Low Discharge Superheat Limit or Compressor Will Not Load RTHD

Question:

Why does RTHD operating mode indicate Low DSH Limit and compressor will not load up?

Discussion:    

RTHD chillers will have AdaptiView / UC800 controls, starter type (AFD or Wye-delta).

DynaView / CH530 controls were used on legacy RTHD chillers; all starter types.

Low Discharge Superheat Limit:

• This is an operating mode that was/is effectively neutered on RTHD chillers with DynaView / CH530 controls AND AdpatiView / UC800 controlled RTHD chillers with Wye-delta starters.  The Low Discharge Superheat Limit setpoint setting for these chillers (mechanical starter type) is set to 0.0°F, effectively neutering this limit.
• With the introduction of RTHD utilizing AFD starter type, a new limit was created...AFD Low Discharge Superheat Limit and Setpoint.  The operating mode that will be seen is Low Discharge Superheat Limit. This limit is intended to help prevent a chiller diagnostic shutdown when the actual discharge superheat approaches the Low Discharge Superheat Cutout setpoint (12°F default) by unloading the slide valve and increasing the speed of the compressor. ( Comment:  The minimum speed [30 Hz] of the compressor with a drive has the tendency to have lower discharge temperatures, thus suppressing the discharge superheat.)

Compressor discharge superheat is a function of compression efficiency. By making the compressor less efficient, superheat is increased.

When observing the Low Discharge Superheat Limit, it does not necessarily mean that there is a problem with the chiller. The chiller can get into this limit mode when the chiller runs unloaded for extended periods of time and/or when oil return needs to be optimized. UC800 controls will react to the limit and cause the discharge superheat to be increased.

Resolution:

It has been reported, when upgrading the UC firmware of the RTHD chiller to version 3.03 or higher, that chillers may not be meeting capacity. The Low Discharge Superheat Limit is observed and the compressor does not speed up even though there is plenty of load on the chiller. The reason for these symptoms is due to background gain setting changes related to the new firmware.

To correct the problem, the Trane service technician will need to connect to the UC800 using Tracer TU and change 3 gain settings from their default state in order to minimize the previously mentioned symptoms. The settings can be found in Utilities \ Equipment \ Field Startup \ Level 4 Gains. There will be a need to go into Level 4 Gains. Access can be achieved by clicking on the bar to the far left of where the Save & Cancel buttons are located.

Red arrow points to where user needs to click for Level 4 Gains access

Settings below are what need to be changed along with the necessary setting:

• Current Limit Proportional Gain                            8.0                  
• Current Limit Integral Time                                   60 sec.
• RLA State Feedback for Water Control                4.0

Additional Notes:

In Configuration within the UC800 Tab, verify the correct starter type is selected after the software upgrade. “AFD” for RTHD with variable speed or “Wye Delta” for fix speed chillers.

For additional questions, please contact Pueblo Technical Service