Primary Secondary Loop vs. Primary Loop Only Systems...
Primary/Secondary-Loop vs. Primary-Loop-Only Systems Comparison of operational modes and performance of two schemes for optimizing chilled-water plants
control), even if flow in the secondary loop (which has its own pump, variable-speed Pump 2) varies significantly.1,2 Pump 1 is sized to maintain water flow between the chiller evaporator’s minimum and maximum allowable values.
By ALEXANDER L. BURD, PHD, PE, and GALINA S. BURD, MS Advanced Research Technology LLC Suffield, Conn.
Typical Control Strategy
S
In the system in Figure 1, the direction of water flow in the decoupling pipe is not controllable and may vary, depending on the ratio of flow in the secondary and primary loops. Various modes of operation of the system were investigated.3,4 The major parameters in the evaluation of energy efficiency were supply- and return-water temperature and flow rate before and after the decoupling pipe separating the primary and secondary loops.
Subdividing various systems into a primary/secondary (P/S) loop via a hydraulically dependant interconnection long has been a standard solution for central chilled-water plants in the United States and Europe. This achieves, at a relatively low cost, reasonably good hydraulic separation of central-water-plant cooling-generation systems (primary loop) from distribution piping Primary loop and terminal units (secondary loop). (generation-system piping) Secondary loop (distribution-system piping) Primary-loop flow is relatively conLoad stant, while secondary-loop flow varies based on load demand. Primary- and VFD secondary-loop water flows are interVFD changeable. ∆P2 t1 t3 In a primary loop, control is achieved B Chiller ∆P1 by maintaining a relatively constant flow 3 2 F2 rate; water temperature may be changed A t4 t2 1 F1 via a reset control. This commonly is referred to as a qualitative control strat1 = Optional constant or variable speed with variable-frequency-drive (VFD) egy. In a secondary loop, control typiprimary-loop pump 2 = Variable-speed secondary-loop pump with VFD cally is achieved by varying water-flow 3 = Non-controllable bidirectional decoupling pipe (AB or BA direction) between primary rate. This commonly is referred to as a and secondary loop F1 = Flow-meter primary-loop flow rate quantitative control strategy. F2 = Flow-meter secondary-loop flow rate Figure 1 depicts a system with a cont1 = Temperature of water leaving chiller t2 = Temperature of water returning to chiller stant- or variable-flow primary loop t3 = Secondary-loop supply-water temperature and a variable-flow secondary loop. t4 = Secondary-loop return-water temperature The dedicated constant-speed Pump 1 ∆P1 = Pressure differential for controlling F1 in Pump 1 ∆P2 = Pressure differential for controlling F2 in Pump 2 maintains practically constant flow in the primary loop (the pump does not FIGURE 1. Optimized control strategy for chilled-water plant with primary (constant- or have variable-frequency-drive [VFD] variable-flow)/secondary (variable-flow) loop.
Alexander L. Burd, PhD, PE, is president of and Galina S. Burd, MS, is a project manager for Advanced Research Technology, an engineering and research consulting firm with offices in Suffield, Conn., and Green Bay, Wis. Alexander (
[email protected]) has 35 years of experience in the design, research, and optimization of HVAC and district energy systems, which includes publication of more than 35 research and technical papers in American and European journals, while Galina (gburd@energyart .net) has more than 25 years of design and research experience in the HVAC and architectural-engineering fields. She has co-authored many technical and research papers published in American journals. 36
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Mode 1. When water flow in the secondary loop (F2) exceeds water flow in the primary loop (F1) because of a load increase, a portion of the water returning from the secondary loop recirculates into the supply-distribution system (A-to-B direction) and mixes with the flow in the primary loop. This mode of operation is represented by the following equations: F2 > F1, t3 > t1, t2 = t4, and (t4 − t3) < (t2 − t1). Mode 2. When water flow in the secondary loop (F2) is less than water flow in the primary loop (F1) because of a load reduction, flow in the decoupling pipe reverses (B-to-A direction). Thus, the excessive flow exiting the cooling-generation system returns to the primary-loop system and the chiller. This mode of operation is represented by the following equations: F1 > F2, t1 = t3, t2 < t4, and (t4 – t3) > (t2 – t1). Mode 3: When the flow in the primary loop (F1) equals the flow in the secondary loop (F2), there is no flow in the decoupling line. All water from the secondary loop returns to the primary loop and chiller, while all water exiting the chiller flows through the secondary loop. This mode of operation is represented by the following equations: F1 = F2, t1 = t3, t2 = t4, and (t2 – t1) = (t4 – t3). Obviously, this mode of operation is the most beneficial from an energy perspective.
