77
If SURGE PROTECTION COUNTS exceed 20 within a SURGE TIME PERIOD and VFD speed has
reached the maximum speed
If IGV, VFD and EC are not able to be operated to do surge protection, when surge count is greater
than 4.
5.11. Envelope Control Valve (EC) Control
The envelope control function is used to artificially load the chiller and keep it running under low load
conditions or to prevent surge conditions. Since this also reduces the performance of the machine, this is
a user selectable option.
There are 3 types of EC valve.
-
Continuous: A valve controlled by 2 contactors, one to open valve, the other to close valve. If both
contractors are opened, the valve will hold its position. This kind of valve has feedback signal of fully
closed or fully opened.
-
On/Off: A valve controlled by only 1 contactor. It can’t hold its current position, and has no position
feedback signal.
-
mA: A valve controlled by 4-20mA signal. It has the most precise control and feedback.
Envelope Control Valve operation will have three different modes when installed (hgbp_opt>0) and
enabled (hgbp_sel>0).
- Envelope Control for surge correction
Most of the inputs to the function are required for determining the operating region for the compressor
map. Each compressor has unique lift characteristics that can be plotted to determine performance. The
controller will determine operating conditions that could result in surging of the compressor and activate
the Envelope Control valve to prevent surge until the chillers operating parameters are in a safe area on
the curve where the valve may be closed again.
- Envelope Control for low load operation
In this condition, envelope control valve will be opened to prevent a recycle shutdown from occurring.
The envelope control valve will remain open until this minimal loading condition has passed and there is
no surge condition present.
- Combination for surge correction and low load operation
When this option is selected, both EC for surge correction and EC for Low Load Operation will be
performed. Surge condition will take higher priority if both conditions are satisfied.
5.12. Economizer Damper Valve Control
The economizer damper control is to maintain the pressure difference between Evaporator Pressure and
Economizer Pressure. Pressure in economizer should be always kept higher than the pressure in
evaporator.
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