Most steam systems have the potential for energy savings. These can be expressed in terms of steam, heat, fuel or their dollar cost. Further savings come via a reduction in CO2 emissions and water consumption. This article outlines some of the savings that may be made, and to quantify them we have assumed a plant generating 4,000kg/hr of steam at 700kPa in a gas-fired boiler, operating 16hr per day, five days per week, 50 weeks per year.
The classic energy saver in steam systems is to fix or replace leaking steam traps. A steam trap audit typically finds that around 10% of installed traps are leaking, wasting steam that can easily account for around 1% or more of the annual fuel bill. Now this might not sound much, but usually the cost of finding and fixing these traps is much less, as the payback period is just a few months or even weeks. The reduction in CO2 emissions can be quite impressive – around 30t less for the plant mentioned.
Another classic energy saver is condensate recovery. Condensate is hot water that will require little or no water treatment, so its place is in the boiler feed tank, not down the drain. If it does go down the drain, the waste is immense – 16,000kl/yr, which would all be recycled if the condensate were recovered, saving up to $32,000 or more a year in fresh water costs. And because the fresh water will be at 20°C instead of the probable 95°C for returned condensate, 314kJ of heat (via the boiler) needs to be input into every litre (kg). Condensate recovery saves this, and it amounts to a staggering 5,024GJ of heat per year (1GJ = 1 million kJ), wasting over 12% of the annual fuel bill, and adding almost 370t unnecessary CO2 to the atmosphere. There’s also the cost of treating the fresh water, which is unnecessary if condensate is recovered.
Even in plants that already have a condensate return system, the value of making sure that every steam trap does properly discharge into it at all times should be clear – the cost could be as little as installing a piece of pipe. If the steam pressure is insufficient to reliably push the condensate back at all times, a condensate pump will solve the problem, especially the ‘steam trap and condensate pump’ combination units (pump-traps) that are available today.
Flash-steam loss is another waster. Flash-steam forms naturally when condensate is discharged by a trap, caused by the fact that the pressure and temperature conditions at a trap’s inlet are invariably much higher than at its outlet (usually atmospheric pressure or close to it). Flash-steam is the result and it’s usually highly visible as an unsightly steam loss from vents on receivers and feed tanks. Proper recovery of this flash-steam (to heat or pre-heat water, air, etc.) typically reduces the annual fuel (and water) bill by about 10% or more, so the savings (of fuel, water and CO2) are similar to those mentioned above for condensate recovery.
The potential for heat recovery from boiler blow-down is much overlooked. It shouldn’t be, because it can easily account for 1.5% or more of the annual fuel bill, and 45t of CO2 (based on a typical blow-down rate of 6% of the boiler’s actual output). Because the blow-down is hot water at steam temperature, the easiest and best way of recovering heat from it is to collect and use the flash steam from it, similar to flash steam recovery from condensate as mentioned above.