Generic CHW Delta T Study - WM Group Delta T...Chilled Water ΔT Study Final Submission 1. PROJECT...

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Chilled Water Delta T Study November 2008 W M Group Engineers Smart solutions that work.

Transcript of Generic CHW Delta T Study - WM Group Delta T...Chilled Water ΔT Study Final Submission 1. PROJECT...

Chilled Water Delta T Study November 2008

WMGroup EngineersSmart solutions that work.

WMGroup Engineers, P.C.

Central Chiller Plant -1- November 2008 Chilled Water ΔT Study Final Submission

TABLE OF CONTENTS 1. PROJECT BACKGROUND

2. SCOPE OF WORK

3. SYSTEM PARAMETERS

4. EXECUTIVE SUMMARY

5. ANALYSIS OF EXISTING SYSTEM

A. Current System Load Profile

B. Current Chilled Water ΔT

C. Current Condenser Water ΔT

D. Current Chiller Staging

E. Estimates of Current Energy Consumption

6. POTENTIAL ENERGY SAVINGS

A. Chiller Energy Savings

B. Chilled Water Pump Energy Savings

C. Condenser Water Pump Energy Savings

D. Cooling Tower Fan Energy Savings

7. CONCLUSIONS AND RECOMMENDATIONS

WMGroup Engineers, P.C.

Central Chiller Plant -2- November 2008 Chilled Water ΔT Study Final Submission

1. PROJECT BACKGROUND Poor chilled water ΔT is a chronic problem in the cooling industry. Poor ΔT reduces chiller operating capacity, requires more chillers to run at part load where the efficiency is not optimum, requires the operation of additional cooling towers and condenser water pumps, and increases energy consumption for the chilled water pumps. Typically, improving the chilled water ΔT on a system requires a relatively small investment and can result in considerable energy savings. At present, the ΔT on the campus central chilled water system is less than ideal. The purpose of this study is to determine the potential energy cost savings available of the ΔT were improved.

WMGroup Engineers, P.C.

Central Chiller Plant -3- November 2008 Chilled Water ΔT Study Final Submission

2. SCOPE OF WORK The scope of work for this project consists of the following: • Prepare an hourly profile of chiller plant operation based on log data

provided by the client (including ΔT for one year.)

• Simulate system operation for actual ΔT based on the logged data. The system simulation shall include energy cost for chillers, chilled water pumps, cooling tower fans and condenser water pumps.

• Simulate system operation assuming ΔT is available per design. The system simulation shall include energy cost for chillers, chilled water pumps, cooling tower fans and condenser water pumps.

• Determine the cost penalty for reduced ΔT.

• Prepare a curve for increase in cost vs. ΔT of 16°F down to 6°F.

• Present findings to the client in a PowerPoint format and submit a written

technical report.

3. SYSTEM PARAMETERS

Certain system parameters must be established for performing calculations based on all available data and good engineering judgment. The following system parameters were used for this study:

Average Cost of Steam: $16.64 per Mlb.

Average Cost of Electricity (Including Demand): $0.16 per kWh

Zero Flow CHW Pressure Drop: 80’

Current Peak Flow CHW Pressure Drop: 160’

Chiller Performance Curves: Per Manufacturer Data

WMGroup Engineers, P.C.

Central Chiller Plant -4- November 2008 Chilled Water ΔT Study Final Submission

4. EXECUTIVE SUMMARY The central chilled water system currently suffers from what is known in the industry as “low ∆T syndrome.” Low ∆T is a common problem for many large central chiller plants, but the problem is often ignored as long as the system continues to provide adequate cooling to the end users. Some of the problems associated with poor ∆T are:

• Excessive chilled water pumping requirements to deliver the required cooling energy using a lower temperature differential

• Higher energy requirements at the chillers due to running additional machines outside of their most efficient operating range (at low loads and high flow rates)

• Higher condenser water pump and cooling tower energy requirements to support the additional chillers

• Excessive equipment wear and reduced lifecycles due to higher run times

Typically, low ∆T problems are relatively inexpensive to correct and provide significant benefits and high return on investment to the system operators.

