This calculator assumes a single group of riser tubes(heat absorbing tubes) being supplied by a single group of downcomer tubes(non-heat absorbing tubes). It should be noted that evaporators do not contain this type of perfect risers and perfect downcomers, rather they are made up of numerous sets of riser and downcomer tubes each with different circulation and flow rate. This calculator is only for learning the processes, as it takes a more powerful computing program than can be realistically achieved with a JavaScript to complete a real life circulation analysis. Please note that this calculator is set up assuming an O-Frame HRSG evaporator with internal downcomers.