7 replies to this topic

[guoyinyanliner]

​"In a cooling water system, I have four shell-and-tube heat exchangers in cooling water service. How can I prevent flow maldistribution across these four heat exchangers

[breizh]

Perform heat and mass balances around your system (HX+ CW). Make sure you have sufficient enough water (flow rate) from the cooling water tower at the right temperature and pressure. Water should be distributed to each HX using a TCV at the inlet of each unit. The temperature at the outlet (process stream) of each HX should control the opening of each TCV .

 

I've attached a simple model produced by Stephen Hall for clarity.

 

Breizh

Attached Files

•  Cooling Water Model.xls   104.5KB   3 downloads

[Pilesar]

Cooling water flow will depend on pressure drop and flow path. A well-designed system should be able to accommodate variations in distribution. Each exchanger should have sufficient water velocity to avoid excessive rate of fouling formation.

[Pilesar]

Note that any TCV (if used) should be on the exchanger outlet (not the inlet) as shown by the spreadsheet Breizh posted. Computer software is available to help model cooling water hydraulic networks. For only four exchangers in a network, it is probably not worth learning a new program.

[breizh]

To add to my previous reply, make sure you have way to vent air which could be trapped in the shell of the HX at the start-up of the unit.

To answer to Pilesar, I've seen both, inlet and outlet of HX. Probably outlet is less troublesome.

Breizh

[guoyinyanliner]

To add to my previous reply, make sure you have way to vent air which could be trapped in the shell of the HX at the start-up of the unit.
To answer to Pilesar, I've seen both, inlet and outlet of HX. Probably outlet is less troublesome.
Breizh

"If the flow distribution is not ideal, is it a viable approach to install a butterfly valve at the heat exchanger outlet to add resistance and balance the flow?"

[breizh]

Yes, you can!

Not easy on operation point of view.

Note: Don't forget that you need to install isolation valves in case of maintenance of HX.

Breizh

[Plantcalckit.forums]

To add to the good points above — one practical approach to quantifying the maldistribution is to calculate the total pressure drop through each branch at your design flow rate. Each branch consists of: (1) piping friction loss from the header to the HX and back, (2) pressure drop across the HX itself (shell side or tube side, whichever carries the cooling water), and (3) any fittings and valves in each branch.

 

If the branches have different pipe lengths or configurations, the branch with the lowest total pressure drop will naturally take the most flow. The difference in pressure drops tells you how much restriction (valve opening) you need to add to each branch to equalize flow.

 

For quick pressure drop estimates on the piping runs, there is a free online calculator at plantcalckit.com/tools/pipe-pressure-drop/ — you can plug in your pipe size, flow rate, and fluid properties to get the friction loss per branch. And if you are sizing TCV or butterfly valves for balancing, the Cv calculator at plantcalckit.com/tools/control-valve-cv/ can help you determine the required Cv for each valve at the desired flow and available pressure drop.

 

This quantitative approach helps you decide whether simple manual butterfly valves are enough, or whether automated TCVs (as breizh suggested) are worth the investment.