10 Big Mistakes to avoid in Plate Heat Exchanger calculation and material selection

In recent years use of Plate Heat Exchangers increased more than other type Plate & Frame Heat Exchangers Market, therefore it's time to focus on mistakes to avoid calculation and material selection.

 #1 Selection of connection size: typical nozzle velocity with water is 5.5 m/s. A major part of PHE design software switches to a larger size if it is higher.

Standard velocity in piping is 2-2.5 m/s so that size of connections is normally smaller and conic reductions are often required. Don't select automatically PHE with connections the same size of piping: double-check velocity in nozzles!

#2 Fouling Factor has been a topic of discussion for years. PHE FF must be 1/10 of Shell&Tube HEX FF as finally API 662 recommended. There is confusion among process engineers on FF to be used in PHE and often EPC Contractor's data sheets are mentioning FF for S&T HE's. Investigations demonstrated that these values are not giving good results in PHE since they often result in oversized units with premature fouling due to reduced velocity and turbulence.

#3 Plate pack size must be considered from a maintenance standpoint. Many times to reduce the impact cost of large frames, the selection is a PHE with plates pack of 400 plates... easy to sell not to open and close!
#4 Diameter of channel plate required is often underestimated. Of course, a pressing depth of 2.2 mm is more efficient than a 4.4 mm, but if an application in Oil & Gas in a refinery in the Middle East long life of PHE is a priority.
If solids or fibers are present, maybe a free flow plate is required channel 11 mm

#5 Pressure drop is a driving force in PHE's creating turbulence to get high U value. In calculation, DP can dramatically change the Heat Transfer Area required.

Furthermore, Process Engineers specified sometimes DP for S&T HE's while typical DP in PHE is 0.7 bar.
Before provide calculation double-check if DP could be increased!

#6 Plate material selection is related to the fluids involved. Stainless steel is common material with water, but often Chloride level ppm is underestimated

316L is not required in PHE because there are no weldings, but 316 instead of 304 can be useful in many applications. Brackish water can require 254 SMO while seawater requires Titanium!

#7 Gasket material is a weak point of PHE especially in case of multipurpose use (like in Pharmaceutical). Try to find a gasket material suitable for different fluids is not so easy... Fluid Compatibility Charts
A wide range of materials is available Viton® Extreme, Aflas® TFE/P, Viton® FKM, EPDM P.C., Buna-n, Silicone, NBR, etc. but if you are in trouble don't hesitate to select a semi welded SPHE or fully welded WPHE!

#8 Heat Recovery is one of the main reasons to select a PHE rather than other types of Heat Exchangers: countercurrent flow, LMTD=1 possible, etc. The LMTD is defined by the logarithmic mean as follows:

where ΔT is the temperature difference between the two streams at end A, and ΔT is the temperature difference between the two streams at end B. With this definition, the LMTD can be used to find the exchanged heat in heat exchanger:

Where Q is the exchanged heat duty (in watts), U is the heat transfer coefficient (in watts per kelvin per square meter) and Ar is the exchange area.
If duty is not District Cooling, industrial applications cost/benefit better ratio in PHE is usually when LMTD is equal to 5.
Therefore if Process Engineer is asking for LMTD 3 show always LMTD 5 option: CapEx will result in more convenient!
#9 Plate Pack mixing: with different chevron angle required thermal length can be achieved (NTU) with consequent minor Heat Transfer Area.

This is good for calculation...what about maintenance? How plate assembly will be correct in the field if mixing is 72/28? Double-check increase of HTA in case of 100% plate H theta or L theta and 50/50 (M channel). Maintenance Manager will be happy and will spread the idea that your PHE's are easy to assembly...

#10 Pressure Vessel Code in Oil & Gas application typical requirement is ASME VIII Div.1 with U stamp and nozzle load calculation according to API 662 table 2 severe conditions. In PHE design this includes extra costs for material (SA-516 Gr.70 for the frame, SA-193 Gr.B7 for bolts, SA-194 Gr. 2H for nuts, etc.) even if you mentioned according to ASME VIII Div. without U stamp.

Verify on data sheets required code, otherwise, you will in trouble!

Taking into consideration these 10 Big Mistakes to avoid you will be ready to design every kind of Plate Heat Exchangers....good calculations!