PLATE HEAT EXCHANGER: AN OVERVIEW
Wrote by Micelli Camargo
This kind of heat exchanger, as the name says is built with plates, most common with stainless steel.
the sealing between the plates is made with a part called gasket that can be glued or cliped, as we call "clip-on gasket".
In the animation the fluids move across the plates in opposite direction, while one "goes up" the other "goes down".
When this happens, the heat exchanger is classified as "Counter-current". Otherwise, as "parallel-current" or "cocurrent", in another words, or both fluid "goes down" or "goes up".
Under the thermal point of view, the counter-current exchanger are more efficienty.
In the heat exchanger, conduction and convection are the most significant forms of heat transfer. Conduction is the heat that passes through the plates and convection, we have fluid movement.
The conduction depends on building materials while the convection significantly depends on the fluid flow, mainly, if the flow is laminar or turbulent.
Laminar flow (LF) is "softer" happening as a kind of layered flow. It happens at lower speeds.
Turbulent flow (TF) is "crazier" or "more chaotic". It happes at higher speeds and it is more commons in industrial environments.
TF promotes more easily the homogenization of the coldest and hottest parts of fluids, so it become being therefore more efficient. Because this, for a heat exchanger, a turbulent flow is desirable.
For a given thermal load, plate heat exchanger are more compact and thermal expansion (increase in capacity) is easier to achieve than shell-tube exchangher. Just increase the number of plates.
However, increasing the number of plates in the heat exchanger, without changing the flow rate, will not improve the overall thermal efficiency of the exchanger.
By placing more plates, the velocity through the plates will decrease and the type of flow may be changed from turbulent to laminar flow.
As a consequence, the heat transfer by convection will decrease significantly, which will change the GLOBAL HEAT TRANSFER COEFICIENT of the heat exchanger and, consequently, the heat exchange.
In conclusing, if you want to increase the heat exchanged, you must, in addition to increasing the amount of plates, change at least, one parameter, for example, the inlet or outlet temperature, or flow rates.
In another hand, under the hydraulic point of view, the turbulent flow has a greater pressure drop than the laminar flow.
Consequently, the pump that you will need to promote the fluid movement, must be as larger as the severe turbulent flow that you have.