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Healthcare for Boilers

Kate Wild of Veolia Water Technologies (Veolia) looks at hospital boiler efficiency

With all the amazing testing, surgery and care that goes on in hospitals it is sometimes easy to forget that none of it would be possible without the humble hospital boiler. It supplies steam for space heating, sterile services, laundry and kitchens and is vital to the operation of the hospital and to patient safety. So it behoves the hospital’s engineers to ensure that their boilers are operating as efficiently as possible.

Modern hospital shell 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 the value set by BS2486 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 sewer 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. When you add in the cost of mains water, fuel, chemicals and sewer disposal it probably costs at the very least about £3 per m3. Improving the make-up water treatment can reduce the volume that has to be dumped to drain thus saving money. 

The blowdown rate is determined by the feedwater (make-up water plus condensate return) TDS. Maximising condensate return, minimises the feedwater TDS and hence the blowdown rate. It’s not always possible to recover condensate, but reducing the TDS of the make-up water by “external treatment” will also help to reduce blowdown. Consider 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 its TDS, so the blowdown will be about 2m3/h costing around £45,000 per annum. Treating the make-up water by reverse osmosis will reduce its TDS by about 95% and the blowdown would be less than 0.1m3/h saving over £40,000 per annum. The annual operating costs of the Reverse Osmosis (RO) plant would be in the region of £15,000 giving a net saving of roughly £25,000 per annum and a payback of about 12 months. Veolia has developed an online tool called recoSMART to evaluate the potential cost savings resulting from alternative external treatment options.

External treatment controls the composition of the feedwater, but equally important is internal treatment or chemical conditioning to control the boiler water chemistry. This needs a number of chemical additives – typically scale inhibitors, dispersants, oxygen scavengers and steam line treatments – and ensuring that their concentrations are correct is critical to efficient operation. Under-dosing exposes the boiler to potential scaling and corrosion problems whilst over-dosing wastes money and, with some chemicals, can cause corrosion. Veolia’s range of chemicals includes both organic and inorganic chemicals formulated to meet the individual customer’s needs, and is supported by automatic control systems and regular site visits by Veolia’s service technicians.

Typical of Veolia’s customers is Borders General Hospital in Melrose. Their boiler system consists of three dual fuel (gas/oil) boilers and a waste heat boiler linked to the incinerator. Direct steam injection into autoclaves, laundry and steam ovens in the kitchen limits the condensate return, but the low TDS of the make-up water means that softening is the only external treatment required. Following a system evaluation, Veolia changed the treatment regime from tannin to a neutralised sulphite programme that allowed the alkalinity levels to be reduced. Automatic, conductivity controlled blowdown system for each boiler ensures consistent boiler water quality rather than the variability inherent with manual blowdown. The revised chemical regime has given the hospital a substantial saving in blowdown and is helping to meet sustainability targets. Veolia’s AQUAService contract provides not only the supply of chemicals together with a programme of fortnightly visits by a chemical technician to carry out chemical testing and dosing adjustment to optimise boiler conditions, but also a biannual full servicing of the water softener and RO plant, which is now operating more efficiently and is generating savings by reducing blowdown and over usage of chemicals.

Visit www.recosolutions.co.uk and use the recoSMART calculator to find out how you can reduce costs by improving boiler efficiency.

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