As stated above, improving the chilled water ∆T has a major effect on the operation of the plant and can reduce energy consumption in all of the major components. The current annual energy consumption at the central chiller plant (including the energy for auxiliaries during free cooling) is broken down as follows:

51%

16%

23%

10%

Annual Chiller Plant Energy

Chillers (Steam and Electric) Chilled Water PumpsCondenser Water Pumps Cooling Tower Fans

Although the chilled water system is capable of achieving reasonable ∆T’s at high loads (up to 12°F) the problems really start to show at lower loads when the ∆T can drop to 2°F or lower. The result of this is that at low loads where only a single machine should be operating, the plant typically operates 3 or 4 chillers to handle

WMGroup Engineers, P.C.

Central Chiller Plant -5- November 2008 Chilled Water ΔT Study Final Submission

the higher flow rates (which can average 18,000 gpm at loads of only a few thousand tons.) Over the course of a year, the average ∆T is only 5.3°F.

Spreading a higher flow rate across multiple chillers at low system loads means that each chiller is operating near the bottom end of its load range. This is where the chiller operates least efficiently, as shown in the typical efficiency curve below. In order to maximize the efficiency of each chiller, flow rates must be properly managed to correctly sequence the chillers in order to keep each chiller operating near the top of its load range and at higher efficiency levels for as much time as possible.

Increasing Load

Increasing

 Efficiency

Chiller EfficiencyCurve

In addition, every time a new chiller is started, the associated condenser water pump and cooling tower fan(s) must operate as well. This can consume a large amount of energy, especially considering that some of the biggest condenser water pumps in the plant are 350 horsepower each.

Reducing the chilled water flow by improving the ∆T will also have a direct impact on the chilled water pump energy requirements. By maintaining a fixed chilled water ∆T throughout the year, the system will have the following annual pump energy consumption:

0

1,000,000

2,000,000

3,000,000

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6°F 8°F 10°F 12°F 14°F 16°F

Chilled

 Water Pum

p En

ergy (kWh/yr)

Chilled Water ∆T (°F)

CHW Pump Energy vs. ∆T

WMGroup Engineers, P.C.

Central Chiller Plant -6- November 2008 Chilled Water ΔT Study Final Submission

With these major factors under consideration, it was possible to use metering data from the chiller plant to calculate the projected energy and cost savings from optimizing the operation of the system. The potential energy savings if all system deficiencies relating to chilled water ∆T are corrected is as follows:

Component Annual Energy Savings

Annual Cost Savings

Chiller Steam 10,887 Mlbs. $181,000

Chiller Electric 2,576,000 kWh $412,000

CHW Pump Electric (12°F ∆T) 2,334,000 kWh $373,000

CW Pump Electric 2,417,000 kWh $387,000

CT Fan Electric 740,000 kWh $118,000

TOTAL $1,471,000

In order to work towards achieving this savings, it is recommended that the campus begin systematically implementing chilled water ∆T improvement projects across campus, starting with the largest buildings. In addition, the campus should also consider repiping some of the headers in the central plant to improve the potential for achieving optimal equipment sequencing. Finally, as the chilled water ∆T is improved the client should periodically investigate the actual operation of the plant and work with engineers to bring it closer to ideal operation.

WMGroup Engineers, P.C.

Central Chiller Plant -7- November 2008 Chilled Water ΔT Study Final Submission

5. ANALYSIS OF EXISTING SYSTEM A summary of the existing operating conditions on the campus central chilled water system is as follows: A. Current System Load Profile The analysis of the chilled water system is based metered log data provided by the Utilities staff covering the one-year period from August 1, 2007 to July 31, 2008. This data included the system ΔT and flow through each chiller, which was used to calculate the tonnage on the system at each point. Since there is a certain margin of error in the flowmeter calibration and certain readings are clearly erroneous, all flow measurements below 100 gpm were assumed to indicate that the chiller was off. In addition, since 2008 is a leap year the readings from February 29 were excluded where required to make a comparison based on a normal 8,760 hour year.

Based on the data which was provided, the following load duration curve was developed:

0

5,000

10,000

15,000

20,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Chiller Plant Loa

d (Ton

s)

Operating Hours

Load Duration Curve 

This load duration curve indicates almost continuous year-round operation for the chiller plant with a peak load of approximately 21,225 tons and a cooling load duration of 2,264 equivalent full-load hours (EFLH.) Typical dormitory and office buildings have a cooling load duration of approximately 1,200 EFLH and hospitals typically have a cooling load duration of 2,000 EFLH, so the reading obtained from the metering system is consistent with what might be expected from a heavily research-oriented institution.

