How to vaporise liquid in a PSHE

Typically, evaporation takes place on the shell side of the Vahterus heat exchanger, where variable nozzle size is possible, and where there is more space for the vapour created. This is important because vapour has greater volume than liquid. The Vahterus PSHE can be customised according to the specific volume flow of each evaporation duty, making it unique among plate heat exchangers.

In some cases where it is seen as beneficial, evaporation takes place on the plate side.

Evaporators most often have vertical heat transfer plates, so the gas can rise up naturally, especially where natural (gravity) circulation is utilised.

Different evaporating duties

External circulation

A typical process is ‘external circulation’, where liquid is fed from the bottom of the shell and a mixture of liquid and gas exits from the top of the evaporator. Liquid is fed from the external drum or receiver that has sufficient static height to allow natural circulation to take place. The heating medium can be water or glycol, amongst other liquids; condensing gases are also frequently used.

Typically, such units are found in refrigeration plants in the form of flooded evaporators, or next to a distillation column working as a reboiler.

Internal circulation

Evaporators can be installed in a system where only gas can exit from the top. These evaporators have the liquid height inside the shell and the evaporation is known as ‘pool boiling’. The plate pack is submerged in the liquid and is heated by liquid, gas or condensing gas.

Such units are typically found in refrigeration plants in the form of a combined type evaporators, and in the process industry in the form of steam generators or next to distillation columns working as a kettle-type reboiler. Kettle-type reboilers or steam generators offer multiple solutions to meet the customer’s process demands.