Overcoming all challenges

In December 2017, the Minn-Dak Farmers Cooperative in Wahpeton, North Dakota, decided to buy a new falling-film evaporator for the first effect of their evaporator station. Their aim: creating better operating conditions for the whole station.

The first effect had previously been equipped with falling-film plate packs, installed years earlier in an old evaporator. But over time, it had become more and more difficult to properly remove the scale that typically forms on plate packs during long campaigns lasting 250 days or more.

Falling-film evaporators are designed to operate with a small temperature dif­ference between heating steam and vapour. Scale build-up leads to a reduced heat transfer and a loss of efficiency, which affects the performance. Another problem is the evaporator’s delayed response to process load changes.

The new falling-film evaporator from BMA has a heating surface of 7,000 m2 and is completely made from stainless steel. It was built in the BMA Greeley, Colorado workshops. The ASME code-compliant design was provided by BMA Germany, who also supplied the integrated impingement separator and the juice distributor.

At the beginning of 2018, sourcing of the necessary materials became very difficult because of the newly imposed US steel tariffs, resulting in delays. To make up time, the workshop started to operate in three shifts – and we managed to keep the promised delivery date!

The evaporator body was shipped in two sections. On site, the calandria section was partially tubed in the horizontal position. It was turned upright once the bottom section had been fitted, and both sections were welded together. The remaining tubes were then assembled and rolled into the tube sheets. Next came installation of the juice distributor and closing of the evaporator head. The subsequent hydro test was also a success.

Despite a number of challenges, the evaporator was ready in time for the campaign and has since been running to the full satisfaction of the Minn-Dak Farmers Cooperative. It responds immediately to process load changes, which makes operating easier and helps to balance the entire evaporator station.

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