I recently entered a boiler house while accompanying a colleague who had to go see a user regarding a steam outsourcing opportunity. So it was none of my business really, but I could not help noticing that the boiler, which was rated at 10 bar steam pressure, was being operated at only 5 bar pressure. My first thought was that the boiler was undersized and could not cope with the required steam demand.

My question to the engineer regarding the steam pressure elicited a most unexpected reply: he was instructed to operate the boiler at low pressure to save fuel. My natural response was: do you succeed in delivering sufficient steam to the production plant? to which he replied in the negative. At best they battled to meet the steam demand, with an oversized boiler pumping out steam at 5 bar pressure.

The steam supply situation is understandable. At 5 bar pressure a specific mass of steam occupies 78% more volume than at 10 bar. This means that to move a certain steam flow at 5 bar requires it to travel at 78% higher velocity through the pipe system than at 10 bar. In this instance the pipe diameters were just too small to cater for the required steam flow and velocity at 5 bar, hence the production processes were starved of steam. In this particular instance the user was prepared to risk production throughput and product quality for the sake of a perceived fuel saving.

Were they saving energy? Nobody could say how much or explain why they should. The laws of physics tell us they should. Reducing the steam pressure also means reducing the saturated steam (and boiler water) temperature and hence the temperature of the flue gas exiting the boiler through the stack. In the case under discussion the temperature reduction is approximately 25 ᵒC (you may check the steam tables to verify this number). Using this temperature difference in my combustion calculator I arrive at a fuel saving of 1,9%.

With lower boiler water temperature one may expect surface radiation and convective heat losses to be less too; however with a well insulated boiler the effect is negligible.

Another risk to be considered is one of condensation and precipitation of certain elements in the flue gas at sufficiently low temperature. When sulphur-bearing fuel is burned, sulphur is converted into sulphur dioxide (SO2) and sulphur trioxide (SO3). If the flue gas is cooled sufficiently, condensation will occur and droplets will appear on surfaces at temperatures below the dew point. The liquid phase will contain highly corrosive sulphuric acid. Depending upon the concentrations of SO3 and water vapour, the dew point temperature can vary from approximately 90 ᵒC to 140 ᵒC. Condensation of these acids results in metal wastage and flue gas duct corrosion. In order to avoid or reduce the cold end corrosion the gas temperature leaving the heat transfer surface in the boiler must be kept at or above 150 ᵒC.

In conclusion: is operating a boiler at reduced pressure really worth the risk of restricted steam flow and cold end corrosion? I would say yes, provided production/operations is not starved of steam and flue gas temperature can be maintained above 150 ᵒC all the way to the point of discharge, even if it means applying thermal insulation to the boiler stack.

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about efficiency of combustion or combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

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