Optimized Control Strategy Figure 1 depicts the optimized control strategy (Mode 3). Unlike a system with an optional constant-speed primary-loop pump, the system has an additional VFD controlling the speed of the primary-loop pump. The rate of water flow in the secondary loop via Pump 2 is dependent on system load. Water-flow rate in the primary loop is a function of water-flow rate in the secondary loop and adjusted to maintain equalized flow. Following are simplified thermal-balance equations applicable for both primary and secondary loops, assuming the specific heat of water does not change appreciably: QPR = QSEC QPR = F1 × (t2 – t1) = F1 × (∆tPR) QSEC = F2 × (t4 – t3) = F2 × (∆tSEC) F1 × ∆tPR = F2 × ∆tSEC F1 = F2 ∆tPR = ∆tSEC
(1) (2) (3) (4) (5) (6)
where: QPR = Primary-loop cooling load, British thermal units per hour QSEC = Secondary-loop cooling load, British thermal
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Circle 166 DECEMBER 2010
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PRIMARY/SECONDARY-LOOP VS. PRIMARY-LOOP-ONLY SYSTEMS
units per hour ∆t PR = Primary-loop temperature differential, degrees Fahrenheit ∆tSEC = Secondary-loop temperature differential, degrees Fahrenheit Equations 5 and 6 essentially represent the algorithm for the control of chilled-water plants. For the system in Figure 1, control is accomplished by varying the speed of Pump 1. The pump’s speed should not be reduced to the extent waterflow rate falls below the allowable low limit or increased to the extent water-flow rate exceeds the allowable high limit. The building-automation system would have to limit VFD-turndown and turn-up ratios to stay within the range of allowable current frequencies correlated to the range of allowable primary-loop water-flow rates. To better match primary- and secondary-loop water-flow rates, two-phase control is suggested. The first phase could consist of quantitative control in both the primary and secondary loops (Equation 5) while the chilled-water-supply temperature remained at a given constant value. The second phase could consist of qualitative control in the primary loop and quantitative control in the secondary loop. (The order in which the control actions are implemented may vary.) Reset water-temperature control (Equation 6) could be realized by varying water temperature (t1) at a given fixed (limited) magnitude of water-flow rate in the primary loop. The change in water temperature would impact flow rate in the secondary loop indirectly; flow via the primary-loop pump could not be changed further because of the aforementioned evaporator-flow limitations. The P/S-loop system with variable flow and temperature control in both loops in Figure 1 is very versatile, allowing the establishment of flow-limiting parameters and temperature set points (t1) over a given time period. 38
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Distribution-system piping loop Load
Generation-system piping loop VFD
∆P2 t3
F2
1
A
t1 ∆P1
2 t4
B
t2
Chiller
F1
1 = Primary-loop-only variable-speed pump with variable-frequency drive (VFD) 2 = Automatic control valve for converting primary-loop-only system to constant primary-loop and variable secondary-loop flow operation (single AB-direction controllable flow decoupling pipe) F1 = Flow-meter generation-system-piping-loop flow rate F2 = Flow-meter distribution-system-piping-loop flow rate t1 = Temperature of water leaving chiller t2 = Temperature of water returning to chiller t3 = Distribution-system-piping-loop supply-water temperature t4 = Distribution-system-piping-loop return-water temperature ∆P1 = Pressure differential for controlling F1 ∆P2 = Pressure differential for controlling F2
FIGURE 2. Primary-loop-only-variable-flow control system.
Primary-Loop-Only-Variable-Flow Control System A primary-loop-only-variable-flow (PLOVF) control system employs a single pump to circulate water through generation- and distribution-system piping loops (Figure 2). This arrangement allows uniformly distributed variable flow throughout entire systems. The generation- and distribution-piping systems have a dependant flow-control arrangement, unlike the generation- and distribution-piping systems in the P/S-loop system in Figure 1, which have an independent flow-control arrangement with two dedicated pumps. When flow in a PLOVF distribution system varies because of a load change, a VFD control varies the speed of the pump. If flow in the distribution system falls below the chiller’s low limit, an automatic control valve modulates to divert flow from the supply line of the distribution system back to the chiller (A-to-B direction). This system, unlike the P/S-loop system in Figure 1, does not have the ability to bypass the chiller if flow in the distribution system exceeds the chiller’s high-limit flow. For this capability to be provided in a PLOVF system, a second controllable decoupling pipeline with automatic
control valve and reversed flow direction would have to be added on the other side of the pump. Otherwise, another chiller, as well as ancillary equipment (i.e., chilled-water pump, cooling tower, condenser pump), would have to be placed online, which would result in increased chiller-plant power demand as soon as flow in the distribution system exceeded the high-level limit. A PLOVF system essentially is capable of maintaining limited total flow rate. Typically, control of systems with flow-rate-limiting devices 5 is achieved by assigning priority status to one of the two loads. For instance, in a district-heating-system customer substation with space-heating and domestic-hot-water loads, priority is given to the domestic-hot-water load. The space-heating load has substantial thermal inertia and the ability to temporarily accept a lower waterflow rate without noticeable impact on air temperature. In a PLOVF system, two systems that typically have low thermal inertia (space cooling) or no inertia at all (chiller-evaporatorwater flow) share the same level of priority control. Control of a PLOVF system is somewhat more challenging than control of a P/S-loop system. This
PRIMARY/SECONDARY-LOOP VS. PRIMARY-LOOP-ONLY SYSTEMS
Chilled-water temperature differential is critical to P/S and PLOVF operations in that it determines chilled-water flow per cooling ton. Impacting chilled-water temperature differential most significantly are the end-users connected to chiller plants. Take, for instance, an air-handling unit (AHU) with the following parameters: • Air is cooled in a counterflow chilled-water coil. • The design cooling load is 28.5 tons. • The design sensible and latent loads are 76 percent and 24 percent of the design cooling load, respectively. • The cooling load varies in direct proportion to outdoor dry-bulb air temperature.
