Equal, uniform distribution of liquid to all tubes is paramount in order to operate as designed. Individual tubes mustbe wetted evenly around their circumference.
Various strategies exists in order to achieve an even liquid distribution over the entire tube sheet. They are very much tied to the experience of manufacturer and process designer of heat exchangers. Some practices are outlined below.
Orifice plate distributors guide the liquid to individual tubes as shown(1),(2)
Another common means of achieving uniform circumferential distribution is to allow the tubes to protrude above the tube sheet. The tubes are slotted in equal distances over the circumference. Liquid is introduced over the circumference over specially designed weirs.
In other applications for each individual tube special conical distributors are designed in order to achieve uniform distribution.
Minimum Tube Wetting Rate
In order to operate the evaporator under stable operation conditions a minimum wetting rate has to be maintained throughout operation. If the minimum wetting rate is not achieved, rivulets separated by dry areas will be formed. This results in dry patches, reducing the heat transfer area and increasing the risk of fouling. Research shows that film break-up has an hysteresis effect, this means stable films are maintained with lower flow rates compared to film which are formed on formerly try surfaced.
In order to apply minimum flow conditions it is paramount that the tubes are aligned perfectly vertical.
(1) MINIMUM WETTING AND DISTRIBUTION RATES IN
FALLING FILM EVAPORATORS
(2) MINTON, P.E. Handbook of evaporation technology. Noyes Publications, Park Ridge,N.J., 1986.