10.3 Condensate Recovery, Boiler Blowdown Control, and Waste Heat Recovery

Condensate Recovery and Blowdown Control

Optimizing a boiler plant requires looking beyond the burner and the steam traps. Tremendous energy savings can be achieved by recovering thermal energy that would otherwise be rejected to the environment. The three primary avenues for this optimization are condensate recovery, rigorous blowdown control, and waste heat recovery systems like economizers.

Condensate Recovery

When steam releases its latent heat of vaporization at the point of use, it turns back into liquid condensate. This condensate is essentially pure, distilled water and it contains a significant amount of sensible heat. A properly designed steam system should aim to return as much of this condensate as possible to the boiler feedwater tank (the deaerator or hotwell).

The benefits of condensate recovery are threefold:

1. Energy Savings: Condensate is hot. Using hot condensate reduces the need to heat cold makeup water from the municipal supply.
2. Water Savings: Every pound of condensate returned is a pound of fresh makeup water that does not need to be purchased.
3. Chemical Savings: Condensate has already been treated. Returning it reduces the volume of chemicals required to treat raw makeup water.

Calculating Condensate Heat Recovery Savings

The thermal energy saved by returning condensate is a function of the difference in enthalpy between the hot condensate and the cold makeup water it replaces.

Btu saved per lb of condensate = h_f,condensate - h_f,makeup

Where:

• h_f,condensate is the sensible heat (enthalpy of saturated liquid) of the returning condensate in Btu/lb. For water, the specific heat is approximately 1 Btu/lb-°F, so h_f can be roughly estimated as (Temperature °F - 32).
• h_f,makeup is the sensible heat of the cold makeup water.

Worked Calculation Example: A facility has the opportunity to return 5,000 lbs/hr of condensate at 180°F. The local makeup water temperature is 60°F. What are the thermal savings in Btu/hr?

First, approximate the enthalpy: h_f,condensate ≈ 180 - 32 = 148 Btu/lb h_f,makeup ≈ 60 - 32 = 28 Btu/lb

Savings per pound = 148 - 28 = 120 Btu/lb

Total Savings = 5,000 lbs/hr * 120 Btu/lb = 600,000 Btu/hr. This represents a substantial reduction in the fuel required by the boiler to heat the incoming feedwater.

Boiler Blowdown Control

As a boiler generates steam, it evaporates pure water and leaves behind the dissolved solids and impurities that were present in the feedwater. If these dissolved solids are allowed to concentrate indefinitely, they will eventually precipitate out, forming insulating scale on the boiler tubes. Scale severely reduces heat transfer efficiency and can lead to dangerous localized overheating and tube failure.

To prevent this, a portion of the concentrated boiler water must be intentionally drained and replaced with fresh, lower-concentration makeup water. This process is called blowdown.

While necessary, blowdown wastes energy because the water being discharged is at the saturation temperature of the boiler operating pressure. Therefore, the goal of the energy manager is to maintain the blowdown rate at the precise minimum required to prevent scaling, but no higher.

Calculating the Blowdown Rate

The required blowdown rate is determined by the concentration of impurities in the makeup water compared to the maximum allowable concentration of impurities in the boiler drum. A common limiting impurity is silica, or Total Dissolved Solids (TDS).

The formula for calculating the required blowdown percentage (as a fraction of total feedwater) is:

Blowdown % = (Concentration in Feedwater / Maximum Allowable Concentration in Boiler) * 100

This can be expressed using specific markers, such as: Blowdown % = (Silica_feedwater / Silica_boiler) * 100

Worked Calculation Example: A boiler's water treatment program dictates that the maximum allowable Total Dissolved Solids (TDS) in the boiler is 2,500 ppm to prevent scaling. The feedwater entering the boiler has a TDS concentration of 125 ppm. What is the required blowdown percentage?

Blowdown % = (125 ppm / 2,500 ppm) * 100 Blowdown % = 0.05 * 100 = 5.0%

This means that 5% of the water fed into the boiler must be blown down to maintain the TDS at safe levels. If a facility is manually blowing down 10% of their feedwater due to lack of automated controls, they are wasting massive amounts of thermal energy and water.

Automated surface blowdown controls, which continuously monitor boiler water conductivity and modulate a blowdown valve, are highly recommended to keep the blowdown rate exactly at the required setpoint.

Waste Heat Recovery

Even with an optimally tuned burner and perfect blowdown control, heat is still lost to the environment. Waste heat recovery systems capture this energy and recycle it back into the process.

Economizers

An economizer is a heat exchanger installed in the exhaust stack of a boiler. It captures sensible heat from the hot flue gases before they exit the chimney and uses it to preheat the incoming boiler feedwater. This directly increases overall boiler efficiency. As a rule of thumb, every 10°F to 11°F increase in feedwater temperature achieved by an economizer yields a 1% increase in overall boiler efficiency.

Blowdown Heat Recovery

Because blowdown water is discharged at boiler pressure and temperature, it contains significant energy. When this high-pressure water is routed to a lower pressure vessel (a flash tank), a portion of it instantly vaporizes into "flash steam."

A blowdown heat recovery system utilizes this flash steam by routing it into the deaerator to help heat the feedwater. The remaining hot liquid from the flash tank is then passed through a liquid-to-liquid heat exchanger to preheat the cold makeup water before it is finally discharged to the drain. This dual-stage recovery can capture up to 90% of the heat that would otherwise be lost in the blowdown process.