to be reduced to about 21 percent of its design magnitude. For the same reduction in cooling-coil capacity to be achieved with the VAFC system, relative chilled-water flow rate would have to be reduced to about 35 percent of its design magnitude. The bottom graph in Figure 3 indicates the CAFC system will increase the relative chilled-water temperature differential by a factor of about 1.9 when the relative cooling load is reduced to 40 percent of its design magnitude. For the same conditions, the VAFC system will increase relative chilledwater temperature differential by a factor of only about 1.2. Thus, the same relative reduction in chilledwater flow via the cooling coil would result in about 1.6-times-higher relative chilled-water temperature differential with the CAFC system than it would with the VAFC system.
Chiller-evaporator low-limit flow control. Various control strategies can be utilized to ensure the sustain-
1.0
Chilled-water-coil relative cooling load
Chilled-Water-System Temperature Differential
• The cooling coil has a two-way chilled-water control valve to vary water flow through the coil to satisfy loads. • The cooling coil is selected for 15°F design chilled-water temperature differential. • The design chilled-water flow via the cooling coil at 40°F inlet water temperature is 45.7 gpm. End-user load control. The authors considered two types of AHU cooling-load-control systems: constant airflow control (CAFC) and variable airflow control (VAFC). Maximum airflow turndown ratio was assumed to be 3.4 at a relative cooling load of 0.29 and lower. The top graph in Figure 3 shows the CAFC system requires a substantially greater change in relative chilled-water flow to achieve the same level of variation in cooling-coil load. For instance, for cooling-coil capacity to be reduced to 40 percent of the design load, relative chilled-water flow would have
0.8 CAFC VAFC 0.6
0.4
0.2 0.2
0.4
0.6
0.8
1.0
0.8
1.0
Cooling-coil relative chilled-water flow rate
Chilled-water-coil relative chilled-water temperature differential
is because a PLOVF system has to maintain two variables that are continually “fighting”: a given set point of pressure differential (∆P2) in the distribution-system loop and a given set point of pressure differential (∆P1) in the generation-system-piping loop. Thus, overall control-strategy execution and system operation may be less accurate and stable, and an allowance for the possible deviation from the control-parameter set point may have to be made. Typically, to avoid a chiller shutting down because of insufficient flow, the set point for low-limit flow via the evaporator is increased. Because any problems with the modulating control valve could impair system performance or even shut down the entire chilledwater system, the application of reset chilled-water-temperature control during off-design conditions (when the chilled-water-flow limitation is in place) is difficult. In comparison with a P/S-loop system, design chilledwater temperature may be constant and elevated in a PLOVF system, which may lead to increased costs for distribution piping, pumping, cooling coils, etc.
2.0 1.8 1.6 CAFC 1.4 1.2 VAFC 1.0 0.2
0.4
0.6 Cooling-coil relative chilled-water flow rate
FIGURE 3. Air-handling-unit-cooling-coil operational parameters with variable (VAFC) and constant (CAFC) airflow control. DECEMBER 2010
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PRIMARY/SECONDARY-LOOP VS. PRIMARY-LOOP-ONLY SYSTEMS
Chilled-water-coil relative cooling load
0.8 0.6
0.4 0.2 0.0
Cooling-coil chilled-water temperature differential, degrees Fahrenheit
ability of a system’s low-limit chilledwater flow. Figure 4 shows the reset water-temperature control required to maintain a given low-limit waterflow level when relative cooling load changes from 0.64 to 0.17 of design. The temperature of water entering the cooling coil increases as load decreases. Limited chilled-water flow is maintained by increasing the temperature of water entering the cooling coil. The variation in relative cooling load from 0.67 to 0.17 requires the temperature of water entering the coil to be increased from 40°F to about 52°F. The bottom graph in Figure 4 indicates chilled-water temperature differential will be reduced from 15°F at the relative cooling load of 0.67 to 3.7°F at the relative cooling load of 0.17.
40
41
42
43 45 46 48 49 50 47 51 44 Cooling-coil inlet chilled-water temperature, degrees Fahrenheit
52
53
16 14 12 10 8 6 4 2 0 40
41
42
43 45 46 48 49 50 47 51 44 Cooling-coil inlet chilled-water temperature, degrees Fahrenheit
52
53
Other factors impacting chilledwater-system temperature differential. Chilled-water temperature
FIGURE 4. Cooling-coil inlet chilled-water temperature, relative cooling load, and temperature differential with low-limit chilled-water-flow control.
differential also can be impacted by deposits on the inside and outside surfaces of cooling coils. Proper and timely cleaning of heat-exchanger heat-transfer surfaces, thus, is important,6 as is cleaning of chilledwater cooling coils and chiller evaporators and condensers. Because of the complexity of real-life conditions, it is unrealistic to expect an increase in cooling-coil chilled-water temperature differential coinciding with a reduction in cooling load. If a system is balanced and well-maintained, temperature differential will remain relatively close to its design value during offdesign conditions.
