Assessing Initial TAB Conditions for Chiller Replacement by Dave Furrer, PE
Without knowing initial conditions, renovating existing mechanical systems can result in flow and performance issues. Often, an existing system may come out of balance over the years or be modified from the original design and incorrectly assumed that replacing failed equipment with like-for-like will resolve issues. It is important for balancers to be hired early in the project to assess initial air and water conditions when parts of the mechanical system will be reused to avoid problems that arise during the last stretch of the project.
At one of our small high school projects in central Oregon, Elevate was hired to provide condensed commissioning services and to witness Testing, Adjusting, and Balancing (TAB) for a replacement air-cooled chiller. The project was small enough that there was no design or architectural team, and the new chiller was provided like-for-like so that it matched the electrical and hydronic characteristics of the old chiller and almost no changes were needed for these systems. Initial installation of the chiller went well, and the controls work was simple since it only required remapping existing points to the Building Automation System (BAS). However, when it came time to start up the chiller, the TAB readings pointed to underlying problems with the system that would not allow the new chiller to work as intended.
To begin balancing, our TAB contractor turned off the chiller and opened all the chilled water (CHW) control valves in the building to simulate maximum cooling demand. To measure flow using an ultrasonic meter, a section of pipe returning from the building was selected so that there was sufficient straight length to avoid turbulence and pockets of entrained air that could affect the reading. A balancing valve was not installed or needed at the pump discharge because it was controlled by VFD. Pressure-Temperature (PT) fittings were installed on the CHW pump at pre-existing taps so the manufacturer’s pump curves could be used as a separate method of calculating and confirming the flow. After taking readings with the pump VFD running at 30 Hz, we determined the CHW flow was only 73 gpm, and needed to be raised to 108 gpm, our new chiller’s minimum flow. Using the equality between pump speed and flow from the pump affinity laws, we can calculate the required pump speed to achieve 108 gpm from our initial measurement:
TAB made follow-up readings to confirm this calculation, and we recorded a minimum flow of 108 gpm at 45 Hz. After this, we raised the pump speed to 60 Hz and found the pump was only capable of 144 gpm while the design flow shown on the chiller submittal is 207 gpm. Unless flow is increased, our new chiller is only capable of 70% the advertised capacity and parts of the building could overheat during the summer when they are starved for CHW. To eliminate the possibility of low flow being caused by debris at key points in the system, we measured the head loss across the chiller and strainer against the manufacturer’s literature and found these readings were normal. Using the affinity law above, we can also calculate pump speed needed to achieve design flow.
However, after reviewing the VFD settings and the pump motor nameplate, we found the pump speed could not be increased from 60 Hz to 85 Hz because were already near the motor’s full load amps and running consistently above this would cause permanent damage to the motor.
In summary, TAB and Elevate recommended upsizing the existing CHW pump so the chiller is supplied with the design flow rate. Until this is addressed, the chiller is only capable of 70% of the design capacity and there may be a few weeks each summer when it struggles to keep the building cool. If our balancer had been able to assess the existing conditions prior to demolition of the existing chiller, they could have identified this problem early and communicated the need for scope changes. This represents cost savings for the owner and a tighter schedule for the contractor. In general I recommend having TAB assess initial conditions any time there are changes to an air or hydronic system that re-uses existing equipment, ductwork, or piping.