ASHRAE Journal - March 2012 - 76

Existing Loop Optimization Current System Plant Name CUP Flow Direction Supply Return Supply Return Flow (gpm) 133 187 293 13 Pressure (psig) 61 43 62 52 Flow (gpm) 286 166 140 32 Scenario 1 Pressure (psig) 59 47 59 52 63% Percentage Of Current Pumping Power Flow (gpm) 286 161 140 38 Scenario 2 Pressure (psig) 59 48 59 52 58% Percentage Of Current Pumping Power

SUP3

Table 2: Existing loop optimization simulation results. used to determine if there are better strategies that will be more reliable and reduce energy consumption. The two methods examined were the Existing Loop Optimization method and the Loop Modification method. For the existing loop optimization method, all existing piping and plant equipment was used; however, the differential pressure at the utility plants was reduced to try to save energy. For the loop modification method, the campus was divided into two zones, so that CUP served a distinct set of buildings and SUP3 served the rest. Again, the differential pressure at the plant was reduced to try to save energy. The simulation flow rate setting was based on building side peak load, which was calculated using trend data from the coldest hour (outdoor temperature) during regular occupancy and ASHRAE DHW consumption standards. For the control strategy simulations, the flow rate remained constant at the maximum building demand to simulate the worst-case scenario. Data was collected from the plant side, as well as from nine buildings prone to DHW problems. These building were chosen to represent the entire loop because of their diverse nature and distance from the plants. The goal of the simulations was to reduce the plant differential pressure without losing building supply pressure, and reduce the building differential pressure without becoming negative.

Loop Modification
For each scenario of the loop modification method, the differential pressure was reduced at the plant while maintaining the necessary building pressure head (non-negative). The modification of pressure settings in each scenario resulted in a reduction of the recirculation pump power necessary. The simulation confirmed that the current system can deliver hot water to campus; however, without automatic valves and pumps with VFD, this requires timely and continuous balancing and maintenance. In the current configuration with the entire system operating as one piping zone, two different plant flows (CUP and SUP3) with different pressures come together in the middle of the system. Based on simulation data, because of the pressure difference and resulting turbulence, this area has the greatest pressure loss in the system. This creates high pressure loss in the piping and causes buildings on the edges of the system to experience low differential pressure (still areas). Scenario 1 represents savings by splitting the loop into two, while Scenario 2 displays the effect of lowering the differential pressure at the plant. The pumping power was reduced by 16% in Scenario 1 and 55% in Scenario 2, relative to the current pumping power required. This pumping power reduction in two-zone operation was realized because pressure head loss was avoided in the center of the zone because the flows no longer mixed. The two zone system also prevented a problem in one part from affecting the whole campus, and would allow issues to be isolated and service maintained. The results of this simulation can be seen in Table 3.

Existing Loop Optimization
The first simulation was run on the existing DHW loop. The current settings were modeled, as well as two scenarios where the differential pressures at the plants were reduced. The simulation showed that as the pressure at the plant decreased, the differential pressure at the buildings, recirculation pump power at the plants, and the required flow rate all decreased. In Scenario 1 and Scenario 2, the recirculation pump power consumption was reduced by 27% and 42% of current pumping power, respectively, due to the addition of VFD drives to the pumps and the reduction of the building differential pressure more in Scenario 2. In each scenario, the differential pressure at CUP and SUP3 were reduced to understand energy savings potential while meeting building demand requirements. The results of this simulation can be seen in Table 2.
76 ASHRAE Journal

Conclusions
The DHW system at Texas A&M University was investigated for inefficiencies and to appease the residents’ complaints. From simulations, field investigations, and data analysis, the following conclusions were drawn: • Automatic controls should be added to plant operations and key points on the loop to eliminate flow problems caused by improper water balance. The current oversized pumps at the utility plants should be replaced with smaller capacity VFD pumps to reduce pumping power by 27% to 42% of the current pumping power. The speed of these pumps without VFDs cannot be adjusted to deliver water flow as needed (demand). Automatic control devices and the pumps with VFDs
ashrae.org March 2012

ASHRAE Journal - March 2012

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