the example chilled-water primary piping system was assumed to vary from about 2.1°F to 22.4°F (Table 1). The magnitude of the temperature differential at the chiller’s design cooling load was limited by the allowable minimum and maximum flow at a given number of
evaporator passes. Low-level flow rate was specified to ensure evaporator operation with sufficient heatexchanger-tube water velocity. Highlevel flow rate was specified to ensure evaporator operation with allowable heat-exchanger-tube velocity and avoid unstable heat transfer and tube
Number of evaporator passes
1
2
3
Low-limit evaporator-water flow rate, gallons per minute
964
482
321
Maximum evaporator chilled-water temperature differential, degrees Fahrenheit
7.5
14.9
22.4
Minimum evaporator specific chilled-water flow rate per cooling ton, gallons per minute per ton
3.2
1.6
1.1
3,473
1,737
1,158
Minimum evaporator chilled-water temperature differential, degrees Fahrenheit
2.1
4.1
6.2
Distribution-piping-system design temperature differential. Distribution-
Maximum evaporator specific chilled-water flow rate per cooling ton, gallons per minute per ton
11.6
5.8
3.9
piping-system design temperature differential impacts the installed and operating costs of central chilledwater systems. For retrofit projects, such as conversion from directexpansion cooling coils to central chilled-water cooling coils, the cost of distribution piping could run as high as 30 percent of the entire system.3,4 Design temperature differential for
Relative evaporator-water-flow-rate variation
3.6
3.6
3.6
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High-limit evaporator flow rate, gallons per minute
Notes: 1. Relative evaporator-water flow rate shows the ratio of maximum to minimum specific chilled-water flow rate at the given fixed number of evaporator passes. 2. Minimum allowable water-flow rate (gallons per minute) via evaporator at a water velocity of 3.3 fps at a given number of passes could be calculated as minimum specific flow rate (gallons per minute per ton) multiplied by design cooling load (tons). 3. Maximum allowable water-flow rate (gallons per minute) via evaporator at water velocity of 12 fps at a given number of passes could be calculated as maximum specific flow rate (gallons per minute per ton) multiplied by design cooling load (tons). 4. Parameters based on the performance of a 300-ton centrifugal chiller.
TABLE 1. Chiller-evaporator design operational parameters.
PRIMARY/SECONDARY-LOOP VS. PRIMARY-LOOP-ONLY SYSTEMS
MCRCL = [1 ÷ (∆TDEMAX ÷ ∆TDEMIN)] × SOSF × 100, % where: ∆TDEMAX = maximum chiller-evaporator design temperature differential ∆TDEMIN = minimum chiller-evaporator design temperature differential SOSF = system operational safety factor, which increases minimum chilled-water flow via a chiller’s evaporator (for PLOVF) to prevent chiller shutdown on low flow The higher water-flow turndown ratios (WFTDRs) in Table 2 are advantageous because they allow optimal operation of a system to satisfy cooling loads.
Chilled-Water Turndown Ratio and Energy Savings Figure 5 shows WFTDR patterns for various control strategies. The system was assumed to use mechanical free cooling at outdoor dry-bulb air temperatures of 55°F and below. Also, system cooling load was assumed to change linearly in direct proportion to outdoor dry-bulb air temperature. The following options, shown in Table 2, were considered:
Option 1 ∆TDEMIN = 15°F, ∆TDEMAX = 22.4°F, WFTDR = 1.5 This means relative chillerevaporator chilled-water flow could be reduced to 67 percent of its
Maximum chiller-evaporator design temperature differential, ∆TDEMAX, degrees Fahrenheit
Minimum chiller-evaporator design temperature differential, ∆TDEMIN, degrees Fahrenheit
Allowable reduction in water-flow turndown ratio (∆TDEMAX to ∆TDEMIN), times
Allowable MCRCL, percent
22.4
15.0
1.5
73.6
22.4
10.0
2.2
49.0
22.4
6.2
3.6
30.5
Notes: 1. Allowable reduction in chilled-water flow rate in distribution piping system the ratio of ∆TDEMAX to ∆TDEMIN. 2. Data adapted from Table 1 (chiller with three-pass evaporator). 3. Minimum allowable flow rate via chiller evaporator increased by 10 percent to ensure chiller in PLOVF system will not be turned down on low-level evaporator flow rate. 4. MCRCL = Minimum chiller relative cooling load, at which Bypass Control Valve 2 in Figure 2 will remain closed.
