Concepts
in Condenser System (Vacuum Equipment)
Condenser System for vacuum creations and its types with design
criteria
Condenser system and vacuum equipment are common to evaporators and
vacuum pans. Nearly all these vessels use direct contact condensers, where the cooling water comes into direct contact with
the vapour to be condensed.
Creation of Vacuum:
a) In
a condenser, we condense
large volume of vapour and
it will only produce a comparatively small volume of water or (condensate) and this water runs out through the
bottom of the condenser down the barometric leg with the waste water.
b) Therefore
if we have condensed the large
volume of vapor into small volume of water in a condenser, the remaining volume or area must be a
vacuum(reduced). Thus we have created
a vacuum.
c) With the aid of an air pump or other, vacuum is produced
in an enclosed vessel called a condenser, which communicates with the vessels to be maintained
under vacuum.
d) Cold
water is pumped in to the condenser to
ensure condensation of vapour coming from pan
or multiple effect evaporator. The
Condenser is placed at a height that
the water after condensation flow out by
gravity together with condensed vapours. The Condenser is a
barometric chamber extended at bottom by a barometric column
dipping into a well open to atmosphere.
The Barometric Column :
There are two dimensions of importance in the
barometric column
Height of the barometric column
Cross-section of the column
Height of the barometric column :
The height of the column represents the sum
of 3 terms:
Ho = head of water corresponding to
the maximum vacuum in the condenser
h = head necessary to give the desired downward velocity of the water
s = margin of safety as provision for abrupt variations in level of the
water in the column.
H = Ho + h + s
Ho is the minimum
head of water required to have maximum vacuum in
condenser. The height of the column should be of the order of 10 (instead of 76
cm): (Vacuum in mtr of Hg x Density of mercury/ Condenser inlet water density)
0.76 m X 13.6 = 10.33
m where 13.6 = density of mercury relative to water.
h =The head necessary to
give desired velocity of water
h= (1 + a) ( V2 / 2g)
h = head of water, in mtr necessary to maintain flow in the
column, at velocity V
V = velocity of flow in the column, in m/sec
g = 9.8 m/sec2
a = coefficient,
The values of coefficient given by Hausbrand as follows
|
Column |
Diameter
of the column in mm |
||||
|
50mm |
100mm |
200mm |
400mm |
800mm |
|
|
Straight |
6.1 |
2.6 |
1.2 |
0.6 |
0.4 |
|
With Double 90o bends at base |
6.7 |
3.2 |
1.8 |
1.2 |
1 |
S is the Safety
margin for sudden variations in vacuum or flow in water. It
may take in general 0.50 mtr.
Cross-section of the column:
The cross-section of the barometric column
requires simply to have sufficient cross-section to assure discharge of the
water.
D =
D = diameter of the column, in mtr
V = velocity of flow of water in the column, in m/sec
Q = weight of vapour to be condensed, in M3/sec
W = ratio of weight of injection water to weight of vapour
condensed,
According to Peter rein More simply,
following the form of equation
D > 1.116 x (Qw )0.4
Qw = Total Water and vapour flow rate in
condenser in M3/sec.
The velocity of water
in the barometric column is maintained at 2 to 3 mtr /sec. So that the air bubbles
do not escape and rise in condenser. They
should pass along with water through tail pipe. For single entry type
condensers it may take upto 4 to 5 mtr/sec.
Types of Condenser System :
From the extraction of the air point of
view the condensers can classified as
Wet air condensers : Condenser with combined vapour and air
extraction.
Dry air condensers : Condenser with separate arrangement for air
extraction.
In Sugar Industry generally used the
following types of condensers
a) Barometric condenser
b) Multi-jet condenser
c) Single Entry Condenser
Barometric condenser
a) In
Barometric condensers having two types. They are Co-Current barometric condenser and Counter Current barometric condenser.
b) Co-Current, barometric condenser water and vapour are introduced to the condenser from top and the worm water is let out removed from the
bottom.
c) Counter
Current barometric condenser. water and vapor are introduced in opposite direction. Baffles with perforation are provided to ensure more
contact time and surface between water and vapour
d) Compared
to the co-current type, counter current type barometric condenser works more
efficiently.
