Boiler Blowdown Heat Recovery

A boiler operating on a high quality feedwater will need very little blowdown, and a short bottom blowdown once per day may be sufficient. On the other hand, if the feedwater is high in solids, alkalinity, or silica such that a relatively higher blowdown rate is required, the daily bottom blow will not be enough; and, in this case, continuous blowdown should be considered. The continuous blowdown, although reduced to as low a rate as possible, still contains significant heating value. As much as 85% or more of this heating value can be recovered.

Flash Steam Recovery:
Fifty percent or more of the heat contained in the blowdown water can be recovered in the form of flash steam. This is accomplished by discharging the blowdown to a flash tank, usually operated at 5 psig. A portion of the blowdown will flash to steam at the lower pressure and the steam is available for use in the deaerator or for other low-pressure steam demands. The actual Btu recovery can be calculated as follows:

  1. % Steam flashed = h1-h2

                                    VL

        Where: h1   =  heat of water leaving the boiler

                     h2   =  heat of water at flash pressure

                     VL  = latent heat of steam at flash pressure

  1. Btu/day recovery = % steam flashed x blowdown

(lbs/day) x h5 where h5 = heat of saturated vapor at flash pressure

                Example: Boiler operating pressure: 235 psig.

          Flash tank pressure: 5 psig.                                 

                Average steam generated: 1,000,000lbs/day

                Blowdown: 5% rate or 52,630lbs/day

                Then: % steam flashed = h1 – h2  =  VL

                     376 – 196 =. 1875 = 18.75%

                                      960

                              and:

Btu/day recovery = % steam flashed x blowdown (lbs/day) x h5 = .1875 x 52,630 lbs/day x 1156 Btu = 11,407,553 Btu/day

Heat Exchange Recovery:

Additional heat can be recovered from the blowdown water remaining after the 5 psig steam is flashed off.  This is accomplished by passing the blowdown water through a heat exchanger to transfer the heat to the incoming makeup water.  In most cases, the heat exchanger will be designed to reduce the temperature of the blowdown water within 20°F of the makeup water temperature.  The additional heat thus recovered can be calculated as follows:

              Btu/day recovery = blowdown (lbs/day) x (1-% flashed) x h2 –h4)

                Where h2 = heat of water at flash pressure

                           h4 = heat of water at heat exchanger outlet temperature

                Example (using previous plant data):

                Btu/day recovery = blowdown (lbs/day) x (1-% flashed) x (h2 – h4)

Btu/day recovery = 52,630 lbs/day x .8125 x 148 Btu = 6,328,758 Btu/day

Summarizing the total heat recovery, we have:

                Flash steam recovery – 11,407,553 Btu/day

                Heat exchange recovery – 6,328,758 Btu/day

                Total recovery – 17,736,311 Btu/day

If we consider that the total heating value in the blowdown is 52,630 x 376 Btu/lb = 19,788,880 Btu/day, in this example we have recovered 89.6% of this heating value.

Summary:

Significant energy savings can be realized in most steam plants by reviewing the plant operation with regard to feedwater quality and the recovery of blowdown heat.  In the next Problem Solver issue we will discuss several other items, which should receive attention.  Included will be an “Energy Conservation Check List” for your convenience in surveying your individual steam system.