TABLE 2. Considered design temperature differentials for chiller evaporators serving primary/secondary and primary-loop-only systems. design value. Relative chilled-water flow via the pump serving the PLOVF system in Figure 2 will follow two straight lines outlined by the triangle ACB in Figure 5 and remain constant at the level designated by straight line CB. Although relative water flow via the distribution-piping system will follow the load change, the bypass valve will be open to ensure the required Primary/secondary and primary-loop-only relative flow rate
erosion. Lower water-flow rates relate to higher chilled-water temperature differentials and vice versa. Table 1 indicates temperature differential is dependent on number of evaporator passes. It shows evaporator relative water-flow rate remains the same (i.e., 3.6—the ratio of maximum to minimum tube water velocities) for all considered number of evaporator passes. Allowable minimum chiller relative cooling load (MCRCL) in Table 2 was calculated using the following equation:
low-limit chiller-evaporator flow is maintained. This means the pump will run at the constant flow rate indicated by the straight line CB. P/S-loop systems with variableprimary and secondary-loop pumping could provide energy savings (compared with the PLOVF-system operation indicated by straight line CB in Figure 5) while dry-bulb outdoor-air temperature ranged from
1.00 A
0.75 B C
0.50 H D
G
F
0.25 E
0.00 55
65
75
85
95
105
Outdoor dry-bulb air temperature, degrees Fahrenheit AC = Secondary-loop relative flow-rate value in systems with constant- and variable-flow primary-loop pumps (∆TDEMIN = 15°F, ∆TDEMAX = 22.4°F) CB = Minimum primary-loop relative flow-rate value in systems with constant- and variable-flow primary-loop pumps (∆TDEMIN = 15°F, ∆TDEMAX = 22.4°F) ACB = PLOVF-system relative flow-rate variation (∆TDEMIN = 15°F, ∆TDEMAX = 22.4°F) AD = Secondary-loop relative flow-rate value in systems with constant- and variable-flow primary-loop pumps (∆TDEMIN = 10°F, ∆TDEMAX = 22.4°F) DH = Minimum primary-loop relative flow-rate value in systems with constant- and variable-flow primary-loop pump (∆TDEMIN = 10°F, ∆TDEMAX = 22.4°F) ADH = PLOVF-system relative flow-rate variation (∆TDEMIN = 10°F, ∆TDEMAX = 22.4°F) AF = Secondary-loop relative flow-rate value in systems with constant- and variable-flow primary-loop pumps (∆TDEMIN = 6.2°F, ∆TDEMAX = 22.4°F) FG = Minimum primary-loop relative flow-rate value in systems with constant- and variable-flow primary-loop pumps (∆TDEMIN = 6.2°F, ∆TDEMAX = 22.4°F) AFG = PLOVF-system relative flow-rate variation (∆TDEMIN = 6.2°F, ∆TDEMAX = 22.4°F) AE = Relative flow-rate variations for secondary-loop pumps for all considered P/S-loop systems and ∆TDEMIN and ∆TDEMAX values
FIGURE 5. Primary/secondary and primary-loop-only relative chilled-water-flow variation. DECEMBER 2010
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PRIMARY/SECONDARY-LOOP VS. PRIMARY-LOOP-ONLY SYSTEMS
55°F to 83°F because of the variable secondary-loop pumping (Pump 2 in Figure 1) depicted by straight line CE in Figure 5.
Option 2
Option 3 ∆TDEMIN = 6.2°F, ∆TDEMAX = 22.4°F, WFTDR = 3.6 This means relative chiller-evaporator chilled-water flow could be reduced to 27.7 percent of its design value. Relative chilled-water flow via the pump serving the PLOVF system in Figure 2 will follow two straight lines outlined by the triangle AFG in Figure 5 and remain constant at the level designated by straight line FG. Although water flow via the distribution-piping system will follow the load change, the bypass valve (2) will be open to ensure the required lowlimit chiller-evaporator flow is maintained. This means the pump will run 42
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100 Cumulative load time-duration factor, percent (at load equal to or lower than shown)
∆TDEMIN = 10°F, ∆TDEMAX = 22.4°F, WFTDR = 2.2 This means relative chiller-evaporator chilled-water flow could be reduced to 44.6 percent of its design value. Relative chilled-water flow via the pump serving the PLOVF system in Figure 2 will follow two straight lines outlined by the triangle ADH in Figure 5 and remain constant at the level designated by straight line DH. Although water flow via the distribution-piping system will follow the load change, the bypass valve (2) will be open to ensure the required lowlimit chiller-evaporator flow is maintained. This means the pump will run at the constant flow rate indicated by straight line DH. P/S-loop systems with variableprimary and secondary-loop pumping could provide energy savings (compared with the PLOVF-system operation indicated by straight line DH in Figure 5) while dry-bulb outdoor-air temperature ranged from 55°F to 74°F because of the variable secondary-loop pumping (Pump 2 in Figure 1) depicted by straight line DE in Figure 5.
Systems with 80-percent PCCLC Systems with 0-percent PCCLC Systems with 26-percent PCCLC Systems with 100-percent PCCLC
Systems with 62-percent PCCLC Systems with 50-percent PCCLC Systems with 38-percent PCCLC Systems with 15-percent PCCLC
90 80 70 60 50 40 30 20 10 0 0
10
20
30
40
50
60
70
80
90
100
Relative cooling load, percent Notes: 1. Constant load percentage based on ratio of constant-cooling-load component that does not change during mechanical cooling season to design cooling load. 2. Relative cooling load shows ratio of current load to design cooling load. 3. PCCLC = Percentage of constant-cooling-load component.
