Chances are the manufacturers used a falling film evaporator.
Falling film evaporators are especially popular in the food industry where many substances are heat sensitive. A thin film of the product to be concentrated trickles down inside of heat exchanging tubes. Steam condenses on the outside of the tubes supplying the required energy to the inside of the tubes. Calculating pressure drops in falling film evaporators has been investigated since the late 1940's. A universal equation is really not agreed upon.