Page 65 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

3.2 Design 61

2
4fG L i 1
i
2gr D i f
DH f;i ¼ 2 (3.18b)
i
Minor pressure losses due to the entrance and exit effects and return bends of each hairpin are
usually estimated in terms of velocity heads. For inner pipes of double-pipe exchangers connected
in series, the bend pressure loss is usually negligible, but the same may
be significant for the annuli.
In an exchanger with N HP number of hairpins connected in series, the
Bend pressure drop
total pressure drop due to direction change is

ð2N HP 1ÞV 2
o
DH f ;o;bend ¼ (3.19)
2g
Where V o is the velocity of the outer fluid.
Inner pipe: With the inlet and exit piping aligned with the inner pipe, the entrance and exit losses
can be neglected. However, in multi-tube exchangers, the losses at the
two tube sheets are taken as one tube velocity head per hairpin for tur-
bulent flow.
Entry and exit losses

Annulus nozzle entry and exit losses are accounted as e
Laminar flow: For Re 100, three velocity heads for head loss in the entry and the exit nozzle
together. For Re < 100, the loss depends on Re.
Turbulent flow: one velocity head for the entry and 0.5 velocity head for the exit nozzle.
For exchangers with internal return bends, nozzle head loss is given by
2ðN HP ÞV 2 n
DH n ¼ ; for turbulent flow (3.20a)
2g
4ðN HP ÞV 2
n
DH n ¼ ; for laminar flow and Re > 100 (3.20b)
2g
Where V n is nozzle velocity.
In case the exchanger has external return bends, the pressure drop is double of the value estimated
by Eq. 3.20.
The total pressure drop in the annular section is:

DP o ¼ DH f;o þ DH f;o;bend þ DH n r g (3.21a)
o
Total pressure Drop

and in the inner pipe is: DP i ¼ DH f;i r g (3.21b)
i