Expanding chiller capacity when retrofitting old chillers

We just completed this chiller design. The existing chillers were two R22, 170 ton chillers that serve three university residence buildings. The client said on hot days, they need to run two chillers. They also said they are having a bunch of problems keeping these chillers running. They are outdoor, air cooled chillers, primary pumps only, with 500 MOP circuit breakers. We were also charged with doing load calculations to determine the correct chiller size. The calculation results required a 265 ton chiller capacity. Eventually we ended up with two 230 ton scroll chillers. Only one chiller will run at a time. The chiller needed 466 MCA and a 500 MOP breaker but if you know how MCA is calculated and how breakers are sized you will understand why we had to change the breaker out to a 100 % rated breaker verses keeping the standard 80 % rated breaker in place. See discussion to understand why.

Every situation is unique but here are some items that worked for us on this project.

  1. Outdoor units were located on concrete pads so we could inexpensively expand the pad foot print for the larger units.

  2. The existing electrical wires were adequate to support the new chiller loads without excessive voltage drop.

  3. The pumps (three of them) can easily produce over 500 GPM. We found this out from our ability to test flow and pressure. In addition we discovered the boiler isolation valves were leaking about 40 GPM of chilled water through the boilers. This is causing condensation inside these cast iron boilers. Our testing services provided documentation for chiller selection and a maintenance issue that need to be addressed.

Another creative solution we proposed was to pipe two 170 ton scroll chillers in series and install a control panel to stage the two chillers. This was proposed due to the 5” pipe limitation. The pipe flow is maxed out at about 400 GPM from a design standpoint. In the past, two 170 ton screw chillers would be brought on line. However, the plant was only delivering 400 gpm so each chiller might see 200 GPM each if flows somehow balanced. More than likely flows are not balanced between the chillers. By putting them in series, we would guarantee both chillers would see 400 GPM and would have up to 10 scroll compressors to stage on (five each chiller). The pressure drop between each chiller was about 9 feet and the existing pumps could handle the extra drop. By pass piping would be put in for chiller maintenance. This is very unusual in chiller design but was supported by the chiller manufacturer. In the end, the client opted for two 230 ton chillers with only one operating at a time.

To make the 230 ton chillers work, the FLA can not be over 80 % of the breaker size. In our case, the 230 ton chiller’s MCA was 466 and the MOP was 500. Some would say this was ok if they sized the breaker using breaker FLA = MCA/1.25 (national electric code). Of special note is that MCA is “not” FLA = MCA/1.25 for air conditioning equipment. MCA for air conditioning equipment is governed by UL 1995 section 36.14. This would also become apparent if you added up all the individual amps for each compressor and condensing fan. If you incorrectly used MCA/1.25 for this application, you would be tripping that breaker on a hot day.