Skip to content
TheBlueprintof Water
Calculators
The steam plant mass balance

Boiler Blowdown Calculator

Set steam demand and return, then read the balance from the drum. Makeup is the primary result; chemistry shows how far cycles can be pushed.

Model 02 · Open to everyone

Operating case

Set the boiler

Steam load, return, and cycles establish the live balance.

lb/hr

50%

Share of the steam that comes back to the feedwater tank. Watch the makeup line answer.

20

Higher cycles reduce blowdown until chemistry reaches its limit.

Advanced assumptionsChemistry, pressure, and fuel
80%
ppm as CaCO3

psig

Sets the saturation temperature the blowdown leaves at.

$8.00per MMBtu
Steam demand20,000 lb/hrSteam and burner respond
Condensate return50%Makeup line answers
Boiler alkalinity1,050 ppm350 ppm over guide
Mass, heat, and chemistryChemistry warning
Boiler mass balance, liveTHE BLUEPRINT OF WATER · BOILER MASS BALANCEPROCESSLOADCONDENSATE RETURN 10,000 lb/hrFEEDWATER TANKPUMPSTEAM DRUM20× concentrated125 psig · 352 °F satFUEL80% fuel to steamHEAT DOWN THE DRAIN138,947 BTU/hr$11,672 of fuel each year
Diagram controlsInspect a live quantity
Live relationship

Current equation

Supporting readoutsSeven measurementsOpen
Blowdown heat138,947BTU/hr$11,672 fuel each year
Annual makeup11,141,053gal
Annual blowdown1,060,204gal

350 ppm over the 700 ppm ABMA guideline for boilers under 300 psi. Dial cycles back, soften the makeup, or return more condensate.

Scenario comparisonCompare a target cycles case20 current → 30 target
Target operating point

The drawing stays on the current 20 cycle case. This target changes the comparison only.

30

Cycle result

20 current30 target

Makeup avoided
365,880 gal
Fuel avoided
$4,025
Target alkalinity
1,550 ppmfrom 1,050 ppm

The target is 850 ppm over the 700 ppm ABMA guideline.

Blowdown fuel cost vs cycles

Drag the plot to set the target. The copper line is where alkalinity hits the ABMA ceiling.

Current20 · $11,672Target30 · $7,647ABMA ceilingpast 13 cycles
Annual blowdown fuel cost versus cycles of concentration. Currently $11,672 per year at 20 cycles. The target is $7,647 per year at 30 cycles. Boiler alkalinity crosses the 700 ppm ABMA ceiling past 13 cycles.

Cycles of concentration

Reference

How the boiler model works

A steam plant is a circuit: feedwater in, steam out to the process, condensate back, and blowdown discharged to hold dissolved solids at the target cycles. Every pound of blowdown also carries heat that was paid for at the burner, so the water balance and the fuel bill are the same calculation seen from two sides.

The instrument above runs that circuit live. The governing relationships are written out below.

The formulas follow Chapter 13 of the book, Heating Things Up.

Feedwater

Feedwater=Steam×CyclesCycles−1\text{Feedwater} = \text{Steam} \times \frac{\text{Cycles}}{\text{Cycles} - 1}

The feedwater pump must supply the steam that leaves as product plus the blowdown that leaves with the concentrated water. As cycles rise, the blowdown share shrinks and the feedwater approaches the steam load.

Steam
Steam load delivered (lb/hr)
Cycles
Cycles of concentration (boiler water vs feedwater)
Assumes
  • Cycles are greater than 1.

Blowdown

Blowdown=SteamCycles−1\text{Blowdown} = \frac{\text{Steam}}{\text{Cycles} - 1}

Blowdown is concentrated boiler water discharged to hold dissolved solids at the target cycles. It drops off sharply as cycles rise, then flattens, the same diminishing return the cooling tower shows.

Steam
Steam load delivered (lb/hr)
Cycles
Cycles of concentration
Assumes
  • Cycles are greater than 1.

Makeup

Makeup=Feedwater−Steam×Condensate Return500\text{Makeup} = \frac{\text{Feedwater} - \text{Steam} \times \text{Condensate Return}}{500}

Makeup is the fresh water bought to cover the feedwater the returned condensate does not. Converted from a mass rate at 500 lb/hr per gpm. More condensate return means less makeup at the same blowdown.

Feedwater
Feedwater flow (lb/hr)
Steam
Steam load delivered (lb/hr)
Condensate Return
Fraction of steam returned as condensate (%)
Assumes
  • 500 lb/hr of water per gpm.
  • Returned condensate carries no alkalinity.

Boiler water alkalinity

Boiler Alkalinity=Makeup Alkalinity×Makeup Fraction×Cycles\text{Boiler Alkalinity} = \text{Makeup Alkalinity} \times \text{Makeup Fraction} \times \text{Cycles}

Alkalinity enters with the makeup, is diluted by the makeup fraction of the feedwater, then concentrated by cycles in the boiler. The 700 ppm ABMA guideline (boilers under 300 psi) is the ceiling to watch.

Makeup Alkalinity
Makeup M alkalinity as CaCO3 (ppm)
Makeup Fraction
Makeup as a fraction of feedwater
Cycles
Cycles of concentration
Assumes
  • ABMA boiler-water alkalinity ceiling 700 ppm for boilers under 300 psi.

Worked example

Take the room's default case: 20,000 lb/hr of steam, 50 percent condensate return, 20 cycles, 80 percent fuel to steam efficiency, 100 ppm makeup alkalinity, and 125 psi operating pressure.

Feedwater
21,053 lb/hr
Blowdown
1,053 lb/hr
Makeup
22.1 gpm
Boiler water alkalinity
1,050 ppm
Heat lost with blowdown
138,947 BTU/hr
Annual fuel cost of that heat
$11,672 per year

Move the condensate return slider and watch makeup, alkalinity, and the fuel line answer together. Returned condensate is water already purchased, treated, and hot.

Common questions

How is boiler blowdown calculated?
Blowdown holds dissolved solids at the target cycles. The feedwater pump supplies the steam that leaves as product plus the blowdown, and blowdown drops off sharply as cycles rise before flattening, the same diminishing return the cooling tower shows.
Why does condensate return matter so much?
Returned condensate is water already purchased, treated, and hot, and it carries no alkalinity back with it. Every point of return displaces makeup, and with it the alkalinity load and the fuel spent heating cold water.
What alkalinity ceiling does the model watch?
The ABMA guideline of 700 ppm boiler water alkalinity for boilers under 300 psi. Alkalinity enters with the makeup and is concentrated by cycles, so the ceiling is what finally caps how high cycles can go.