Plate & Frame Heat Exchangers: Tips & Tricks for Purchasing

 

Comparison among different quotations of Plate & Frame Heat Exchangers is not so easy...There are several criteria to take into consideration.

Pressure Drop

DeltaP is the driving force to generate turbolence in PFHE and design assuming 10 mwg rather than 5 mwg as mentioned in data sheets could reduce dramatically Heat Transfer Area (HTA). Furthermore estimation of max DeltaP  available is based on thumb rule (piping plus valves plus elbows plus height plus PFHE head of the pump should be 20 mwg let's assume 30 mwg to be safe ). That means if a Vendor calculation is based on 10 mwg after installation nobody will realize that DeltaP of PFHE is higher than expected.

Connections size

General criteria is to get max 7 m/s velocity in connections, otherwise design software will select model with larger nozzles. Sometime Vendor to be more aggressive on price  is offering a smaller frame with several plates... After installation you will get whistles on connections due to high speed.

Plate material 

Typical material is stainless steel (AISI 304,316,316-Ti,316 L). Sometimes to buy 316 could be cheaper than 304 due to larger quantities used by PFHE Vendor.

316-Ti is common use in Germany, but there no significant advantage to use, because there are no weldings on pressed plates. Same concept with 316 L (low Carbon less than 3%) No weldings no need of low C conternt.

Sometimes data sheets are mentiioning materials that can not be pressed (like AlX6) 

therefore alternative material should be selected (like 254 SMO). Again larger amount of Titanium used by Vendor in Marine applications could be a cheaper option.

AHRI PFHE Certification

The Air Conditioning, Heating and Refrigeration Institute (AHRI) Standard 400 certification is an independent, third-party verification of thermal performance

To certify a product to AHRI standards, the manufacturer submits specifications and performance data to AHRI for performance evaluation and potential certification.

Therefore if a Vendor is certified there are more guarantee on performance in comparison with other Vendors.

Pressure Vessel Code

Often Vendor is mentioning in offer "according to ASME VIII Div.1" without U Stamp.

This includes mechanical calculations and materials according to the Code, but in Oil & Gas U stamp is a must and could be an extra cost if not included in the  offer. 

Futhermore delay in delivery time must be considered due to Inspector Audit in Vendor workshop and paperwork.

Spare Parts

Sometimes thare is some misunderstanding on spare parts denomination: 

" Complete set of gaskets" from Vendor standpoint is consisting of 4 nozzle gaskets for commisssioning and start up for Buyer standpoint is including 400 gaskets of installed flow plates pack...This occurs because specifications are copy & paste of Shell & Tube HX where gaskest where only on connections...It is better to clarify before Purchase Order otherwise big discussion will start.

Extra margin

Data sheets are mentioning often 10% on flowrate and 15% on surface (HTA).

But nobody is mentiong if DeltaP should increased as well. If a Vendor is calculating with same DeltaP, Pressure Drop will be the limitating factor on PFHE performance and comparison of two Vendors will be incorrect.

Plate arrangement

When Temperature profile is long theta, sometimes is cheaper to quote a multipass unit rather than a single pass with tall plate dimensions. The disadavantage of multipass is that connections are on pressure plate of frame, so that every time service is required opening plate pack will require disconnect piping.

Particle size

If process fluids are involving particles, fibers or other suspended solids, some Vendors are quoting standard channel plates (3-4 mm) mentioning that turbolence will avoid fouling while other Vendors will quote wide gap channel plates (6-12 mm) to guarantee long time operation. This could be the significant difference in prices...

Shear Stress  

API 662 prescribed that Shear Stress Tau must be higher than 50Pa in OIl & Gas applications. Not every Vendor has this feature included in  design software, therefore if PFHE data sheet is not including Shear Stress it means that Vendor is not used to work in Oil & Gas.

Connections Type

The standard  connection on PFHE is rubber lined or st.steel with stubolts. In Oil & Gas applications in the Middle East Welding Neck is required with API 662 Nozzle Load according to Table 2 Severe Conditions. If Vendors have different type of connection impact on prices will be relevant.

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