Fouling factor for plate & frame HXs 1

[mjcook]

I'm in a debate over fouling factors for plate & frame heat exchangers. A customer requirement is 0.005 fouling factor on all heat exchangers for a project, but one HX is a plate and frame unit, so the fouling factor doesn't apply. The plate and frame suppliers talk in terms of % excess area. The customer's engineer is trying to translate fouling factors into % XS area for the plate & frame HX, and is coming up with large numbers that the plate & frame supplier thinks are too large. It looks to me like to keep overall pressure drop reasonable, the manufacturer increases the number of passes and lowers velocities as %XS area increases, which ends up making the unit foul faster. Anyone have references/advice/experience in applying traditional fouling factors to plate & frame exchangers?

 

[speco]

Generally, plate and frame manufacturers refuse to use fouling factors. Fouling factors for shell and tube exchangers often result in and overall U (heat trasfer coefficient) which is half of the clean U. That is, the surface provided is on the order of twice the surface required without fouling. This depends, of course, on the design if the exchanger and the fouling factors used. In some cases the additional surface is not nearly this high.

The plate and frame designs have much higher overall U factors due to the turbulence in the fluid flow. For exchangers with water on both sides, this overall coefficient can sometimes get to be in the 1200 range (that's in BTU/hr-sq. ft- deg. F). If you were to factor in a .005 fouling factor one side only) into an exchanger with this high a coefficent, the U would drop to 171. If the fouling facor is applied to both sides the U would drop to 92. This may be an extreme case, but you should get the idea.

The manufacturers of plate and frame exchangers typically do no use much of a safety factor in their designs. If they use any, it is seldom more than 20% in excess surface. If they are trying to be competitive, they will sometimes cheat to the point where the design is as much as 20% undersurfaced. They always claim to guarantee the performace, by the way. If they get caught, they will just give you more plates to add to your existing undersized exchanger.

If I were specifying a plate and frame exchanger, I would suggest adding a safety factor to the heat duty required and not telling the vendors that it's in there. If you don't ask for a safety factor, they will probably not include one. If you specify a fouling factor, they will probably either ignore it or take exception to it.

A vast majority of plate and frame exchangers are single pass on both sides. To add surface, there are two choices: (1) add more plates or (2)use bigger (longer) plates.

If you add plates, the fluid velocities go down, reducing the U, and requiring still more surface. Longer plates are preferable where available for this reason, but may have higher pressure drop.

There are also different types of plates which can be used. Some plates have more ridges or ones which are more perpendicular to the flow pattern. These have higher coefficients and higher pressure drop, and are sometimes called "hard" plates. "Soft plates" have lower coefficients and lower pressure drop. The point here is that the manufacturer can usually change a number of factors in his design to accommodate both heat transfer and pressure drop, but higher U is generally accompanied with a higher pressure drop. If you want more safety factor in your exchanger, it is best to allow more pressure drop with it.