WMGroup Engineers, P.C.

Central Chiller Plant -8- November 2008 Chilled Water ΔT Study Final Submission

B. Current Chilled Water ΔT The chilled water ΔT on the central chilled water system varies considerably throughout the year, ranging from a low of less than 2°F to a maximum of approximately 12°F. Over the course of the year, the average chilled water ΔT was approximately 5.3°F. Plotting the typical ΔT’s at each load level against the load duration curve shows that the ΔT tends to become lower as the system load becomes lower:

0°F

2°F

4°F

6°F

8°F

10°F

12°F

0

5,000

10,000

15,000

20,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Chillled Water ΔT (F)

Chiller Plant Load (Ton

s)

Operating Hours

Load Duration Curve with CHW ΔT

A regression analysis shows that the historic chilled water ΔT and chiller plant load have a 96.4% correlation coefficient:

0°F

2°F

4°F

6°F

8°F

10°F

12°F

14°F

0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000 20,000

Chilled

 Water ΔT (F)

Chiller Plant Load (Tons)

Chilled Water ΔT Regression Analysis

WMGroup Engineers, P.C.

Central Chiller Plant -9- November 2008 Chilled Water ΔT Study Final Submission

Note that the system ΔT tends to fall more rapidly as the load becomes less than 4,000 tons. The reduction of ΔT at lower load conditions is a common occurrence in systems with “low ΔT syndrome” and is symptomatic of the following two issues:

• Control issues with managing flow across multiple chillers at low load conditions (possible chiller staging problems)

• Inefficient control valve operation at the air handlers

C. Current Condenser Water ΔT The condenser water temperature differential on the chillers for the central cooling system varies considerably throughout the year at each load level. At higher loads, the average condenser water ΔT is approximately 9°F, but this drops to 6°F or less as shown in the 24-hour moving average on the chart below:

0°F

2°F

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6°F

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10°F

12°F

0

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0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Cond

enser Water ΔT (F)

Chiller Plant Load (Ton

s)

Operating Hours

Load Duration Curve with CW ΔT

Examining the condenser water ΔT is one method of evaluating the load level that each chiller typically operates at. Since the condenser water pumps are constant speed, the heat rejection of each chiller to the condenser water system (and therefore the load on each chiller) is therefore directly proportional to the ΔT.

The theoretical full-load condenser water ΔT for a steam turbine chiller at design conditions should be approximately 15°F. Since the average condenser water ΔT is only 6°F for most of the year, the average load on each chiller is therefore approximately 40%.

WMGroup Engineers, P.C.

Central Chiller Plant -10- November 2008 Chilled Water ΔT Study Final Submission

D. Current Chiller Staging There are currently 12 main chillers on the central chilled water system:

Chiller Capacity (Tons) Prime Mover

CH-1 1,500 Steam Turbine

CH-2 1,500 Steam Turbine

CH-3 4,000 Steam Turbine

CH-4 4,000 Steam Turbine

CH-5 5,000 Steam Turbine

CH-6 5,000 Steam Turbine

CH-7 2,000 Electric

CH-8 2,000 Electric

ECP-1 1,000 Electric

ECP-2 1,000 Electric

ECP-3 1,000 Electric

ECP-4 800 Electric

It should be noted that chillers CH-1 and CH-2 are currently slated for replacement in the near future. However, since a final selection has not been made at the time of this study and the existing system performance data is based on the existing system, this study will assess performance improvements based on the existing plant configuration. In addition, several of the machines may have derated capacity due to age. Since this data is not available, it will be assumed that each chiller is capable of operating at its full rated capacity.

The number of chillers which were operating in the central chilled water system at each load level was evaluated over the 1-year period covered by the metering data. The actual number of chillers operating at each load level varied substantially at different times, presumably due to a variety of operational factors. In order to help evaluate the trends of how many chillers are typically operating, a 24-hour moving average was applied to filter through the noise in the data:

WMGroup Engineers, P.C.

