Persuading people on plate exchangers v Spirals

[mrtangent]

Dear Sirs,

I would like to ask some advice on persuading my mechanical engineer co-workers that plate exchangers reliability is not a significant issue. I have recently tried to convince my co-workers of using these but came up with the following obstacles.

Duty was oily fluid cooling from 130 to 40 deg C with process water.

1) The plates are only 0.5mm thick - so how do you know they wont stress crosion crack ?, because spirals are thicker it is thought they would be ok?
> This meant we needed to consider titanium or hastalloy - getting two complext and expensive.

2) Plates suffer from sealing issues. Theres a risk that they wont seal and contrinue to drip.

3) Plate exchangers can just "Ping" open - with the whole plate pack just opening - this is a safety consern. Because just 4 bolts are holding the pack together !. I've not heard of this but they apparently think this has happened. So with 130 deg C oil an obvious safety consern !.

4) Process water fouling. In the past at our facility we have fouled process water exchangers due to high wall temperatures (above 50°C)- with a plate this is thought to be worse - is this true ? In addition conserns are raised about on-line cleaning - it is thought that using an acid cleaner could cause localised corrision in dead spots on the exchanger ??

I would appericate any feed back or links to other sites.

Thanks



James Bruce

 

[Montemayor]

James:

I think you've laid out the basis of a qualitative comparison between both types:

1) The Spiral (you haven't said if it is closed or open, with gaskets) may not be comparable with plates because, if closed (seal welded), it may not be accessable for cleaning inside the spiral.

2) The Spiral is going to be more expensive [assuming the same wetted materials of construction (MOC)are used]. I don't believe anyone who has used both types can counter that. This is probably the strongest argument in favor of a PHE.

3) The argument about stress corrosion is invalid if both units are made of the same MOC; metal thickness doesn't infer immunity from corrosion - just time waiting for the failure.

4) You state plates suffer from sealing issues; I think you mean that gasket failure is a possibility. While that is a possibility, there are gaskets that can be selected to withstand the 130 oC oil. They may be expensive and drive up the costs - and this should be investigated. This is a valid concern with hot oil. Protective shrouds are available for protection from fluid leaks in PHEs. This may be the real argument that eliminates the PHE as a candidate - the personnel safety issue.

5) I've never seen or heard of PHEs failing open as you describe. I doubt this could happen if operated at rated loads. However, the gaskets can fail if not properly selected nor suited for the service. Gasket failure is more of a possibility.

6) Fouling on the water side should be more of a concern for a closed Spiral unit than it would be for a PHE. The PHE will normally be essentially fouling free on the water side due to the high turbulence and Reynolds Number designed into the unit for performance. The argument here goes against the Spiral, in my opinion.

I've used both and had success with both although I've experienced gasket failures due to mis-directed maintenance instructions. I love Spirals, principally because they are compact, efficient, & good looking. However, in all fairness, practical engineering often points to the less expensive and also efficient PHE. Be aware, however, that the prior statement does not include a SAFETY value! PHEs are, inherently, physically bigger in foot print, use a lot of gaskets, and are sometimes placed in awkward places for maintenance. This doesn't necessarily make them bad; it means they require more foresight and planning.

PHEs are excellent choices in warm water applications. I don't think any other type can compete with them there. When you start to increase the temperature and change the fluid, you start to increase the demands on the integrity of the gasket system and its effectiveness. This is an area that is dependent on experience and the degree of safety concern(s). Your evaluation is not an easy one because of this latter dilemma, in my opinion.

I would recommend you analyze your application with the safety aspect included and make a decision based on what your company's value for safety (after making a fair risk evaluation based on location, probability, service, etc.).


Art Montemayor
Spring, TX

 

[saxon]

james, Check with APV-CREPACO Inc. and ask for their "Heat Transfer Handbook" Design and Applications of Paraflow Plate Heat Exchangers. It covers everything you ever wanted to know plate exchangers and then some.

Hope this helps.
Saxon