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Heat Exchanger Technology

Wednesday, February 18, 2009

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Tubular Exchanger Manufacturers Association (TEMA) has developed a series of fouling factor for Shell and Tube (S&T). These fouling factor are generally higher than Plate Heat Exchanger (PHE), why not use S&T fouling factors to size PHE ?

Isn't this approach conservative and guarantee the performance ?

Reason being...
  • Oversized Plate Heat Exchanger (PHE) required extra CAPEX and extra Space for oversized PHE
  • Tubulence minimise fouling tendencies in correctly sized PHE. Oversized PHE results low actual velocity and increase potential fouling and inefficient heat transfer
  • HTRI studies showed PHE fouling significant lower than Shell & Tube Heat Exchanger (S&T)... factor of 6.7
GUIDELINE : Do not oversized PHE more than 25% against required area.

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posted by Webworm, 12:14 PM | link | 0 comments |

Thursday, August 21, 2008

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Plate Heat Exchanger having narrow channel most probably "not recommended" for fluid contains solid and/or slurry services. However, there are still many success story of using PHE in abovementioned services.



There are some rule-of-thumbs to gage if a plate heat exchanger suitable for fluid contains solid or slurry :

i) 80% of the particles are less than 70% of the interplate gap on the heat exchanger.

ii) 100% of the particles are less than 90% of the interplate gap

iii) Flowing velocity is below erosional velocity (causes premature plate failure)

iv) The minimum wall temperature is above a point where crystal growth is expected.

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posted by Webworm, 11:44 PM | link | 0 comments |

Wednesday, August 20, 2008

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As the price of electricity, natural gas and other fossil fuels continues to climb, chemical processors are more closely examining high-temperature operations and heat-transfer systems to see if more efficiency can be had. In many cases
it can, and, as a result, heat-transfer projects are not only justifiable, but downright attractive.

There was a dialog among a few heat exchanger specialist from Alfa Laval, Paul Muller, Exergy LLC, etc. The dialog mainly discussed on strategy to improve heat transfer efficiency, heat recovery and efficient process control during this high energy price arena.

A few tips have been present :
  • The energy crisis results high prices all of the time shorten paybacks. Energy efficient is one of the way to minimize cost
  • Energy efficient heat transfer equipment such as Plate Heat exchanger, Gasketed Heat exchanger, Bonded Heat exchanger, etc is one of the option.
  • For a service using Shell & Tube (S&T), the overall heat transfer coefficient (HTC) is around 300 Btu/h ft2°F. However, the overall heat transfer coefficient (HTC) for a compact heat exchanger can be improved 3-4 times (~1000 to 12000 Btu/h ft2°F).
  • With lower overall heat transfer coefficient, this may translate into less space, smaller installation and handling cost. Capital cost may not be low as the fabrication cost for compact heat exchanger is high.
  • Gasketed Heat exchanger good for maintenance. However shall take additional attention on the compatibility between gasket and fluid.
  • All welded or Bonded heat exchanger may be considered if there is gasket & fluid compatible problem
  • For laminar flow, heat transfer rate is only the function of fluid thermal conductivity. Operate heat transfer equipment at lamina flow during turndown could significantly reduce it heat transfer rate
  • Compact heat exchanger promote turbulence. High turbulence increase heat transfer rate and reduce fouling (read more)
  • Thus plant releasing hot exhaust gas from burner, boiler, gas turbine, etc to atmosphere may take the opportunity to recover heat
  • Improve temperature control in process system would reduce energy usage
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posted by Webworm, 9:55 PM | link | 0 comments |

Monday, July 28, 2008

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"Plate Type Heat Exchangers V/s STHE - We are planning to replace an existing Shell & Tube HE with a Plate Type HE as the existing one is not offering the desired effect. The duty of the HE is attached. Since one of the fluid handled contains solids, can a PTHE handle it? I would like to know if similar retrofits have been carried out and what are the precautions to be taken while opting for such a change ? "



First of all is to interpret "fluid handled contains solids" :

The process fluid contains solid can be
(a) high concentration, small particle size or;
(b) high concentration, large particle size or;
(c) low concentration, small particle size or;
(d) low concentration, large particle size

Above combination may end-up with different heat exchanger which best suit their own need. Thus, whenever specify solid in fluid, concentration (wt% or ppmw) and particle size (micron or mm) are required to be spelt out so that other (vendor , consultant) can provide the most cost effective solution whilst meeting process demands.

Can PHE take fluid with solid in it ?

PHE is having a very unique feature where narrow channel and proper engineered cross-sectional areas promote turbulence and high heat transfer (convective) capabilities. High turbulence has the potential of removing scale stick on the plate. However, present of solids in fluid promote pluggage in the narrow channel and ultimately results NO flow.


(Click image for better view)

What are the option can be suggested here ?

Probably first and ordinary option is relook into the root-cause of under-performed STHE by analyses the STHE construction, review maintenance frequency and methodology, etc and check if there is oppurtunity to modify existing STHE to increase heat exchange performance.

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posted by Webworm, 3:34 PM | link | 0 comments |