Flash steam recovery delivers large energy savings

Affco NZ has been producing prime meat products since 1904. At its Wairoa site, a large rendering addition was made to avoid trucking offsite. During the design phase, and with energy conservation in mind, one of the key components was to ensure condensate was recycled to the boiler house.

The site’s existing 11.3 MW coal-fired boiler had sufficient capacity to supply the new rendering process equipment. The majority (12,000 kg/h) is used for two rotary driers, drying meat and bonemeal from approximately 70% to 10% moisture for end use as dry chicken feed or dog biscuits.

3-port secondary side control on the heat exchangers gives rapid response to the variable hot water demands. Image: Spirax Sarco.

At differential pressures (between steam use in process and the backpressure of the condensate) higher than 4 bar, a conventional condensate pump return system is not very effective, wreaking more than 6% flash steam energy in the condensate lines and often boiling the boiler feedtank water.

“I’ve worked with another operation where we couldn’t control the flash steam - it just went to waste,” said Plant Engineer Paul Sullivan. “We worked with Spirax Sarco to ensure that we had minimal wasted energy in the steam and condensate loop.”

The solution has paid off in just over a year and, according to Sullivan, “The project achieved all of our expectations, plus more.”

After several site visits and a review, Spirax Sarco put forward a proposal. Spirax suggested splitting condensate into high- and low-pressure systems and utilising energy from the dryer condensate at high pressure by installing a flash vessel. In addition, flash steam energy would be recovered and used for a new ‘on-demand’ hot water system comprising two heat exchangers in series using low-pressure condensate for preheating and high-pressure flash steam. This was ideal as the existing hot water system was due for an upgrade.

Excess flash steam beyond hot water demand was used to heat the make-up water storage tank, while all spent condensate was returned to the vented receiver and pumped back to the boiler.

Flash steam is recovered and utilised in the plate heat exchanger. Image: Spirax Sarco.

“Dropping the pressure and temperature of the condensate to 80°C was ideal. All the flash steam is run through the main heat exchanger and serves almost the entire washdown hot water requirements,” Sullivan said. “The only time we use direct boiler steam for hot water is at start-up from 6.00 to 9.00 am and it is minimal as the storage tank is preheated to about 65°C from the prior day from excess flash steam.”

Controlling the condensate return temperature at 80°C (subcooling) ensures maximum energy recovery. All spent condensate is returned to the condensate receiver and pumped to the boiler feedtank, enabling a reduction in boiler chemicals and make-up water required.

Spirax’s solution combined Affco’s need for condensate return and a hot water system upgrade into a large energy-savings project. Sullivan is enthusiastic about the project’s success: “We are not wasting energy; we are saving in the order of $100K per year and could not have asked for a better turnout.”