CH530 Condenser Pump Prestart

Issue:

CH530 Condenser Pump Prestart

Resolution:

The water regulating valve controller on an RTHC was not integrated with UCP2 and operated completely independently based on shell DP alone.   This was accomplished with add-subtract logic control that did not interact with the rest of the chiller controls.   In the integration of water regulator valve control into the CH530 control platform we attempted to address the many issues that the independent control scheme has demonstrated in use.  These include cold slugging after startup when starting with a closed valve, inability to prove condenser flow during startup due to valves being too far shut to consistently actuate the flow switches commonly used, trapping of air pockets in the condenser during shutdown and the lack of information about the operating mode of the chiller VS valve control response.    As a new product the controls were changed to alleviate as many of the issues as possible and are not a one for one duplication of the old control logic system on a new platform.

We exercise the valve for a variety of reasons:
     - allow purging of any accumulated air pockets
     - blending of water in system into the condenser to reduce the incidence of cold slugs of water after starting the chiller
     - allow flow proving devices to actuate as pump protection
     - reduce the effects of scale on valves when the valve consistently throttles to keep the chiller on line.   In these cases the condenser may also be prone to silting and or poor heat transfer due to low velocities.

If the issue is the rapid cycling of the control signal slamming the valve or system the valve should have its maximum slew rate set to prevent this.   If the chiller is attempting to start while the valve is exercising the pre-run time may need to be increased for the pump(s).  The prestart timer should be set to a value higher than zero for those applications having startup problems.  This allows the system to stabilize prior to starting the compressor.

While RTHD's are much more resistant to poor tower water control schemes there are limits to the algorithm's ability to start an inverted system.   If the evaporator loop is allowed to circulate over a weekend and is very hot the chiller may not be capable of making differential even if there were no water flowing through the condenser at startup.  If the system is running in free cooling then being switched over to mechanical cooling the operational switchover logic should consider how the system temperatures exist when switching out of free cooling.   Depending on the plumbing and heat exchangers used the approach may be high enough to invert the chiller and create trips.

Inverted start failures are a system level problem that can be helped by chiller controls but cannot be solved by the chiller.   Allowing a system to vary the return temperature from the tower during low wet bulb conditions can also take a chiller into a head pressure control mode during running.  This is generally a function of an over sized tower for the conditions being run and a cyclic fan control or other step change in water temperature coming back from the tower.

Notes:

Changes:

Other Question/Problem/Cause Statements:

• CH530 Condenser Pump Prestart

Facts:

• RTHD