Estimating Methods for Determining End-Use Water Consumption

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The Federal Building Metering Guidance specifies buildings with water using processes and whole building water consumption that exceeds 1,000 gallons per day must have a water meter installed. Below are methods for estimating daily water use for typical end-uses that drive building-level, end-use water consumption.

Batch Processes/Manufacturing

You may have batch processes/manufacturing that use water. To estimate the water use from these applications you need the following:

  • The approximate number of units daily washed, processed, or manufactured
  • The estimated amount of water needed per unit
  • The percentage of water recycled or reused.

The following calculations can be used to estimate the average daily water use for batch processes/manufacturing applications.

Daily Use (gal/day) = (N × W) × (1 - R)

(Where N = Number of daily units washed, processed, or manufactured; W = Gallons consumed per unit; R = Percentage recycled or reused, expressed as a decimal, i.e., 40% is expressed as 0.4 in the equation)

Evaporative Cooling Systems

Open-recirculating cooling tower systems use large volumes of water through the process of evaporation to provide process or comfort cooling. In addition to evaporative losses, cooling towers also lose water to blowdown, system leaks, and drift (water droplets that escape the cooling tower structure as a result of system air flow and wind-effect air flow).

To estimate daily water use for a cooling tower, you'll need:

  • Chiller tonnage (nameplate)
  • Cooling tower cycles of concentration.

The table below provides total daily water usage for a system that operates a full load 24 hours per day, based on chiller tonnage and cooling tower cycles of concentration.

Daily Cooling Tower Water Use at Full Load (Gallons per Day)

Cycles Of Concentration
Chiller Tonnage (Nameplate)345678
1005,4804,9304,6604,3804,3804,110
20010,9609,8609,3208,7708,4908,490
40021,92019,73018,36017,53017,26016,710
50027,40024,38023,01021,92021,37021,100
60033,15029,32027,40026,58025,75025,210
80044,11039,18036,71035,34034,25033,700
100055,07049,04046,03044,11042,74041,920
150082,74073,42068,77066,03064,38063,010
2000110,14097,81091,78088,22085,48083,840
2500137,810122,470114,790110,140107,120104,930
3000165,210146,850137,810132,330128,490126,030
3500192,880171,510160,550154,250149,860146,850
4000220,270195,890183,560176,160171,510167,950
5000275,340245,480229,590220,270214,250209,860

 

The usage levels presented in the table are for 24-hour operations. Most systems do not operate 24 hours per day. To make this daily water use more accurate, you will need to incorporate the number of hours the system operates. This can be done prorating the full load value in the table. Divide the typical daily operating hours by 24 and multiply this number by the full load value in the table.

Below is an example of a cooling tower operating at three cycles of concentration rejecting heat from a 400-ton chiller that operates typically six hours per day. The full load daily water use from the table is 21,920 gallons per day.

Daily Use (gal/day) = Full Load Water Use 21,920(gal/day) × 6 hours / 24 hours

Daily Use (gal/day) = 5,4 80

Plumbing Fixtures

Most buildings have restrooms with plumbing fixtures. Daily water used by plumbing fixture is dependent on the number of occupants in the building. As a reasonable rule of thumb for a typical nine-hour work day, people use between eight and 12 gallons per day in restrooms. The low-end estimate is typical for buildings with water-efficient fixtures. The high-end estimate is more appropriate for buildings with higher water consuming fixtures.

The following calculations can be used to estimate the range of water used based on the typical occupancy of your building.

Low Estimated Daily Use

Daily Use (gal/day) = Average Number Daily of Occupants × 8 (gal/person)

High Estimated Daily Use

Daily Use (gal/day) = Average Number Daily of Occupants × 12 (gal/person)

Steam Heating Systems

Steam generating boilers use water to offset losses from the system from boiler blowdown and leaks or losses in the steam distribution system and condensate return. If the makeup water to the boiler is not metered, the quantity can be estimated by knowing either softener performance or steam generation rate.

Softener Performance

If your steam boiler system has a water softener you can estimate the amount of water provided to the boiler using the volume of water provided by the softener between regenerations and the frequency the softener system is regenerated. The softener system will be programmed to provide a certain volume of conditioned water. This volume may be recorded in the manual or provided by the softener supplier.

This value is the expected volume of water per regeneration cycle based on site specific water hardness. The frequency of regeneration is also required to estimate daily water use. The frequency of regeneration is a calculated value that represents the amount of softening needed over a given period of time to remove the hardness from the water. The typical frequency of regeneration can be requested from the softener supplier. Using these two items the daily water use for the steam boiler system can be estimated using the following equation:

Daily Use (gal/day) = Regeneration Volume (gal/regen) × Typical Regeneration Frequency (regeneration/week) ÷ 7 (days/week)

If you don't have a softener system or cannot reasonably estimate the water passing through the softeners, the next methodology can be used as an alternative.

Steam Generation Rate

Most large steam generating systems track steam production on per hour or per day basis. The following steps provide a methodology to determine the average daily water use of a steam boiler system.

  • Step 1: Convert Steam Generation Rate into Gallons

    Your steam generation rate is likely tracked in terms of pounds per unit of time (hours or days ). Therefore, you'll need to convert this into gallons using the following formula:

    Steam Generation Rate (gal/day) = Steam Rate (pounds/day) ÷ 8.314 (pounds/gal)

  • Step 2: Estimate Feed Water Rate using the Steam Generation Rate and Cycles of Concentration

    The feed water supplied to the boiler is equal to the sum of the steam leaving the boiler and the blowdown discharged from the boiler. The feed water rate can be estimated using the following equation:

    Feed Water Rate (gal/day) = Steam Generation Rate (gal/day) ÷ [1 – (1 ÷ Cycles of Concentration)]

  • Step 3: Estimate Boiler System Makeup using the Feed Water Rate and Estimated Percent of Condensate Returned

    Now that you have the feed water rate, you need to determine the amount of condensate return coming back from your heating or process steam system. This will be a fraction of the steam generation rate. Use the following formula to estimate the boiler system makeup, which is the daily use:

    Daily Use (gal/day) = Feed Water Rate (gal/day) – [Steam Generation Rate (gal/day) × % steam returned]

Vehicle Wash Systems

Depending on the type of vehicle wash station and reclaim used, estimated daily water use can be calculated the from the following tables by multiplying the number of vehicles washed daily by the factor provided in the table.

Vehicle Wash Daily Water Use Estimate without a Reuse System (Gallons per Vehicle)

Car Wash TypeLowAverageHigh
Self Service12.0 x N15.0 x N18.0 x N
In-Bay Automatic16.9 x N42.9 x N68.9 x N
Conveyor19.0 x N34.0 x N49.0 x N

(Where N = number of vehicles washed daily)

Vehicle Wash Daily Water Use Estimate with a Reuse System (Gallons per Vehicle)

Car Wash TypeLowAverageHigh
Self Service3.5 x N7.5 x N12.7 x N
In-Bay Automatic4.9 x N21.3 x N48.5 x N
Conveyor5.5 x N16.9 x N34.5 x N

(Where N = number of vehicles washed daily)