• ML/WT
• KT
• HLT 

A. Have an independent whirlpool process in a saved space;
 

B. Or when the boiling kettle is heated by direct fire,

the local high temperature causes bubbles that will effect the

whirlpool process, so a separated whirlpool is better.

C. Or for electric heated kettle, a separated whirlpool vessel has better whirlpool result.

Meanwhile a stacked vessel (ML/WT+KT) is cheaper than totally single WT (MLT+KT+WT).
 

But C depends on different brewers, many brewers can accept the whirlpool result
when the KWT is with heating elements inside, as the elemens inside the KWT is
round cirle shape, so not a big effection,

And the effection is more concerned in Bigger brewery.   

• MLT/HT
• KWT

• MKT
• L/WT
• HLT