Determining Softener Size for Boiler Feedwater

June 10, 2020 - Nathan Olszak

How to Determine Softener Size for Boiler Feedwater

Softener Sizing

In the process of selecting a proper water softener for boiler feedwater treatment, several considerations must be reviewed: water analysis; the boiler horsepower; and information pertaining to the recovery of the steam.

Water Hardness Analysis

Hardness is made up of calcium and magnesium. Hardness within natural waters will vary considerably, depending upon the source from which water is obtained. Sections of the country that have limestone formations generally have a high hardness content in the water. Since surface waters are diluted by rainfall, well water in the same area will normally have a much higher hardness than surface water since the flow is underground over rock layers.

The degree of hardness at any location should never be assumed. Every effort should be made to obtain a water analysis at the site of installation. This will assure accuracy in your selection process.

In order to determine the size of a water softener, the first procedure in the selection process is to determine the amount of hardness. If the water analysis reports express total hardness in parts per million (ppm), or mg/L hardness must be converted to grains per gallon (gpg).

Determining Make-up Volume

In order to determine the amount of water used to feed a boiler, calculations are necessary to convert the rating of the boiler to the maximum amount of make-up in gallons. Boiler ratings are provided in several forms. However, all can be and should be converted to a common factor of horsepower. For each horsepower, a feedwater volume of 4.25 gallons per hour is required. To convert other boiler ratings to horsepower, the following table should be referenced:

Boiler Rating Factor Used to Convert
to Horsepower per hour
Pounds of Steam Per Hour Divide by 34.5
BTU’s Divide by 33,475
Square Foot Area – Water Tube Divide by 10
Square Foot Area – Fire Tube Divide by 12

Upon determining the boiler horsepower rating, twoadditional factors are needed to be known in order to obtain the net amount of make-up water required in a 24 hour period. The first of these is to determine the amount of condensate return to the boiler. This information is generally known by the boiler operator or design engineer. The amount of condensate returned is subtracted from the maximum amount of boiler water make-up.

A very accurate method in determining the net amount of make-up water per hour, or the percent of condensate returned, can be simply calculated on existing operations by comparing a water analysis of the water from the condensate receiver tank and the raw make-up water. In comparing these two waters one can be very accurate in the amount of condensate returned to the system.

Example: A condensate receiver tank with a water containing 300 ppm of total dissolved solids (TDS) and a know factor of 600 ppm TDS in the raw water make-up supply would indicate a 50% condensate return. As described earlier in this publication, condensate is near perfect water (zero TDS) when it enters the condensate receiver. Therefore, when a raw water supply of 600 ppm TDS is diluted with 0 ppm TDS water at a one-to-one ratio, the result would be 300 ppm TDS. This would represent a dilution of 50% or a condensate return of 50%.

The final step in our gathering of data for the softener selection process is to obtain the number of hours in a day the boiler is operated. This is not only important in order to determine total make-up volume, it is also information required to determine the design of our softener system. A boiler operating 24 hours per day will require soft water at all times. Therefore, the design will require the use of two or more units. On systems operating 16 hours per day, the use of a single softener may meet the needs of the operation. Typically the time required to recharge a softener is approximately three hours.

Softener Selection

The following represents a typical boiler plant from which we can calculate the demand for a softener.

  • 1. Determine Water Hardness
    Analysis received or taken is in parts per million (ppm).
    Convert to grains per gallon (gpg). 342 ÷ 17.1 = 20 gpg
  • 2. Determine Boiler Horsepower
    Boiler rating is in pounds per hour of steam.
    Convert to horsepower per hour.
  • 3. Determine Maximum Gallons Per Hour Make-up
    Convert H.P. per hour to gallons per hour make-up.
    H.P. per hour × 4.25 = gallons per hour make-up.
  • 4. Determine Amount of Condensate Returned to System and Calculate Net Make-up Requirement
    Deduct the condensate returned from the
    total make-up water to determine net gallons make-up water per hour.
  • 5. Determine Total Daily Water Use Requirements
    Multiply the hours of operation by net make-up water per hour.
  • 6. Determine Total Grains of Hardness to be Removed Daily
    Daily water use × grains of hardness per gallon.
  • 7. Determine Continous Flow Rate
    Divide hours net make-up water by 60 minutes to
    get gpm.

Softener Capacity Ratings

Softener should be sized to enable removal of daily amount of grains with half the maximum capacity of the system.

The continuous flow should be achieved at around 5 gpm per cubic foot of resin or less.

The stringent requirements to meet very low hardness levels usually dictate larger systems than non-boiler applications.

To meet the needs of a boiler system having a continuous demand of 24 hours per day, the selection of a softener system must be of a multi-operation. The use of two or more softeners provides 24 hour soft water.

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