October 2012
6-27
6.5
Boiler and Steam Systems
In both types of steam boiler configurations, as the steam is distributed, its heat is
transferred to the ambient environment and, as a result, it recondenses to water. This
condensate is then either discharged to the sewer or captured and returned to the
boiler for reuse. If the condensate is discharged to the sanitary sewer, most codes
require it to be cooled to an acceptable temperature before discharging. The hot
condensate is typically tempered with cool water to meet the temperature discharge
requirements.
As the water is converted to steam, dissolved solids, such as calcium, magnesium,
chloride, and silica, are left behind. With evaporation, the total dissolved solids
(
TDS) concentration increases. If the concentration gets too high, the TDS can cause
scale to form within the system or can lead to corrosion. The concentration of TDS
is controlled by removing (i.e., blowing down) a portion of the water that has a high
concentration of TDS and replacing that water with make-up water, which has a
lower concentration of TDS. Some boiler operators practice continuous blowdown by
leaving the blowdown valve partially open, requiring a continuous feed of make-up
water.
From a water-efficiency standpoint, installing and maintaining a condensate recov-
ery system to capture and return condensate to the boiler for reuse is the most effec-
tive way to reduce water use. Recovering condensate:
••
Reduces the amount of make-up water required.
••
Eliminates or significantly reduces the need to add tempering water to cool con-
densate before discharge.
••
Reduces the frequency of blowdown, as the condensate is highly pure and adds
little to no additional TDS to the boiler water.
In addition, since the steam condensate is relatively hot, when it is added back to the
boiler, it requires less energy to reheat to produce steam again.
Proper control of boiler blowdown water is also critical to ensure efficient boiler op-
eration and minimize make-up water use. Insufficient blowdown can lead to scaling
and corrosion, while excessive blowdown wastes water, energy, and chemicals. The
optimum blowdown rate is influenced by several factors, including boiler type, oper-
ating pressure, water treatment, and quality of make-up water. Generally, blowdown
rates range from 4 to 8 percent of the make-up water flow rate, although they can be
as high as 10 percent if the make-up water is poor quality with high concentrations
of solids.
31,32
Blowdown is typically assessed and controlled by measuring the conductivity of the
boiler make-up water compared to that in the boiler blowdown water. Conductivity
provides an indication of the overall TDS concentration in the boiler. The blowdown
percentage can be calculated as indicated in Equation 6-6. The boiler water quality is
31
U.S. Energy Department (DOE), Energy Efficiency & Renewable Energy (EERE). January 2012.
Minimize Boiler Blowdown
.
32
DOE, EERE. January 2012.
Return Condensate to Boiler
.
.