FIGURE 6. System relative cooling load and cumulative time-duration factor. at the constant flow rate indicated by straight line FG. P/S-loop systems with variableprimary and secondary-loop pumping could provide energy savings (compared with the PLOVF-system operation indicated by straight line DH in Figure 5) while dry-bulb outdoor-air temperature ranged from 55°F to 67°F because of the variable secondary-loop pumping (Pump 2 in Figure 1) depicted by straight line FE in Figure 5.
Cooling-Load Profile Cooling-load profile depends on the ratio of constant load (e.g., internal heat gain from lights, equipment, people, etc.) to variable load (e.g., ventilation, heat gain from building envelope, etc.). Cumulative relative cooling load and time-duration factor for eight constant- and variable-load components in New England are given in Figure 6. The design cooling load for the process area of a manufacturing facility is close to 57 tons. The actual constant-cooling-load component was near 35 tons, or 62 percent of the design load. The actual variable-load
component was close to 22 tons, or 38 percent of the design load. Applied in energy analysis, Figure 6 is illustrative of relative coolingload variations when the percentage of constant-cooling-load component (PCCLC) changes from 0 percent to 100 percent of total cooling load.
P/S- and PLOVF-System Electrical-Energy Usage Chilled-water-pumping electricalenergy savings. The top graph in Figure 7 shows potential annual chilled-water-pumping electricalenergy savings for a P/S-loop system with variable-flow-rate control in both loops. The electrical energy consumed by pump motors was assumed to vary in direct proportion to changes in motor speed by a power of 2.5. This is lower than the theoretical value of 3 recommended for centrifugal pumps. These savings will be realized when distributionsystem water flow is equal to or lower than the allowable chiller-evaporator low limit (Figure 5). In calculations of annual electrical-energy use, the pump serving the PLOVF system was assumed to have a design flow rate
PRIMARY/SECONDARY-LOOP VS. PRIMARY-LOOP-ONLY SYSTEMS
Calculated MCRCL = 73.6 percent Calculated MCRCL = 49 percent Calculated MCRCL = 30.5 percent Calculated MCRCL = 73.6 percent
Linear (approximated MCRCL = 73.6 percent) Linear (approximated MCRCL = 49 percent) Linear (approximated MCRCL = 30.5 percent) Linear (approximated MCRCL = 73.6 percent)
Primary/secondary-loop-system pumping annual electrical-energy savings, percent
70 60 50 40 30 20 10 0
P/S-loop-system pumping and reset chilled-water-temperature-control annual electrical-energy savings, percent
equal to the design flow rate of the P/S-loop pumps. The pump serving the PLOVF system also was assumed to have a design pressure head equal to the cumulative design pressure head of the P/S-loop pumps. With a MCRCL of 73.6 percent (Table 2), P/S-loop-system annual electrical-energy savings vary from 5.8 percent (calculated PCCLC of 62 percent) to 60 percent (calculated PCCLC of 0 percent). Annual pumping electrical-energy usage for the considered P/S and PLOVF systems will be equalized at a PCCLC greater than or equal to 75 percent (Figure 7). With a MCRCL of 49 percent (Table 2), P/S-loop-system annual electrical-energy savings vary from 10.4 percent (PCCLC of 26 percent) to 41 percent (PCCLC of 0 percent). Annual pumping electrical-energy usage for the considered P/S and PLOVF systems will be equalized at a PCCLC greater than or equal to 35 percent (Figure 7). With a MCRCL of 30.5 percent (Table 2), P/S-loop system annual electrical-energy savings vary from 2.8 percent (PCCLC of 15 percent) to 9.5 percent (PCCLC of 0 percent). Annual electrical-energy usage for the considered P/S and PLOVF systems will be equalized at a PCCLC greater than or equal to 21.5 percent (Figure 7). With an MCRCL of 73.6 percent and a PCCLC varying from 0 percent to 40 percent, the P/S-loop system with constant-primary-loop and variable-secondary-loop pumping control consumes less electrical energy annually than the PLOVF system. These savings are identified in Figure 7 as the difference in Y-axis values to the left of the intersection of the dashed lines. The savings could be as high as 22 percent at a PCCLC of 0 percent. In calculations of electrical-energy savings, primary- and secondarypump usage was assumed to be about 6 percent of chiller-plant annual electrical-energy consumption. The data in the top graph of Figure
0
20 40 60 70 30 10 50 Percentage of constant-cooling-load component in relation to overall design cooling load, percent
Single-chiller MCRCL = 73.6 percent Two-chillers MCRCL = 55.2 percent Single-chiller MCRCL = 49 percent
Two-chillers MCRCL = 36.7 percent Single-chiller MCRCL = 30.5 percent Two-chillers MCRCL = 22.9 percent
6 5 4 3 2 1 0 0
40 60 70 50 30 20 10 Percentage of constant-cooling-load component in relation to overall design cooling load, percent
Notes: 1. Savings calculated for chiller plant operating 24 hr a day, seven days a week in Hartford,. Conn. 2. MCRCL = minimum chiller relative cooling load (Table 2), at which Bypass Control Valve 2 in the primary-loop-only-variable-flow system (Figure 2) remains closed. 3. Single-chiller operation considered in top graph only. 4. Dashed red line in top graph indicates pumping savings over primary (constant-flow)/secondary (variable-flow) system. 5. Dashed lines indicate savings over the primary-loop-only system. 6. Annual pumping savings in top graph compared with chiller-plant annual pumping electrical energy only. 7. In bottom graph, pumping and reset-chilled-water-temperature-control annual electrical-energy savings compared with annual electrical energy of entire chiller plant.
