What Does This Boiler Blowdown Calculator Do?
When a boiler makes steam, pure water leaves and solids stay back. Over time TDS in the boiler drum climbs. High TDS causes foaming, priming and carryover into steam lines. You have to throw out some boiler water and replace it with cleaner feedwater. That is blowdown.
This calculator helps you rate the blowdown flow you need and estimate how much water and heat you lose in your system. You enter steam flow, boiler water TDS limit and feedwater TDS. If you do not know feedwater TDS, you can configure it from makeup water TDS and condensate return rate. The tool then gives blowdown rate, makeup water rate and heat loss so you can set your blowdown valve or automatic controller correctly.
If you are sizing a new feedwater system, you can also configure pump head and flow on our main boiler feed pump sizing hub to match the extra feedwater needed for blowdown.
Boiler Blowdown Formula
It is a simple mass balance. Feedwater brings in solids, blowdown removes them. Steam is assumed zero TDS.
Feedwater × Feed TDS = Blowdown × Boiler TDSBlowdown Flow = Steam Flow × Feed TDS ÷ (Boiler TDS − Feed TDS)
Feed TDS = (Makeup TDS × Makeup % + Condensate TDS × Condensate %) ÷ 100
Rate % of Steam = Feed TDS ÷ (Boiler TDS − Feed TDS) × 100
Rate % of Feed = Feed TDS ÷ Boiler TDS × 100
Heat Loss = Blowdown Flow × (h_f at boiler pressure − h_f feedwater)
Boiler pressure matters only for heat loss. TDS balance does not need pressure. That is why many plants rate blowdown only by conductivity.
How to Use the Calculator to Configure Blowdown
- Enter steam flow. Use average hourly flow, not peak. This sets base water balance.
- Enter boiler water TDS limit. Take from maker or water treatment company.
- Configure feedwater TDS. If you know it, enter directly. If not, enter makeup TDS and condensate return rate to let the system estimate it. Example: 300 ppm makeup, 60% condensate return at 10 ppm gives about 124 ppm feedwater.
- Enter feedwater temperature. Used only to estimate heat loss in blowdown.
- Click Calculate Blowdown Rate. No result shows until you click. The right panel will then show flow rates and heat loss.
Understanding the Results
Blowdown Rate % of Steam
This is the number most operators use. If it says 6.5%, you need to blow down 6.5 kg for every 100 kg steam. Many plants try to keep this under 5% by improving condensate return or makeup quality.
Blowdown Flow Rate
Actual kg/h or lb/h you must dump. Use it to size blowdown valve and blowdown vessel. If you have automatic conductivity control, this is average flow when valve modulates.
Feedwater and Makeup Rate
Feedwater = steam + blowdown. Makeup = feedwater - condensate returned. If condensate return drops, makeup and blowdown both go up and your system water cost rises fast.
Heat Loss
Blowdown water is at boiler temperature, so you lose sensible heat. For a 10 bar boiler, each 100 kg/h blowdown is about 12 kW heat loss. Heat recovery from blowdown can recover 60-80% of this.
Worked Example
Plant runs at 2000 kg/h steam, boiler TDS limit 2500 ppm, makeup TDS 300 ppm, condensate return 60% at 10 ppm, feedwater 85°C.
| Step | Calculation | Result |
|---|---|---|
| Estimate feedwater TDS | (300×40 + 10×60)/100 | 126 ppm |
| Blowdown flow | 2000×126/(2500-126) | 106 kg/h |
| Rate % of steam | 106/2000×100 | 5.3% |
| Feedwater flow | 2000+106 | 2106 kg/h |
| Makeup flow | 2106 - 2000×0.6 | 906 kg/h |
| Heat loss | 106×(763-356) kJ/h | ~12 kW |
So you need about 106 kg/h continuous blowdown. If you improve condensate return to 80%, feedwater TDS drops to 68 ppm and blowdown rate falls to 2.8% - almost half water and heat loss.
What Values Should You Enter?
Use lab tested values from same day. TDS can swing a lot if water source changes.
Steam Flow
Average flow from flow meter. If no meter, use boiler rating × load factor. Higher steam flow raises blowdown flow proportionally.
Boiler TDS Limit
Get from boiler manual. Lower limit means more blowdown. High pressure boilers need much lower TDS to avoid carryover.
