-
8/11/2019 Calculate Saturated-Gas Loads for Vacuum Systems
1/3
r,
Calculate
Saturated-Cas
l,oads
for
r/acuum
Systems
Use
this
procedure
tu
find
the amount
of
noncondensable
gas
saturated
uitb
condensable gas
being
discbarged.
,r,
by
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sys
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COMMON
PROBLEM
in
designing
vacuum
systems
for chemical
processing applications
is the
calculation
of load
requirements. This
is es-
pecially
true
when dealing
with
ex-
isting
process
equipment. One of
the
key
factors is
an
accurate
deter-
mination
of operating conditions.
The
calculation of load is rela-
tively straightforward. To illustrate
the appropriate
procedure,
we
will
look
at a
representative case:
a
dis-
tillation
column
with
an
overhead
reflux condenser.
(The
reflux con-
denser may 0r may not be
followed
by
a
vent
condenser, see Figure l.)
The operating
parameters
needed
to size the vacuum
system
are
l.
the load
going
to
the vacuum
system
from
the
reflux
or
vent
con-
denser (if
present)
in
mass
flow
terms,
2.
the
pressure of the
load,
and
3. the
temperature of
the
load.
The
load
can be
considered as the
amount of noncondensable
gas
sat-
urated with the condensable gas be-
ing discharged from the
reflux
or
vent
condenser.
First calculate the pressure and
temperature
of
the gas
going
to
the
vacuum
svstem.
In
a
new
system,
this can
be cletermined
from
the
condenser design.
For
an
existing
system,
actual
measurements
can
be
taken.
C)nce
this
information
is
known,
the
nontondensable
gas
load
must
be
EDWARD
B.
MYERSON,
STOI(IS
VACUUM,
INI
calculated.
Unless a
reaction
occurs glands.
The procedure
is
as
follon's:
or some
carrier
gas
is injected into l. Evacuate
the systern to
about
the
system,
the amount of
noncon- 125 mm Hg
abs.
densable
gas
in the process
stram
2.
Isolate the
system
from the
vac-
can be
considered equivalent
to
the
uum
source
and
turn off the
vac-
amount
of air
leakage into
the
uum source.
equipment.
There are
several
meth- 3.
Record
the time
required for
a
ods
of determining
air leakage. change
in
pressure
(the
pressure
They
include
change
must be large enough
to
al-
l. a
rate-of-rise test
to
find
the air low
for
an accurate
time reading);
leakage
of an
existing
system
(1),
do
not
allow
the system pressur
to
2.
measurement
using an
air-leak- rise
above
380 mm
Hg
abs.
age meter
(1),
4.
Given the system
volume
,
the
3.
determination
via standard pressure change, and the elapsed
charts
based on system
size and op- time, calculate the air-leakage
rate
erating
pressure
(1)
or,
(at
70"F') via:
4.
calculations
based
on system
size,
fittings,
openings, and operat- L
--
O.39'lVLPlt
(1)
ing pressure
(2).
The
rate-of-rise
method
yields
a
reasonable
representation
of
the
air
leakage
of
a
system.
The
test,
how-
ever,
can
only
be
done on
a
system
that
can
be taken off
line, emPtied
of
process
fluids, and
then evacu-
atecl
to
a pressure
of
125
mm Hg
abs.
or
less. The test should
be
per-
f
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8/11/2019 Calculate Saturated-Gas Loads for Vacuum Systems
2/3
noncondensable-gas
florv
rate
are
empirical
and.
thus,
not
as
rigorous'
ir-leakage
curves
are
Published
bv
the
Heat
Exchange
lnstitute,
see
Figure
2.
To
use
thse
t'urves.
both
system
volumc
and
operattng
I)res-
sure
data
are
reqtrired.
\\'hcn
using
ordinary
shaft
sels,
add
up
tn 5
lbih
to
allon'
for
additional
leakage'
The
final
method
is
basecl
on
both
system
size
and
the
number
and
sizes
of
flanges,
ports,
etc.
The
pro-
cedure
coniists
of
two
stePs
(2)'
First,
estimate
the
air-leakap;e
rate
according
to
vesset
size
using
the
fol-
lowing
equations:
Lr
=
l'2ArDL.P'26
(for
100
to
l0
tcrr)
be
understood;
the
concePt
states
that
the
total
pressure
is equal
to
the
sum
ofthe
paitial
pressures'
and
the
total
pressure
times
the
mole
frac-
tion
of
a gas
is
equal
to
its
Partial
Dressure.
'
T., .alculate
the
saturation
of
a
noncondensable
with
two
condensa-
bles,
first
calculate
the
partial
pres-
sure
of
the
noncondensable:
NOMENCLATURE
D :
seal
diarneter
(in.);
S
:
flow
of
saturated
vapor
(lb/h);
L
:
air
leakage
into
s1'stem
(lb/h):
L,
-
specific
leak
rate
tlb/h/in.):
Ll,
=
leaLage
rate
ol
fittirrgs.
ralre:'
seals,
etc.,
(lb/h/in
)
M
:
molesl
Mu,
:
molecular
weightl
N
:
low of
noncondensables
(lb/h):
P
:
pressure
(torr,
lnm
Hg
abs
):
=
rinre
(mlll)r
l'
:
sl'stem
vrtlume
(tr):
W
:
mass
flow
(Ib,,,/h)l
avg
=
average
c
=
condensablest
i
=
comportent
ii
n :
n()Ircotrdertsablcs:
p
=
partial:
t
:
t{)tal:
\'
:
\'ap()r
(cottclerrsables
at
tcllPer-
ature
].).
