Blow Down Rate

Overview

Engineering reference data for Blow Down Rate in steam condensate.

Key Formulas

Steam Quality

$x = \frac{m_v}{m_{total}}$

Mass fraction of vapor in two-phase mixture.

Enthalpy of Wet Steam

$h = h_f + x \cdot h_{fg}$

Specific enthalpy of wet steam.

Flash Steam

$m_{flash} = m_{liquid} \frac{h_f - h_{f2}}{h_{fg2}}$

Steam generated when condensate flashes to lower pressure.

Condensate Load

$m_c = \frac{Q}{h_{fg}}$

Condensate generated by heat transfer.

Variables

References

• Original Source

Blowdown Rate Formula

The blowdown rate for a boiler can be calculated using the formula:

$q_{BD} = \frac{q_S \cdot f_c}{b_c - f_c}$

Where:

• $q_{BD}$ is the blowdown rate (kg/h)
• $q_S$ is the steam consumption (kg/h)
• $f_c$ is the Total Dissolved Solids (TDS) in the feed water (ppm)
• $b_c$ is the maximum allowable Total Dissolved Solids (TDS) in the boiler water (ppm)

Factors Affecting Blowdown Rate

The blowdown rate is influenced by the following factors:

• Steam consumption: The amount of steam used in the process and not returned as condensate to the boiler.
• Feed water impurities: The concentration of impurities in the feed water, measured as TDS (ppm).
• Boiler water limits: The maximum allowable TDS in the boiler water to prevent scale and corrosion.

Practical Considerations

When calculating blowdown rate, consider these operational relationships:

• Steam consumption (qS): Directly proportional. Higher steam demand increases required blowdown.
• Feed water TDS (fc): Higher impurity concentration in feed water increases blowdown rate.
• Boiler water TDS limit (bc): Tighter allowable limits (bc) increase blowdown rate to maintain concentration.

The denominator (bc - fc) represents the allowable concentration rise before blowdown is necessary. If feed water TDS approaches the boiler limit (fc → bc), blowdown rate increases dramatically, indicating poor water treatment efficiency.

Blowdown Rate Calculation Example

To demonstrate the application of the blowdown rate formula, consider a boiler with the following operating parameters:

• Steam consumption ($q_s$): 1000 kg/h
• Feed water TDS concentration ($f_c$): 100 ppm
• Maximum allowable boiler water TDS ($b_c$): 3000 ppm

The required blowdown rate ($q_{bd}$) is calculated as:

$q_{bd} = \frac{q_s \times f_c}{b_c - f_c} = \frac{1000 \times 100}{3000 - 100} = \frac{100000}{2900} \approx 34.48 \text{ kg/h}$

This result indicates that approximately 34.48 kilograms of water must be continuously discharged per hour to maintain the boiler water concentration within safe operational limits.

Relationship Between Blowdown Rate and Efficiency

The blowdown rate directly impacts boiler efficiency and operating cost. A higher blowdown rate, while necessary for water quality control, results in:

• Increased thermal energy loss from the discharged hot water
• Higher water and chemical treatment consumption
• Potential need for additional heat recovery systems

Optimal blowdown control requires balancing water quality maintenance with energy conservation. Modern systems often employ automated TDS monitoring to minimize unnecessary blowdown.

Blowdown Variables Reference

Core Blowdown Rate Formula

The fundamental formula for calculating the required blowdown rate to maintain a desired boiler water concentration is:

$q_{BD} = q_S \cdot \frac{f_c}{(b_c - f_c)}$

Where:

• $q_{BD}$ = Blowdown rate (kg/h)
• $q_S$ = Steam consumption (kg/h)
• $f_c$ = TDS in the feed water (ppm)
• $b_c$ = Maximum allowable TDS in the boiler water (ppm)

This equation is derived from a mass balance of dissolved solids in the boiler system. It shows that the blowdown rate is directly proportional to steam production and the ratio of feed water concentration to the allowable concentration range.

Factors Influencing Blowdown Requirement

The need for blowdown and its frequency are primarily determined by three operational factors:

1. Steam Consumption ($q_S$): Higher steam usage increases the rate at which impurities are left behind, requiring more frequent or higher-volume blowdown.
2. Concentration of Impurities in Feed Water ($f_c$): Feed water with higher dissolved solids loads the boiler with impurities faster, increasing blowdown demand.
3. Maximum Allowable Boiler Water TDS ($b_c$): This limit is set by boiler design and water treatment. A stricter (lower) limit requires more blowdown to maintain safe concentration levels.

Practical Note on Blowdown Frequency

The blowdown is a routine operation. The calculated rate ($q_{BD}$) is typically integrated into a continuous blowdown system or used to determine the frequency and duration for intermittent manual blowdown to achieve the same daily or weekly mass of solids removed.