Hello,
Our new Trane rotary water cooled chiller is apparently oversized. It s a 250 ton machine (Trane RTHD-250), but the max load thus far has been 90 tons on a design day, and many days have been much less. Consequently, the machine constantly short cycles. Trane has stated that this will prematurely wear the machine. Trane also states that the machine should not be operated at loads less than 30% of it's capacity, or 75 tons. We have many days where the load is only 50 tons.

Currently we operate the machine to make 50°F chilled water. I'm thinking that in the shoulder seasons, we could lower the chilled water temp (effectively reducing the capacity of the machine, which would lower the 30% min loading).

We are also wondering if we could add an adjustable frequency controller to the compressor, not to control the machine on loading, but to reduce the capacity of the compressor. Also thinking about adding hot gas bypass... Anyone have any experience adding AFC or hot gas to a rotary machine?

Much appreciated!

Do you mean "centrifugal" for "rotary"?

I'm not familiar with the controls on your machine, but I wonder if Trane has a control parameter on that machine for monitoring the temperature difference between water entering and leaving the cooler, and keeping the machine online until a minimum difference is reached.

Example: chilled water supply setpoint is 45 degrees F. If delta between entering and leaving water approaches, say, 1.5 degrees F, the machine shuts down, but not until then, even if it is delivering 45 degree supply chilled water.

I have a similar situation as yours in that my chillers are oversized, but I do not have a short cycling problem. Part of this is due to year round process loads the chiller must handle, plus the control strategy discussed above. That said, I do real time measurement of my chiller tonnage output and there are times in winter where it's running at 50 tons...chiller is rated for 220 tons.

For your machine I would imagine you need to work closely with Trane and a technician well schooled in the particulars of your machine.

It's not a centrifugal, the compressor is a screw. Trane says adding hot gas bypass, which would artifically load the machine under low load conditions, would probably work, however, they have not tested it on this machine, and won't provide any service or support hencefourth. They stated the same sentiment when asked about a AFC.

For now, we are changing the loop temperature parameters, to give the machine more to chew on when it is on, and allow time for more energy to build up in the loop, thus allowing the machine to stay off for a longer period of time. The trick is to make it work when the load is really small.

What is your temperature deadband between when the machine shuts off and then starts again? Does it also have a minimum off time setting before it can restart, even if the deadband is exceeded within that timeframe?

The small load is obviously a reason for the short cycling. However, have you looked at your total chilled water system volume? If low in relation to your chilled water flow rate, the chiller will cycle. So for a standard 250 Ton machine and assuming you have a 3 GPM per ton your system flow would be around 750 gallons. Even a system volume of 1,000 gallons would result in a turn-around of chilled water in 1.3 minutes. Trane recommends a 2-minute cycle rate on the chilled water system for stable operation. I usually push for the 3 to 5 minute range. However your second post alludes to the fact that you do have some system volume as you are 'storing' energy in the loop.

See page 9 of following...

http://www.trane.com/download/equipmentpdfs/RLC-PR...

A buffer tank and changing the control parameters like yourself and shophound suggests would probably provide more stable operation.

If you have sufficient volume I think your best option is to tweak the control parameter and set points.

I don't know if you machine is under any factory warranty but I would bet your warranty would be voided w/ the inclusion of a drive or hot gas bypass? That's a question for Trane.

P.S.... I believe this guy is a water cooled jobber.. Do you have means of increasing condenser water temperature back to the unit at low loads and thereby increasing compressor lift....???? Messes w/ your efficiency but the goal here I believe is to keep the baby online longer at part loads.... Just a thought... I haven't thought thru that option very far....

My question would be, what is going to change over the life of the building (and the chiller)? Honestly, you'll be money ahead in the long run to admit a mistake was made selecting the chiller, and either replace it with something smaller, or add a new smaller chiler to carry the low loads. A cold wateer storage tank might band-aid the system, but probably costs more than doing it right.

If the building potentially will increase its loads dramatically, into the preferred range of the 250-ton machine, then hot gas bypass or other false loading schemes may have merit.