use of heated perimeter for HX calcs

[simchuck]

In the design/analysis of a double-pipe heat exchanger, is it correct to use the "heated perimeter" for the annular flow, or should the "wetted perimter" be used?

For example, I have ethylene-glycol flowing in the annular space, with water flow in the inner pipe. I am looking to find the heat transfer coefficient on the outside of the inner pipe (i.e., the ethylene-glycol side).

I have the approach - determine Re, Pr and Nu, and use the Nusselt form (Nu=0.023 x Re^0.8 x Pr^0.3). Just not sure about which perimeter to use for the Re calculation.

Can anyone give me a brief explanation of when the use each is appropriate?

 

[hxspecialist]

Simchuck,

Calculation of the annular side of the heat exchanger requires the use of the hydraulic diameter. The hydraulic diameter is equal to 4 times the cross sectional area divided by the wetted perimeter. The surface area on the annular side is the outside perimeter of the inner tube. Computation of Reynolds number, Nusselts number must be based on the hydraulic diameter defined above.

 

[daubery]

Hi 'Simchuck',

When calculating the 'Hydraulic Diameter (HD)', it will be different for heat-transfer & pressure drop.

1. HD heat-transfer - use perimeter of inner tube ie. the actual surface at which heat-transfer takes place.

2. HD friction (pressure drop) - use perimeter of inner tube + perimeter of inner face of outer tube ie. the actual surface over which friction occurs.

This is as per 'Kern'... & other reputable heat-transfer texts.

If you reuire additional help just ask.

Regards,

Des Aubery...
(adTherm Technology -

http://www.adtherm.com

- info@adtherm.com )

 

[Tremolo]

Simchuck,

For the tube outside heat transfer coefficient, the Dittus-Boelter equation depends on the Reynolds number, as you have written. The Reynolds number is used to characterize the fluid flow and should be calculated based on the hydraulic diameter = 4 x cross sectional flow area divided by the wetted perimieter, as mentioned by hxspecialist.

Also as mentioned by hxspecialist, the Nusslet number should use the same hydraulic diameter.

Tremolo.

 

[25362]

See also thread124-75085.