Central Chiller Plant -11- November 2008 Chilled Water ΔT Study Final Submission

0

1

2

3

4

5

6

7

8

0

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10,000

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20,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Num

ber o

f Chillers Ope

rating

 (24

‐hou

r Moving Av

erage)

Chiller Plant Load (Ton

s)

Operating Hours

Number of Chillers Operating 

This analysis indicates that although the chillers stage to follow the load at higher load levels, at load levels below approximately 10,000 tons the ability of the plant to stage chillers is reduced. On average, the plant operates approximately 3 or 4 chillers at lower load levels. This is clearly related to the low ΔT’s at low loads, and correcting the ΔT at the buildings will allow improved control of the flow to reduce the number of chillers which operate.

A reduction in the number of chillers which operate will save energy by:

• Allowing chillers to be selected to run as close as possible to their ideal design points

• Reducing the number of condenser water pumps which are operating

• Reducing the number of cooling tower fans which are operating

Although the chilled water ΔT improvements at the buildings are required to optimize the number of chillers which are operating, additional modifications to the piping in the plant will also be required to fully achieve the potential benefits. These piping replacements would include installing headers between the chilled water pumps to allow better control of the flow across multiple chillers.

WMGroup Engineers, P.C.

Central Chiller Plant -12- November 2008 Chilled Water ΔT Study Final Submission

E. Estimates of Current Energy Consumption The majority of the energy consumption for the central chilled water system includes the following individual components:

• Chillers

• Chilled Water Pumps

• Condenser Water Pumps

• Cooling Tower Fans

The energy consumption of smaller components such as purge pumps, condensate pumps for the steam turbine chillers, and other items are not evaluated.

CHILLERS

Although there are some gaps and inconsistencies in some of the data points, metering data is available for the energy consumption of the chillers. The annual energy consumption and output for each of the chillers according to the data is as follows:

Chiller Ton-hrs per Year Energy Use per Year

Average Efficiency

Chiller #1 604,552 3,648 Mlbs. 6.0 lbs/ton-hr Chiller #2 530,442 4,079 Mlbs. 7.7 lbs/ton-hr Chiller #3 2,363,011 16,848 Mlbs. 7.1 lbs/ton-hr Chiller #4 5,707,238 38,949 Mlbs. 6.8 lbs/ton-hr Chiller #5 15,668,547 131,525 Mlbs. 8.4 lbs/ton-hr Chiller #6 18,590,511 191,267 Mlbs. 10.3 lbs/ton-hr Chiller #7 2,737,427 -- kWh -- kW/ton Chiller #8 1,167,175 -- kWh -- kW/ton

ECP-1 445,953 20,937 kWh 0.047 kW/ton ECP-2 301,235 13,038 kWh 0.043 kW/ton ECP-3 0 12,529 kWh -- kW/ton ECP-4 0 144,193 kWh -- kW/ton

The expected performance benchmark for a steam turbine centrifugal chiller would be quite a bit higher than the 6 to 8 lbs/ton-hr listed for some of the machines and the expected performance benchmark for an electric centrifugal chiller would be at least a factor of 10 higher than what is calculated for ECP-1 and ECP-2. Since the performance according to the metering data is so far out of range due to data gaps and errors in the energy monitoring (which implies less energy consumption than

WMGroup Engineers, P.C.

Central Chiller Plant -13- November 2008 Chilled Water ΔT Study Final Submission

actually occurs over the course of a year) manufacturer performance data approved by the client for a previous chiller plant configuration study will be used instead.

Performance data used for the chillers is as follows:

