Blowdown heat recovery systems can recover about 90% of the blowdown thermal value. The value varies with volume, boiler pressure, method/frequency of blowdown and fuel cost. Continuous blowdown systems use 5 – 10% boiler capacity. Therefore, if estimating a value, 5% of average boiler capacity is reasonable. The blowdown BTU value varies with boiler steam pressure, the greater the boiler pressure the increased amount of higher value flash steam. Assume 25% flashes to steam and remainder is hot condensate for boilers under 300 psi.
Blowdown heat recovery systems offer a rapid payback, depending on blowdown volume. Several boilers can be connected to a single heat recovery unit, reducing capital costs. Typical payback is usually under 12 months.
A blowdown heat recovery system should be considered when:
- ~ 5% of boiler water is make-up (smaller boilers or lower percentage for larger boilers)
- ~ 500 lbs/hour steam is blown down
- Continuous blowdown systems of at least 1 gpm
General formula provided by the department of energy:
Average Boiler Capacity in pounds per hour of steam x 5% = Blowdown volume
- (Blowdown volume x 25% x 1,200 BTUs per Pound) + (Blowdown volume x 75% x 140 BTUs per Pound) = Total BTUs
- Total BTUs x 90% Efficiency / (1,000,000 BTUs x Boiler Efficiency) = Millions of BTUs per Hour Recovered MMBTUs x $ per MCF = Hourly Savings
It doesn’t matter whether blowdown is from the surface or bottom of the boiler, heat is lost along with the solids. Depending on boiler operating pressure, steam and/or flash steam provides the most intense heat, but condensate heat recovery provides additional heating. The colder the incoming water (such as with cold make-up water and limited or no condensate return) the higher the overall efficiency.
All steam boilers must be blown down to reduce the amount of Total Dissolved Solids (TDS) in the boiler water. However, along with the solids, boiler chemicals and thermal energy is lost; blowdown heat recovery systems cannot recover the chemicals, but they do recover up to 90% of the heat energy that would otherwise be lost down the drain. The recovered heat is used to pre-heat boiler make-up water before it enters the deaerator. This low-pressure steam is used to heat water inside the deaerator, which reduces the cost to run the deaerator and improves overall boiler efficiency.
Reducing the temperature of the blowdown before it reaches the sewer drain is a typical Code requirement. Therefore, a heat recovery system also eliminates the need to dilute blowdown with cold water before it enters the sewer.