Makeup TDS
TDS of softener or RO outlet that feeds your system. If you use raw water, rate will be high.
Condensate Return
Percent of steam you get back. Good system is 70-90%. Leaks, traps failed open and process loss drop this rate.
Feedwater TDS
If you measure feedwater conductivity, enter directly and the calculator will not estimate. That is more accurate than makeup calculation.
Feedwater Temp
For heat loss only. Higher feedwater temp reduces net loss but total water loss stays same.
Factors That Change Blowdown Rate in a System
Makeup Quality
RO or DM water at 10 ppm makeup needs very little blowdown. Hard water at 500 ppm needs 10 times more rate.
Condensate Return
Returning condensate is cleanest water you have. Improve return to cut makeup and blowdown both.
Boiler Pressure
Higher pressure needs lower boiler TDS, so rate goes up even if feedwater stays same. Configure limits per pressure.
Steam Purity Need
Process like food or turbine needs lower carryover, so you run boiler TDS lower and accept higher blowdown rate.
Control System
Manual blowdown often over blows. Automatic conductivity based control holds steady TDS and lowers average rate by 30-50%.
Heat Recovery
Flash vessel and heat exchanger can recover heat from blowdown water to heat makeup. Does not cut rate but cuts fuel loss.
Common Mistakes When You Configure Blowdown
- Using raw water TDS as feedwater TDS: feedwater is mix of makeup and condensate, so its TDS is lower than makeup. Estimate correctly or you will over blowdown.
- Setting boiler TDS too high: to reduce rate, some raise limit above maker spec. That causes carryover and scale, much costlier than water.
- Only bottom blowdown: bottom is for sludge, not TDS control. You need continuous surface blowdown to control TDS rate properly.
- Not accounting for condensate loss: if return drops from 70% to 40%, makeup doubles and feed TDS climbs fast. Recalculate rate each month.
- Ignoring heat loss: 100 kg/h blowdown at 10 bar is about 12 kW continuous. Over 8000 hours that is 96 MWh fuel waste.
How to Reduce Blowdown Rate and Save Water
Once you estimate current rate, you can try to cut it:
- Fix condensate system: repair leaks, failed traps, insulate lines. Each 10% more return cuts blowdown about 15%.
- Improve makeup treatment: RO permeate at 20 ppm vs softener at 300 ppm can cut blowdown by 90%.
- Install automatic TDS controller: keeps boiler TDS near limit without over blowing. Pays back in months.
- Recover heat: use blowdown heat exchanger to heat makeup water. Configure flash tank if rate is above 200 kg/h.
- Monitor daily: log feedwater TDS, boiler TDS and condensate return. Trend rate weekly to spot changes in your system.
Frequently Asked Questions
What is a good boiler blowdown rate?
For low pressure firetube with softened makeup and 60% condensate return, 4-8% of steam is common. With RO makeup and high condensate return, 1-3% is achievable. Below 1% may mean TDS meter error.
How do you calculate blowdown from TDS?
Blowdown flow = steam flow × feed TDS ÷ (boiler TDS - feed TDS). Rate % of steam = feed TDS ÷ (boiler TDS - feed TDS) × 100.
Is blowdown % of feedwater or steam?
Both are used. % of steam = B/S×100, % of feedwater = B/(S+B)×100 = Feed TDS/Boiler TDS×100. Always note which you report.
How to estimate feedwater TDS if not measured?
Estimate from makeup system: Feed TDS = (makeup TDS × makeup % + condensate TDS × condensate %) ÷ 100. Makeup % = 100 - condensate return %.
Can I reduce blowdown by increasing boiler TDS limit?
Only up to maker limit. Going above causes carryover and scale. Better to improve makeup quality or condensate return to lower feed TDS and naturally cut rate.
What is difference between continuous and bottom blowdown?
Continuous surface blowdown controls TDS rate. Bottom blowdown is intermittent, for sludge removal at bottom drum. You need both in a system.
How much heat is lost in blowdown?
Heat loss = blowdown flow × (enthalpy at boiler pressure - feedwater enthalpy). At 10 bar, 100 kg/h blowdown loses about 10-13 kW if not recovered.
Basis: TDS mass balance as per ASME and BS 2486. Boiler water TDS limit per maker. Saturated water enthalpy interpolated from steam tables 1-100 bar. Heat loss estimate assumes no flash recovery unless you configure recovery system.