Strategies to Achieve Flow Reversal for Back Flushing Heat Exchangers. - Watco Group

Strategies to Achieve Flow Reversal for Back Flushing Heat Exchangers.

Flow reversal:

an effective method to avoid the accumulation of fouling

Regular backflushing to minimize fouling build-up can be applied to various kinds of heat-exchangers. It can be achieved by patented intelligently engineered flow reversal valves with minimal pressure losses, crossover, and installation impact. Alternatively, standard 2, 3 or 4-way valves may be used for lower capex (and higher opex) applications. Read about the different strategies below.

Fouling build-up in heat exchangers connected to cooling towers is a major issue that impacts performance of the heat exchanger and (chiller) condensers. A regular and automatic backflush of the installation is a smart strategy that helps getting rid of the deposited matter before it settles and crystallizes and that maintains optimal performance through maximum heat exchange efficiency. The backflow just needs to last for a short period, approximately 15-30 seconds, and doesn’t noticeably affect the heat exchange process. The flow to the cooling tower, naturally, cannot be reversed.

Since the backflushing takes place while the heat exchanger is in operation, the cost of manpower, cleaning equipment, and standstill or production losses are minimized. Also, it extends the interval between cleaning sessions.

In plate heat exchangers, backflushing washes away debris stuck at the cooling water inlet of the unit. For shell and tube heat exchangers, the back-flush works best in combination with brushes that travel through the tubes in response to the reversed flow. They nestle in catch baskets, designed for minimal flow restrictions, at the end of each tube during normal operation.

Below we introduce several methods to accomplish such back-flush: the reversal of the flow over the heat exchanger without affecting the flow direction to the cooling tower. Each strategy comes with its specific set of merits and demerits in practical workability, complexity in technical realization and cost involved.

Flow reversal valve

Our recommended solution: the patented Eqobrush Swingbox Reversal Valve. A slightly higher investment (CAPEX) pays back in years of worry-free operation with minimal pressure loss and Cross-over of cold water to the warm water stream. 

Pressure loss only occurs during the cleaning cycle of the system, a total of about 5 minutes per 24 hours of operation. 

Alternative solutions:

4-Way valve

The 4-Way valves is difficult to fit in to piping solutions due to its shape.  

  • OPEX issue: Constant pressure loss due to piping elbows. 
  • Footprint increase
  • Limited CAPEX advantage over Flow Reversal Valve

3-Way valves

The set of 3-Way valves need to be synchronized to reverse flow without the water coming to rest over a too long period of time. 

  • Control issues, 1 defect actuator leads to system failure. 
  • Footprint increase
  • Capex (2 actuators of equal power required) 

Butterfly valves

4 Butterfly valves need to be synchronized to reverse flow without the water coming to rest over a too long period of time. 

  • Control issues, 1 defect actuator leads to system failure. 
  • Footprint increase
  • Capex (4 actuators of equal power required) 
You get what you pay for...

Aspects to consider in decision making

1. Size Limitations:

The forces applicable within the valves are directly proportional to the water volumes flowing through the pipes. They are expressed as a square function of the pipe diameter, based on constant water speed. These forces limit the application of the complex valves, such as the Eqobrush swing box flow reversal valve.
Also, the width of the valve body will, above certain pipe diameters, no longer be practical to consider. The practical limitation is set to a (cooling tower to heat exchanger or condenser) pipe diameter of 600 mm.

2. Process Management:

Obviously, when applying a solution that involves 2 or 4 valves, the coordination and synchronization become an issue. The process management system should provide precautions to avoid the water flow coming to a halt or the heat exchanger from being bypassed for an extended period.

3. Technical Realization:

The above schematic drawings make it clear that more valves require a complex pipe layout. The practical aspects of this in terms of available space as well as the cost of installation may prove burdensome, not in the least in the case of retrofitting existing installations.

4. Pressure Loss:

Simpler solutions cause lower pressure loss. Complex pipework means more pressure loss, subsequently the energy consumption of the system pump increases. This particularly applies to the 4-way valve.

5. Cross-Over:

Old fashioned flow reversal valves and standard of the shelf 2-3 or 4-way valves may come with the defect of substantial cross-over: the leaking of cool water (from the cooling tower) into the return hot water stream. This basically reduces cooling tower capacity, as the less cool water reaches the process. In practice, up to 5% of the capacity may be lost via cross-over.

6. Capex:

Simple valve solutions can be cheaper in material investment. However, this may be countered by cost of installation and the cost of the process management system.

Conclusion:

For critical processes where lots of gains can be achieved, the “short-term gain, long-term pain” principle may apply to cheap solutions. We recommend careful consideration of the best solution in terms of return on investment (ROI) and worry-free operations over a long period of time.

Reading tip on backflushing of heat exchangers:

https://www.chemicalprocessing.com/articles/2012/backflush-away-fouling/

3D pipe layout for a 4-way valve in line with a chiller condenser.

See the complexity to fit a 4-way valve into the piping configuration. Every angle involves additional pressure loss which needs to be compensated by pump power. 

 

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