100% 90% 80% 70% 60% 50% 40% 30% 20% 10%

CH-1 (lbs/ton-hr) 10.0 9.7 9.8 10.2 11.0 12.1 13.6 15.4 17.6 20.2

CH-2 (lbs/ton-hr) 10.0 9.7 9.8 10.2 11.0 12.1 13.6 15.4 17.6 20.2

CH-3 (lbs/ton-hr) 9.5 9.2 9.3 9.7 10.4 11.5 12.9 14.7 16.8 19.2

CH-4 (lbs/ton-hr) 9.6 9.3 9.4 9.8 10.5 11.6 13.1 14.8 16.9 19.4

CH-5 (lbs/ton-hr) 8.5 8.3 8.3 8.7 9.3 10.3 11.6 13.1 15.0 17.2

CH-6 (lbs/ton-hr) 8.6 8.4 8.4 8.8 9.4 10.4 11.7 13.3 15.2 17.4

CH-7 (kW/ton) 0.694 0.658 0.620 0.595 0.580 0.490 0.498 0.527 0.576 0.713

CH-8 (kW/ton) 0.705 0.669 0.631 0.605 0.589 0.498 0.506 0.536 0.586 0.725

ECP-1 (kW/ton) 0.750 0.711 0.671 0.644 0.627 0.529 0.539 0.570 0.623 0.771

ECP-2 (kW/ton) 0.750 0.711 0.671 0.644 0.627 0.529 0.539 0.570 0.623 0.771

ECP-3 (kW/ton) 0.750 0.711 0.671 0.644 0.627 0.529 0.539 0.570 0.623 0.771

ECP-4 (kW/ton) 0.675 0.640 0.604 0.579 0.564 0.476 0.485 0.513 0.560 0.693

Chillers 4, 5 and 6 are also capable of refrigerant migration for free cooling when cold condenser water is available. The periods of free cooling were determined by evaluating when these chillers were providing useful cooling with little or no steam consumption. Conditions suitable for free cooling occur for approximately 2,650 hours per year during the winter months.

Based on these parameters, the projected energy consumption for the chillers based on the annual chilled water production is as follows:

Chiller Ton-hrs per Year Energy Use per Year

Average Efficiency

Chiller #1 604,552 8,771 Mlbs. 14.5 lbs/ton-hr Chiller #2 530,442 6,246 Mlbs. 11.8 lbs/ton-hr Chiller #3 2,363,011 26,863 Mlbs. 11.4 lbs/ton-hr Chiller #4 3,239,398 40,956 Mlbs. 12.6 lbs/ton-hr Chiller #5 12,937,982 122,737 Mlbs. 9.5 lbs/ton-hr Chiller #6 15,697,978 154,563 Mlbs. 9.8 lbs/ton-hr Chiller #7 2,737,427 1,538,125 kWh 0.562 kW/ton Chiller #8 1,167,175 661,949 kWh 0.567 kW/ton

ECP-1 445,953 255,721 kWh 0.573 kW/ton ECP-2 301,235 176,589 kWh 0.586 kW/ton ECP-3 0 0 -- kW/ton ECP-4 0 0 -- kW/ton

Free Cooling 8,090,937 N/A N/A

WMGroup Engineers, P.C.

Central Chiller Plant -14- November 2008 Chilled Water ΔT Study Final Submission

Using this method for evaluating chiller energy, the results all fall within expected ranges. The current energy consumption for the chillers is therefore projected to be approximately 360,000 Mlbs. of steam per year and 2,632,000 kWh of electricity per year.

CHILLED WATER PUMPS

The chilled water pump head requirement on the system can be calculated from the total flow rate by deriving a formula using the following assumed constraints:

• 80’ TDH pump head requirement at zero flow

• 160’ TDH pump head requirement at current peak flow

The system curve follows a second-order polynomial bounded by these constraints, so it is easy to calculate the head requirements at varying system flow rates:

60

80

100

120

140

160

180

200

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000

System

 Head (Fee

t)

Chilled Water Flow (gpm)

Chilled Water System Curve

Peak Flow

Once the pump head requirement is known for each flow rate the chilled water pump energy can be calculated as a function of flow rate:

WMGroup Engineers, P.C.

Central Chiller Plant -15- November 2008 Chilled Water ΔT Study Final Submission

0

250

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2,250

2,500

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000

Pump Energy Req

uiremen

ts (hp

)

Chilled Water Flow (gpm)

Chilled Water Pump Energy

The current flow rates recorded by the system, matched to the load duration curve, are as follows:

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30,000

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0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Chillled Water Flow (gpm

)

Chiller Plant Loa

d (Ton

s)

Operating Hours

Load Duration Curve with CHW Flow Rates 

Based on the flow rates recorded by the metering system, the current total chilled water pumping energy is calculated to be approximately 4,048,000 kWh per year.

WMGroup Engineers, P.C.

Central Chiller Plant -16- November 2008 Chilled Water ΔT Study Final Submission

CONDENSER WATER PUMPS

The condenser water pumps are sized with one pump for each chiller, and the condenser water pumps are all constant speed. It is assumed for the purposes of this study that each condenser water pump operates at its full rated motor horsepower whenever its associated chiller is operating. As was done for the chiller staging analysis, it is assumed that a chiller is operating when the flow meter records that water is flowing through the machine.

