Efficient boiler operation requires maintaining water quality within specific parameters. As water boils into steam, dissolved solids (TDS) remain in the boiler, increasing in concentration. If these solids exceed specific limits, they can cause foaming, carryover, scale formation, and corrosion, leading to equipment failure or hazardous conditions.
Boiler blowdown is the process of intentionally removing a portion of this concentrated water and replacing it with fresh feedwater to keep the Total Dissolved Solids (TDS) within acceptable limits.
The Blowdown Formula
The calculation is based on a mass balance equation for the dissolved solids. Since the steam produced is assumed to have negligible TDS (0 ppm), all solids entering with the feedwater must eventually be removed via blowdown to maintain equilibrium.
Blowdown Rate (B) = (S × Cf) / (Cb – Cf)
Where:
S = Steam Generation Rate (kg/hr or lbs/hr)
Cf = Feedwater TDS (ppm)
Cb = Maximum Allowable Boiler TDS (ppm)
How to Use This Calculator
Steam Generation Rate: Enter the amount of steam your boiler produces per hour. You can use kg/hr or lbs/hr, as long as you are consistent. The result will match this unit.
Feedwater TDS: Enter the conductivity or Total Dissolved Solids of the water entering the boiler (after treatment/softening) in parts per million (ppm).
Maximum Allowable Boiler TDS: Enter the manufacturer's recommended limit for TDS in the boiler water. Common limits range from 2,000 to 3,500 ppm for low-pressure fire-tube boilers, but high-pressure water-tube boilers require much lower limits.
Why Is This Calculation Critical?
Calculating the correct blowdown rate is a balancing act:
Insufficient Blowdown: Leads to scale deposits on heat transfer surfaces, reducing efficiency and potentially causing tubes to overheat and fail. It can also cause "priming" or "foaming," where water enters the steam line.
Excessive Blowdown: Wastes tremendous amounts of energy (hot water is discarded), water, and treatment chemicals. Minimizing blowdown without exceeding TDS limits is key to energy conservation.
Blowdown Percentage
The calculator also provides the Blowdown Percentage. This represents the percentage of feedwater that is discharged as blowdown. It is calculated as:
Blowdown % = (Feedwater TDS / Boiler TDS) × 100
This percentage helps operators understand the efficiency of their water usage. A lower percentage generally indicates better water efficiency, often achieved by improving feedwater quality (lowering Feedwater TDS).
function calculateBlowdown() {
// 1. Get input values
var steamRate = document.getElementById("steamRate").value;
var feedwaterTDS = document.getElementById("feedwaterTDS").value;
var maxBoilerTDS = document.getElementById("maxBoilerTDS").value;
var errorDiv = document.getElementById("error-message");
var resultDiv = document.getElementById("result-area");
// 2. Clear previous errors and results
errorDiv.style.display = "none";
errorDiv.innerHTML = "";
resultDiv.style.display = "none";
// 3. Validation
if (steamRate === "" || feedwaterTDS === "" || maxBoilerTDS === "") {
errorDiv.innerHTML = "Please fill in all fields.";
errorDiv.style.display = "block";
return;
}
var S = parseFloat(steamRate);
var Cf = parseFloat(feedwaterTDS);
var Cb = parseFloat(maxBoilerTDS);
if (isNaN(S) || isNaN(Cf) || isNaN(Cb) || S < 0 || Cf < 0 || Cb = Cb) {
errorDiv.innerHTML = "Feedwater TDS must be lower than Maximum Boiler TDS. If Feedwater TDS is equal to or higher than the limit, you cannot operate the boiler safely without infinite blowdown.";
errorDiv.style.display = "block";
return;
}
// 4. Calculation Logic
// Formula: Blowdown Rate (B) = (S * Cf) / (Cb – Cf)
var blowdownRate = (S * Cf) / (Cb – Cf);
// Total Feedwater (F) = Steam (S) + Blowdown (B)
var totalFeedwater = S + blowdownRate;
// Blowdown Percentage (based on Feedwater) = (B / F) * 100
// Or simply (Cf / Cb) * 100 mathematically
var blowdownPercent = (blowdownRate / totalFeedwater) * 100;
// 5. Display Results
// Rounding to 2 decimal places for clarity
document.getElementById("res-blowdown-rate").innerHTML = blowdownRate.toFixed(2) + " (mass/hr)";
document.getElementById("res-percent").innerHTML = blowdownPercent.toFixed(2) + "%";
document.getElementById("res-feedwater").innerHTML = totalFeedwater.toFixed(2) + " (mass/hr)";
resultDiv.style.display = "block";
}