Boiler Bowdown Considerations

Boilers require periodic blowdown in order to maintain effective operation, provide good equipment life, and reduce maintenance time and expense. In this sense, "blowdown" refers to the removal of boiler water in order to maintain an acceptable level of Total Dissolved Solids (TDS). Blowdown has an economic impact since the water that is removed has been heated from the inlet temperature to the steam saturation temperature and the energy used to heat this water comes from the fuel burned in the boiler. In most cases, blowdown with a Clayton Steam Generator is significantly less than with other conventional industrial steam boilers. This reduction often results in equally significant fuel savings.

Dissolved solids primarily are introduced into the boiler through make-up water. Yet, some solids are introduced through chemical treatment or, in upset conditions, through condensate. Dissolved solids brought into the steam boiler water tend to stay in the boiler water and do not get removed with the steam. Hence, the level of solids (usually expressed in parts per million [ppm]) in the boiler water increases (concentrates) over time.

As previously mentioned, blowdown is the process of removing boiler water that has approached a maximum acceptable level of concentration. When this happens, the water that is blown down is replaced by make-up water with a much lower TDS level. This dilutes the boiler water, lowering the TDS concentration. The higher the TDS level in the blowdown water, the lower the amount of water that must be removed in order to dilute the remaining water to any given level.

The amount of water that must be blown down for any given application depends upon:

  • The TDS level in the make-up water—the higher the level, the greater the amount of blowdown.
  • The amount of make-up water vs. the amount of condensate returned—the greater the percent of make-up the greater the amount of blowdown.
  • The maximum acceptable TDS level in the boiler water—the lower the level the greater the amount of blowdown.
  • The TDS in the blowdown water—the higher the TDS, the lower the amount of blowdown.
  • The average load level for the boiler (BHP or lbs of steam per hour).

As illustrated above, applications with high make-up levels, high TDS in the make-up water and extended operation at high load levels result in high levels of blowdown, creating a high economic impact.

The TDS of the blowdown water is significant because the higher the TDS, the lower the volume of water that must be removed in order to remove the same amount of solids, in terms of pounds, and achieve the same level of concentration after dilution with make-up water.

Clayton Steam Generators provide a fuel savings from reduced blowdown because of two factors:

  1. 1. Since the Steam Generator is a forced circulation boiler, it can tolerate relatively high TDS levels in the feedwater—as high as 8,550 ppm.
  2. 2. Water that is blown down is separator trap return water that has been concentrated in the separator by a factor of, typically 4 to 5 times. (See How We Make Steam for an explanation of the Clayton system and trap return water.)

Considering these two factors, blowdown water for a Clayton Steam Generator typically has a concentration of 24,000 to 40,000 ppm. This is comparable to typical values for conventional boilers in the range of 2,500 to 3,500 ppm. Since our blowdown water has TDS concentrations that are 7 to 16 times higher than conventional boilers, the volume of blowdown water with Clayton Boilers is 1/7th to 1/16th that of a conventional boiler. We will show that this can amount to a major savings in fuel cost.

In addition to the factors that determine the volume of blowdown water, the economic impact depends upon:

  • The temperature of the condensate and make-up water—the lower the temperatures, the greater the economic impact.
  • The steam pressure—the higher the pressure affecting the temperature, the greater the economic impact.
  • The cost of fuel—the higher the fuel cost, the greater the economic impact.
  • The number of hours of operation.
  • The efficiency of the boiler.

These savings can be illustrated by the following example:

Make-up water TDS Level: 300 ppm
Make-up water temperature: 60°F
% of Make-up: 100%
TDS level in conventional boiler: 3000 ppm
TDS level in Clayton feedwater: 6500 ppm
Average boiler load: 300 BHP
Boiler Efficiency: 80%
Steam pressure: 125 PSIG
Hours of operation per day: 16 hours per day
Days of operation per year: 312 days per year
Cost of natural gas: $4.00 per MMBTU

In this case, the savings in blowdown is 860 lbs of water/ hour, which equals an annual savings (based on the assumptions shown) of nearly $6,400. This could easily represent greater than 6% of the installed cost of the new equipment and, most significantly, these savings are ongoing.

As noted, savings with a Clayton Steam Generator that result from reduced blowdown can be significant. These savings are application dependent. Please contact us for an estimate for your application.

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Boiler Blowdwn Considerations