Blowdown Heat Recovery: Recapturing Liquid Gold

Boiler-Blowdown-Heat-Recovery-SystemBoiler blowdown is a necessary evil of every boiler system and allows precious BTU’s to escape down the drain. However, you don’t have to let that liquid gold disappear into the sewer. A blowdown heat recovery system allows you to capture this heat and put it to use. In this article we cover the basics of a blowdown heat recovery system.

Continuous Blowdown

Use a blowdown heat recovery systems when there is a continuous surface blowdown occurring in a boiler. When blowdown is continuous it easy to justify the costs and the return on investment are rapid.

To recover heat from boiler blowdown you should use a flash vessel, a heat exchanger or a combination of the two. The type of system you use will depend on the pressure and size of your boiler.

Flash Vessel

Flash vessels separate the flash steam from liquid condensate. This creates low pressure steam that carries heat to other areas of the boiler room. This steam is most commonly piped to heat the boiler feedwater tank. It can also be piped to heat combustion air or used for a number of other heating applications in the boiler room.

Heat Exchanger

A heat exchanger takes the hot liquid condensate and runs it in close proximity to the colder boiler make-up water. A flow control valve regulates the flow rate of the hot condensate inside the heat exchanger. This serves two main purposes.

One, it allows for pre-heating of boiler make-up water. This improves boiler efficiency while recovering energy that would otherwise be wasted.

Two, it lowers the temperature of the condensate before it goes down the drain. This is important because many state laws do not allow for water over 140 degrees to enter the sewer system.

Blowdown Heat Recovery Spirax Sarco

Picture from Spirax Sarco

Blowdown Heat Recovery Example Scenario

The US Department of Energy discusses heat recovery in a steam tips sheet. They shares a good example of annual savings that is achievable by installing one of these systems. They look at a boiler that is blowing down continuously at a rate of 3,200 lb/hr. The boiler produces 50,000 lb/hr of 150-pounds-per-square-inch-gauge (psig) steam. In the steam tips sheet they perform some calculations. Ultimately, they are able to show that a blowdown heat recovery system would result in an cost savings of $68,000 per year.

With savings like that, you can see why heat recovery systems are so important.

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