Waste heat recovery

Reduced fouling and maintenance

Self-cleaning effect in spiral heat exchangers:

  1. Self-cleaning effect in spiral heat exchangers
  2. If a fouling deposit starts building up 
  3. The flow velocity increases over the deposit
  4. The deposit is scrubbed off and performance is not affected
  5. Performance stays at an optimum level
     

Shell-and-tube heat exchangers gradually lose efficiency in applications with heavy fouling. The result is lower energy transfer and high maintenance costs.

Spiral heat exchangers on the other hand have a self-cleaning design, making them much less prone to these problems and very suitable for handling highly fouling fluids. Examples of where spiral heat exchangers are used include oil refineries, pulp and paper mills, mineral processing plants, and petrochemical plants, typically in services most suffering from fouling.

Example

Oil refinery visbreaker cooler/interchanger

This example is based on input from a European oil refinery, where spiral heat exchangers replaced shell-and-tubes in a visbreaker cooler service (interchanger) in an existing plant. There are usually multiple heat exchangers operating in parallel in this service, but this example shows a one-to-one comparison between the two technologies.

The increased heat recovery in this case originates from two factors; reduced downtime and better performance due to less fouling build up. Reduced fouling also means reduced cleaning frequency and lower cleaning costs.

The two alternatives in this example represent two different maintenance philosophies; making few or many stops for cleaning.

Improved performance and reduced cost when using spiral heat exchanger

Reduced fouling

Assumptions

  • Heat transfer at startup = 1 MW (3.4 MMBtu/h)
  • Operating hours per year = 8,400
  • Working days per cleaning = 1.5
  • Cost per day for cleaning = €4,000
  • Installed cost of spiral heat exchanger = €200,000

Shell-and-tube cleaned twice per year

  • Annual cleaning cost = €12,000
  • Average performance loss = 30%
  • Annual heat recovery loss due to cleaning = 0.86%
  • Total performance loss = 31% or 0.31 MW (1.1 MMBtu/h, 0.21 barrels of oil/hour)

Shell-and-tube cleaned 12 times per year

  • Annual cleaning cost = €72,000
  • Average performance loss = 7.5%
  • Annual heat recovery loss due to cleaning = 5.14%
  • Total performance loss = 12.5% or 0.125 MW (0.43 MMBtu/h, 0.1 barrels of oil/hour)

Spiral heat exchanger cleaned once per year

  • Annual cleaning cost = €6,000
  • Average performance loss = 2.9%
  • Total performance loss = 2.9% or 0.029 MW (0.10 MMBtu/h, 0.02 barrels of oil/hour) 

Reduced fouling in the self-cleaning of spiral heat exchangers

The self-cleaning design of spiral heat exchangers make them the ideal choice for fouling duties.

Shell-and-tubes versus spiral heat exchangers

Performance in heavy fouling services
Heat transfer efficiency for spiral heat exchangers and shell-and-tubes in oil refinery visbreaker cooler.

Click on the image to see a larger view

 

Payback period

Replacing existing shell-and-tube with spiral heat exchanger

Payback period when changing from shell-and-tubes to spiral heat exchanger. 

Payback period (years)

Click on the image to see a larger view