FIGURE 7. Potential annual electrical-energy savings for primary/secondary-loop system with variable flow. 7 are related to single-chiller plants. The use of multiple chillers will increase WFTDR and reduce MCRCL. This will reduce annual pumping electrical-energy savings. Correc-
tion factors for two- and three-chiller plants are given in Table 3.
Reset-chilled-water-temperature electrical-energy savings. The P/Sloop system with variable-flow
Single-chiller plant
Two-chiller plant
Three-chiller plant
1.00
0.75
0.50
Notes: 1. Single-chiller-plant loading factor assumed to be 100 percent. 2. Multiple-chiller-plant loading factor assumed to be 75 percent. 3. Minimum allowable chiller relative cooling load increased by 10 percent.
TABLE 3. Correction factors for adjusting pumping annual energy savings to account for multiple-chiller operations. DECEMBER 2010
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electrical-energy savings for pumping and reset chilled-water-temperature control expressed as a percentage of total chiller-plant annual energy consumption are shown in the bottom graph of Figure 7. The annual savings of a single-chiller plant with an MCRCL of 30.5 percent and a PCCLC of 62 percent and a singlechiller plant with an MCRCL of 73.6 percent and a PCCLC of 0 percent vary from 0.5 percent to 5.2 percent. The annual savings of a two-chiller plant with an MCRCL of 22.9 percent and a PCCLC of 62 percent and a twochiller plant with an MCRCL of 55.2 percent and a PCCLC of 0 percent vary from 0.4 percent to 4.1 percent.
900 800 700 600 500 400
Primary-loop-only-chilledwater-system relative flow rate via bypass valve, percent Primary/secondary-chilled-water-systemloop flow rate, gallons per minute
Cumulative annual electricalenergy savings. Cumulative annual
Primary-loop-only-system total water-flow rate (F1 in Figure 2) Primary-loop-only-system distribution-piping flow (F2 in Figure 2)
55
60
70 65 75 Outdoor dry-bulb air temperature, degrees Fahrenheit
80
85
55
60
70 65 75 Outdoor dry-bulb air temperature, degrees Fahrenheit
80
85
80
85
50 40 30 20 10 0
Secondary-loop water-flow rate (F2 in Figure 1) Primary-loop water-flow rate (F1 in Figure 1)
900 800 700 600 500 400
Hourly electrical-energy usage by pumps in primary/secondary and primaryloop-only systems, kilowatt-hours
control also has savings associated with reset chilled-water-temperature control. While considering these savings, the authors assumed, based on manufacturer data, that approximately 2 percent of the chiller’s input energy would be saved per degreeFahrenheit increase in chilled-water temperature. The increase in chilledwater temperature also would increase secondary-loop water flow, which, in turn, would reduce the electrical-energy savings associated with the secondary-loop pump employing variable-flow control. The authors incorporated that reduction in their calculations. The chiller’s power demand was assumed to be about 78 percent of the total plant power demand.
Primary-loop-only-flow-rate chilledwater system, gallons per minute
PRIMARY/SECONDARY-LOOP VS. PRIMARY-LOOP-ONLY SYSTEMS
55
60
70 65 75 Outdoor dry-bulb air temperature, degrees Fahrenheit
Primary-loop-only-system pump hourly electrical-energy usage Primary/secondary-pump cumulative hourly electrical-energy usage
7 6 5 4 3
55
60
70 65 75 Outdoor dry-bulb air temperature, degrees Fahrenheit
80
85
FIGURE 8. Primary/secondary- and primary-loop-only-system hourly trend data.
Converting From PLOVF to P/SLoop-With-VFD-Control System A central chilled-water system with 2,500-ton chillers serving multiple AHUs at a New England manufacturing facility was converted from PLOVF (similar to that shown in Figure 2) to P/S-loop operation (nearly identical to that shown in Figure 1). The system had a design cooling load of approximately 930 tons and a design secondary-loop chilled-water flow of about 1,600 gpm. About 50 percent of the cooling load was con44
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stant (PCCLC of 50 percent). The P/S-loop system had two dedicated water-flow meters to separately measure the flows in the primary and secondary loops, as well as in the distribution- and generation-piping loops of the PLOVF system. To compare the hourly operational modes of the systems, the authors selected a two-day sample with approximately the same dry-bulb outdoor-air temperature for both systems. The reset chilled-water-temperature control
for the P/S system was not activated during the two days. The outdoor dry-bulb air temperature ranged from 57°F to 83°F. The average daily dry-bulb air temperature was close to 69°F, while the cooling load ranged from 250 tons to 450 tons. Only one chiller was running. The high-limit chiller-evaporator flow was 1,200 gpm (i.e., ∆TDEMIN was 10°F). The lowlevel chiller-evaporator flow was 670 gpm (i.e., ∆TDEMAX was 17.9°F). Thus, WFTDR was 1.79.
