Why is the blowdown rate important?

Insufficient blowdown causes foaming, carryover of water with steam and scaling in the boiler; excessive blowdown causes unnecessary heat and water loss. The correct ratio is determined according to the feed water quality.

Can the heat lost in blowdown be recovered?

Yes. With the blowdown flash tank, the flash steam can be sent to the deaerator, and with the heat exchanger, the heat of the remaining water can be transferred to the make-up water.

Blowdown Calculation – Steam Boiler Blowdown Rate and Flow

As water evaporates in the steam boiler, dissolved solids (TDS) accumulate in the boiler water. To keep it within limits, some of the boiler water is regularly drained off; this is called blowdown. This tool calculates the required blowdown rate, blowdown flow rate, and blowdown heat loss.

Formula

Blowdown rate (%) = F ÷ (B − F) × 100
Blowdown rate = Steam flow rate × F ÷ (B − F)

F: feed water TDS, B: allowable TDS (ppm) for boiler water. Heat loss is calculated from the difference in enthalpy of saturated water and make-up water at boiler pressure.

Example

If the feed water TDS is 150 ppm, the allowed boiler water TDS is 3,000 ppm and the steam flow rate is 5,000 kg/h, the blowdown rate is 5.26% and the blowdown flow rate is 263 kg/h.

For details about water treatment and blowdown, you can refer to our boiler water treatment article.

Boiler Blowdown Rate Calculation

Formula

Example

Frequently Asked Questions