Electric gas cooling meets university's cooling demands at lower cost
A recent chiller plant improvement at the school involved the design and installation of a combination chiller plant, with both electric centrifugal chillers and a gas-fired absorption unit. With this hybrid system, the university said it will be in a favorable negotiating position with energy suppliers, and has increased system reliability.
Texas Lutheran University was founded in 1891 and offers degrees in 36 academic disciplines and pre-professional programs. Located about 20 miles east of San Antonio, the school has an enrollment of about 1,300 and occupies an open, landscaped campus surrounded by lakes, parkland, and the Guadalupe River.
The campus includes 19 buildings, including classroom facilities, residence halls, a student union, library, chapel, field house, and administrative and service facilities, totaling 570,000 sq. ft.
Older plant insufficientCampus buildings receive central chilled water service and hot water service from a central plant building.
"We had only 950 tons of usable chiller capacity, and on hot days we were sending out water at up to 58¿F. We could only barely meet sensible load," Dodgen said.
The chiller plant consisted of two older centrifugal chillers.
The university worked with BMW Engineering, Corpus Christi, Texas, and with the Trane San Antonio sales and service organization to evaluate options for plant improvement. BMW identified a hybrid system of electric centrifugals and gas-fired absorption chillers as an attractive option because of the availability of favorable off-peak rates for both natural gas and electricity.
The university natural gas supplier is Reliant Energy of Houston and the electric supplier is the City of Seguin, which buys energy through the Lower Colorado River Authority. The campus receives three-phase electric service that is primarily metered at 7,200 vac at a campus substation before it is distributed on a mostly underground campus electric loop. The electric chillers operate at 600 volts.
The equipment chosen included two Trane CenTraVacTM Model CVHF centrifugal chillers rated at 500 tons each, and one Trane HorizonTM direct-fired absorption chiller rated at 500 tons, for a total capacity of 1,500 tons.
The CVHF units are high-efficiency two-stage machines with hermetic compressors that use low-pressure R-123 as a refrigerant. The Horizon features an integrated gas-fired steam generator and it matches up well with the centrifugal machines, Trane officials said.
Both types of machines operate in the same temperature ranges, so the chilled water outputs are identical.
The three chillers are designed to operate in any combination and the chiller plant provides chilled water at 44¿F, with the chilled water normally returning at 54¿F.
The system is designed as a primary-secondary loop system with a plate-and-frame heat exchanger at the mechanical plant. Condenser water from the three chillers is piped to ground-level cooling towers adjacent to the mechanical plant. Under design conditions, condensing water goes to the towers at 95¿F and returns at 85¿F.
Trane officials said the performance of both the centrifugal and absorption machines can be optimized an analyzed with the company's UCP2TM unit control panel. Chiller plant control is provided by a Trane Tracer SummitTM system, which interfaces with a campus-wide control systems that include both Tracer Summit and Siemens control elements.
The central plant delivers chilled water to a mixture of constant-volume air handlers and UniTraneTM horizontal concealed fan coils. Some of the newer campus buildings are equipped with Trane Modular Climate ChangerTM central station air handlers and VariTraneTM VAV distribution boxes.
"There were some hiccups at the beginning - not surprising with an all-new plant - but we worked through them. Trane and BMW Engineering did everything they could to make the transition as seamless as possible," Dodgen said.
"With the new lower temperatures of chilled water, there was a need to rebalance most of the air handlers and terminal boxes. This is a time-consuming process, but it had been expected. Ultimately, the new chiller plant was available to run at full capacity by graduation time," said Ken Dixon, an existing building sales consultant from Trane's San Antonio office.