Centrifigal chillers

[lfg2007]

I have 4 Carrier Centrifigal chillers under commissioning phase. Recently the units (4) shut down while starting on "VFD over temp" alarm. I was told this was caused by the condenser water being warmer than the chilled water. This situation was caused by running the secondary loop without the chillers operating. This was solved by adjusting the condenser bypass valve by the Carrier tech. Question 1) How does modulating the condenser valve help the situation. 2) Should the head pressure control valve be modualted by the unit?

 

[imok2]

Sounds to me like the cooling tower water was being bypassed and not being cooled in the cooling towers thus creating hi condensing water tenperature and tripping on hi head. The bypass control is normally controlled by a sensor in the condenser water supply

 

[ozmosis]

I don't see how an overtemp in the VFD would be affected by the condenser temp. An overtemp of a VFD is simply that and either the ambient temp increased surrounding the VFD, a VFD fan failed or a sensor failed in the VFD. An alarm in a VFD of this nature is looking at the temp in the VFD itself, not what it is controlling.
stopping a VFD will cool it down and the alarm will go away for a while but it may come back. I would monitor the VFD to ensure it is ok itself.
The only possibility is if the coolant/refrigerant of the chiller is used to directly cool the VFD. In this case it could possibly be connected.

 

[gepman]

lfg2007
I agree with imok2 and ozmosis in that I don't believe that your description of the fault is correct. The condenser (heat sink) water would normally be warmer than the chilled water. The VFD temperature wouldn't have much to do with the condenser temperature except if the VFD was in an overheated space normally cooled by your centrifugal chiller.

Carrier doesn't put too much information on their centrifugal chiller line on their website (I have looked recently) but if their centrifugal chiller line is similar to the ComfortLink 30HXC (screw compressor series) then there is an interlock for the condenser water pump (see page 58 of form 30HX-8PD). I interpret this drawing to mean that if the chiller shuts down it will stop the cooling water flow. Also the fan on the cooling tower is usually either controlled by an AquaStat (on/off control) or a wet-bulb sensor which controls the degree of approach of the cooling tower water to the wet-bulb (which is a better control scheme). According to page 21 of the same form the unit will not start if the condenser water is 55 deg. F or below and once operating should be maintained above 70 deg. F. I believe that this is what caused the failure to start, low condensing water temperature. The 30HXC controller can control a tower pump VFD (don't go below minimum flow for the cooling tower), a bypass valve, flow regulating valve (don't go below minimum condenser water flow). I would actually recommend controlling the cooling tower fan speed with the same VFD that controls the degree of approach mentioned above. Just back off the fan speed if the cooling tower water gets too cold.

 

[lfg2007]

All

Thanks for the responses. I love this forum. As far as VFD over temp I am trying to follow on ozmosis's comments. The reason the condenser water was cooler than the chilled water is that the secondary pumps were circulating the chilled water while the chillers were off. This was done in an effort to maintain some cooling to critical areas while the chillers were down. This is a critical faclity and I thought that part of an emergency scenario would have been to keep the cooling going thru the cooling capacity of the water in the secondary loop when chillers were down.
We also have the requirement that the head pressure control valve on the chiller be capable of modulating condenser flow (pressure) to allow a chiller to start with the condenser water temp as low as 45' F. So i am still curious what the pressure of the condenser water affects the chiller ops.

 

[Keith53]

Nope.....

What happened was there was no water being sent to upper hot water basins in cooling tower, so condensor water temps soared.....The VFD then sped up the fan in an attempt to bring down condensor water return temps and VFD then tripped on overheat....If VFD had not tripped, then next trip would have been high condensor pressure/temp trip.

Sending water to upper basin via bypass valve corrected high condensor water temps and allowed VFD to then operate fan in normal range.

Have your tech check that tower bypass valve is controlling properly to maintain return temps correctly. Also have VFD tech reprogram VFD to ensure safety shutdowns aren't too close to max expected operating fan speed.