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  1. #1
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    How to calculate chilled water usage

    Large condo building that gets chilled water from Austin Energy used for cooling. Each condo has their own HVAC unit. Is it possible to determine chilled water usage based on CFM? For example, one unit spec is: 1,100 CFM on low fan speed with a 1” filter.

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    Quote Originally Posted by seththomas View Post

    Large condo building that gets chilled water from Austin Energy used for cooling.

    Each condo has their own HVAC unit.
    Is it possible to determine chilled water usage based on CFM? YES

    For example, one unit spec is: 1,100 CFM on low fan speed with a 1” filter.
    ___ ___ ___ Q. Total = 4.5 * CFM * Enthalpy Difference { ED }
    ____ Enthalpy in and Enthalpy out are dependent on the temperature and humidity
    ______ ED = E in - E out

    Presume the difference in Air energy = Water energy

    ...
    Or determine the annual energy use based on local Coolng Degree Days (CCD)
    and an applicable ACCA Manual J calculation for each condo unit.
    However, the room temperatures are not going to be the same.

    I presume you are investigating a utility billing method for each tenant.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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    Yes, I am trying to determine if there is a better way to improve the current utility billing method. The chilled water is billed based on Ton Hours. To simplify the discussion, let's assume ED = 0. So using the formula for a user operating their unit for 1 hour that is 1,100 CFM, the result would be 4.5 * 60 * 1,100 = 297,000. What does this represent in terms of Ton Hours?

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    Quote Originally Posted by seththomas View Post

    Yes, I am trying to determine if there is a better way to improve the current utility billing method.
    The chilled water is billed based on Ton Hours.


    To simplify the discussion, let's assume ED = 0.


    So using the formula for a user operating their unit for 1 hour that is 1,100 CFM,
    the result would be 4.5 * 60 * 1,100 = 297,000.
    _______________ ...... 0 = ______ ... 0

    What does this represent in terms of Ton Hours?
    Assume ED = 0

    Q. Total = 4.5 * CFM * Enthalpy Difference
    Q. Total = 0
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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    T room = 75'F
    R.H. = 50%
    E in = 28.149 BTU/lbm from Psychrometric Chart

    T supply = 55'F
    R.H. = 90%
    E out = 22.205

    ED = 5.944
    Q. Total = 4.5 * 1,100 * 5.944
    ......... = 29.423 BTU/HR
    ............ = 2.45 Tons

    A/C runs ~ 300 hours per month
    ........... _______ > 100 & < 500 depending whether
    ........................................ 1. Condo is in Vancouver, Phoenix or Miami
    ........................................ 2. Size and construction of the condo
    ...………………………………………...……...…….. Condo built in 1918 or 2018
    ........................................ 3. T-stat set-point at 71'F or 79'F

    ............ 735 Ton Hours / Month ... > 245 & < 1,225
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
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    AUSTIN TX
    MET DATA
    COOLING DEGREE DAYS / MONTH

    CDD MONTH RUN TIME
    .................. HOURS / MONTH
    CDD ____ ..RUN TIME .. TON HOURS
    10 JAN 9.25 __ 22.7
    21 FEB 18.93 46.4
    58 MAR 50.52 123.9
    154 APR 129.76 318.2
    348 MAY 285.21 699.3
    517 JUN 418.05 1025.1
    593 JUL 477.28 1170.3
    616 AUG 495.15 1214.1
    429 SEP 349.10 856.0
    198 OCT 165.42 405.6
    47 NOV 41.23 101.1
    14 DEC 12.80 31.4
    3005 ANNUAL 2452.70 6014.0

    IF T-STAT set at 72'F, Add 25% - 40%
    IF T-STAT set at 78'F, Subtract 20%
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  7. #7
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    Supply Air temp and RH

    Quote Originally Posted by dan sw fl View Post
    T room = 75'F
    R.H. = 50%
    E in = 28.149 BTU/lbm from Psychrometric Chart

    T supply = 55'F
    R.H. = 90%
    E out = 22.205

    ED = 5.944
    Q. Total = 4.5 * 1,100 * 5.944
    ......... = 29.423 BTU/HR
    ............ = 2.45 Tons

    A/C runs ~ 300 hours per month
    ........... _______ > 100 & < 500 depending whether
    ........................................ 1. Condo is in Vancouver, Phoenix or Miami
    ........................................ 2. Size and construction of the condo
    ...………………………………………...……...…….. Condo built in 1918 or 2018
    ........................................ 3. T-stat set-point at 71'F or 79'F

    ............ 735 Ton Hours / Month ... > 245 & < 1,225
    How is the supply air temperature and relative humidity determined when the chilled water is pumped through the coils of the HVAC unit and most of the supply air is from inside the unit with only a small percentage of fresh air.

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    Thread Starter

    How does the temperature of the chilled water factor into the equation

    Quote Originally Posted by seththomas View Post
    How is the supply air temperature and relative humidity determined when the chilled water is pumped through the coils of the HVAC unit and most of the supply air is from inside the unit with only a small percentage of fresh air.
    Doesn't the temperature of the chilled water that runs through the coils factor into the determination?

  9. #9
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    Quote Originally Posted by seththomas View Post

    How is the supply air temperature and relative humidity determined
    when the chilled water is pumped through the coils of the HVAC unit
    and
    most of the supply air is from inside the unit with only a small percentage of fresh air.
    Equipment Performance data is available from vendors though their AHU Selection software.

    + KNOWN Real Life situations.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  10. #10
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    Quote Originally Posted by seththomas View Post
    Doesn't the
    temperature of the chilled water (CW) that runs through the coils factor into the determination?
    YES, Standard CW temperature = 45'F.

    Coil Approach temperature is in the ballpark of 10'F
    depending the number of coil rows.

    -------
    http://www.ref-wiki.com/technical-in...mperature.html
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  11. #11
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    EERE Technical Fact Sheet
    __________ ______ SIZING
    ________________ Equipment Selection
    ……………………………….. Life-Cycle Analysis

    http://www.advantekinc.com/downloads...tSelection.pdf

    JANUARY 2002
    _____________ is or is becoming OutDated in several aspects.
    Attached Images Attached Images  
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  12. #12
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    Quote Originally Posted by seththomas View Post
    Large condo building that gets chilled water from Austin Energy used for cooling. Each condo has their own HVAC unit. Is it possible to determine chilled water usage based on CFM? For example, one unit spec is: 1,100 CFM on low fan speed with a 1” filter.
    Probably easier and more accurate too just install flow meters for each user!

  13. #13
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    Thread Starter

    How does the temperature of the chilled water factor into the equation

    [QUOTE=dan sw fl;25805305]YES, Standard CW temperature = 45'F.

    Coil Approach temperature is in the ballpark of 10'F
    depending the number of coil rows.

    -------

    I checked with Austin Energy and the CW temperature is 44'F. I assume this factor does not directly enter into the equation, but only affects how long the blower needs to run for the temperature in the unit to reach the set temperature. In other words, if the CW temperature were warmer, the blower would need to operate for a longer period of time to reach the set thermostat temperature, correct?

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