The most critical aspect of managing condenser water pump energy is therefore expected to be the staging of the chillers. It was demonstrated in the chiller staging analysis that the current sequence is less than ideal, so there is definitely the potential for improvement if the chiller sequencing can be adjusted by improving the chilled water ΔT and completing piping and controls modifications in the plant.

Based on the metering data, the current energy consumption of the condenser water pumps is projected to compare to the load duration curve as follows:

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0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Cond

enser W

ater Pum

p Load

(kW)

Chiller Plant Loa

d (Ton

s)

Operating Hours

Condenser Water Pump Electrical Load 

Based on this analysis, it is calculated that the condenser water pumps currently consume approximately 5,919,000 kWh of electricity per year.

WMGroup Engineers, P.C.

Central Chiller Plant -17- November 2008 Chilled Water ΔT Study Final Submission

COOLING TOWER FANS

The cooling towers are configured in a similar manner to the condenser water pumps, with tower cells sized for one or two cells to be matched with each chiller. Although the cooling tower fans have dual-speed motors, it is a bit difficult to model when the fans are operating at each speed level. Therefore, it is assumed for the purposes of this study that when a cooling tower cell is on then its fan is operating at its full rated motor horsepower. Just as with the condenser water pumps, it is assumed that the cooling tower cells which are paired with a particular chiller will be operating when the metering system records flow through the chiller.

Based on the metering data, the current energy consumption of the cooling tower fans is projected to compare to the load duration curve as follows:

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700

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20,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Cooling Tower Fan

 Loa

d(kW)

Chiller Plant Loa

d (Ton

s)

Operating Hours

Cooling Tower Fan Electrical Load 

Based on this analysis, it is calculated that the cooling tower fans currently consume approximately 2,663,000 kWh of electricity per year.

WMGroup Engineers, P.C.

Central Chiller Plant -18- November 2008 Chilled Water ΔT Study Final Submission

6. POTENTIAL ENERGY SAVINGS A. Chiller Energy Savings By improving the ΔT on the central chilled water system, it will be possible to reduce the chilled water flow rate to a more acceptable range to stage off more chillers, particularly at lower loads. There are two primary benefits which will be obtained from improved chiller staging:

• Energy consumption by the chillers will be reduced by keeping each machine operating closer to its most efficient point

• Keeping less machines operating will reduce the total run-hours on most of the machines, reducing maintenance requirements and extending equipment life

In order to determine the future operation of the chillers in an ideal staging plan, a model developed by WM Group for a previous chiller configuration study was used to model the energy consumption of the plant. Chillers were staged in the following order determined by both chiller efficiency and the historic staging order seen in the log data over the past year:

1. Chiller #5 (5,000 ton steam turbine) 2. Chiller #6 (5,000 ton steam turbine) 3. Chiller #3 (4,000 ton steam turbine) 4. Chiller #4 (4,000 ton steam turbine) 5. Chiller #7 (2,000 ton electric) 6. Chiller #8 (2,000 ton electric)

The resulting comparison of chiller production, energy use, and efficiency is as follows:

Chiller Current Operation Future Operation

Ton-hrs per Year

Energy Use per Year

Average Efficiency

Ton-hrs per Year

Energy Use per Year

Average Efficiency

CH-1 604,552 8,771 Mlbs. 14.5 lbs/ton-hr 0 0 -- lbs/ton-hr

CH-2 530,442 6,246 Mlbs. 11.8 lbs/ton-hr 0 0 -- lbs/ton-hr

CH-3 2,363,011 26,863 Mlbs. 11.4 lbs/ton-hr 4,121,521 38,271 Mlbs. 9.29 lbs/ton-hr

CH-4 3,239,398 40,956 Mlbs. 12.6 lbs/ton-hr 1,907,135 17,967 Mlbs. 9.42 lbs/ton-hr

CH-5 12,937,982 122,737 Mlbs. 9.5 lbs/ton-hr 21,175,749 188,129 Mlbs. 8.88 lbs/ton-hr

CH-6 15,697,978 154,563 Mlbs. 9.8 lbs/ton-hr 12,093,857 104,882 Mlbs. 8.67 lbs/ton-hr

