Sheet1DEAERATOR VENT STEAM HEAT RECOVERY

The amount of steam / air mixture that needs to be released can be estimated by considering the effects of Dalton's Law of partial pressures and Henry's Law.

Consider the feasibility of installing a deaerator. Prior to installation, the boiler plant is fed by feedwater from a vented feedtank operating at 80C. This essentially means that each 1 000 kg of feedwater contains 5 gram of air. The proposed deaerator will operate at a pressure of 0.2 bar g, which corresponds to a saturation temperature of 105C. Assume, therefore, that all the air will be driven from the water in the deaerator. It follows that the vent must reject 5.9 gram of air per 1 000 kg of feedwater caThe proposed deaerator will operate at a pressure of 2.75 bar g, which corresponds to a saturation temperature of 140C. Assume, therefore, that all the air will be driven from the water in the deaerator. It follows that the vent must reject 5.9 gram of air per 1 000 kg of feedwater capacity.Total pressure in the deaerator 3.6bar aTemperature of the vapour in the deaerator130deg C130C corresponds to a saturation pressure2.73bar aTherefore, from Dalton's Law:-If the vapour space in the deaerator were filled with pure steam, the vapour pressure would be3.6bar a. As the vapour space has an actual temperature of130C, the partial pressure caused by the steam is only2.73 bar a.

The partial pressure caused by the non-condensable gases (air) is therefore the difference between these two figures = 0.87bar aThe proportion to volume of air to steam in the mixture 32%Therfore every litre of released air acompanied by 2.1379310345litres of steamThe density of air at 130 deg C0.862grams/LThe density of steam at 130 deg C0.543grams/LTherefore 0.862 grams of air release with1.1608965517g of steamand, 5.9 grams of air is released with7.9458116649g of steamTherfore the total mixture of air and steam released per 5.9 g of air can be calculated.5.9 g + 7.94581166493319 g =13.8458116649332 grams of air/steam mixture per 1000 kg of deaerator capacityAlthough the theory suggests that 22.4 grams of steam/air mixture per tonne of deaerator capacity is required, in practice this is impossible to monitor or regulate successfully.Therefore, based on practical experience, deaerator manufacturers will tend to recommend a venting rate of between 0.5 and 2 kg of steam/air mixture per 1 000 kg / h of deaerator capacity to be on the safe side. It is suggested that the deaerator manufacturer's advice be taken on this issue.

The practical total vent air and steam mixture is (Both HRSG's and Export in full load)300Kg/hr

Heat loss640.1913875598Kcal/hr

By dirct mixing heat exchanger the heat recovery is more than 90%576.1722488038Kcal/hr

The cost of fuel saving per year in terns of IFO (Assuming Boiler eff 90% and IFO calorific value 10000kcal)507.0315789474Kg

Cost saving17746.1052631579Per year

Approx cost of heat recovery equipment25000

Payback period1.4087598168Years

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