
which can either be pre-set or defi ned by the user. pRack pR300 accepts
the envelope control settings for the Bitzer CSH series compressors, and
these simply need to be enabled in main menu branch C.a.g.
For all other series of compressors, the envelope can be managed by
enabling and setting all the related parameters in main menu branch C.a.g.
The following parameters need to be set in order to manage the envelope:
•
Defi nition of the points (maximum 30)
•
Defi nition of the zone (maximum 12). Each zone can be made up of
one or more polygons (total maximum 14, which must be closed and
convex)
•
Defi nition of the behaviour of the compressor in the diff erent zones
(capacity and duration)
The meaning of the parameters is shown in the fi gure:
P1
P
6
P
7
(P
o
l
.
3
)
(P
o
l
.
2)
Z
one
1
(P
o
l
.
1)
P4
P
8
P
5
P2
P
3
Su
c
tion
pressure
D
is
ch
arge
pressure
Z
one
2
Fig. 6.r
pRack pR300 can also manage the variation in the envelope as the
capacity delivered changes, for example in the case of variation in
frequency for inverter-driven compressors.
Note
: for further details on confi guration of the envelope, contact
Carel.
6.3.12 Bitzer CRII modulating compressor
pR300 can manage a system fi tted with CRII compressors. Up to 2
CRII compressors can be confi gured (one on each line), each as fi rst
compressor in L1 and L2.
CRII is set as a capacity controlled piston compressor in which the
capacity steps can be activated/deactivated rapidly (5 s). In addition,
the CRII compressor can be ON with the capacity steps deactivated, so
as to respond faster to demand. This condition can remain active for a
maximum set time, after which capacity control will be activated, in turn
for a set time, to prevent compressor malfunctions.
Compressor capacity control is achieved by valves managed using SSR
digital outputs, due to the high number of cycles involved. Up to 3
capacity steps can be confi gured on for each line (check the number of
SSRs available).
Management by the pR300 of the diff erent capacity steps allows
complete compressor capacity modulation.
Capacity-control activation/deactivation
Capacity-control refers to the valve installed on the compressor. For 2
capacity steps, the compressor will be fi tted with 2 valves and pRack
needs to manage 2 SSR digital outputs. Capacity-control activation/
deactivation refers then to energising/de-energising the valve, so as to
open or close the fl ow of refrigerant. A step is activated when demand
exceeds the corresponding capacity, vice-versa it is deactivated when
demand falls to the lower step. The following fi gure represents an
example of a CRII with 2 capacity steps (50-100). The fi rst steps is activated
as soon as demand exceeds 50%, and the second when reaching 100%.
When demand falls, on the other hand, the second step is deactivated as
soon as this falls below 50%, and the fi rst at 0%.
Summary of Contents for PRK300D0E0
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