CH-7 2,737,427 1,538,125 kWh 0.562 kW/ton 88,717 54,447 kWh 0.614 kW/ton

CH-8 1,167,175 661,949 kWh 0.567 kW/ton 3,582 2,358 kWh 0.658 kW/ton

ECP-1 445,953 255,721 kWh 0.573 kW/ton 0 0 -- kW/ton

ECP-2 301,235 176,589 kWh 0.586 kW/ton 0 0 -- kW/ton

ECP-3 0 0 -- kW/ton 0 0 -- kW/ton

ECP-4 0 0 -- kW/ton 0 0 -- kW/ton

WMGroup Engineers, P.C.

Central Chiller Plant -19- November 2008 Chilled Water ΔT Study Final Submission

One of the biggest changes which is immediately observed by comparing the future operation to the current operation is that only 6 chillers are needed to take care of all the current loads on the campus for the entire year. In fact, for approximately 5,500 hours out of the year, the entire campus can be served by a single chiller, CH-5. This is a dramatic shift from the current mode of operation.

Another observation is that the average efficiency of the machines which are operating has generally increased dramatically, with the only exception being the two electric machines which are used for only a relatively few number of hours per year. This is due to the fact that the machines are all spending more time out of the year operating closer to their most efficient design points.

A summary of the annual steam and electric consumption of the current and optimized operating scenarios is as follows:

Scenario Annual Electric Consumption

(kWh)

Annual Steam Consumption

(Mlbs.)

Current Operation 2,632,384 360,136

Optimized Operation 56,805 349,249

ANNUAL SAVINGS 2,575,579 10,887

At an average electric cost of $0.16 per kWh, the annual electric cost savings would be $412,000 per year. At an average steam cost of $16.64 per Mlb., the annual steam cost savings would be $181,000 per year. The total projected energy cost savings for fully optimizing chiller staging would therefore be $593,000 per year.

WMGroup Engineers, P.C.

Central Chiller Plant -20- November 2008 Chilled Water ΔT Study Final Submission

B. Chilled Water Pump Energy Savings The energy required to pump chilled water around the campus is one of the things which will be most immediately and directly reduced by an improvement in chilled water ΔT. Since the flow at a given load is inversely proportional to the ΔT, doubling the ΔT at a given load would effectively cut the flow requirements in half. The relative comparative flow requirements at different temperature differentials as a function of the load duration curve is as follows:

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Chilled

 Water Flow (gpm

)

Operating Hours

Projected Chilled Water Flow ‐ Constant ΔT

16 deg F 14 deg F 12 deg F 10 deg F 8 deg F 6 Deg F The difference in chilled water pump energy requirements between the ΔT ranges is so great that it must be plotted on a logarithmic chart for clarity:

10

100

1,000

10,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Chilled

 Water Pum

p Energy (hp)

Logarithmic Scale

Operating Hours

Projected CHW Pump Horsepower ‐ Constant ΔT

16 deg F 14 deg F 12 deg F 10 deg F 8 deg F 6 Deg F

WMGroup Engineers, P.C.

Central Chiller Plant -21- November 2008 Chilled Water ΔT Study Final Submission

A comparison of the annual chilled water pump energy requirements for different constant ΔT scenarios vs. the current pump energy requirements is as follows:

Experience with other large systems has shown that a 12°F constant ΔT is a very conservative and achievable target for system-wide improvements. Therefore, the client can realistically expect to achieve approximately $373,000 per year in energy savings on the chilled water pumps through ΔT improvements.

Scenario Annual Energy Consumption

(kWh)

Annual kWh Savings vs.

Base

Annual Cost Savings vs. Base

($0.16/kWh)

Current Operation 4,048,000 -- --

6°F Constant ΔT 5,016,000 No Savings No Savings

8°F Constant ΔT 3,068,000 980,000 $157,000

10°F Constant ΔT 2,197,000 1,851,000 $296,000

12°F Constant ΔT 1,714,000 2,334,000 $373,000

14°F Constant ΔT 1,409,000 2,639,000 $422,000

16°F Constant ΔT 1,199,000 2,849,000 $456,000

WMGroup Engineers, P.C.

