Heat Exchanger World Magazine September 2023 | Page 28

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Thermal Design

Suspension

Inlet / outlet

Distribution area

Leak chamber

Main heat transfer area

High theta plate

Heat transfer plate design

Maldistribution Plate heat exchangers are known by efficient heat transfer since their compact structure . Flow distribution area is the guide for streams flow through channels . The fluid flows by this guidance to outlet ports . Yet this path might result a problem for viscous fluids . Maldistribution is our decisive parameter in such cases . Definition of the maldistrubution is blind spots exist in a plate . A high percentage of maldistrubtion with the fluids having viscosity over 20 cP may cause far less level of heat transfer than required .

vc

* 2 2

× � Nc × Ach md % = ( ×

2 × �pc

( Aport

(

Wall shear stress and fouling relation Wall shear stress is the force the fluid exerts on the plates to keep the molecules suspended in the flow . Naturally , it increases with the increasing presssure losses . On the other hand , the nature of fouling depends on the working conditions and must be taken into account . Fouling factors are used all around the world , and are provided by different associated organizations and institutions . The traditional fouling factor is '' 0.0005 '' and it will cause 30-40 % increase of the effective heat transfer area . This might seem safe and effective , but it will result in an increased number of channels . As an outcome , velocities will decrease and the vortex effect will become less impactful . Besides usage of excess area , a more useful way to prevent fouling is to check shear stresses on the wall . As a numerical limit value , 50 Pa would be enough to design a heat exchanger able to self-clean with the sweeping caused by the high shear stress on wall . If the fluid is so viscous that it is not possible to reach this value , then our minimum level of shear stress should be at least 30 Pa . Higher shear stress means higher self-cleaning capabilities . Also , another topic to consider in this situation is operating the heat exchanger according to its design parameters . Operating the heat exchanger with half capacity will cause

Low theta plate

( fouling over time since the channel velocities become lower and self-cleaning is less effective .

* Sh ( wall ) = �pc × b 2 × Lp

Differential pressure Differential pressure is the pressure between the opposite sides of the plate . A heat transfer plate holds steady while fluids flow through each side without mixing . During continuous flow , the forces applied by fluids effect the plate from both sides . Alongside the working pressure values , the difference between the values is critical as well . From a general perspective , we could say that the limit for differential pressure is 5 bar for a standard liquid-to-liquid heat exchanger , but it is not possible to determine for sure without knowing plate geometry and material . A designer must consider these factors or it could lead to fatal effects for the heat exchanger , such as bending of the plate .

Channel arrangement Plate heat exchangers have numerous types of configurations thanks to having low and high theta heat transfer plates , chosen for whether they gain more heat transfer or fewer pressure losses . A channel formed with both high theta plates has high heat transfer , and the same goes for L theta plates . A formed channel has lower pressure losses in this case . When high theta and L theta plates form as a channel , the channel has mixed characteristics of both and is referred to as the M channel . The channel arrangement depends on process data . The designer should consider permissible pressure drops , temperature profile , viscosity of fluids , phase changes , and many other factors , in order to deal with possible future problems . To choose the best configuration , an optimization program would be useful . An optimization algorithm can be easily created since our main goal is lower the costs . To do that , the algorithm should minimize the number of plates among all possible configurations created with a combination of different plates and passes . Also another topic should be mentioned is channel arrangements importance for after sales works . Complex channel arrangements would cause complications for customer and cause problems for periodic maintenance . If there is not other obligation it is desirable to have channel arrangement for full of H theta or L theta channels . If it is not possible , channel arrangement should be symmetrical for M , H or L theta channels on primary and secondary circuits

28 Heat Exchanger World September 2023 www . heat-exchanger-world . com