2 replies to this topic

[STEVENLEE]

Dear All,

I am completing a specification sheet for heat exchanger application. I am needing help on determining the TEMA Type for the heat exchanger. The heat exchanger will heat compressed air at 85 psig from 60 *F to 140*F using saturated steam at 150 psig. I would appreciate your thoughts and suggestions. If you need more details, let me know.

Thanks.
STEVEN

[Art Montemayor]

STEVEN:

Depending on the rest of the basic data, I am prone to NOT specify TEMA construction for a compressed air application. Allow me to explain.

You have not told us the flow rate of each stream, whether either of the two fluids is fouled, dirty, or contains possible fouling contaminants, the allowable pressure drops, orientation, value placed on exchanger foot print, maintenance accessibility requirements, etc.

I doubt you have a fouling problem with either fluid. I also doubt you have a relatively large air flow rate – say larger than 300 Scfm. If you do, let us know.

If the two assumptions made above are true, then I would definitely not apply a TEMA type of design. I would apply a spiral or helical type of tube arrangement – such as that designed and made by Graham in their Heliflow exchangers. I cite my reasons and the logic employed below:

• Regardless of whether fouling is expected or not, the air should be place in the tubes. If there is no fouling the need for tube clean-out becomes academic and of no practical value;
• TEMA type exchangers are relatively more expensive because of their construction and the standards imposed on such construction. A Heliflow exchanger will not only be less expensive in Stainless Steel tubes, it will also be much more efficient and thus, occupy less foot print and space volume. The Heliflow is also routinely found to be “on the shelf”, and their fabrication time is much less than most other exchangers.
• The Heliflow can be mounted in a variety of positions without jeopardizing its efficiency or operation;
• The natural, spiral layout of the Heliflow tubes allow for natural tube expansion without any stresses or thermal forces to act on them or on the rest of the exchanger; the tubes expand naturally and do not impose any excessive stress on any component;
• You will find that a Heliflow unit will occupy approximately less than half of the volume space required by a TEMA unit. There are less components, but the spiral flow inside the tubes lends a much higher film coefficient to the heat transfer operation.
• The Helifow unit is bolted together and can be disassembled for inspection and shell cleanout – just like a normal BEU TEMA unit – except that you don’t need overhead cranes or lifting equipment to handle a normal sized Heliflow;
• The pressure drops that I have experienced in a Heliflow are comparable or frequently LESS than those obtained in a TEMA unit due to the fact that the change of direction on the tube side is smooth and gradual for the Heliflow, while the TEMA unit undergoes straight 90 and 180 degree turns;

If you are decided on applying a TEMA type of exchanger – heater, in this application – then I would recommend that you apply a BEU unit. A simple U-tube arrangement should do nicely and be much less expensive than any other TEMA type. Unless you come up with a fouling fluid, I see no reason for any other type. The air, of course, should remain on the tube side.

I suggest that you download my Specification Sheets Excel Workbook that I have introduced in one of the Forum threads – I forget which one, at this moment – and that you fill in the Specification Sheet for your exchanger in the appropriate Form I provide. Then you can upload the filled-in form into one of the posts in this thread so that we can quickly and efficiently peruse through the application that you have before you.

All specification sheets should be done on an Excel Form – much as I’ve done. This is much more efficient for engineering work because it allows for accurate notation, quick compilation of data, and easy transmittal and sharing of the resultant specification across a world-wide, recognized platform. Feel free to ask me any questions on the Specification Sheet and also look for and download the TEMA type Excel workbook that I also uploaded into this Forum some months ago. There, you will find some useful tips on exchanger applications.

Await you response.

[STEVENLEE]

Art,

You supplied a wealth of information. Allow me to include the missing heat exchange variables in my first posting. The maxiumum compressed inlet air flow rate is 336 scfm (10,443 lb/hr) @ 85 psig. To raise the compressed air temperature from 60*F to 140*F it will require 243 lb/hr of sat. steam @ 150 psig (based on a latent heat of 858 BTU/lb and steam inlet temperatrue of 366*F). It would be safe to assume that fouling would not be an issue for this application.

I visited Graham website, and the Heliflow exchanger may work for this application. I will check the Heliflow's heat transfer coefficient for steam/gas and compare it with other heat exchangers such as the U tube exchanger. Overall the heat tranfer coefficient is relatively low for steam/gas applications. When specifiying the Heliflow exhanger, I will disregard the TEMA type and focus more on the ASME code requirements. As you suggested, I plan to upload your spec. sheet and include the data related to my application so that everyone will have better understanding of the the heat exchanger application I am working on. Thanks for steering me in the right direction.

Steven