ASHRAE Journal - March 2012 - 73

One study by Goldner found that constant speed pumps controlled by a recirculation line aquastat reduce water consumption, decrease delivery time, increase temperature, and save money.2 Another method for resolving flow problems is to retrofit valves on the loop. A study concluded that for variable water flow, three way valves at remote points on the loop can maintain minimum flow; however, correctly sized pumping is a better solution.3 The best improvement for a DHW system would be a combination of improved insulation and reduced recirculation flow rate.4 Before time and money are invested in retrofitting equipment, such as the automatic control device and water pump with variable frequency drive, hydraulic simulation software was used to analyze the DHW system. The specific characteristics of piping, joints, and heating equipment can be entered, and the simulation can show flow and temperature at fixtures.5 A study was performed on the TAMU DHW system in 2005 that identified problems in the loop and presented several possible control strategies.6 This investigational article will build upon the previous paper’s analysis using new simulation software to model different scenarios and conditions, which will help to determine the most reliable and cost effective control strategy.

300

250

Flow Rate (gpm)

200

150

100

0 12 a.

3 a.

6 a.

9 a.

12 p.

3 p.

6 p.

9 p.

CUP Supply CUP Recirculation

SUP3 Supply SUP3 Recirculation

Figure 1: Daily flow data. gallons/year (15.8 million L/year) were consumed based on the one-year trended data. During times of the year with peak campus occupancy (September through April), the supply flow rate increased while the recirculation flow rate decreased due to increased demand. During the same time period, the supply and recirculation pressure were reduced from the increased load on the system. In the summer (May through August), decreased occupancy and hot weather resulted in the supply flow from both CUP and SUP3 decreasing while the recirculation flow and pressure increased for both plants. The supply and recirculation flows and their pressure values at both plants fluctuated due to demand or system issues. The daily flow data, shown in Figure 1, was examined for Feb. 9, 2011, which was representative of the most demanding day in the heating season. In the early morning (2 a.m. to 5 a.m.), few faucets were open, essentially making the DHW loop a closed system. The supply flow decreased and the supply and recirculation pressures increased because both domestic cold water (makeup water) and domestic hot water had little usage or consumption. The supply and recirculation pressure increased at the same rate because the recirculation pump ran at a constant speed. As residents began to use the faucets and showers around 6 a.m., the flow increased, the DHW system became an open system, and the supply pressure and recirculation pressure both decreased.

Site Information
More than 60 years ago, the DHW Main Campus loop was designed to support only a few buildings at Texas A&M University. As the campus expanded over the decades, the size and complexity of the DHW system has also grown. The DHW loop has a total piping length of over 12 miles and serves 71 main campus buildings. The buildings on the DHW loop include 41 residence halls, two dining halls, and 28 office and lab buildings making up more than 6 million ft2 (557 418 m2). Two interconnected utility plants are on the loop: the central utility plant (CUP) and the satellite utility plant 3 (SUP3). In 2010, total DHW production was approximately 46 billion Btus (48.5 billion kJ), 21 billion Btus (22 billion kJ) at CUP and 25 billion Btus (26.3 billion kJ) at SUP3. The DHW setpoint is 140°F (60°C) at CUP and SUP3. There is more than 100% redundancy when comparing the estimated total peak load of the building side to plant capacity, which means that the plant capacity is sufficient. To verify the capacity of the loop and building piping system, maximum hourly DHW consumption was estimated for each building based on ASHRAE DHW consumption standards.7 Piping size requirements were then determined for supply and recirculation pipes with recirculation flow rate estimated at 2 gpm (0.12 L/s) per building. The capacity of existing supply and recirculation pipes was found to be sufficient by comparing the calculated and existing pipe sizes.

Plant Operation
The DHW system is much more complex than the campus heating hot water and chilled water loops, which operate as closed systems all the time. The DHW system alternates between an open system and a closed system based on usage, which causes it to be more complicated. Additionally, the DHW system is unique because the recirculated water joins the makeup water via a recirculating pump before entering the boiler or heat exchanger.
ASHRAE Journal 73

Data Analysis, Simulations, and Results Production Profile
Supply and recirculation flows in FY 2010 were 1.6 million gallons/year (5.9 million L/year) and 1.2 million gallons/year (4.3 million L/year), respectively. Consequently, 41.8 million
March 2012

ASHRAE Journal - March 2012

Table of Contents for the Digital Edition of ASHRAE Journal - March 2012