PRIMARY/SECONDARY-LOOP VS. PRIMARY-LOOP-ONLY SYSTEMS
Low-limit chiller-evaporator flow was set at 700 gpm during P/S-loop operation; during PLOVF-loop operation, it was set at 770 gpm. An attempt to lower low-limit flow during PLOVF-loop operation was unsuccessful because of evaporator-induced chiller shutdown on low flow. Total chilled-water flow via the pump in the PLOVF system (Point F1 in Figure 2) was relatively stable, varying from about 720 to 860 gpm (Figure 8). Meanwhile, chilledwater flow via the distributionpiping system varied from 439 to 847 gpm. Chilled-water flow via the chiller’s evaporator was maintained at its allowable low-limit magnitude through bypass-control-valve modulation. Chilled-water flow via the bypass control valve, which represents the difference between flow via the pump and flow via the distribution-piping system, varied from about 10 to 285 gpm. Flow via the bypass valve, expressed as a percentage of total flow via the pump, varied from about 2 percent (at higher cooling loads) to 39 percent (at lower cooling loads). Any time— and to any extent—the bypass valve (2) was open, VFD speed and pump flow remained constant, wasting electrical energy. Chilled-water flow in the P/S-loop system varied from 700 to 816 gpm (Figure 8). Flow in the secondary loop varied from 420 to 820 gpm. Chilled-water flow via the distribution-piping system varied independently of water flow via the chiller evaporator pump. Chilled-water flow via the distribution system was reduced following the load reduction in the cooling system. This led to reduced electrical-energy use by Pump 2. Chilled-water flow via the primary-loop pump and chiller evaporator remained relatively constant, satisfying low-limit water-flow requirements via the evaporator. The same 100-hp pump (design water flow 2,400 gpm) was used for the PLOVF system and the secondary
loop of the P/S-loop system. The primary loop of the P/S-loop system had its own 15-hp pump (design water flow 1,200 gpm). The long operational hours of the PLOVF system, with nearly constant flow via the pump necessary to maintain low-limit flow via the chiller evaporator, resulted in wasted electrical-energy power demand (kilowatts) and usage (kilowatt-hours). Secondary-loop pump electrical-energy usage decreased and increased following cooling-load variations. Electrical-energy savings for the P/S-loop system varied from about 3 percent to 50 percent, averaging around 20 percent per day. The authors have observed similar electrical-energy savings following other chiller plants’ conversion from PLOVF to P/S-loop operation.
Summary This article compared the operational modes and performance of P/S-loop and PLOVF systems. The operating efficiency of chilledwater systems with a primary (constant-flow)/secondary (variableflow)-loop arrangement can be improved by applying a two-phase optimized control strategy consisting of variable flow and temperature control for primary-loop pumps and variable-flow control for secondaryloop pumps. PLOVF-with-VFD control wastes energy when primary-loop flow is constrained by the chiller-evaporator low limit and chilled water is diverted via decoupling line from the supply pipe to the return pipe of the generation system. The magnitude of annual energy savings (pumping and reset chilled-water temperature) of P/S-loop control depends on multiple variables, such as chillerevaporator-flow turndown ratio, minimum chiller relative cooling load, load composition, number of chillers, etc. For New England weather conditions, annual electrical-energy savings could be 0.5 to 5.2 percent for a single-chiller plant and 0.4 to 4.1 percent for a two-chiller plant.
P/S-loop-with-variable-flow control reduces chiller-plant power demand by resetting chilled-water temperature when flow in the distribution piping exceeds a high-limit value for the chiller evaporator and the system diverts water via decoupling line from the return pipe to the supply pipe of the distribution system. This prevents or delays the addition of a chiller and associated ancillary equipment and saves energy. Compared with PLOVF systems, P/S-loop systems with variable flow also have the advantage of moreaccurate, flexible, and stable control and operational conditions.
References 1) Burd, A.L. (1994). Optimizing customers’ heating systems to reduce capital and operating cost in district heating with cogeneration. Proceedings of International District Heating and Cooling Association Annual Conference, pp. 237-252. 2) Burd, A.L. (1993). Computer design of terminal heating substations for district heating. ASHRAE Transactions, 99 (2), 245-265. 3) Burd, A.L., Burd, G.S., & De Maio, M. (2005, March). Smith & Wesson: The story of a chilled-water retrofit (part 1). HPAC Engineering, pp. 38-47. 4) Burd, A.L., Burd, G.S., & De Maio, M. (2005, July). Smith & Wesson: The story of a chilled-water retrofit (part 2). HPAC Engineering, pp. 24-37. 5) Burd, A.L. (1997). Deferred heat supply for space heating using a capacity-limiting device – A beneficial approach for district heating. ASHRAE Transactions, 103 (2), 23-31. 6) Burd, A.L., & Burd, G.S. (1996). Optimal cycle for plate heat exchanger cleaning in customer substation. Proceedings of International District Energy Association Annual Conference, pp. 177-192. Did you find this article useful? Send comments and suggestions to Scott Arnold at
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