Waste heat recovery

Reduced need for chilled water

Chilled water is often used as a cooling medium instead of regular cooling water in difficult condensation services. This can often be avoided by using compact heat exchangers instead of shell-and-tubes as condensers.

Compact heat exchangers offer an economical way of working with crossing temperatures and a very close temperature approach. This means a compact heat exchanger can perform required cooling with cooling water of a much higher temperature than a shell-and-tube condenser.

Eliminating the use of chilled water leads to lower operating costs, and cuts investment costs if you are building a new plant.

Example

In this example an acetone condenser cools the incoming gas from 38°C (100°F) to 22°C (72°F). An AlfaCond compact heat exchanger can accomplish this with 20°C (68°F) cooling water, whereas a shell-and-tube requires chilled water at 12°C (54°F). Figure 3.24 shows the temperature programmes when using chilled water and regular cooling water.

Switching to regular cooling water saves 200 kW of electricity. In addition, investment costs would drop by roughly €170,0000 if this was a new plant being built, since no chillers need to be installed. This compares to the estimated total investment cost for the plate heat exchanger of €172,000. For a new plant, the whole heat exchanger investment is more or less offset by avoiding chiller investment.

The diagram in figure 3.23 shows the payback period for different electricity prices. It shows a revamp scenario, i.e. reduced installation costs are not included in the calculations.

 

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Thanks to the possibilty of working with crossing temperatures and a close temperature approach, a compact heat exchanger can often use regular water for cooling wheras a shell-and-tube would require chilled water.

Results

 

Results - reduced need for chilled water

Assumptions

  • AlfaCond heat exchanger
  • 8,400 operating hours per year
  • Chilled water completely replaced by regular cooling water
  • Negligible difference in required pumping power for chilled water and regular cooling water
  • Revamp scenario
  • Cost for heat exchanger: €43,000
  • installation factor: 3
  • Total installed cost: €172,000
  • Chiller COP = 4
  • Condenser capacity: 800 kW (thermal)

Payback period

Eliminating the need for chilled water

Payback period for investment in more efficient cooler. 

Payback period (years)

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