(6)
a
If
Lr
:
1i'DLsPosa
(for
10
to
I
torr)
(7)
The
total
air
leakage
is:
L,:L+[Lr
(8)
Once
the
air-leakage
rate
and non-
conclensable-gas
flow
have
been
de-
termined,
the
saturation
can
be
eas-
ilv
calculated.
For
a
simple
problem
o
air
or
other
dry
gas
saturated
with
Vstuum
Syslem
Its*,
tJ.,*m
ryrlem
lor
dirtillotion
column
wilh
ovetheod
telux
condenser'
L
:
O.l
06pl^,06
(2)
a
single
condensable.gas,
the
satura-
(forTti0tol00torr)tionamountlS(.al(.ulateovla
S
:
'Mzu,P,lMw"(P
-
P')
(9)
L
:
0.072po.o26yoorr
(3)
(for 100
to
l0
torr) When
there
is more
than
one
non-
condensable,
the
average
molecular
f
:
0.026P{'{r34y006
(4)
weight
of
the
noncondensables
(for
r0 ro
I
rorr)
(+)
"Hfii:
*
:1^l:
i::iil:
weight
is
t alculaterl
b1
'fht'
nt'xt
step
is to
determittc
the
air
leakace
frt.'ln
atl
valves,
'ittings,
u',,*
:
ll(14',lLIu/1\t'
,eals,
t..
'I-his
is
done
for
each
item
+
...
+ \4t,lN1trt,lW,)
(10)
usilrg
thc
frrllorn
ing
equati('n\
and
f
inrling
the
sllctific
leakage
rate.
L.,
For
applicati.ns
in'ol'ing
rnore
frorn
-lablc
1 :
than
one'condensable
and
at
least
one
n()n('ontlensablc,
the
technique
L,
:
3.98ri'Dls
(5)
is
sorncwhat
clif{erent'
lt
reqtrires
(for
760
to
100
torr;
that
the
concept
of partial
prejsut'c
Toble
l.
Eslimoles
or
speciic
leok
roles.
ComPonenl
Stotir
Seols
Threoded
(onnedions
(onvenlional
Gosket
Seols
0-tings
Thermolly
(ycled
Goskels
r
100"
Rolary
Seols
Potking
Glonds
lilethonicol
Seols
lsolotion
lolves
Plug
(ock
Bolt
Globe
Gole
Throllling
Yolves
Aaess
Porls
Yiew
Vtlindows
Specillt
leol
tole
(lb/h/in.)
0.015
0.005
0.002
0.005
0.018
0.03?
0.25
0.t0
0.01
0.02
0.02
0.01
0.2s
0.02
0.015
\l\R(
ll
l )l)l
-
8/11/2019 Calculate Saturated-Gas Loads for Vacuum Systems
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ar::-ii,:
.:i
-
.15'
{5
t:. it:
.i;:.il.:,;,,;,;
tigure
2.
ir-leokoge volues
or
commerciolly
lighl
syslems'
P,:
Prn
*
P*r
* P*:
-l-he
partiaf
pressure
of
a
condensrtbLe
when
saturated
is equal
to the
vaPor
pressure
of the
condensable
at
the
saturation
temperature
in a
mixture
of multiple
condensables.
So,
P,",
:
P,*r
(12)
the
n,
Fu,,
-
P,
-
P,,*,
- P,sr
(13)
Next.
calculatc
the
number
of rnolcs
of noncondensablcs:
(
I I
)
Third,
calculte
the
total
number
of
moles
Present:
Pr"lP,:
M,IM,
(15)
M,:
M"(P,lPu,,)
(16)
vent
condenser
to
the
vacuum
sls-
tern
because
both
the air
leakagc
and the saturatcd
\aPor
loads
arc
knon'n.
along
r+ith
thc prcssrrre
arrd
temperature.
To
fi
nish
vacuuln-svs-
tem-sizine
sim pl,v
involves
determin-
ing the
nutnber, tvpe,
and
size
o
lhe
required pumps.
r
LtTERATtTRE
CITEI)
l.
"Starrdarrls
ir
Steanr
.fct
Vat'rtrtttt
Systcnrs."
,1tlr
Etl..
Hcat
Exchange
II'rstitrrtc,
(.lcr
cllrrd
1
I
{)8t11.
2.
Rvans,
J.
L., and
D,
L. RoPer'
"Prrxtss
\';rt'ttunr
Systettt
l)esigtt
ancl
()pcrirtiorr."
l\'lc(irau-Hill^
Ncu
\irrk
(ll)tl(i).
then,
Fourth,
calculate
the
number
of
moles
of
each
condensable
present:
h,ilM,
:
P,;1P,
...
(17)
Finallr,
calt
ulate the
salttrali()ll
am()unt
of each condensable:
l{"'
: t14.75t''"
(18)
'I'his
procedure
applies
regardlcss
o{'the
quantity
of condensables
present.
It
allows
for
the
dett'rmitra-
iion
o'
the
load
frorn the
refltrr
or
lr|,,:
W,,lhLu"
(
l4)