On a recent project there were two buildings served by a common medium temperature chilled water system (CHWS). The medium temperature chilled water system consisted of two pumps, two heat exchangers and a building loop for each building. The pumping system and heat exchangers provided medium temperature chilled water to building loops, serving the active chilled beams in each building. The pumping system controlled to the differential pressure sensors located in each building loop. Throughout the construction process, one of the building’s schedules began to lag behind the other by several months. While Building 1 was nearing completion, Building 2 was still under heavy construction, lacking automatic controls for the chilled beams, building pressurization was unable to be maintained and safeguards against potential condensation issues were not in place. The piping, control valves for the chilled beams and control components for Building 2 were in place. The question then became, “How can we complete testing on the Building 1 while safeguarding Building 2?”
Building 1 had all control components in place, the terminal units serving the active chilled beams were calibrated and balanced on the air side, the building pressurization was positive and being maintained and all safeguards against potential condensation issues were in place. In addition to the issues previously mentioned, Building 2 also had isolation valves for the main supply and return lines. With all components in place, it was possible to perform a proportional balance of the medium temperature CHWS. The key issue at hand was protecting Building 2 from condensation issues and managing to test the active chilled beam air and water sides for Building 1.
A meeting was held between EAB, the controls contractor and the general contractor to discuss the balance of the medium temperature CHWS. The issues at hand were presented and gone over in detail. EAB presented the option of closing the isolation valves on the chilled water side of the heat exchangers and allowing the medium temperature chilled water pumps to continue operation. The thought process being that the medium temperature chilled water would reach the ambient temperature of the spaces served and not pose a risk of condensation at that point. This would allow a full readout of the medium temperature CHWS and proportional balance of the coils served. This was deemed the best option available for the construction schedule and allowed for testing to continue.
The chilled water isolation valves for the heat exchangers were closed and the pumps were allowed to continue in operation. Once the temperature in the medium temperature chilled water loop reached ambient conditions, the controls contractor was directed by the general contractor to manually open all control valves for the medium temperature CHWS in Building 2. The control valves for Building 1 were commanded through the BAS and a readout was performed for each building loop. The medium temperature chilled water was then proportionally balanced at each device. A final readout was then performed to verify the proportional balance of the system. Once the devices were verified to be proportionally balanced, the medium temperature chilled water differential pressure sensors calibration were verified and the readings recorded. The pressures in Building 1 were determined to be 18.0 PSI at Sensor 1 and 15.0 PSI at Sensor 2. The pressures for Building 2 were determined to be 14.0 PSI at Sensor 1 and 12.0 PSI at Sensor 2.
With the differential pressures and coils at design flow in each building, a differential pressure setpoint was able to be set in the controls. The challenge now shifted to the testing of the active chilled beams for air and water temperatures in Building 1. The control sequence called for the pumps to maintain the differential pressure setpoint at the lowest sensor in the loop. To test the active chilled beams for Building 1, Building 2 would have to be isolated from the system. Building 2 was isolated from the system at the building isolation valves. To properly test the system, the differential pressure control would need to be limited to the differential pressure sensors for Building 1. The controls contractor altered the system program to eliminate the differential pressure sensors for Building 2. This allowed the pumps to control to the differential pressure setpoint of Building 1 only. The differential pressure setpoint was then adjusted to 15.0 PSI to maintain the flow at the chilled beams in Building 1. The chilled water side of the heat exchangers was then opened at the isolation valve and allowed to control the loop temperature. Testing of the chilled beams for Building 1 was allowed to commence at this point.
Once testing was complete on Building 1 chilled beams, the building was released to control under automatic operation. Building 2 was left in an isolated condition until it met the requirements to circulate the medium temperature chilled water. When Building 2 met the requirements to circulate the medium temperature chilled water, the isolation valves were then opened. The differential pressure sensors for Building 2 were re-enabled back into the control system and the differential pressure setpoint adjusted accordingly. The chilled beams for Building 2 were then tested. With both buildings tested the heat exchanger performance could now be verified. A test was performed under a full flow scenario in which both buildings medium temperature chilled water control valves and associated terminal units were placed in a full cooling scenario. Capacity testing of the heat exchangers was then able to be performed.
This system presented unique challenges regarding the phasing of both projects. The result of this process was both buildings were able to be tested and completed on schedule while preventing condensation issues. In TAB we are always striving to test under the best conditions available. When those conditions are not available, being able to execute a plan that results in the best outcome possible is key.
Written by Durwood Lassiter, III, TBE.