Recovering Heat from a Boiler Blowdown Recovering Heat from a Boiler Blowdown

A boiler must blowdown periodically to reduce harmful material found in the boiler water. When the blowdown occurs not only are solids eliminated (or reduced) but also boiler chemicals and thermal energy. The way to recover the loss of thermal energy that occurs during the blowdown is with the use of a blowdown heat recovery system.

What a Blowdown Heat Recovery System Does

A blowdown heat recovery system can recover up to 90 percent of the thermal energy that would otherwise be loss during the blowdown process. The heat recovery system used in a blowdown takes the pre-heat boiler make-up water and creates a low pressure steam in order to heat water in a device known as a deaerator. The steam reduces the cost to operate the deaerator, improving the boiler’s efficiency.

Requirements for the Blowdown Discharge

The discharge that results from a blowdown has to be cooled according to applicable local codes before it enters a sewer system. The blowdown heat recovery system reduces the temperature of the blowdown and does away having to mix the blowdown with cold water prior to its entry into the sewerage system.

When a Blowdown Heat Recovery System is Used

A blowdown heat recovery system is used when either 5 percent of the boiler water is the make-up in smaller boilers (the percentage is even lower for larger boilers) or the blowdown consists of 500 pounds of steam per hour.

How the Blowdown Operation Works

Regardless where the blowdown takes place in the boiler (either from the surface or the bottom of the boiler) heat is lost along with the solid materials. The boiler operating pressure whether steam or flash steam determines the amount of heat loss that will occur. The heat recovery replaces up to 90 percent o the heat loss that occurs. This is done through a condensation process that uses cold water. The colder the water that is used in the condensation process the greater the heat recovery and efficiency of the boiler.

Determining the Savings from the Blowdown

As mentioned the blowdown heat recovery system recover about 90 percent of the thermal energy that is lost. Determining the actual value of the loss depends on the volume, pressure of the boiler, the method of blowdown employed and its frequency as well as the cost of fuel. A blowdown system that is continuous or ongoing uses a boiler capacity of between 5 and 10 percent of the boiler’s capacity. Other assumptions we can use in the formula for estimating savings includes accounting for the BTUs (British thermal units, a measurement of energy) value created by the boiler steam pressure (higher  boiler pressure results in higher flash steam). We will use a flash steam rate of 25 percent with 75 percent going to hot condensate for a boiler operating under 300 psi.

The determining formula is:

Average boiler capacity in pounds per hour of steam x 5 percent = Blowdown volume

The amount of flash steam that is produced by the blowdown recovery can exceed the amount of steam needed by the deaerator device resulting in a reduction of the saving potential for the blowdown heat recovery system.

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