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Water Side Pressure Drop in Shell and Tube Heat Exchangers

Posted August 26, 2014 1:00 AM by larhere

In previous posts I have presented the methods and equations used to design flooded evaporator and water cooled condenser tube bundles for specific heat transfer conditions. These water chiller heat exchangers operate with water on the tube side and refrigerant on the shell side. They are designed for water velocities in the range of 3 to 10 FPS. The water flow rate is usually one of the design inputs.

Both heat transfer performance and water pressure drop increase with increased water velocity. It is important to design for the highest water velocity that will have an acceptable pressure drop in the application. The method for calculation of pressure drop is shown below.

For the tube bundle, determine the water flow area.

Af=Nt*(π*Di2/4)/Np

Where

Nt=number of tubes

Di=inside tube diameter, ft

L=length of tube, ft

Np=number of passes

For internally enhanced tubes, a good approximation of L is the length of the enhanced surface, as that will be the majority of the friction.

Water side pressure drop (ref ASHRAE Handbook-HVAC Systems & Equipment, Chapter 35):

∆P=Np*(KH+f*L/Di)*ρ*V2/2*g

And

V=GPM/(448.8*Af)

Where

∆P=pressure drop, psi

Np=number of tube passes

KH=entrance & exit flow resistance & flow reversal coefficient, number of velocity heads (V2/2*g)

f =friction factor

ρ =fluid density, lb/ft3

V=fluid velocity, fps

g=gravitational constant=32.17 lbm*ft/(lbf*s2)

For smooth tubes the friction factor, f, can be obtained from the Moody Diagram.

For internally enhanced tube and Reynolds Numbers greater than 20,000 the friction factor is usually in the form

f=C* ReD

where

Re= V* Di* ρ/µ

and

µ=viscosity, lbf*sec/ft2

The coefficients C and D can be provided by the enhanced tube manufacturer.

Flooded evaporators and water cooled condensers are major components of water chillers. As such they represent a large part of the chiller cost. The tube bundle design determines the length of the chiller and is a major factor in its width.

Using the heat transfer and water side pressure drop calculation methods, designers can perform iterative analyses to vary the tube bundle lengths, number of tubes and number of passes. This will result in the heat exchangers with the desired performance, lowest cost and chiller dimensions that best meet the requirements of the market.

Editor's Note: CR4 would like to thank Jim Larson, GEA Consulting Associate, for contributing this blog entry.

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