New NPLV Rating Works “Weather” Your Plant Has a Single or Multiple Chillers In December 1998, the Air-conditioning and Refrigeration Institute (ARI) issued a revised Standard, ARI 550/590-98, which addressed efficiency measurements for centrifugal, screw and reciprocating chillers. The major change involved the formula used to calculate average chiller efficiency. The formula was revised to improve its accuracy, especially at off-design conditions. The goal was to give chiller buyers better information with which to make energy comparisons. However, the revised formula was derived from analysis of a single-chiller system. This has raised concerns about its applicability to multiple-chiller systems, which make up the majority of chiller plants. This worry is groundless when the difference between multiple-chiller systems and single-chiller systems is understood, along with how the formula works for both types of systems. Background The formula used in ARI Standard 550/590-98 to calculate average chiller efficiency involves a set of four Operating Conditions. Each Condition consists of a “% Design Load” and a “Head.” The Head is represented by either an outdoor dry-bulb (DB) temperature for air-cooled chillers, or an entering condenser-water temperature (ECWT) for water-cooled chillers. For watercooled chillers, the four Conditions are 100% load @ 85°F ECWT, 75% load @ 75°F ECWT, 50% load @ 65°F ECWT, and 25% load @ 65°F ECWT. These temperatures were derived by averaging weather data from around the United States, and utilizing a typical cooling-tower approach.
The formula makes some assumptions about chiller operating hours. It assumes that a chiller spends 1% of its operating hours at 100% load and 85°F ECWT simultaneously, 42% of operating hours at 75% load and 75°F ECWT, 45% of operating hours at 50% load and 65°F ECWT, and 12% of operating hours at 25% load and 65°F ECWT. These values are based on weather data, recognizing the major impact of weather on both chiller loading and efficiency. The result of the formula is a chillerefficiency number, expressed in kW/ton. If the chiller design conditions are the standard ARI conditions (44°F leaving chilled-water temperature, 85°F ECWT or 95°F outdoor DB, 2.4 gpm/ton evaporator flow, 3.0 gpm/ton condenser flow, and standard fouling factors), then the efficiency number is known as the Integrated Part-Load Value (IPLV). But what about different chiller design conditions? For departures from standard ARI conditions (i.e., 83°F ECWT or different flow rates), the efficiency number is known as the Non-standard Part-Load Value (NPLV). Because IPLV is a specialized subset of NPLV, which is used primarily in manufacturers’ catalogs, we will focus on NPLV in this Update. The ARI recognizes that an NPLV rating can’t predict exactly what the absolute chiller efficiency would be in an actual installation. NPLV does, however, provide a meaningful way of comparing the relative efficiency of different chiller models. The actual efficiency may differ from the NPLV by a few percent, but each chiller model will differ by a similar amount. The Multiple-Chiller Question Standard 550/590-98 states that NPLV applies to single-chiller systems and that full energy analyses should be used for multiplechiller systems. The ARI (of which YORK is a member) recognized that multiple-chiller
systems make up the majority of chiller plants. However, single-chiller plants are easier to analyze, so ARI started there. It’s important to note that no analysis was ever done to determine whether the new Standard would work for multiple-chiller systems. Multiple-chiller plants are now under study in an ARI committee. What makes multiple-chiller plants different from single-chiller plants, as far as chiller efficiency is concerned? In a single-chiller plant, the chiller sees the full range of building cooling loads: from 100% design load down to 10%, when the chiller shuts off. In multiple-chiller systems, on the other hand, chillers cycle off as the buildingcooling load gets lower, and the load on the remaining chillers increases. The result is that the individual chillers see higher loads, on average. In fact, the more chillers there are in the system, the higher the average chiller load. Table 1 illustrates this phenomenon. Table 1 — Average Chiller Loads in Multiple-Chiller Systems (parallel chillers) Number of Chillers in the System
Building Total Load (Tons)
Building Average Load (Tons)
Building Average Load (%)
Chiller Average Load (%)
1
4000
2000
50
50
2
4000
2000
50
67
3
4000
2000
50
75
4
4000
2000
50
83
A multiple-chiller system appears to breach the assumptions used in calculating NPLV. The weather assumptions would be fundamentally unchanged, but the chillerloading assumptions would change, with more hours being spent at higher loads. Wouldn’t this make the Standard and NPLV inapplicable for multiple-chiller systems? The Multiple-Chiller Answer Surprisingly, no. NPLV is also valid for multiple-chiller systems, despite the difference in average chiller loading. YORK was one of the developers of the Standard, and is deeply involved in the study of multiplechiller systems for the ARI. In analyzing these systems, YORK has found that the Standard does accurately predict chiller efficiency for multiple-chiller systems as well. How can that be, if the assumptions used in the formula have changed so radically? To find the answer, we need to look at the
