aci eRCM Express, Manual

Page: 81 / 97

mCore Firmware Ver 01.10 Ver 2019-Oct-29 Page 81 of 97
Reciprocating compressors allow for significant loading and unloading of the driver
(engine or motor) based on actuating/de-actuating devices (such as clearance pockets and
end deactivators).
At times, multiple hardware configurations may require the same power requirements but
generate different flow rates. As such, at times configuration X will require less load than
configuration Y, but then at a different compression ratio configuration X can require more
load than configuration Y. When this happens, it is commonly known as “curve-crossing”.
Ideally, when a PLC requests NSU (Next Step Up) it will be a load step that will increase
load (or increase flow) on the unit, not decrease load (or flow). Thus, when curve-crossing
is present, the complexity of selecting the NSU and NSD increases somewhat.
Furthermore, end users do not generally desire lots of unloading devices to engage at once.
If the control lines have insufficient actuation pressure, then the result can be multiple
devices actuating late (2 seconds later after others). This can cause the dynamic load on the
driver to jump up and down. Often, some engines cannot handle too much jumping of load
and will shut down. Thus, part of picking the NSU and NSD may involve consideration for
load step efficiency, and number of devices engaging at the same time.
Different types of unloading sequences relative to the unit’s operating map are better
handled by different load step selection methodologies. One method does not handle all
unit arrangements. Thus, eRCM Express provides multiple methods for load step selection.
The Applications Engineer that created the eRCM Viewer model for use in the eRCM
Express should have reviewed the load steps, the operating map, and any concerns, and
subsequently set the model to use the best load step selection method for it.
Currently, eRCM provides nine (9) load step selection modes. Each one has its own pros
and cons.
Option #31 and Option #41 cannot be changed via the PLC, as these modes are for specific
hardware implementations.
When in doubt, set unit to
Load Step Selection Mode to 3 , and set the
MinLoadFlowPercChange to 1 (1%). These settings are good, albeit not necessarily most
efficient, for most (but not all) models.