You are probably familiar with the situation – a user of steam boilers returns a low percentage of condensate to the hot well, and is concerned that the temperature of the feed water is relatively low. He somehow knows that boiler efficiency is linked to the temperature of the feed water, and it just seems so logical that heating the feed water would increase efficiency – a fairly low cost high yield intervention.

A few elementary calculations, based on a number of realistic assumptions, reveal that a 40 ᵒC feed water temperature increase may increase boiler efficiency by almost 12%, or in terms of fuel spend the bill will be 12% lower than before. Sounds too good to be true? Well, it probably is.

Engineers battling every day to cut steam cost to the minimum know how much it takes in terms of management, supervision, operation and technological input to improve efficiency by a single percentage point. Twelve percent improvement is huge in any engineer’s language.

On two occasions during the past six months I engaged in debate with engineers regarding the perceived benefits of steam heating of the hot well, and to this day I am not sure if my counterparts have been convinced by what I have said. Maybe the time has come to put pen to paper and to analyze the situation in the light of simple logic.

Let us assume a scenario where a boiler is operated at 10 bar (g) pressure, using feed water at 40 ᵒC from the hot well. The steam demand is 1000 kg/h under these conditions. The engineer decides to inject steam into the hot well to raise the water temperature to 85 ᵒC, aiming to improve boiler efficiency and to save fuel in doing so. A steam heating system is installed to the hot well and put into operation.

For the hot well to be heated to the target temperature of 85 ᵒC steam energy to the tune of (4.2 kJ/kgᵒC*40 ᵒC*1000 kg/h) = 168000 kJ/h is required. Assuming an overall thermal efficiency of 70% the quantity of steam required to meet the heating requirement = (168000 kJ/h/2000 kJ/kg/0.7) = 120 kg/h. Although the feed water temperature has now been raised to a level where 12% efficiency is gained, the steam consumption has also increased by 12% to 1120 kg/h to provide the additional steam required to increase the feed water temperature. So whatever fuel savings were gained by the higher feed water temperature were lost to the higher steam flow required to improve the efficiency.

We are obviously running in a circle here, which is to be expected, since the system is merely circulating energy within itself – energy is taken from the system at one point and introduced back into it at another.

Energy efficiency improvement can only be achieved if energy losses from the system can be prevented or reduced (e.g. steam leaks, stack heat loss, shell heat loss, etc.), or if lost energy from the system can be recovered (e.g. feed water heating in an economizer, condensate return, etc.). Sorry, no short cut to real energy savings!

But steam heating of the hot well is not all in vain and must be encouraged as a means to reduce oxygen in the feed water. As the temperature of the water increases, so the solubility of oxygen therein reduces and so the concentration of oxygen scavenger required to chemically treat the boiler water is also reduced. So by circulating energy in the system some water treatment cost savings can be gained.

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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