Heat Exchangers: Fundamentals and Design Analysis
Condensers with in-tube condensation
Prof. P. K. Das.
Mechanical Engineering
IIT KHARAGPUR
1
In-tube condensation:
A large number of condensers allow condensation of vapour inside the
tube. Air cooled condensers as used in air conditioning plant of an
automobile is an example.
vapour Condensation inside the tube is much different from condensation
outside. from outer surface of a tube condensate film is drained by
gravity. Gravity drainage of condensate is possible for vertical
configuration of the condenser. However, the condensate film becomes
thicker as it flows down and restricts the flow passage. This may limit the
length of the condenser. The counter current flow of vapour and
condensate may also cause some problems.
In horizontal tube condensate movement is possible by vapour shear.
Different regimes of condensate and vapor distribution may be seen. This
condensate is shown in the next slide.
In-tube condensation in vertical tube
Prof. P. K. Das
Department of Mechanical Engineering 2
Prof. P. K. Das
Department of Mechanical Engineering 3
In this case the average heat transfer coefficient is
determined employing Nusselt’s theory of film
condensation.
Ø
- Φ - Φ -
htotal = hΦ + 1- hB
π π
- -
Since due to thick through hΦ >> hB
(a) (b)
- Φ-
htotal = hΦ can be approximated
π
In tube condensation in horizontal tube
(a) Idealized (b) Actual
Prof. P. K. Das
Department of Mechanical Engineering 4
This heat transfer coefficient can be evaluated as
14
- ρ2gh k 3
hΦ =F(Φ) L fg L
μL ΔTsatd
F is a function of Ø depending on flow rate as
34
π Φ sin2(π-Φ)
F(Φ)=0.728 +
Φπ 2π
Which is valid for
π
Φπ
2
Prof. P. K. Das
Department of Mechanical Engineering 5
14
- ρ2gh k 3
htotal =0.728ε L g L
μL ΔTsatd
Where ɛ is the void fraction given by
-1
1-n* ρ 2 3
ε= 1+
G
n* ρL
Annular or bubbly flow: in this region heat transfer coefficient can be calculated from
12
τ 1
h= w ρL CPL +
ρL T
where T+ =8.5Re0.1Pr0.57
M
Re= Where b= tube circumference
bμL
Prof. P. K. Das
Department of Mechanical Engineering 6
Some comments on Intube Condensation
1.Condensation in horizontal tubes may involve partial or total condensation of the
vapor.
2. Depending on the application, the inlet vapor may be superheated, equal to 1.0 or
below 1.0.
3. Hence, the condensation process path may first begin with a dry wall
desuperheating zone, followed by a wet wall desuperheating zone, then a saturated
condensing zone and finally a liquid subcooling zone.
4. The condensing heat transfer coefficient is a strong function of local vapor quality,
decreasing as the vapor quality decreases.
5. The condensing heat transfer coefficient is also a strong function of mass velocity,
increasing as the mass velocity increases.
6. Opposed to external condensation, intube condensation is independent of the wall
temperature difference (Tsat-Tw) for most operating conditions, i.e. except at low
mass flow rates in stratified types of flows.
Prof. P. K. Das
Department of Mechanical Engineering 7
Thank You!!
