Savings possible? Blow me down

Boilers need to be fed with water which is, at least, softened, to remove calcium and magnesium salts which would otherwise lead to scale formation. It then has to be dosed with a variety of 'conditioning chemicals' to remove oxygen, prevent corrosion and condition sludge. As water turns to steam in the boiler it leaves behind all the salts that were dissolved in the feedwater so the boiler water becomes more concentrated. If the total dissolved solids (TDS) content of the boiler water exceeds a value set by the boiler manufacturer – typically about 3500mg/l for a 10bar shell boiler – it could lead to carryover of water droplets into the steam, adversely affecting steam purity.

To control the TDS in the boiler, some of the concentrated boiler water is discharged to drain as 'blowdown'. This blowdown water was originally purchased from the mains supply, treated in the make-up water treatment plant, dosed with expensive conditioning chemicals and heated to boiling point. It is usually discharged to sewer, so, when you add in the cost of mains water, fuel, chemicals and sewer disposal it probably costs at the very least about £3/m3 – and that’s money down the drain. The blowdown water can be passed through a heat exchanger to heat the make-up water, and that will recover some of the heat content, but improving the make-up water treatment can actually reduce the volume that has to be dumped.

The blowdown rate is determined by the feed water TDS. Feed water consists of make-up water and condensate return, so maximising condensate return, which is distilled water, will help to minimise the feedwater TDS. Blowdown rates typically range from 5 to 10% of boiler feedwater flow rate, but can be as high as 30% when makeup water has a high solids content or condensate return is low. It’s not always possible to recover condensate, but reducing the TDS of the make-up water will also help to reduce blowdown.

Let’s assume you have a 20tph boiler with 40% condensate return and make-up from a 500mg/l TDS London mains water. A water softener will remove calcium and magnesium ions from the make-up water but will not reduce all of its TDS, so the blowdown could be about 2m3/h which equates to approximately £45,000 per annum. Treating the make-up water by reverse osmosis, on the other hand, will reduce its TDS by up to 95%. That would reduce the blowdown to less than 0.1m3/h which could save over £40,000 per annum. Taking into consideration the cost of the reverse osmosis unit, you could receive a payback within 12 months.  

An ion exchange deioniser will remove virtually all of the TDS, including silica, and may even be lower in capital costs than an RO system.  However, during regeneration there is a downtime in the production of de-ionised water, but this can easily be solved by using a duty standby system - this cost of is offset by not requiring a water softener upfront.

These numbers illustrate the potential savings; clearly each system has to be assessed in terms of steam rate, make-up water analysis, condensate return and boiler water quality control parameters. ELGA Process Water has developed an online tool called rEcoSMART to evaluate the cost savings you could make, before you invest time and money into implementing any changes. You simply enter on the interactive screen your raw water analysis, steam rate and condensate return along with water, discharge and energy costs, and rEcoSMART will show the cost savings you will make by installing the various make-up water treatment options. This means you can immediately assess the payback. And, of course, one of ELGA Process Water’s Engineers is always available to help and advise you with the decision making.