Central Chiller Plant -22- November 2008 Chilled Water ΔT Study Final Submission

C. Condenser Water Pump Energy Savings The potential to save energy on the condenser water pumps is largely dependent on the ability to improve chiller sequencing. The current sequencing of condenser water pumps is based exclusively on which chillers are operating. From a theoretical standpoint, the idealized condenser water pump energy should be roughly proportional to the load on the system since the heat rejection to the cooling towers is proportional to the load on the system. However, in reality the condenser water pumps will follow the same new sequencing as the chillers. A plot of the current condenser water pump energy requirements and the possible future condenser water pump energy requirements against the load duration curve is as follows:

0

250

500

750

1,000

1,250

1,500

0

5,000

10,000

15,000

20,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Cond

enser W

ater Pum

p Load

(kW)

Chiller Plant Loa

d (Ton

s)

Operating Hours

Condenser Water Pump Electrical Load 

Current CWP Energy Idealized CWP Energy CWP Energy Matched to Chiller Staging The condenser water pump energy requirements of the current operation, “idealized” operation (theoretical based on ideal staging of all available pumps) and the likely future operation matched to the chiller staging is as follows:

Scenario Annual Energy Consumption

(kWh)

Annual kWh Savings vs.

Base

Annual Cost Savings vs. Base

($0.16/kWh)

Current Operation 5,919,000 -- --

“Idealized” Operation 2,911,000 3,008,000 $481,000

Matched to Chiller Staging 3,502,000 2,417,000 $387,000

WMGroup Engineers, P.C.

Central Chiller Plant -23- November 2008 Chilled Water ΔT Study Final Submission

D. Cooling Tower Fan Energy Savings Just as with the condenser water pumps, the potential to reduce energy consumption for the cooling tower fans is largely dependent on the ability to improve chiller sequencing. The idealized cooling tower energy requirements will roughly follow the load duration curve, but in reality the cooling tower fans will follow the same new sequencing as the chillers. A plot of the current cooling tower fan energy requirements and the possible future cooling tower fan energy requirements against the load duration curve is as follows:

0

100

200

300

400

500

600

700

0

5,000

10,000

15,000

20,000

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Cooling Tower Fan

 Loa

d(kW)

Chiller Plant Loa

d (Ton

s)

Operating Hours

Cooling Tower Fan Electrical Load 

Current CT Energy Idealized CT Energy CT Energy Matched to Chiller Staging

The cooling tower fan energy requirements of the current operation, “idealized” operation (theoretical based on ideal staging of all available towers) and the likely future operation matched to the chiller staging is as follows:

Scenario

Annual Energy

Consumption (kWh)

Annual kWh Savings vs.

Base

Annual Cost Savings vs. Base

($0.16/kWh)

Current Operation 2,663,000 -- --

“Idealized” Operation 1,112,000 1,551,000 $248,000

Matched to Chiller Staging 1,923,000 740,000 $118,000

WMGroup Engineers, P.C.

Central Chiller Plant -24- November 2008 Chilled Water ΔT Study Final Submission

7. CONCLUSIONS AND RECOMMENDATIONS The following is concluded as a result of this study:

• The central chilled water system currently suffers from “low ∆T syndrome” especially at low load levels

• Low ∆T is resulting in excess chilled water pumping and non-ideal sequencing of the chillers

• Inefficient chiller sequencing results in:

- Chillers operating outside of their most efficient ranges

- Wasted energy due to operation of too many condenser water pumps and cooling towers

- Excess run-time on the equipment which reduces life expectancy

• The following annual energy savings is potentially achievable on the central chilled water system:

Component Annual Energy Savings

Annual Cost Savings

Chiller Steam 10,887 Mlbs. $181,000

Chiller Electric 2,576,000 kWh $412,000

CHW Pump Electric (12°F ∆T) 2,334,000 kWh $373,000

CW Pump Electric 2,417,000 kWh $387,000

CT Fan Electric 740,000 kWh $118,000

TOTAL $1,471,000

Therefore, the following is recommended:

• The client should begin systematically implementing chilled water ∆T improvement projects across campus, starting with the largest buildings

• The client should consider repiping some of the headers in the central plant to improve the potential for achieving optimal equipment sequencing

• As the chilled water ∆T is improved, the client should periodically investigate the actual operation of the plant and work with engineers to bring it closer to ideal operation