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

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Transcript of Generic CHW Delta T Study - WM 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 T 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 T C. Current Condenser Water T 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 T Study Final Submission

    1. PROJECT BACKGROUND Poor chilled water T is a chronic problem in the cooling industry. Poor T 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 T on a system requires a relatively small investment and can result in considerable energy savings. At present, the T 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 T were improved.

  • WMGroup Engineers, P.C.

    Central Chiller Plant -3- November 2008 Chilled Water T 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 T for one year.)

    Simulate system operation for actual T 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 T 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 T.

    Prepare a curve for increase in cost vs. T of 16F down to 6F.

    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 T 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%

    AnnualChillerPlantEnergy

    Chillers(SteamandElectric) ChilledWaterPumpsCondenserWaterPumps CoolingTowerFans

    Although the chilled water system is capable of achieving reasonable Ts at high loads (up to 12F) the problems really start to show at lower loads when the T can drop to 2F 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 T 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 5.3F.

    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.

    IncreasingLoad

    Increasing

    Efficiency

    ChillerEfficiencyCurve

    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

    4,000,000

    5,000,000

    6,000,000

    6F 8F 10F 12F 14F 16F

    Chilled

    WaterPum

    pEn

    ergy(kWh/yr)

    ChilledWaterT(F)

    CHWPumpEnergyvs.T

  • WMGroup Engineers, P.C.

    Central Chiller Plant -6- November 2008 Chilled Water T 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 (12F 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 T 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 T 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

    ChillerPlantLoa

    d(Ton

    s)

    OperatingHours

    LoadDurationCurve

    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 T Study Final Submission

    B. Current Chilled Water T The chilled water