factors that determine the efficiency of a refrigeration compressor. There are two factors: Load and Head. Load is the amount of refrigerant gas that the compressor must handle over time. Head is the pressure difference against which the compressor must operate. In a water chiller, the lower pressure is determined by the evaporator temperature. The higher pressure is determined by the condensing temperature, which is determined primarily by the weather conditions: the outdoor dry-bulb temperature entering the air-cooled condenser, or the wet-bulb temperature of the air entering the cooling tower. Which has the greater impact on compressor efficiency: Load or Head? Let’s look at water-cooled centrifugal chillers, the most common type of chiller used in large plants. Figure 1 shows the energy performance of an “average” centrifugal chiller. The performance was simulated by averaging together the most recent performance curves published by YORK, Carrier and Trane in their catalogs. The curves show the relationship between “% Design Load” and “% Design kW” for various ECWTs. If we hold the Head constant (by using only the 85°F ECWT line) and vary the Load, what happens to chiller efficiency? We find that the kW/ton varies relatively little — only 5%. This is illustrated in Figure 2, using the same curves as in Figure 1. A simple analogy may help clarify why the change is so small. Imagine a catapult (representing the compressor) throwing a 10-lb rock (representing the Load) up a 20foot cliff (representing the Head). Say that the power required for this operation is 1 HP. If the rock is reduced to only 5 lb, then the machine needs only 0.5 HP. Is the machine becoming more efficient in terms of HP/lb? No. If we divide power (HP) by Load (lb), we get the same answer in both cases: 1÷10 = 0.5÷5 = 0.1 HP/lb. Alternatively, if we hold the Load constant (at 100%, for example) and vary the Head, we find that change in chiller efficiency can be as much as 30%. This is illustrated in Figure 3. Let’s use our same simple analogy to examine this case. Imagine the rock (representing the Load) stays at 10 lb, but the cliff (representing the Head) is lowered to 10 feet. Now the catapult would require 0.7 HP to accomplish the operation. Has the catapult become more efficient? Yes. If we
Figure 1 — Off-design Performance of “Average” Water-cooled Centrifugal Chiller 100%
80%
% Design kW
divide the power (HP) by the Load (lb), the answer is now 0.7÷10 = 0.07 HP/lb, which is 30% better than when we only lowered the Load in Figure 2. We said earlier that the chiller-loading pattern in a multiple-chiller plant is different than a single-chiller plant. However, the weather stays the same, no matter how many chillers are in the plant. Because Head (determined by the weather) will have the major impact on chiller efficiency, and Load (determined by the number of chillers) will have a minor impact, it stands to reason that ARI 550/590-98 will accurately predict chiller efficiency in multiple-chiller plants. Let’s analyze a multiple-chiller plant and see if that is true.
60%
40%
75°F ECWT 20%
0%
Description of System (all constant-speed)
1
One 3600-ton unit
2
Two 1800-ton units
3
Three 1200-ton units
4
Four 900-ton units
5
Five 720-ton units
6
Six 600-ton units
Group Number
Description of Operating Schedule
1
24 hr/day, 7 days /wk, chiller shut off below 0°F outdoor DB
2
24 hr/day, 7 days /wk, chiller shut off below 55°F outdoor DB
3
12 hr/day, 5 days /wk, chiller shut off below 0°F outdoor DB
4
12 hr/day, 5 days /wk, chiller shut off below 55°F outdoor DB
20%
30%
40%
50%
60%
70%
80%
90%
100%
% Design Load
Figure 2 — Chiller Efficiency Changes with Variable Load /Constant Head 0.583 kW/ton 100%
80%
% Design kW
System Number
65°F ECWT 55°F ECWT
10%
Analysis of a Multiple-Chiller Plant Six systems were analyzed, addressing the same building load, using from 1 to 6 chillers each, using average U.S. weather data, and run with 4 different operating schedules.
85°F ECWT
kW/ ton varies by only 5.1%
0.553 kW/ton
60% 0.557 kW/ton 40%
20%
0% 10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
% Design Load
Figure 3 — Chiller Efficiency Changes with Variable Head /Constant Load
The Building Load Parameters:
0.583 kW/ ton
100%
Design Building Cooling Load = 3600 tons Design Outdoor Temperatures = 100°F DB/78°F WB
Design Efficiency for All Chillers = 0.583 kW/ton NPLV for All Chillers = 0.488 kW/ton Design Leaving Chilled-Water Temperature = 44°F Minimum Entering Condenser-Water Temperature = 65°F Average Internal Load = 40% of Peak
% Design kW
Design Cooling-Tower Approach = 7°F
kW/ ton varies by 30.0%
80%
0.408 kW/ ton 60%
40%
20%
0% 10%
20%
30%
40%
50%
60%
70%
% Design Load
80%
90%
100%
TABLE 2 — Average Efficiency of the Six Chiller Systems (kW/ton) Group
System 1
System 2
System 3
System 4
System 5
System 6
1
0.493
0.476
0.480
0.481
0.482
0.483
2
0.487
0.486
0.489
0.492
0.493
0.496
3
0.494
0.482
0.487
0.487
0.489
0.491
4
0.491
0.490
0.494
0.496
0.498
0.500
Average
0.491
0.484
0.488
0.489
0.489
0.492
From this information, 24 analyses were done. These were created using the ASHRAE Temperature-Bin Method for calculating energy. The results are summarized in Table 2 above. The results show that the average kW/ton for single- and multiple-chiller systems are the same, within reasonable accuracy. The new NPLV rating tracks efficiency for multiple-chiller systems as well as single-chiller systems.
P.O. Box 1592, York, Pennsylvania USA 17405-1592 Copyright by YORK International Corporation 1999
Summary ARI Standard 550/590-98 states that NPLV is only applicable to single-chiller systems, because of its chiller-loading assumptions. However, it turns out that the Standard is equally applicable to multiple-chiller systems, because chiller loading (between 50% and 100% load, where individual chillers operate in multiple-chiller systems) only marginally impacts chiller efficiency. The major factor in chiller efficiency is Head, which is determined by the weather. And the weather doesn’t care how many chillers a plant contains.
Subject to change without notice. Printed in the USA ALL RIGHTS RESERVED
