Recipients of federal research grants once were reimbursed for utility costs based on estimates, such as of the square footage of buildings. Recently, however, the federal government began requiring data. To satisfy that requirement and to help pinpoint energy waste and maintenance problems, Johns Hopkins University in Baltimore, the largest university recipient of federal-research-grant money, installed meters to monitor the use of steam, chilled water, and electricity in more than 40 buildings. A few larger meters monitor steam and chilled-water production in the campus boiler room and chilled-water plants. Purchased electricity is monitored at the incoming feeders and submetered at each building.
The campus powerhouse has four steam boilers: one rated at 80,000 lb per hour and three rated at 48,000 lb per hour. Output steam pressure is 125 psi. On cold winter days, the powerhouse may generate a peak load of 80,000 to 85,000 lb per hour. During the summer, the rate may drop to about 18,000 to 25,000 lb per hour.
For chilled-water production, the powerhouse contains a new 2,700-ton duplex centrifugal chiller and two 1,500-ton centrifugal chillers, while another location on the north side of the campus has a 1,500-ton centrifugal chiller and two nominal 1,200-ton centrifugal chillers. Although the latter two primarily make ice, they also can be used to make chilled water.
Steam and chilled water are distributed through a network of pipes installed primarily in utility tunnels. Some parts of the distribution system are buried directly underground. As steam leaves the powerhouse, its pressure is reduced to 70 psi; downstream of the distribution flow meters, its pressure is reduced further.
For chilled-water production and distribution, the university selected dual-turbine flow meters. The monitoring system takes a reading by averaging the outputs of the turbines.
For steam, the university picked TRIO-WIRL vortex and swirl flow meters from ABB Inc., which offer accuracies of 1 and 0.5 percent of rate, respectively. Because the meters have no moving parts, maintenance and downtime are minimized. And because they can be installed horizontally, vertically, or at an angle, the cost of installation piping is relatively low. Digital-signal processing makes the meters virtually immune to vibration and electrical noise, while increasing their sensitivity to low flow rates.
Both meter types incorporate local or remote read-out displays for instantaneous or totalized flow.
Both the vortex and swirl meters require careful sizing. In the experience of Johns Hopkins, meters in distribution systems often can be reduced one to two sizes from the piping-line size to ensure good range. The range of these steam meters is about 20-to-1. Without proper sizing, the meters may give inaccurate results at the low steam flow rates experienced by the campus during summer.
Because the steam is not superheated, its temperature is a constant for any given pressure. Pressure sensors mounted on the steam lines provide sufficient information for determining mass-flow rate.
A feed-water control valve regulates the flow of water to the boilers, based on boiler-water level and the mass-flow rate of steam exiting the boilers.
The steam flow meters are serviced and calibrated annually. Technicians check the wave-form shape of the meters' piezoelectric sensing elements for various output frequencies. Then, they electrically disconnect the meters and impress a signal frequency on the electronics.
Output signals from the steam flow meters, pressure sensors, and chilled-water meters are sent to a control unit that collects the data, determines mass flow, and converts flow values to engineering units. The steam, chilled-water, and electricity energy values from the various buildings travel via a local-area network to central servers that maintain a database of usage in each facility.
Personal computers tied to the network can access the databases for reports, which typically include: total monthly energy use by building, a 12-month spreadsheet of steam and chilled-water use in each of the 18 buildings in which sponsored research is conducted, and a spreadsheet showing a monthly breakdown of chilled-water, steam, and electric-energy use in each monitored building for the university's fiscal year to date.
The reports allow the plant-operations department to study trends and histories, as well as note unusual values that may indicate the need for maintenance. They also serve as verification of energy costs for the buildings in which sponsored research is conducted.
The university recently embarked on a large energy-conservation project. The flow meters will play an integral role, helping to verify results of measures to save energy and eliminate waste.
The flow meters also will play an integral role in the university's efforts to balance production and distribution values. For example, the university can account for about 95 percent of the produced chilled water though measurements in the distribution system. By contrast, the distribution measurements account for only 80 percent of the steam produced. Presumably, buildings without meters and distribution losses make up the remaining 20 percent. More flow meters will be added to help bring distribution values in line with production values.
In the future, the university may elect to bill campus departments for energy use based on actual metered values. By that time, the plant-operations department should have a good history of energy usage and have built confidence in the measured values indicated by the flow meters. Lastly, the university plans to investigate a more sophisticated and efficient form of boiler control based on the steam mass-flow rates measured in the powerhouse.
Information and photos courtesy of Johns Hopkins University.
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