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. <\/p>\n\n\n\n
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. <\/p>\n\n\n\n
The steam supply situation is\nunderstandable. At 5 bar pressure a specific mass of steam occupies 78% more\nvolume than at 10 bar. This means that to move a certain steam flow at 5 bar\nrequires it to travel at 78% higher velocity through the pipe system than at 10\nbar. In this instance the pipe diameters were just too small to cater for the\nrequired steam flow and velocity at 5 bar, hence the production processes were\nstarved of steam. In this particular instance the user was prepared to risk\nproduction throughput and product quality for the sake of a perceived fuel\nsaving. <\/p>\n\n\n\n
Were they saving energy? Nobody\ncould say how much or explain why they should. The laws of physics tell us they\nshould. Reducing the steam pressure also means reducing the saturated steam\n(and boiler water) temperature and hence the temperature of the flue gas\nexiting the boiler through the stack. In the case under discussion the\ntemperature reduction is approximately 25 \u1d52C (you may check the steam tables to\nverify this number). Using this temperature difference in my combustion\ncalculator I arrive at a fuel saving of 1,9%. <\/p>\n\n\n\n
With lower boiler water\ntemperature one may expect surface radiation and convective heat losses to be\nless too; however with a well insulated boiler the effect is negligible. <\/p>\n\n\n\n
Another risk to be considered is\none of condensation and precipitation of certain elements in the flue gas at\nsufficiently low temperature. When sulphur-bearing fuel is burned, sulphur is\nconverted into sulphur dioxide (SO2) and\nsulphur trioxide (SO3). If the flue gas is\ncooled sufficiently, condensation will occur and droplets will appear on\nsurfaces at temperatures below the dew point. The liquid phase will contain\nhighly corrosive sulphuric acid. Depending upon the concentrations of SO3 and water vapour, the dew point\ntemperature can vary from approximately 90 \u1d52C to 140 \u1d52C. Condensation of these\nacids results in metal wastage and flue gas duct corrosion. In order to avoid\nor reduce the cold end corrosion the gas temperature leaving the heat transfer\nsurface in the boiler must be kept at or above 150 \u1d52C.<\/p>\n\n\n\n
In conclusion: is operating a\nboiler at reduced pressure really worth the risk of restricted steam flow and\ncold end corrosion? I would say yes, provided production\/operations is not starved\nof steam and flue gas temperature can be maintained above 150 \u1d52C all the way to\nthe point of discharge, even if it means applying thermal insulation to the\nboiler stack. <\/p>\n\n\n\n
This post was compiled by\u00a0Ren\u00e9 le Roux<\/strong>\u00a0for 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.<\/em><\/p>\n\n\n\n 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.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":" 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. … <\/p>\n