Height of the condenser:
The condenser is a device for exchange of
heat between vapour and cold water. The exchange will be the more complete as
contact between these two fluids is more intimate.
Height of Condenser
is decided as per the time that
is necessary for the water to remain
in contact with steam. While proper arrangement of baffles in
condenser to increase the contact area than the overall height of 3.65 mt ( 12
ft) is sufficient .
Cross-section of the condenser.
The cross-section will depend directly on the
quantity of vapour to be condensed.
S = 0.16 m2 / ton
of vapour to be condensed per hour. where S =
horizontal cross-section of the body of the condenser.
Condenser volume :
The condenser volume of 0.75 m3/ 1000 kg of vapour is recommended however 0.5 m3/ 1000
kg vapours can be considered as a reasonable figure for rain
type condenser. however, has determined experimentally that 0.6 m3/ 1000kg per
hour was sufficient in most cases. In some designs provide maximum effective contact area of water to
vapour in that design it will take 0.4 to 0.3 m3/ 1000kg per hour.
Also can be cross checking by Volume of condenser =
Cross sectional area x Height of the condenser
The cone at the bottom of
the barrel should have a slope of 70° to the horizontal.
Diameter of cooling water pipe :
The cooling water may be supplied by a pump.
Its effective head, h, will
be equal to the geometrical head at the entry to the condenser, increased by
the vacuum in head of water. We have then:
V = a x
V = water velocity in the inlet pipe, in m/sec
a = coefficient depending on the length of the pipe, its
bends, valves and other obstructions to flow. In general, ” a ” is of the order
of 5
g = 9.8 m/sec2
h = head of cold water, at entry to condenser, in mtr (h
= pressure Of water at inlet X head at mean sea level)
Multi Jet Condensers ( Wet air barometric condensers) :
Principle :
Jet condensers were first placed on the
market by Schutte – Koerting about 1930. They are based on the dynamic effect of jets of water which, penetrating into the body of water in the
barometric column, enter with
them, by friction, the air contained in the condenser. If the cross-section of the barometric column is
small enough to ensure a suitable velocity, the bubbles of air do not rise into
the condenser and are evacuated to the well at the foot of the column.
a) The main difference with the Multi Jet
condenser is that, it does not require air pump, and although
it requires more water than the dry air type condenser, it is more economical
to operate and maintain.
b) This
type of condenser has high pressure water jet nozzles fitted in the lower
section of the condenser, directed straight at the outlet or barometric leg.
The vapor inlet to the condenser is from the top of the condenser much above
the level of the jet.
c) The
nozzles are called as jet nozzles and they create vacuum in the system. The jet
is given such manner, that the jet of water will flow exactly through the
center of the tailpipe. On the top portion of the jet box another set of
nozzles is fitted circumferential through which the flow of pressurized water
is flowing towards the center of the box. This part of water is responsible for
the condensation of vapour, which is called as spray nozzles.
d) As
against the dry air type, the Multi Jet differs (wet type) in principle that
due to the high velocity of water passing down the barometric leg, air or gas
bubbles will not rise but instead it will be drawn away through the outlet with
the waste water.
e) The
pressure of water in both the parts to be employed as between 0.5 to 0.7 kg/cm2
for observing the pressure the gauges are fitted to inlet pipes. The proportion
of the quantity of water is in ratio of 40% water to jet and 60% to spray.
