How to Calculate Boiler Blowdown Rate

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Boiler Blowdown Rate Calculator

Calculation Results

Blowdown Percentage: 0%

Blowdown Flow Rate: 0 kg/hr

Note: The total feedwater required is the sum of steam rate and blowdown rate.

Please ensure Max Boiler TDS is significantly higher than Feedwater TDS.
function calculateBlowdown() { var f = parseFloat(document.getElementById('feedwaterTds').value); var m = parseFloat(document.getElementById('maxBoilerTds').value); var s = parseFloat(document.getElementById('steamRate').value); var resultBox = document.getElementById('resultDisplay'); var errorBox = document.getElementById('errorMessage'); if (isNaN(f) || isNaN(m) || isNaN(s) || m <= f) { resultBox.style.display = 'none'; errorBox.style.display = 'block'; return; } errorBox.style.display = 'none'; // Formula: Blowdown % = (F / (M – F)) * 100 // Where F = Feedwater TDS and M = Max Boiler TDS var percentage = (f / (m – f)) * 100; // Flow Rate = (Percentage / 100) * Steam Generation Rate var flowRate = (percentage / 100) * s; document.getElementById('blowdownPercentageOutput').innerText = percentage.toFixed(2); document.getElementById('blowdownFlowOutput').innerText = flowRate.toFixed(2); resultBox.style.display = 'block'; }

How to Calculate Boiler Blowdown Rate

Understanding and calculating the boiler blowdown rate is critical for maintaining the efficiency and longevity of your industrial steam system. Blowdown is the process of intentionally wasting a small portion of boiler water to remove accumulated impurities and dissolved solids.

Why is Boiler Blowdown Necessary?

As water evaporates into steam, it leaves behind dissolved solids (TDS) such as calcium, magnesium, and silica. Over time, the concentration of these solids increases. If not controlled through blowdown, these solids will:

  • Form scale on heat transfer surfaces, reducing efficiency.
  • Cause "carryover," where water droplets contaminate the steam, potentially damaging turbines and valves.
  • Lead to corrosion and pitting within the boiler shell and tubes.

The Boiler Blowdown Formula

The standard formula used to determine the percentage of blowdown required is based on the concentration of Total Dissolved Solids (TDS):

Blowdown Rate (%) = [Feedwater TDS / (Max Allowable Boiler TDS – Feedwater TDS)] × 100

Example Calculation

Suppose you have the following operating parameters:

  • Feedwater TDS: 200 ppm
  • Max Allowable Boiler TDS: 3,500 ppm
  • Steam Generation Rate: 10,000 kg/hr

Using the formula:

  1. Subtract Feedwater TDS from Max Boiler TDS: 3,500 – 200 = 3,300.
  2. Divide Feedwater TDS by that result: 200 / 3,300 ≈ 0.0606.
  3. Multiply by 100 to get the percentage: 6.06%.
  4. To find the flow rate: 0.0606 × 10,000 = 606 kg/hr.

Types of Boiler Blowdown

There are generally two methods used in industrial settings:

  1. Surface (Continuous) Blowdown: This involves a constant discharge of water from the surface of the boiler water. It is the most effective way to control TDS levels and can be automated using a conductivity sensor.
  2. Bottom (Intermittent) Blowdown: This involves opening a valve at the bottom of the boiler for a short duration to remove sludge and sediment that has settled at the base.

Best Practices for Efficiency

To optimize your blowdown rate, consider installing a Heat Recovery System. Since blowdown water is at the same temperature and pressure as the boiler water, it contains significant thermal energy. A flash tank or heat exchanger can recover this heat to pre-heat the incoming makeup water, significantly reducing fuel costs.

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