Diameter of spray nozzle (Ds)
Ds =
As (area of the each spray nozzle ) =
Qs / Vs
Qs = Quantity of water per each spray nozzle
in M3/Sec = Quantity of water / No. of spray nozzles
Vs = Velocity of water at nozzle in m/sec
=
h = Pressure of water at spray nozzle in
kg/cm2 x head at mean sea level
Diameter of jet nozzle (Dj ):
Dj =
Aj (area of the each jet nozzle) = Qj /
Vj
Qj = Quantity of water per each jet nozzle in
M3/Sec = Quantity of water / No. of jet nozzles
Vj = Velocity of water at nozzle in m/sec
=
h = Pressure of water at jet nozzle in kg/cm2 x head
at mean sea level
Condenser Vapour pipe dia :
Quantity of vapour to condensed (Q) = Heating
surface of the equipment x Evaporation rate.
Cross sectional area of the vapour pipe =
Quantity of vapour in M3/sec / Velocity of vapour in m/sec.
Condenser Diameter :
Cross sectional area of the condenser =(
Quantity of vapour in M3/sec + Quantity of inlet water to the condenser in
M3/sec) / Velocity of vapour in m/sec.
Single Entry Condenser:
a) Single
entry condensers are having only one water distributing
box for spray and jet nozzles. Out of
total water about 60 to 80 % water is used
at Spray nozzles and 20 to 40 % water for
jet nozzles.
b) Vapours
are condensed by forming fine mist
inside the condenser, increasing surface area
of contact and finally minimum requirement
of water.
c) The
difference between approach temperature ( difference
between vapour temp. and condenser water tail
pipe temp) does not exceed 5 to 60C.
d) The
Specially designed high efficiency centrifugal spray nozzles fitted on the jet
box create micro fine atomized spray, resulting into wider surface contact
with incoming vapours.
e) The
difference between water outlet temperature and inlet temperature of 10°C is
achieved. Under automation the difference can achieved upto 15°C.
f) Single
entry and Multi jet condensers are of
Parallel flow type wet air condenser. The
Single entry condenser requires less water
than conventional Multijet condenser due to the following
reason.
g) In
the single entry condenser design the spray nozzles are fitted in nozzle box
which fitted at the center of the condenser. Thus the spray water covers
all the available area inside the condenser hence the spray water particles
contact area for the vapour condensation is more than the multi-jet condenser
system. Another impotent parameter in this design tail pipe
diameter and velocity of water in the
tail pipe .
h) The
nozzles dia calculation same as multijet condenser. The velocity of warm water
(outlet water ) goes upto 4 to 5 m/sec. The design of bottom cone, its reducer
and venture were placed important role.
Condenser Water Requirement
In a condenser the vapours entering, transfer
heat to the cold injection water, the heat transmission depending on the
temperature and quantity of water. Based on the heat balance the equation for
arriving at the quantity of injection water is.
Cooling water ratio ( W ) i n Kg of injection water per Kg
of vapour =
I = Total heat of vapour in Kcal/kg (
For 55 oC temperature of vapour the total heat having 621
Kcal/kg)
For saturated online steam table purpose go
through the link Online Steam
Table for Saturated Steam
Ti = temperature of injection water in °C
To = temperature of condenser outlet water in
°C.
According to E.Hugot the optimum value of
this Ti and To difference is 10°C.
The difference in temperatures of vapour
being condensed and the outlet water termed as approach should be minimum since the efficiency of the condenser operation
depends on the fact that minimum quantity of water should absorb all the latent
heat of vapour.
Under normal conditions
the cooling water ratio to vapour condensed in around 50 to 60 for barometric
condensers, 60 to 70 for Multi jet condensers and 40 to 50 for single entry
type condensers.
How to
calculate the number of spray nozzles and jet nozzles to be provide
I want you to explain more about this
h = Pressure of water at spray nozzle in kg/cm2 x head at mean sea level
Can you give an example?
for example
water pressure at spray nozzle = 0.25 kg/cm2
Head at mean sea level = 0.760 m of Hg x 13.6 (Density of mercury) = 10.33 m
then h = 0.25 x 10.33 = 2.6
Please explain to me
For example:
Water inlet temperature: 33 °C, Vapor temperature: 52 °C, temperature of condenser outlet water: 47 °C
So: Ratio W = (619.7-47)/(47-33) = 40.9
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